Drive Engineering - Leakage Currents of Frequency Inverters.pdf

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Drive Engineering – Practical Implementation

Leakage Currents of Frequency Inverters – Electrical Safety Standards – Compatibility of Residual Current Devices – Frequency Inverters in IT Systems

Edition 06/2011 19294026 / EN

Introduction

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Drive Engineering – Practical Implementation – Leakage Currents of Frequency Inverters

Introduction This documentation has been drawn up to support the work of the sales teams by answering general ques-tions on the technology and project planning phase of products.

A selection of applicable regulations is considered and general recommendations are provided based on existing experiences.

In special applications, additional or different requirements may apply.

Please do not hesitate to contact the author if you have any questions or suggestions.

Frank Glasstetter SEW-EURODRIVE GmbH & Co KG Ernst-Blickle-Straße 42 76646 Bruchsal P.O. box: Postfach 3023 • 76642 Bruchsal

Phone +49 7251 75-2322 Fax +49 7251 75-502322 http://www.sew-eurodrive.com [email protected]

This document has been prepared on the basis of the current status of knowledge. Consequently, subse-quent discoveries may lead to different statements. As a result, the possibility of misinterpretations ormistakes in the technical data cannot be ruled out.

Table of Contents

Table of Contents

1. Introduction ............................................................................................................................................ 5

2. Leakage Currents .................................................................................................................................. 6

2.1. What types of leakage current can occur? .............................................................................. 7 2.2. Leakage currents form two circuits ........................................................................................ 10 2.3. Modern frequency inverters can generate higher leakage currents ...................................... 11 2.4. Frequency range of the leakage current of a frequency inverter ........................................... 12 2.5. Measuring leakage currents ................................................................................................... 13

3. Electrical Safety in Case of Increased Leakage Currents ............................................................... 14

3.1. Protection against dangerous shock currents in connection with leakage currents .............. 14 3.2. Mounting plate as second PE conductor ............................................................................... 15 3.3. EMC ground strap as second PE conductor .......................................................................... 15 3.4. Second PE conductor for decentralized products in the food industry .................................. 16

4. Additional Protective Measures ......................................................................................................... 19

4.1. Operator protection with RCD ................................................................................................ 19 4.2. Damp and wet areas .............................................................................................................. 23 4.3. Protection against fire ............................................................................................................ 25 4.4. Plant protection ...................................................................................................................... 25 4.5. Overview of necessary protective measures ......................................................................... 26 4.6. Availability and costs of additional protective measures ....................................................... 26

5. Residual Current Devices ................................................................................................................... 27

5.1. Availability of systems with RCDs .......................................................................................... 27 5.2. RCDs in TN systems .............................................................................................................. 27 5.3. Time-delayed RCDs ............................................................................................................... 29 5.4. Trip characteristics of an RCD ............................................................................................... 30 5.5. Selective RCDs; DIN VDE 0100 part 530; from 2005-06 ...................................................... 32

6. IT Systems ............................................................................................................................................ 33

6.1. DIN VDE 0100 part 410; "Protection against electric shocks"; from 2007-06 ....................... 33 6.2. Insulation monitor ................................................................................................................... 34 6.3. Insulation resistance of a plant .............................................................................................. 35 6.4. SEW frequency inverters in "IT design" ................................................................................. 36 6.5. EMC in an IT system .............................................................................................................. 38

7. Inverters in "IT Design" in a TN System with RCD .......................................................................... 39

7.1. Control cabinet inverters in "IT design" on an RCD ............................................................... 39 7.2. Decentralized frequency inverters in "IT design" on an RCD ................................................ 40

Drive Engineering – Practical Implementation – Leakage Currents of Frequency Inverters 3

Contents

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8. Measures Against False Tripping of an RCD ................................................................................... 41

8.1. Data inquiry for customer consulting ..................................................................................... 41 8.2. Measures to prevent false tripping when the inverter is connected to the supply

system .................................................................................................................................... 42 8.3. Measures to prevent false tripping during operation ............................................................. 42 8.4. Measures to prevent false tripping when the inverter is disconnected from the supply

system .................................................................................................................................... 46 8.5. Operating principle of a dual-winding transformer ................................................................. 46 8.6. Compensation of 150 Hz leakage currents using a LEAKCOMP® filter ................................ 48 8.7. Applications with optimized leakage currents ........................................................................ 50 8.8. Values gained from practical experience ............................................................................... 51 8.9. Summary of monitoring and protection devices ..................................................................... 51

Contact ........................................................................................................................................................... 52

Introduction

1. Introduction Leakage currents are playing an ever increasing role today.

On the one hand, the number of units that produce leakage currents is increasing (units with switched-mode power supply). On the other hand, more and more insurance com-panies (fire protection) and institutions for statutory accident insurance and prevention (occupational safety) are pushing for the use of residual current devices (RCD), which causes many problems to plant manufacturers.

RCDs are recommended only for very few industrial applications, because the availability of a plant decreases with the installation of RCDs. In most cases, the plant manufacturer has other options to increase the electrical safety of a plant without compromising the plant availability. In this regard, the customer should be made aware of the applicable standards (see chapter 4).

In Germany, it is not difficult to suggest another solution to the customer on the basis of the relevant standards. In Spain, however, institutions for statutory accident insurance and prevention have a strong influence on the customers, which is why it is difficult to dissuade these customers from using RCDs. For this reason, it should be clarified for each project that will be implemented in Spain whether the supply of the plant is equipped with an RCD.

If it is known in advance that the use of an RCD is mandatory, the plant must be planned with reduced leakage currents (see chapter 8.7). Subsequent conversions at the custom-er site are very time-consuming and expensive, and they leave a bad impression.

In case of modernizations, note that modern frequency inverters can produce a higher leakage current than older frequency inverters (see chapter 2.3). While older frequency inverters could be operated on an RCD without false tripping, it might not be possible to operate the same number of modern frequency inverters on this RCD.


Leakage Currents

2. Leakage Currents A controlled drive system consisting of a frequency inverter, a motor and a motor cable generates cable-conducted, high- and low-frequency interferences. EMC measures re-duce these interferences by partly dissipating them as leakage currents to earth.

Although leakage currents can be measured, it is not possible to specify fixed values for certain frequency inverters. This is because the level of the leakage currents also de-pends on the entire plant structure.

The leakage currents of a plant change continuously as the operating conditions of the plant and the supply system conditions change, e.g. subject to utilization and unbalanced loads.

The level of the leakage currents depends on:

• Supply system quality (load unbalance) • EMC measures (filters, chokes) • Number and power rating of the frequency inverters • Pulse frequency • Cables (length, unshielded, shielded, low-capacitive)

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Leakage Currents

2.1. What types of leakage current can occur? The following figure shows the different leakage currents that occur in a system with a frequency inverter:

Types of leakage currents in a system

2.1.1. Stationary leakage currents at the EMC filter

An EMC filter consists of a choke and capacitors that are connected to ground. In case of an ideal, balanced supply system with a sine-wave voltage, the leakage currents to ground are 0. However, this is never the case in practice.

Due to supply voltage distortions, there is a capacitive overall current that is constantly drained off via the protective earth conductor, hence the name stationary leakage current.

The higher the supply voltage distortion,

the higher the stationary leakage currents!

The stationary leakage current is present even when the frequency inverter is in "control-ler inhibit" status; it has frequency components of 50 Hz to 1 kHz and can have an ampli-tude of several hundred mA.

Simple and cheap EMC filters with low inductances and large capacitors cause high lea-kage currents and may accidentally trip the RCD.

Never connect a single-phase consumer to the output of a three-phase EMC filter (filter without neutral conductor connection). The unbalanced load on the filter further increases the leakage currents, which significantly reduces the filter effect.


Leakage Currents

2.1.2. Single-phase frequency inverters

This is why, if using several single-phase frequency inverters, you must make sure to connect them to the 3 phases as evenly as possible in order to prevent the RCD from tripping.

Single-phase frequency inverters generate higher leakage currents in a three-phase supply system than three-phase frequency inverters.

When using single-phase frequency inverters on a three-phase supply system, you should install a 4-conductor filter (3 phases + neutral) as a collective line filter.

2.1.3. Leakage currents during operation

If a motor is operated on a frequency inverter, there are additional frequency components above 1 kHz in the overall leakage current.

A long motor cable with grounded shielding acts like a capacitor that is connected to ground and drains off leakage currents accordingly (about 0.2 mA – 1 mA per meter of the cable). In addition, leakage currents are dissipated to ground via the motor capacity.

Motor cable

The higher the pulse frequency, the higher the leakage currents.

In conjunction with universal current-sensitive RCDs, you should choose a pulse frequen-cy greater than 4 kHz as the sensitivity of most universal current-sensitive RCDs signifi-cantly decreases above 4 kHz. Observe the trigger frequency curve of the RCD (see chapter 5.4).

Pulse frequen-cy

In unfavorable cases, there can be resonances between the line filter and the motor cable that can multiply the leakage current. This is especially likely in a combination of line fil-ters with low leakage current, IT frequency inverters (without Y capacitors) and a pulse frequency of 4 kHz. In systems with reduced leakage currents, it is generally recom-mended to increase the pulse frequency to 8 kHz.

Resonance

At creep speed, e.g. 10 Hz, the total leakage current increases significantly. The lowest leakage currents are achieved in the medium control range of the inverter (40 – 60 Hz).

Speed / creep speed

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Leakage Currents

2.1.4. Dynamic leakage currents during power-on/power-off

An RCD trip is most likely in the moment an inverter is connected to the supply system. At this point, the large X capacitors in the DC link are charged. This means that very high currents flow for a short time, which also cause leakage currents via the Y capacitors of the DC link to ground (see following block diagram).

Inverter

When connecting or disconnecting the supply voltage with switches without snap action mechanism, the phases are not switched simultaneously. This causes an unbalance in the supply voltage. As long as not all three phases are energized, an increased leakage current flows to ground via the filter capacitors of the already connected lines.

Switching con-tacts

A system should not be disconnected from the supply system under load, because this amplifies the effect described above. This also causes large contact-breaking sparks at the switching contacts, which generate more high leakage currents.

These dynamic leakage currents can have several amperes and trip RCDs without delay with 300 mA.

A short-time-delayed RCD (10 ms) is largely unaffected by these dynamic leakage cur-rents as they have usually decayed after 10 ms (see chapter 5.3).

Solution

The leakage current of a system is the result of many individual, interdependent factors, which cannot be determined in advance.

Conclusion


Leakage Currents

2.2. Leakage currents form two circuits During inverter operation, leakage currents flow from the inverter output via the motor cable and via the motor; they should completely flow back to the inverter, if possible. Leakage currents in this circuit are not detected by the RCD. Good low-resistance groun-ding is extremely important to prevent the leakage currents from taking another path. An additional protective earth connection (PE) between the motor and the inverter may be useful.

Another circuit is formed between the supply system and the inverter input. Leakage cur-rents flowing here towards the supply system will trip the RCD.

The inductance of a line filter works against the leakage current in the kHz range and feeds part of it back to the inverter via the Y capacitor.

Leakage currents form two circuits

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Leakage Currents

2.3. Modern frequency inverters can generate higher leakage currents Modern frequency inverters are equipped with faster switching and more powerful IGBTs. As shown in the illustration below, a faster switching IGBT causes a steeper voltage rise. This reduces the power losses of the IGBT, resulting in less heating. This is why modern IGBTs can carry more load, which means that they can switch more power while retaining the same size.

A steeper voltage rise causes more electromagnetic interference, which must be reduced by more complex EMC measures (filters). This is why newer, smaller frequency inverters produce higher leakage currents than older, larger inverters.

If you want to modernize a plant which is protected via RCDs, you must take into account that the new, smaller inverters generate a higher leakage current while providing the same power output as the old ones.

The risk is that the RCD, which could be operated with the older inverters without any false tripping, can no longer be operated with the same number of modern frequency inverters.

This could require a change in the installation regarding the leakage currents.

• Fewer power losses => less heating => more power provided by the same frame size!

• More electromagnetic interference => more complex EMC measures => higher lea-kage currents

• Modern frequency inverters => higher leakage currents!


Leakage Currents

2.4. Frequency range of the leakage current of a frequency inverter The leakage current of a frequency inverter is made up of the following frequencies.

Frequency range of the leakage current

50 Hz on the supply system end of the frequency inverter

50 Hz leakage currents can usually be neglected, as they are very low.

They flow continuously depending on the supply system quality.

150 Hz from the DC link

150 Hz leakage currents are extremely high for a short time the moment the supply sys-tem is connected.

They also flow during operation.

kHz range at the output of the frequency inverter

• The higher the load, the higher the high-frequency leakage currents • Depends on the pulse frequency (8 kHz) • Increases with long, shielded cables • The motor winding also dissipates leakage currents with the selected pulse frequency.

The 150 Hz leakage currents from the DC link of the inverter are the main problem for the RCD.

Practice

With MOVIDRIVE® B or MOVITRAC® B inverters in IT design (without Y capacitors in the DC link), the 150 Hz problem does not occur (see chapter 7).

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Leakage Currents

2.5. Measuring leakage currents A simple measurement of the effective leakage current is not very conclusive; a mea-surement using an oscilloscope or a leakage current analysis using a LEAKWATCH® system is recommended instead.

2.5.1. Leakage current measurement using an oscilloscope

The three supply phases and the neutral conductor (if present) are routed through a cur-rent clamp in order to measure the total residual current flowing to ground.

Measuring the leakage current at the PE conductor of the supply does not measure the total leakage current as leakage current can also be dissipated through another path.

However, the leakage current measurement of an RCD cannot be compared to an oscil-loscope measurement. The measurement of an RCD is much slower. Current peaks or high-frequency leakage currents above 4 kHz are less significant.

2.5.2. Analysis of leakage currents

In a leakage current analysis, the leakage currents of the different frequencies must be considered. This shows where the leakage currents come from, and what causes them. The result helps you to select a suitable countermeasure. A correct analysis of the fre-quency ranges is the prerequisite for a perfect reduction of the leakage currents. The LEAKWATCH® system from the company EPA allows for a precise leakage current anal-ysis.

Countermeasures to reduce leakage currents:

• 50 Hz ND line choke, NF line filter with low leakage current • 150 Hz IT design, LEAKCOMP® by EPA • kHz range NF line filter, HD output choke, unshielded cable

The tripping curves of all commercially available RCDs are stored in the LEAKWATCH® system. It also shows the utilization of the selected RCD in percent during the leakage current analysis.

Leakage current analysis with LEAKWATCH®


Electrical Safety in Case of Increased Leakage Currents

3. Electrical Safety in Case of Increased Leakage Currents As explained in chapter 2, increased leakage currents caused by frequency inverters cannot be avoided. This is why, according to EN 61800-5-1, the operating instructions suggest using a second PE conductor (double safety), or a 10 mm² PE conductor (me-chanically fixed).

The following excerpts do not contain the full text from the standards. For detailed infor-mation, refer to the applicable edition of the respective standards.

3.1. Protection against dangerous shock currents in connection with leakage currents

Poor or interrupted grounding can result in high, fatal leakage currents flowing to ground via a person. The following standards deal with this issue:

3.1.1. Product standard for frequency inverters; DIN EN 61800-5-1; VDE 0160-105-1; from 2008-04

At least one of the following measures shall be applied unless the leakage current can be shown to be less than AC 3.5 mA or DC 10 mA.

Paragraph 4.3.5.5.2

• PE conductor at least 10 mm² Cu or 16 mm² AL (mechanically fixed according to the standard).

• Automatic cut-off of the power supply when PE is interrupted. For example: PE conductor monitoring unit "PECON+" from EPA. This unit only moni-tors the supply ahead of the monitoring unit. The line after the unit is not monitored.

• Second PE conductor in parallel via separate terminals (double safety). If there is no second PE terminal at the input,

− the second PE may be connected to the shielding plate at the output. − You may also use one of the 4 mounting screws of the inverter as a PE connection

screw. In this case, you have to provide for a grounding symbol on the grounded mounting plate.

• Connection with an industrial connector according to IEC 60309 (CEE connector) and a minimum PE conductor cross section of 2.5 mm² as a part of a multi-conductor power cable. Strain relief must be provided on both ends. (An RCD is not mandatory here)

In order to comply with the standard requirements regarding increased leakage currents, the safety of the PE conductor must be increased. This can be achieved by:

Conclusion

• A second PE conductor (double safety) • A 10 mm² PE conductor (mechanically fixed) • A connection with a plug connector for industrial applications (CEE connector)

with a minimum cross section of 2.5 mm².

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3.2. Mounting plate as second PE conductor IEC 60364-5-54; DIN VDE 0100-540;

Protective conductors and protective bonding conductors; 2007-06

If the system includes housings of low-voltage switchgear combinations (→ control cabinet) or metal-enclosed conductor rail systems, the housings or structural parts (→ mounting plate) made of metal can be used as protective conductors, provided they meet all of the following three requirements:

Paragraph 543.2.2

The electrical continuity of the connection must be ensured by the design or by appropri-ate connections to protect the connection from deteriorating due to mechanical, chemical, or electromechanical influences.

a)

They meet the requirements of paragraph 543.1 (minimum cross section). b)

It must be possible to connect other protective conductors at each intended point of con-nection.

c)

In practice, the grounded, galvanized mounting plate is also used as PE (minimum cross section for grounding the mounting plate with 10 mm²) or as second PE within the control cabinet. A mounting plate can be integrated in the protective measures, because the risk of the mounting plate breaking can be excluded.

Practice

Frequency inverters with a back panel made of metal must be bolted together with a galvanized mounting plate over a large surface for EMC reasons.

In practice, this connection is often used as a second PE connection.

3.3. EMC ground strap as second PE conductor According to EN 60204-1, ground straps (HF litz wire) can also be used as PE conduc-tors when the connection points are marked with the ground symbol.

3.3.1. EN 60204-1; chapter 13.2.2; 2006

Where the PE conductor can be easily identified by its shape, position, or construction (e.g. a braided conductor, uninsulated stranded conductor), color coding throughout its length is not necessary, but the ends or accessible locations (connection points) shall be clearly identified by the grounding symbol or by the bicolor combination green-and-yellow.

In practice, a ground strap (HF litz wire) is often used as a second PE conductor. Practice



3.4. Second PE conductor for decentralized products in the food industry According to DIN EN 61800-5-1 (product standard for frequency inverters) and DIN EN 60204-1 (electrical equipment of machines), increased leakage currents require a PE conductor of 10 mm² or a second PE conductor.

MOVIFIT® FC or the SNI master and MOVIGEAR® have two separate PE terminals inside and outside the housing. This allows plant manufacturers to connect the PE conductors stipulated by DIN EN 61800-5-1.

Example for MOVIFIT® FC

[1] Conductive connection over a large area between MOVIFIT® and the mounting rail

[2] PE conductor in the supply cable

[3] Second PE conductor via separate terminals (double safety for leakage currents > 3.5 mA according to EN 61800-5-1)

[4] EMC-compliant equipotential bonding via ground strap (HF litz wire), if necessary.

However, in the food industry, the connection of a second PE conductor poses some problems (moisture, bacteria, corrosion, bad connection).

SEW-EURODRIVE offers the following option for such cases.

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3.4.1. Connection with a prefabricated plug connector for industrial applications.

MOVIGEAR® can be connected without a second PE conductor with a prefabricated plug connector from SEW-EURODRIVE.

MOVIGEAR® is available with a M23 plug connector for industrial applications in line with EN 60309 (identical with IEC60309). In connection with a 2.5 mm ² cable, a second PE conductor is not necessary according to DIN EN 61800-5-1 4.3.5.5.2.

[1] MOVIGEAR® with M23 circular connector, straight

[2] MOVIGEAR® with M23 circular connector, angular

3.4.2. Second PE of external 24 V supply

The PE conductor of an external 24 V supply can be used as second PE connection if the cross section is the same as that of the power supply.

If, for example, a MOVIFIT® FC requires an external 24 V supply, the PE conductor of this 3-core cable can be used as the second PE connection. This second PE connection should then be marked as such.



3.4.3. Machine structure as second PE

According to DIN EN 60204-1, 8-2-1 from 2007 (electrical equipment of machines), cus-tomers can also use the conductive machine structure as PE system or as second PE connection. Comply with the following prerequisites (section 8.2.3):

• A direct PE connection between the ground bus in the control cabinet and the conduc-tive machine structure must be ensured.

• With respect to EMC, an additional HF-capable connection between the ground bus or the metal back panel of the control cabinet and the machine structure is necessary.

• If a part is removed for any reason (e.g. unit replacement), the PE system must not be interrupted.

• The continuity of every PE system of a machine must be tested (test 1) according to section 18.2.2 and monitored. Low junction resistance due to correct, paint-free mounting over a large area is necessary to prevent that the measurement value of the loop impedance measurement (test 2) is exceeded.

• Metallic cable ducts may not be used as PE conductors for reasons of electric safety.

• With respect to EMC, a low-resistance connection as equipotential bonding between the control cabinet, metallic cable duct and motors is beneficial, because a cable duct is always in parallel with the installed cables and is easy to check for interruptions.

Example: Grounding concept

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Additional Protective Measures

4. Additional Protective Measures

4.1. Operator protection with RCD If a customer demands a RCD, you should clarify in advance why the customer needs it.

Operator protection, fire protection, or plant protection.

If an RCD is not stipulated by a standard, it is not recommended to use one, be-cause this would decrease the availability of a plant.

The following summary of the most important, latest standards regarding this issue is to assist you with your assessment.

4.1.1. Protection against electric shocks; IEC 60363-4-41; DIN VDE 0100 part 410; ; from 2007-06

RCDs are required as "third safety element" for extraordinary hazards.

The concept of "protective measures against dangerous shock currents" is based on the principle of dual safety. A third safety level is required for extraordinary hazards.

A difference is made between:

1. Protection against direct contact (basic protection) 2. Protection in the event of indirect contact (fault protection) 3. Protection in the event of direct contact (additional protection) = fault current

30 mA

Additional protection is only permitted as an addition to basic protection.

Additional protection is obligatory for extraordinary hazards.

Corresponding specifications in VDE 0100 group 700.

For example: Training rooms with test stands, exhibitions, shows, trade fairs, plants in the open or in wet areas, etc.

Additional protection with RCDs with a rated residual operating current of 30 mA as additional protection in case basic and/or fault protection fails.

Paragraph 415.1.1

Additional protection provided by supplementary protective equipotential bonding if fault protection fails.

Paragraph 415.2 note 1



Overview of the concept of "protective measures against electric shocks"

Protection against electric shocks

1. Basic protection (insulation) Protection against direct contact

2. Fault protection (fuse) Protection in the event of indirect contact

In a fault-free system In case of the first fault, when basic protection fails

3. Additional protection as third safety element Protection in the event of direct contact

Additional protection provided by RCDs, as supplementary protection if basic and/or fault protection fail.

Additional protection provided by supplementary protective equipotential bonding, as additional protection in case fault protection fails.

"Additional protection" with RCDs must be provided for: Paragraph 411.3.3

• Socket outlets with a rated current not exceeding 20 A that are intended for general use by ordinary persons. Exemptions are:

− Socket outlets for use under the continuous supervision of skilled or instructed persons, e.g. in some commercial or industrial locations.

− Socket outlets provided for connection of a particular item of equipment.

− Recommendation: Unique plug or fixed connection.

• Only for Spain and Ireland: From the international standard IEC 60364-4-41 Socket outlets with a rated current not exceeding 32 A that are intended for use by ordinary persons.

• Final circuits for mobile equipment with a current rating not exceeding 32 A (no excep-tions) for use outdoors.

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Paragraph 411.1

Residual current monitors (RCMs):

Residual current monitors (RCMs) are not protection devices with regard to operator pro-tection.

Note 2

However, they may be used to measure the residual currents in electrical plants.

RCMs may be used as protection devices in agricultural and horticultural organizations (VDE 0100 part 705).

Exception:

4.1.2. Exhibitions, shows, trade fairs, and stands; DIN VDE 0100 part 711; from 2003-11

Cables supplying temporary structures should be protected at their origin by an S-type, 300 mA RCD to ensure discrimination regarding the RCDs protecting the final circuits.

Paragraph 711.48

The recommendation to use a 300 mA RCD is based on the increased risk due to cables and lines.

Comment

All socket outlet circuits up to 32 A and all final circuits apart from those for emergency lighting must be protected by a 30 mA RCD.

Paragraph 711.481.3.1.4

In practice, the 230 V stand supply (lights, socket outlets) is equipped with a 30 mA RCD. Practice

In agreement with the trade fair electricians, you can usually request a second 400 V machine supply without RCD for the operation of a machine on a frequency inverter. In this case, a minimum protective equipotential bonding of 10 mm² is necessary.

It is rare that a machine supply without RCD is not accepted.

The only option in this case is to supply the machine

• via a dual-winding transformer (see chapter 8.4) • or a LEAKCOMP® leakage current compensation unit (see chapter 8.6).

Neither of these two is provided by the trade show company.



4.1.3. Classrooms with experimental equipment; DIN VDE 0100 part 723; from 2005-06

This standard applies to electrical installations in classrooms with equipment designed for experiments with dangerous voltages.

Paragraph 723.1.11

It does not apply to classrooms with experimental installations in which only such elec-trical equipment is installed which is fully protected against direct and indirect contact.

Protection against direct contact

Additional protection via RCD: Paragraph 723.412.5 If a TN or a TT system is used for supplying experimental equipment, a 30 mA, type A

RCCB must be installed.

Protection in the event of indirect contact

"Additional equipotential bonding": Paragraph 723.413.1.2.2 Any touchable, conductive, external parts in classrooms with experimental equipment

must be interconnected via equipotential bonding conductors and connected to the PE conductor of the power supply. The cross section must be at least 4 mm² Cu.

4.1.4. Rooms containing a bath tub or shower; DIN VDE 0100 part 701; from 2008

as well as swimming pools and other basins; DIN VDE 0100 part 702; from 2003

• Equipment with appropriate IP degree of protection

• Additional equipotential bonding

• One of the following protective measures

− 30 mA RCD − Protective separation for just one piece of equipment − Low voltage

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4.2. Damp and wet areas

4.2.1. Damp and wet areas and rooms; DIN VDE 0100 part 737; from 2002

In damp and wet areas and rooms, electrical equipment must at least be drip-proof. Paragraph 5.1

Drip-proof → degree of protection IPX1 according to DIN EN 60529 (VDE 0470 part 1).

In areas and rooms in which water jets are used and water is usually not directly pro-jected against electrical equipment for cleaning purposes, the equipment must at least be protected against splashing water, → degree of protection IPX4.

Paragraph 5.2

The protection provided by the degree of protection IPX5 (→ water jets) does not allow for the equipment to be cleaned with pressurized water, e.g. hosing down the equipment or using a high-pressure cleaner on it.

Comment

High-pressure/steam cleaning is defined as IPX9K in DIN 40050 part 9.

MOVIFIT® in Hygienicplus design and SEW gearmotors in ASEPTIC design meet the requirements of degree of protection IP69K to DIN 40050-9 as standard.

SEW solution

MOVIFIT® Hygienicplus

This means that MOVIFIT® Hygienicplus units in connection with SEW gearmotors in ASEPTIC design are ideally suited for areas that are common in the food and beverage industries.



Professional and careful installation is essential in damp and wet areas. Installation in-structions, such as cable routing with drip loops, must be observed.

Installation

Approved cable routing with drip loop Non-approved cable routing

4.2.2. Outdoor installations; DIN VDE 0100 part 737; from 2002

In protected installations in the open, equipment must at least be drip-proof (IP X1). Paragraph 6.1

In exposed installations in the open, equipment must at least be protected against spraying water (degree of protection IP X3).

Paragraph 6.2

4.2.3. Comment taken from the VDE publication "67B"; chapter 4.8.1; first edition 2003

Regarding: "Damp and wet areas and rooms"

In contrast to DIN VDE 0100-701 (rooms with bath tubs or showers) or DIN VDE 0100-702 (swimming pools and other basins), no special IP rating is stipulated for damp and wet areas and rooms for protection against electric shock.

Additional protection with 30 mA RCDs is not mandatory for "damp and wet areas and rooms".

This is based on the assumption that a person in such a room usually does not handle electrical equipment without clothes on and with moist skin, in contrast with a person in a bathroom. However, if this is expected, we recommend implementing protection against electric shocks analogously in accordance with DIN VDE 0100-701 and 702.

In practice, "additional equipotential bonding" is often implemented on a voluntary basis in damp and wet areas.

Practice

• This increases the safety. • The availability of the system is not reduced.

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4.3. Protection against fire

4.3.1. Fire protection equipment...; DIN VDE 0100 part 530; from 2005-06

Measures for precautionary fire protection and for limiting the effects of a potential fire include:


• Residual current devices (RCDs) • Residual current monitors (RCMs) • Fire alarm systems • Fire extinguishing systems • Cables and lines with special behavior in case of a fire

according to DIN VDE 0100-482.

If using a residual current device, you should use a type B RCD with a trip current of 300 mA.

Paragraph 532.2

If these RCDs cannot be used (e.g. operating current too high), it is also possible to use RCMs in connection with a switching device with disconnection function.

Paragraph 532.3

4.3.2. RCD type B+ for preventive fire protection

For heightened fire protection, the prestandard DIN VDE 0664-110 from 2009 defines a trigger current of 420 mA and disconnection times up to 20 kHz.

4.4. Plant protection

4.4.1. Protection against electric shocks; DIN VDE 0100 part 410; from 2007-06

RCMs are not protective devices, but they may be used to monitor residual currents in electrical installations. RCMs produce an audible and visual signal when a preselected value of residual current is exceeded.


RCMs monitor residual currents in electrical installations, show the actual value and indi-cate when parameterized trip values are exceeded.

Explanation

RCMs can be used for preventive maintenance in order to protect the plant.

• To protect the plant in the event of a fault, RCMs generate an according signal to be evaluated by qualified personnel.

• The system is not disconnected.

RCMs increase the availability of a plant as creeping insulation faults are detected at an early stage. They are ideally suited for plant protection.

Conclusion



4.5. Overview of necessary protective measures

Overview of necessary protective measures

4.6. Availability and costs of additional protective measures

Additional equipotential bonding

• The availability of the system is not affected by this. • This is a low cost option.

Residual current device

• Reduces the availability of the system, see chapter 5.1. • RCDs that are suitable for frequency inverter operation are expensive. Often, expen-

sive options must be installed to reduce leakage currents.

Residual current monitor

• RCMs increase the availability of a system, as creeping insulation faults are detected at an early stage (highly recommended for plant protection).

• Only permitted for operator protection in agricultural and horticultural companies. • RCMs are expensive.

Protective measure Availability Costs

Additional equipotential bonding Consistent availability Low costs

Residual current device Reduced availability High costs

Residual current monitor Increased availability High costs

Summary

26


Residual Current Devices

5. Residual Current Devices

5.1. Availability of systems with RCDs

• An RCD can trip even during normal operation of frequency inverters, because it can-not distinguish operational leakage currents from an inverter and fault currents. This is why increasing the plant and operator safety by using an RCD will inevitably reduce the availability of the plant.

• RCDs are limited to a rated current of 125 A in the product standards, which is why most manufacturers offer them only up to 125 A.

• However, in systems with frequency inverters of this size, leakage currents of several amperes are flowing. This is why it makes no sense, from a technological and eco-nomical point of view, to use RCDs in systems of this size.

5.2. RCDs in TN systems The RCD can only take over a protective function when the PE conductor and the neutral conductor are routed separately upstream from the RCD.

RCD in a TN system



5.2.1. Electronic equipment for use in power installations; DIN EN 50178; VDE 0160; from 1998-04

An RCD can be used for additional protection if the leakage current is low enough so that the RCD is not triggered unintentionally.

Paragraph 5.2.11.2

Type B for three-phase frequency inverters

If smooth residual direct currents are expected in a system, only universal current-

sensitive RCDs of type B are permitted.

Smooth residual direct currents have no connection with neutral. They occur in connec-tion with three-phase frequency inverters with a six-pulse bridge.

Six-pulse bridge circuit

iL = Load current

iF = Residual current

A DC component in the residual current can significantly reduce the sensitivity of a type A RCD.

Universal current-sensitive RCDs of type B trip safely even in case of non-pulsating direct current.

Type A for single-phase frequency inverters

For single-phase frequency inverters, you can use RCDs of type A, because smooth re-sidual direct currents do not occur here.

Two-pulse bridge circuit

iL = Load current

iF = Residual current

For three-phase frequency inverters, only universal current-sensitive RCDs of type "B" may be used.

Conclusion

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5.3. Time-delayed RCDs

Universal current-sensitive (type B) RCD with short time delay (K)

When a frequency inverter is connected to the supply system, the large DC link capaci-tors are charged. At this moment, high leakage currents flow to ground. To prevent false tripping, it is highly recommended to use an RCD with time-delay.

RCDs with short-time delay K must not trip in the event of a residual current pulse of 10 ms. The maximally permitted break time of 300 ms for operator protection is ob-served. These devices are used for electrical consumers that briefly generate high lea-kage currents when they are switched on due to suppression capacitors.

Short-time de-lay

Selective S or time delayed RCDs must not trip in the event of a residual current pulse of 40 ms. The maximally permitted break time of 500 ms for fire protection is observed.

Time-delayed

According to the standard, time-delayed RCDs are only available for currents higher than 30 mA (i.e. 300 mA, 500 mA, etc.). They are not permitted for operator protection.

There are adjustable RCDs that allow you to set the trip time and the rated residual cur-rent. However, they are not considered an additional protective measure, but merely an optional plant protection element.

Adjustable

In connection with a frequency inverter, you should only use an RCD with a time delay or short-time delay.

Conclusion



5.4. Trip characteristics of an RCD

• RCDs are subject to a certain tolerance. According to the standard, they are permitted to trip when 50% of the rated residual operating current is reached (e.g. 30 mA => 15 mA).

• In practice, an RCD often trips at an rms residual current of 25 mA.

• RCDs detect leakage currents of different frequencies with varying sensitivity. An RCD is highly sensitive to leakage currents of a low frequency, e.g. 150 Hz, whereas lea-kage currents in the kHz range can be much higher.

• The cardiologic effect (ventricular fibrillation) is much more dangerous in case of low-frequency leakage currents than with high-frequency leakage currents. Added to this are the thermal and electrochemical effects of a current on the human body.

The following diagram shows the hazard limit values for RCDs of type B in relation to the frequency [Hz] of the current [mA].

Hazard limit values for RCDs of type B

The limits for the trip values are defined in the equipment standards DIN VDE 0664-100 and DIN VDE 0664-200 (RCCBs, RCBOs) up to a frequency of 2 kHz.

For fire protection, 300 mA is the general limit for all frequency ranges.

Some RCDs were specifically developed for operation with frequency inverters. They follow the trip curves described in the standards, which means they have a higher trip value at higher frequencies.

30



Example: Trip level of the RCDs "B NK" and "B SK" from the company Doepke.

Trip level B NK B SK

0 – 100 Hz Rated residual current 30 mA Rated residual current 30 mA

100 – 1000 Hz Rated residual current 300 mA Rated residual current 300 mA

1 kHz – 1 MHz 300 mA 2 A

Scope of protection Operator protection and fire protection Operator protection

5.4.1. Leakage current analysis using a MOVIAXIS® inverter as an example

• This leakage current measurement shows that the leakage currents at 150 Hz are close to the red trip curve (top left).

• In contrast, the leakage currents of the selected clock frequency of 4 kHz of this appli-cation are no problem at all for the RCD.

To reduce the risk of false tripping, it is essential to select the RCD in accordance with the trip curves.

Conclusion:

Low-frequency leakage currents will trip a frequency inverter faster.



5.5. Selective RCDs; DIN VDE 0100 part 530; from 2005-06 Usually, RCDs trip without delay. That means you cannot connect such devices in series in order to achieve a selective disconnection. In order to achieve discrimination, the units connected in series must be scaled with respect to trip time as well as to the rated residual current.

Paragraph 535.2.2

A selective RCD must only be used with a universal current-sensitive RCD if the selective RCD is universal current-sensitive itself.

Selective or time delayed RCDs must not trip in the event of a residual current pulse of 40 ms. The maximum break time is 500 ms.

Selective RCD

32


IT Systems

6. IT Systems Automatic cut-off protection in IT systems is based on a power source insulated to ground. This means no dangerous current can flow via a person in the event of the first fault (fault to frame or ground fault).

The advantage of an IT system is the supply guarantee as the first fault does not have to trigger a cut-off, but merely a fault message. If the first fault is corrected before a second fault occurs, there should be no downtime.

6.1. DIN VDE 0100 part 410; "Protection against electric shocks"; from 2007-06

6.1.1. Monitoring and protection devices

The following monitoring and protection devices may be used in IT systems: Paragraph 411.6.3

• Insulation monitoring devices (IMD) • Residual current monitors (RCM) • Insulation fault detection devices • Overcurrent protection devices • Residual current devices (RCD)

In Germany, an insulation monitoring device is obligatory for IT systems. Paragraph 411.6.3.1

As each IT system requires an insulation monitoring device, a residual current monitor is not required.

Paragraph 411.6.3.2

6.1.2. RCDs in IT systems:

If faults occur in two different pieces of equipment in different line conductors, a cut-off triggered by an RCD is only assured if each consumer has its own RCD.

Comment

If an RCD is used, a cut-off triggered by the RCD due to capacitive leakage currents can-not be ruled out when a first fault occurs.

Comment

Although RCDs are permitted as additional protection devices for IT systems, they are not recommended for the reason stated above.

Conclusion


IT Systems

6.2. Insulation monitor

• For frequency inverter operation, SEW recommends a "pulse-code modulated insu-lation monitor" in connection with SEW frequency inverters in "IT design".

• Even when using frequency inverters in "IT design", leakage capacitances and DC voltage components are still present, which can cause false tripping in a basic insula-tion monitor.

6.2.1. Selection and erection of electrical equipment; DIN VDE 0100-530; from 2005-06

Insulation monitoring devices (IMDs) must be able to detect the insulation resistance of the system even when DC components in the residual current are superimposed by elec-tronic equipment, e.g. frequency inverters.

537.3.1 IT systems

6.2.2. Operating principle of a pulse-code modulated insulation monitor

A pulse-code-modulated insulation monitor measures the leakage capacitance of the system before the actual insulation measurement and adjusts its parameters to it.

This allows the monitor to discriminate a leakage capacitance from an insulation fault.

When using standard frequency inverters (with Y capacitors to ground), a pulse-code-modulated insulation monitor charges the Y capacitors with a pulsed DC voltage before the insulation measurement. Once the capacitors are charged, the insulation measure-ment is performed. The pulse-code-modulated insulation monitor compares several mea-surements before it issues a signal.

If several standard frequency inverters are installed downstream from an insulation monitor, the total capacitance increases. The pulse-code-modulated insulation monitor must now charge a much higher capacitance before performing the measurements. Due to the longer charging time for the capacitors, the response time of the insulation monitor is prolonged significantly. This is not a problem for a small number of small standard fre-quency inverters.

However, in case of a greater number of large standard frequency inverters, this can take several minutes. Otherwise, the insulation monitor is not able to charge the capacitors properly and cannot perform a correct insulation measurement. In this case, it would show an insulation resistance that is too low.

Connecting one or several large frequency inverters to the supply system can cause distortions in the system depending on the system conditions and impedance, which might lead to false tripping of a basic, standard insulation monitor. A pulse-code-modulated insulation monitor compares several measurements, which means it is time-delayed (4 – 5 s). This means a pulse-code-modulated insulation monitor is immune against such short-time supply system distortions.

An ideal combination would be a pulse-code-modulated insulation monitor, e.g. IRDH 275B from the company Bender, in connection with frequency inverters in "IT design".

Conclusion

However, if a certain number of frequency inverters switched in parallel is ex-ceeded, this combination also delivers unreliable insulation values that are too low. In this case, the components must be distributed to additional insulation monitors.

34


IT Systems

6.2.3. Trip value setting for insulation monitoring devices

Trip values can usually be set on the unit from 50 kΩ to 500 kΩ.

Minimum insulation values

The values stated in the following standards are minimum values. In insulation monitoring devices, this value should be increased by 50 % to allow for the tolerance of the units. For example, if 40 kΩ are required, the unit should be set to a trip value of 60 kΩ.

• DIN VDE 0420 – A2; 1990-06 Specifies 30 kΩ as the minimum value for insulation fault trips in IT systems.

• DIN VDE 0107; 1989-11 ‚ In hospitals and rooms used for medical purposes, an insulation monitoring device is required for every IT system. It must issue a signal when the insulation value decreas-es to 50 kΩ.

• DIN VDE0105-100 – 5.3.101.3.3 (e); Operation of electrical installations; 2009-10 In IT systems, an insulation resistance of 50 Ω / V nominal voltage is sufficient in all cases.

DIN VDE 0100-600 - C) – Note 6; 2008-06; The trip value of the insulation monitoring device is usually set to 100 Ω / V of the nominal line voltage.

6.3. Insulation resistance of a plant The entire insulation resistance of a plant is the sum of the parallel connection of all oh-mic connections to ground.

The insulation resistance of newly erected plants and equipment is usually very good. During operation of the plants, the insulation resistance usually deteriorates.

Reasons for the deterioration:

• Electrical influences

− Overvoltage, overcurrent, voltage waveform

• Mechanical influences

− Vibrations, shocks, cable bending

• Environment

− Temperature, moisture, chemical influences, dirt, accumulation of dust, oil, aggres-sive atmospheres

• Ageing

− Cables and equipment

If a plant is idle for a long time, it is possible that the insulation resistance decreases due to moisture. Once the plant has been started up again, the insulation resistance usually increases automatically, as the moisture is dissolved by the operating temperature.


IT Systems

6.4. SEW frequency inverters in "IT design" In SEW control cabinet inverters in "IT design (05)", all Y capacitors of the DC link are removed. Measuring resistors to ground (2.5 MΩ) and smaller Y capacitors according to the frame size remain on the supply system end, but they do not have any negative ef-fects.

The Y capacitors of SEW inverters can also be removed at a later time, but only by the SEW Service.

Exception:

In MOVITRAC® B inverters of size 0 (up to 4 kW), the Y capacitors can be deactivated.

Block diagram of a frequency inverter

Cy Y capacitors

6.4.1. Variants of SEW control cabinet inverters in "IT design":

MOVITRAC® B (not size 0) and MOVIDRIVE® B inverters are available in "IT design" → SK05.

MOVITRAC® B, MOVIDRIVE® B

Power section 05 + control unit 00 → complete unit SK50

IT variant in combination with technology variant → 5T.

Power section 05 + technology variant control unit 0T → complete unit SK5T

In MOVITRAC® B inverters of size 0, the Y capacitors can be deactivated with insulating disks.

MOVITRAC®B size 0

In MOVITRAC® LTE inverters, the Y capacitors and a varistor can be deactivated via two screws on the left side of the housing.

MOVITRAC® LT

All MOVITRAC® LT inverters can be ordered with EMC filter and without EMC filter (iden-tical with IT variant).

MOVIAXIS® A MOVIAXIS® inverter can be operated with a pulse-code-modulated insulation monitor. MOVIAXIS® is not available in "IT design".

36


IT Systems

6.4.2. Variants of decentralized SEW frequency inverters in "IT design"

In decentralized SEW frequency inverters, not all Y capacitors are removed completely.

In a MOVIMOT® in "IT design" → SK 16, the Y capacitors of the DC link are only removed partly, as fault-free operation of MOVIMOT® cannot be ensured otherwise.

MOVIMOT®

In a MOVIFIT® FC in "IT design" → 51, the Y capacitors of the DC link are removed com-pletely. The Y capacitors on the supply system end are all still active, but they do not cause any significant leakage currents (150 Hz).

MOVIFIT® FC

MOVIGEAR® An IT variant is not available.

6.4.3. Standard inverter (with Y capacitors) in case of a ground fault in an IT system

A ground fault in an IT system does usually not trip the inverter.

Operating an inverter on an IT system with ground fault can "inflate" the DC link of other inverters if they are inhibited and equipped with Y capacitors.

This means an unloaded DC link can be charged excessively if it is equipped with capaci-tors that are connected against ground. The inverter trips.

Ground fault in an IT system with frequency inverters

Aside from the already described increased total capacity, this is another reason why only inverters in "IT design" (without Y capacitors) should be used for IT systems with several inverters.


IT Systems

6.5. EMC in an IT system A frequency inverter without Y capacitors has a much weaker filter effect against interfe-rence emission, which means no EMC class requirements are met. In an IT system, EMC classes are not prescribed since EMC measurements are not possible without reference to ground.

To ensure fault-free operation, SEW-EURODRIVE recommends a "line filter for IT sys-tems" at the input and an HD ring core choke at the output.

Leakage currents cannot flow back to the transformer in an IT system. However, leakage currents can flow within the drive system in case of good equipotential bonding, which will achieve a certain filter effect even in an IT system.

Leakage currents of an IT system

IT line filter An IT line filter (e.g. FN 258 HVIT from the company Schaffner) has a higher insulation resistance and a lower capacity than a standard line filter.

Due to electrical safety reasons, IT line filters have two Y capacitors connected in series, which are designed for higher voltages. In case of a first fault (short circuit, phase-to-ground), a continuous current flows via the Y capacitors for a longer period of time, which places great strain on these capacitors.

38


Inverters in "IT Design" in a TN System with RCD

7. Inverters in "IT Design" in a TN System with RCD

7.1. Control cabinet inverters in "IT design" on an RCD The control cabinet inverters MOVIDRIVE® B (MDX) and MOVITRAC® B (MC07B) in "IT design" (05) are often used together with RCDs in TN systems.

MOVIDRIVE® / MOVITRAC®

Since all Y capacitors of the DC link have been removed in the "IT variant", the leakage currents (150 Hz) of the frequency inverter are reduced to a tenth.

The leakage currents from the motor cable and the motor (kHz range) are not reduced by this, of course.

MOVITRAC® B and MOVIDRIVE® B inverters in "IT design" are suited for use with a 30 mA RCD.

In order to achieve good electromagnetic compatibility, we recommend to combine a line filter with a low leakage current (see chapter 8.3.1) and an HD output choke (see chapter 8.3.2).

EMC of "IT variants"

The following application has been optimized with respect to leakage currents. Even con-nected to a bad, unbalanced supply system, the leakage currents are very low.

Application with optimized leakage currents

MOVIAXIS® is not available in "IT design". MOVIAXIS® is not suitable for operation with a 30 mA RCD.

MOVIAXIS®

Practical experience shows that MOVIAXIS® can be operated on a 30 mA RCD in con-nection with a LEAKCOMP® unit. A LEAKCOMP® unit is an active leakage current filter from the company EPA, see chapter 8.6.


Inverters in "IT Design" in a TN System with RCD

7.2. Decentralized frequency inverters in "IT design" on an RCD A MOVIMOT® size 1 in "IT design" → SK16 was optimized for operation on an RCD with respect to leakage currents (as of unit version 72). Even when the MOVIMOT® size 1 is disconnected from the supply system under load, the RCD usually does not trip.

MOVIMOT® size 1

MOVIMOT® size 1 in "IT design" is ideally suited for use with a 30 mA RCD.

MOVIMOT® size 2 is currently not suited for use with a 30 mA RCD. MOVIMOT® size 2 in "IT design" → SK16 will be optimized with respect to leakage currents at a later date.

MOVIMOT® size 2

In general, MOVIFIT® FC behaves like a control cabinet inverter regarding leakage cur-rents.

MOVIFIT® FC

In a MOVIFIT® FC in "IT design" → SK51, the Y capacitors of the DC link are removed completely. The Y capacitors on the supply system end are all still active, but they do not cause any significant leakage currents.

MOVIFIT® FC in "IT design" is suited for use with a 30 mA RCD.

40


Measures Against False Tripping of an RCD

8. Measures Against False Tripping of an RCD Measures to prevent false tripping of an RCD must always be tailored to the plant. There is no all-encompassing solution that always works reliably and is not too expensive. The plant should already be designed with low leakage currents.

8.1. Data inquiry for customer consulting The following information is required to offer competent advice to customers regarding the compatibility of an RCD.

1. Why is an RCD used? What is to be protected? − Operator protection = 30 mA − Protection against fire = 300 mA − Plant protection = 500 mA, 1 A

2. Data of the RCD − Is a type B RCD used? − Is it a time-delayed RCD?

3. Electrical structure of the plant − Number and power rating of the frequency inverters − Speed of the frequency inverters

(avoid creep speed, e.g. 10 Hz, see chapter 2.1.3) − Cable lengths (shielded or unshielded) − Are filters or chokes already installed in the plant?

4. At what point does the RCD trip? − When connecting the drive inverter to the supply system. − During operation (start, stop). − When disconnecting the frequency inverter from the supply system at standstill. − When disconnecting the frequency inverter from the supply system during opera-

tion. This should be avoided, see chapter 2.1.4.



8.2. Measures to prevent false tripping when the inverter is connected to the supply system

• Use a time-delayed RCD! • Do not start up several frequency inverters downstream of an RCD simultaneously.

8.2.1. ND line choke

• An ND line choke mainly counters surges that are caused by switching on a frequency inverter with Y capacitors.

• If several frequency inverters are switched on at the same time, an ND line choke should be installed.

• A line choke can have a positive effect especially in connection with an RCD without time delay (which is not recommended).

8.3. Measures to prevent false tripping during operation

• Increasing the pulse frequency to 8 kHz; higher-frequency leakage currents are not that critical for RCDs.

• Observing the trip frequency curve of the RCD. • Splitting the electric circuits among several RCDs

8.3.1. A line filter reduces the leakage currents during operation

A line filter reduces leakage currents mainly in the frequency range above 4 kHz, which is why a higher pulse frequency, e.g. 8 kHz, should be set on the inverter.

Shared line filter

Use only one shared line filter for several inverters. This one filter has a lower leakage current than the sum of the individual filters.

4-conductor filter

If you use several inverters with integrated EMC filters, you can significantly reduce the leakage currents by using an upstream 4-conductor filter.

When using single-phase frequency inverters, you should install a 4-conductor filter as a collective line filter.

Low leakage current line filters (not available from SEW) cause very low leakage cur-rents. As opposed to a standard line filter, a line filter with a low leakage current has a higher inductance and a smaller Y capacitor. This means there is less low-frequency leakage current from an unbalanced supply system drained off via the Y capacitor to ground.

Low leakage current line filters

Even in connection with a bad, unsymmetrical supply system, the leakage currents are very low thanks to this specific line filter.

A line filter with a low leakage current has a smaller effect regarding EMC.

42



If low leakage currents are expected, the FN 258L line filters with low leakage currents from the company Schaffner or the NF-KC-16-LL from the company EPA are recom-mended.

If high leakage currents are expected, e.g. due to

• a long, shielded motor cable, • a larger group of frequency inverters, • or creep speed (e.g. 10 Hz),

the FN 3268 line filter with low leakage currents from the company Schaffner is recom-mended. This line filter also offers beneficial EMC effects.

8.3.2. Ferrite core choke (HD output choke)

Using a ferrite core choke in the motor cable makes a shielded cable unnecessary. The shield of a motor cable increases the leakage current in the kHz range.

Advantage

• There is no voltage drop at an HD choke. • No leakage current flows to ground at the HD choke.

If the sum of the 3 phases equals 0, the HD choke is not working. If the sum of the 3 phases is not 0, the HD choke is working and heats up. When the temperature rises, the inductance of the HD choke decreases. The HD choke can heat up to 145 °C; above this, it is saturated and no longer has an effect. This means it is intrinsically safe. As a worka-round, you can install a second choke.

Operating prin-ciple



Install the motor phases with the output choke as follows: Installation 1. Take the three motor phases into one hand. 2. Secure the beginning of the three phases with a cable tie. 3. Wind the three phases together around the ring core five times.

Now all three phases are routed in parallel around the ring core.

Note the following:

• Wind all three phases in the same direction. • Do not mix up the beginning and the end of the phase. Otherwise this will cancel the

effect of the choke. • If you wind each phase around the ring core individually, there is a risk that the

winding direction or the beginning and end of the phase are mixed up.

The following figure shows how to connect the output choke:

Group drives carry increased capacitive reactive currents. The unbalanced component (leakage currents) of these reactive currents saturates a ferrite core choke very quickly, rendering it ineffective.

Group connec-tion

For group drives, a sine filter should be installed directly downstream from the inverter.

Use an HD output choke for the motor cable. Recommenda-tion If this one heats up, install a second HD output choke in series.

• 1 HD choke => 5 windings • 2 HD chokes => 3 windings

44



8.3.3. Motor cable

Note the following points during project planning:

• Use a low-capacitance cable. • Length of motor cable as short as possible. • Use unshielded cables, if possible. Install alternative EMC measures, e.g. HD choke, if

necessary. • If you can do without an existing shielding, disconnect it on both ends and provide for

a good insulation. • If you cannot do without a shielded cable, only use a low-capacitance cable in con-

junction with a low leakage current line filter for high leakage currents (e.g. FN 3268 from the company Schaffner).

8.3.4. Sine filter (HF output filter)

An output filter without DC link connection converts the pulsed inverter output voltage to a sine voltage in case of phase-to-phase connection.

Without DC link connection

In case of phase-to-ground connection, it has a reduced filter effect. The pulsed vol-tage remains.

An output filter with DC link connection converts the pulsed voltage into a sine voltage in case of phase-to-phase and phase-to-ground connection. The leakage currents are fed back directly into the DC link via the DC link connection.

With DC link connection

However, the reverse currents that flow from the output filter to the DC link are a major disadvantage. The inverter has to provide these reverse currents, but the motor cannot use them. The reverse current to the DC link via the capacitors is about 90 % at 4 kHz and about 30 % at 12 kHz. This is why the inverter can no longer be operated at a pulse frequency of 4 kHz and a utilization of 125 %.

With DC link connection, the pulse frequency must be increased to 12 kHz.

At this pulse frequency, the inverter can only provide about 70% of its power.

In case of group drives (several motors on one frequency inverter), an output filter offers the advantage that shielded motor cables are not necessary and should be avoided. The unshielded cable reduces leakage currents significantly.

Group drives

It was shown in practice that an HF sine filter does not reduce leakage currents signifi-cantly. Its only advantage is that an unshielded cable can be used.

Practice

With regard to the leakage current, a much cheaper HD choke should be preferred to an HF output filter.

This does not apply to group drives; an output filter must be used here.



8.4. Measures to prevent false tripping when the inverter is disconnected from the supply system

• Do not disconnect frequency inverters from the supply system during operation, but only when the drive has come to a standstill, see chapter 2.1.4.

• False tripping can also occur when the inverter is disconnected from the supply sys-tem with an unbalanced cut-off. Remedy:

− Main switch without snap action mechanism. − Replace irregularly worn switch contacts. − Switch RC elements or varistors via the main contacts. Not to be confused with RC

elements in the control circuit in parallel to the contactor coil. RC elements have a longer service life than varistors.

8.5. Operating principle of a dual-winding transformer The measures mentioned so far can counter leakage currents only to a certain extent. They cannot guarantee to prevent false tripping in each and every case. A reliable solu-tion is to install a dual-winding transformer between the RCD and the inverter, in accor-dance with EN 50178-5.2.11.1 and EN 60204-1-8.4.

In contrast to the isolation transformer principle, the PE conductor is looped through the dual-winding transformer and connected to the secondary winding of the isolation trans-former and the consumer. The consumers must be protected on the secondary side. An isolation transformer according to EN 61558-2-4 with the neutral point of the secondary connected to ground is only one component of a dual-winding transformer.

Block diagram of a dual-winding transformer in accordance with the standards EN 50178-5.2.11.1 and EN 60204-1-8.4.

This is not an isolation transformer used as a safety device!

Here, the customer TN system is galvanically separated from the second, newly estab-lished TN system. This keeps leakage currents away from the RCD. The leakage currents produced by the frequency inverter now only flow on the secondary side, because the circuit of leakage currents is closed only here. This prevents false tripping caused by leakage currents. As residual currents no longer flow to the primary side, the protective function of the RCD is lost!

46



Another advantage of this solution is that an RCD of type A is not negatively affected by the smooth leakage currents of the frequency inverter (saturation). This means it is not necessary to replace a type A RCD with a type B RCD.

For example, for a supplier who wants to use the supply system of the customer, which is equipped with a type A RCD, this is the only permitted solution.

SEW-EURODRIVE uses the following portable dual-winding transformers in two sizes from the company SBA Trafo Tech to connect frequency inverter models:

Example: Transformers – SEW

Power Output current Base area Height Weight Type

500 VA 3 x 0.72 A 400 mm x 200 mm 250 mm 5 kg GTV Order no. 216-0405

1500 VA 3 x 2.17 A 500 mm x 200 mm 300 mm 2 kg GTV Order no. 211-0432

Dual-winding transformer from the company SBA

The dual-winding transformer is a good solution for plants with a small power rating. For higher power ratings, the transformer would be too large, too heavy, and too expensive. The use of a LEAKCOMP® leakage current filter is a simple and cost-effective solution in this case, see chapter 8.6.

The following table shows examples of transformers with separate windings for installa-tion in control cabinets. These data are just to give an overview. Transformers are not available from SEW-EURODRIVE.

Power Output current Base area Height Weight Price

400 VA 1 x 1.74 A 120 x 100 mm 110 mm 5 kg About EUR 50

3.2 kVA 3 x 4.6 A 300 x 170 mm 330 mm 35 kg About EUR 400

6.5 kVA 3 x 9.5 A 360 x 200 mm 370 mm 55 kg About EUR 600

10 kVA 3 x 14 A 360 x 230 mm 370 mm 60 kg About EUR 800





NOTICE! • This is not an isolation transformer that can be used as a safety device!

Conclusion

• The only purpose of a dual-winding transformer is to prevent the RCD from tripping. • The RCD no longer provides protection and persons who work with the plant/unit

must be instructed accordingly.

A dual-winding transformer is a good solution for small power ratings. False trip-ping is prevented. An RCD of type A can still be used.

8.6. Compensation of 150 Hz leakage currents using a LEAKCOMP® filter A "LEAKCOMP®" unit is an active leakage current filter from the company EPA. It meas-ures operational leakage currents from 150 Hz to 750 Hz and compensates them. The determined leakage currents are feed back to the system offset by 180°.

Leakage currents below 150 Hz are dangerous for people, which is why they must not be compensated.

Leakage currents in the kHz range cannot be compensated by a LEAKCOMP® filter. They must be reduced via line filters and output chokes.

For larger plants that cannot be equipped with SEW control cabinet inverters in IT design, this is a convenient solution.

The combination with a LEAKCOMP® filter has already proven successful many times, in particular for the operation of a MOVIAXIS® unit on a 30 mA RCD.

Wiring diagram of an application with a LEAKCOMP® leakage current filter

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A LEAKCOMP® filter has about the same size as an RCD. It can be installed right next to a universal current-sensitive RCD on the mounting rail. The three line conductors and the neutral conductor are routed through the unit without the PE conductor.

Wiring diagram of a LEAKCOMP® leakage current filter

A LEAKCOMP® filter is mostly installed in larger plants of higher value, as it is usually too expensive for smaller plants (approx. € 1100). For plants with a smaller power rating, a dual-winding transformer is a cost-effective alternative.

Practice



8.7. Applications with optimized leakage currents In an application with optimized leakage currents, the focus is on the leakage currents, of course. But even when designing such a plant, it is possible to take EMC aspects into account as well.

Regarding RCD compatibility, the 150 Hz leakage currents from the DC link of each fre-quency inverter are the most important factor. 150 Hz leakage currents cannot be sup-pressed by filters and chokes. Leakage currents in the kHz range that flow on the motor end are quite easily suppressed.

To significantly reduce the 150 Hz leakage currents of a plant, you must use a MOVI-DRIVE®, MOVITRAC®, MOVIFIT®, or MOVIMOT® inverter in "IT design", see chapter 7.

Another option to reduce the 150 Hz leakage currents significantly is to install a "LEAK-COMP®" leakage current filter from the company EPA, see chapter 8.6.

If you have to use an RCD, the following measures will reduce the leakage current:

• Residual current monitor

− Operator protection → RCD type B with short time delay K. − Protection against fire → selective S RCD type B. − Plant protection → Residual current monitors (RCM)

• Line filter with low leakage current

− Low leakage currents → FN 258L line filter − Higher leakage currents → FN 3268 line filter

• MOVIDRIVE®, MOVITRAC®, MOVIFIT® → "IT variant" • Avoid creep speed • Pulse frequency → 8 kHz • Unshielded cable in connection with HD output choke

The following figure shows an example for 30 mA operator protection with low leakage current line filter, as small leakage currents are expected.

Application with optimized leakage currents

50



8.8. Values gained from practical experience Practical experience shows that in case of a high-quality supply and leakage current-optimized installation, a standard MOVIDRIVE® or MOVITRAC® inverter can be installed in the following way.

• 1 standard frequency inverter on a 30 mA, type B RCD with short time delay (K).

• 4 to 7 standard frequency inverters on a 300 mA, selective (S), type B RCD.

• Compared with a standard inverter, MOVIDRIVE®, or MOVITRAC®, and MOVIFIT® FC in "IT design" have only about 1/10 of the 150 Hz leakage current from the DC link.

• 1 MOVIAXIS® 10 kW inverter with 4 drives in connection with a LEAKCOMP® filter on a 30 mA, type B RCD with short time delay (K).

Reworking on site cannot be ruled out!

Operation without false tripping is only guaranteed with the described dual-winding trans-former, which is only suitable for smaller power ratings.

8.9. Summary of monitoring and protection devices

Use Residual current monitor Residual current Delayed cut-off Maximum cut-off time

Operator protec-tion

Type B RCD with short time delay K

30 mA 10 ms 300 ms

Protection against fire

Selective S type B RCD

300 mA 40 ms 500 ms

Plant protection Residual current monitor Parameterizable No cut-off Signal only

Supply system Insulation monitoring Trip value Delayed Inverter

IT system Pulse-modulated insulation monitor

Adjustable Usually 100 Ω/V

Plant-specific In "IT design"


Contact

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Contact Germany

Headquarters Production plant Sales

Bruchsal SEW-EURODRIVE GmbH & Co KG Ernst-Blickle-Straße 42 76646 Bruchsal P.O. box Postfach 3023 • 76642 Bruchsal

Phone +49 7251 75-0 Fax +49 7251 75-1970 http://www.sew-eurodrive.com [email protected]

Service Competence Center Central Gear units/motors

SEW-EURODRIVE GmbH & Co KG Ernst-Blickle-Straße 1 76676 Graben-Neudorf

Phone +49 7251 75-1710 Fax +49 7251 75-1711 [email protected]

Central Electronics

SEW-EURODRIVE GmbH & Co KG Ernst-Blickle-Straße 42 76646 Bruchsal


North SEW-EURODRIVE GmbH & Co KG Alte Ricklinger Straße 40-42 30823 Garbsen (near Hanover)


East SEW-EURODRIVE GmbH & Co KG Dänkritzer Weg 1 08393 Meerane (near Zwickau)


South SEW-EURODRIVE GmbH & Co KG Domagkstraße 5 85551 Kirchheim (near Munich)


West SEW-EURODRIVE GmbH & Co KG Siemensstraße 1 40764 Langenfeld (near Düsseldorf)


Drive Service Hotline / 24 h hotline +49 180 5 SEWHELP +49 180 5 7394357

Additional addresses for service in Germany provided on request.

SEW-EURODRIVE—Driving the world

SEW-EURODRIVEDriving the world

www.sew-eurodrive.com

SEW-EURODRIVE GmbH & Co KGP.O. Box 3023D-76642 Bruchsal/GermanyPhone +49 7251 75-0Fax +49 7251 [email protected]

Documents

Drive Engineering - Leakage Currents of Frequency Inverters.pdf