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PowerPoint PresentationMultirestraint differential system
Overcurrent differential protection
Other protection systems
It is of essential importance that the instrument transformers can withstand the rated primary currents and the fault currents that can arise. If the CT breaks down, the entire associated equipment will be left unprotected since no information then will be supplied to the protective relays.
If the relays don’t get any signals the protection system is out of order and severe damages can occur in the HV net.
© ABB Power Technology 1_114Q07- * -
Multirestraint differential system
Introduction
A bus is a critical element of a power system, as it is the point of convergence of many circuits, transmission, generation, or loads.
The effect of a single bus fault is equivalent to many simultaneous faults and usually, due to the concentration of supply circuits, involves high-current magnitudes.
High-speed bus protection is often required to limit the damaging effects on equipment and system stability or to maintain service to as much load as possible.
The bus protection described refers to protection at the bus location, independent of equipment at remote locations
Ct saturation and its solutions on bus protection
Differential protection is the most sensitive and reliable method for protecting a station bus.
The phasor summation of all the measured current entering and leaving the bus must be 0 unless there is a fault within the protective zone.
For a fault not in the protective zone, the instantaneous direction of at least one current is opposite to the others.
However, a current transformer saturation problem can result from the large number of circuits involved and the different energization levels encountered in these circuits for external faults.
For example, if there is an external fault on one circuit of a six-circuit bus, five of the current transformers may supply varying amounts of fault current, but the sixth and faulted circuit must balance out the total of all the others.
Consequently, this circuit is energized at a much higher level, near saturation or with varying degrees of saturation, giving rise to possible high false differential currents.
For the same reasons, dc saturation also is unequal.
Dc saturation is much more serious than ac saturation because a relatively small amount of dc from an asymmetrical fault wave will saturate the current transformer core and appreciably reduce the secondary output.
The L/R ratio of the power-system impedance, which determines the decay of the dc component of fault current, should strongly influence the selection of the bus protective relaying.
Typically, the dc time constants for the different circuit elements can vary from 0.01 sec for lines to 0.3 sec or more for generating plants.
The nearer a bus location is to a strong source of generation, the greater the L/R ratio and the slower the decay of the resulting dc component of fault current.
Ct saturation and its solutions on bus protection
Of the several available methods for solving the unequal performance of current transformers, four are in common use:
Eliminating the problem by eliminating iron in the current transformer [a linear coupler (LC) system]
Using a multi-restraint, variable-percentage differential relay, which is specifically designed to be insensitive to dc saturation
Using a high-impedance differential relay with a series resonant circuit to limit sensitivity to ct saturation
Using a restraint differential relay with moderately high impedance to limit sensitivity to ct saturation
Information required
When local bus protection is applied, the following information is required for the scheme selection, relay selection, and setting calculations:
Bus configuration information.
Current transformer information, including
There is one set of bus relays per bus section.
Use a dedicated ct for bus differential protection.
If possible, the connection of meters, auxiliary cts, and other relays in differential-type bus schemes should be avoided since these devices introduce an additional burden into the main circuit.
Lead resistance, as well as ct winding resistance, contributes to ct saturation. Therefore, the length of secondary lead runs should be held to a minimum.
Usually, the full-ct secondary winding tap should be used. This has two advantages. It minimizes the burden effect of the cable and, second, leads by minimizing the secondary current and makes use of the full-voltage capability of the ct.
Normal Practices on Bus Protection
Normally, there is no bus relay required for the transfer bus on a main-and-transfer bus arrangement, because the transfer bus is normally deenergized and will be included in the main bus section when it is energized.
No bus relay is required for a ring bus because the bus section between each pair of circuit breakers is protected as a part of the connected circuit.
Special arrangements should be considered if there is any other apparatus, such as station service transformers, capacitor banks, grounding transformers, or surge arresters, inside the bus differential zone.
There is no simple scheme available for a double-bus-single-breaker arrangement , because its current transformers are normally located on the line side.
The linear coupler scheme provides a highly reliable bus protection.
Of the four systems commonly in use,
it has a fast operating time;
is the easiest to apply, set, and maintain;
can readily accommodate switching or changes in the bus layout.
Since iron is eliminated, an air-core transformer, a linear coupler, is required.
Adding a linear coupler can be a disadvantage, particularly in existing installations where adequate current transformers exist.
Linear couplers are air-core mutual reactors wound on nonmagnetic toroidal cores.
Linear couplers are designed to fit into the same space as a conventional current transformer.
Bushing or wound-type units are available for most of the voltage classes.
They are usually mounted in a circuit breaker or transformer bushing.
The single conductor through the center of the unit forms the primary of an air-core reactor and provides a definite linear relationship between the primary current and secondary voltage.
The linear coupler differential system
The linear couplers have a negligible dc response, so only the steady-state conditions need be considered.
The linear coupler method of differential protection is a voltage differential scheme in which a series circuit is used.
All the linear coupler secondaries of a particular phase are connected in series with one relay to form a closed loop.
Under normal conditions or when external faults occur, the induced voltages in all the linear couplers add to 0.
On the internal faults, the net voltage will operate the relay.
The linear coupler bus protection system can easily accommodate system changes and future expansion. In addition, it can be applied to an unlimited number of circuits.
Since the linear couplers do not contain iron, there are no saturation or transient problems.
The setting is calculated using only Ohm's law.
The operating time is between 32 ms and 16 ms.
Relays require minimum panel space.
The operating voltages are safe for personnel and well within the insulation limits of all connected apparatus.
Since linear couplers may be open- or short-circuited with complete safety, circuits can be switched among several bus sections much more easily than conventional current transformers.
In connecting linear couplers, the four wires from the star-connected couplers should be transposed with respect to all other circuits and carried in the same conduit or duct.
If a multiconductor cable is used, the other conductors should not be employed unless there is no possibility of their inducing tripping voltages into the linear coupler circuits.
Manual test auxiliaries are used to check the scheme during installation and at regular test intervals.
To check for correct connections and a shorted coupler, three high-resistance voltmeters are connected across the relays.
With load currents flowing, these voltages should be very low or 0.
Since the circuit might also be open at some point, a second test, which requires opening the trip circuit, is also applied. A low series voltage of 0.6 or 1.2 V is introduced into the differential source. Approximately half this test voltage will appear across the voltmeter; the remainder will appear across the rest of the loop. An open circuit will cause the voltmeter to register 0 voltage or full test voltage, depending on whether the coupler or relay circuit is open.
Multirestraint differential schemes use conventional current transformers, which may saturate on heavy external faults.
For this reason, the secondary current output may not represent the primary.
In a differential scheme, the current transformers and relay function as a team.
When the current transformers do not perform adequately, the relay can within limits make up for the deficiency.
For this scheme, a more complex relay is required than that described for the linear coupler bus protection system.
More elaborate application rules are also necessary, since there is a limit of current transformer performance beyond which the relay cannot compensate.
The multi-restraint differential scheme uses the variable-percentage differential relay, which consists of three induction restraint units and one induction-operating unit.
Two of the units are placed opposite each other and operate on a common disc.
In turn, the two discs are connected to a common shaft with the moving contacts.
All four of the units are unidirectional; that is, current flow in either direction through the windings generates contact-opening torque for the restraint units or contact-closing torque for the operating unit.
Each restraint unit (called R, S, and T) also has two windings to provide restraint proportional to the sum or difference, depending on the direction of the current flow.
If the currents in the two paired windings are equal and opposite, the restraint is cancelled.
Thus, the paired restraint windings have a polarity with respect to each other. With this method six restraint windings arc available.
In addition to providing multiple restraint, the variable-percentage characteristic helps in overcoming current transformer errors.
At light fault currents, current transformer performance is good, and the percentage is small for maximum sensitivity.
For heavy external faults, current transformer performance is likely to be poor, and the percentage is large.
The variable-percentage characteristic is obtained by energizing the operating unit through a built-in saturating autotransformer.
The saturating autotransformer also presents a high impedance to the false differential current, which tends to limit the current through the operating coil and to force more equal saturation of the current transformers.
On internal faults, in which a desirable high differential current exists, saturation reduces the impedance.
A further advantage of the saturating autotransformer is that it provides a very effective shunt for the dc component, appreciably reducing the dc sensitivity of the operating units.
At the minimum pickup current of 0.15 ± 5% A, the restraining coils are ineffective.
Figure on the left may be used if only three circuits are involved. The term circuit refers to a source or feeder group.
When several circuits exist and the bus can be reduced to four circuits, then the second scheme may be used.
For example, assume a bus consists of two sources and six feeders, and that the feeders are lumped into two groups.
The bus now reduces to four circuits.
In paralleling current transformers, each feeder group must have less than 14-A load current (restraint coil continuous rating).
If the bus reduces to more than four circuits, then the scheme should be used.
Each primary circuit must be identified as either a source or feeder.
Next, a number of feeders are lumped into a feeder group by paralleling feeder current transformers.
Each feeder group must have less than 14-A load current and not contribute more than 10% of the total phase or ground fault current for a bus fault.
Then connect the “source” and “feeder groups” alternately as shown in Figure.
Overcurrent Differential Protection
The differential scheme is obtained by paralleling all the current transformers per phase with an induction disc overcurrent relay across their output.
It is permissible to use auxiliary ct's to match ratios.
It is most desirable for all ct's to have the same ratio on the tap used so that auxiliary ct's are not required.
Overcurrent Differential Protection
Relays must be set above the maximum false differential current for an external fault.
That is, very little saturation can be allowed if any degree of internal fault sensitivity is to be obtained.
A certain amount of dc or ac saturation can be tolerated, because
(1) the operation of induction disc relays on the dc component is less efficient, and
(2) the relay operation is not instantaneous.
To increase the response of these schemes, the dc time decrement must be short.
This requirement virtually limits applications to substation buses remote from large generating stations.
Although the relay cost is low, the engineering cost is usually high, since considerable study or experience is required to assure correct operation.
Improved Overcurrent Differential Protection
The sensitivity of the overcurrent differential scheme can be improved by externally connecting a series resistor with each overcurrent relay, as shown in Figure a.
These resistors are called stabilizing resistors.
Although the high-impedance differential scheme also uses conventional current transformers, it avoids the problem of unequal current transformer performance by loading them with a high-impedance relay.
This arrangement tends to force the false differential currents through the current transformers rather than the relay operating coil.
Actually, the high-impedance differential concept comes from the above “improved overcurrent differential” approach. It uses a high-impedance voltage element instead of “a low impedance overcurrent element plus an external resistor.”
The high-impedance differential relay consists of an instantaneous overvoltage cylinder unit (V), a voltage-limiting suppressor (varistor), an adjustable tuned circuit, and an instantaneous current unit (IT).
On external faults, the voltage across the relay terminals will be low, essentially 0, unless the current transformers are unequally saturated.
On internal faults, the voltage across the relay terminals will be high and will operate the over voltage unit. Since the impedance of the over voltage unit is 2600 ohm, this high voltage may approach the open-circuit voltage of the current transformer secondaries. The varistor limits this voltage to a safe level.
High-impedance differential system
Since offset fault current or residual magnetism exists in the current transformer core, there is an appreciable dc component in the secondary current.
The dc voltage that appears across the relay will be filtered out by the tuned circuit, preventing relay pickup.
The IT current unit provides faster operation on severe internal faults and also backup to the voltage unit.
The relay has successfully performed operations up to external fault currents of 200 A secondary and down to an internal fault current of 0.27 A secondary. Its typical operating speed is 25 msec.
They combine the advantages of high-impedance and percentage restraint differential characteristics in one unique operating principle, which provides reliable operation for internal faults and secure restraint on external faults.
They can accommodate a large range of line ct ratios, other relays may be included with the same ct circuit, and the bus arrangement can easily be changed or added to without concern.
The relay generally connects to the system with a special auxiliary ct required for each restraint circuit when 5-A-rated cts are used.
One restraint circuit is required for each phase of each circuit or combination of circuits connected to the bus.
The auxiliary ct permits the use of unmatched ct ratios to bring the overall ratios into agreement and permit the possible use of other burdens in the ct circuits.
Moderately high-impedance relay
The relay operates on the principle of a differential comparison between all incoming and out-going lines to the bus.
Circuits L1, L2, ... Lx, are connected to auxiliary ct's TM1, TM2, .... TMx, respectively, which balance the main ct ratios.
Current fed into the relay becomes IL1, IL2, ... ILX, combining for a total input of IT at terminal K.
The comparator circuit is made up of resistors RS, Rd3, Rd1, and transformer TMD.
Resistor Rs, across which is developed the restraint voltage VS, is composed of two equal resistors.
Operate voltage Vd3 is developed across resistor Rd3. The differential resistor Rd1 is a series connection of resistors that are connected depending on the characteristics of the ct´s and the required total circuit resistance.
The TMD is a toroidal core transformer. The voltage developed across the primary Vd1 is proportional to the differential current Id1. This produces the transformer secondary voltage Vd2. Secondary current Id2 flow in such a way as to develop operating voltage Vd3 is greater than the restraint voltage VS.
The toroidal reactor TMZ and associated diodes D1 and D2 make up the voltage-limiting circuit. The start function is used to supervise the trip.
Protecting a bus that includes a transformer bank
Ideally, when the bus includes a power transformer bank, separate protection should be provided for the bus and transformer, even though both protection schemes must trip all breakers around the two units.
Such a system offers maximum continuity of service, since faults are easier to locate and isolate.
Also, using a bus differential relay for bus protection and transformer differential relay for transformer protection provides maximum sensitivity and security with minimum application engineering.
However, economics and location of current transformers often dictate that both units be protected in one differential zone.
Protecting a bus that includes a transformer bank
A typical application, shown in Figure, protects a three-winding transformer bus with four circuits.
Protecting a double-bus single breaker with bus tie arrangement
The double-bus single breaker with bus tie provides economic and operating flexibility comparable to the double-bus double-breaker arrangement.
However, the ct's are normally on the line-side location, which results in increased differential relaying problems.
Protecting a double-bus single breaker with bus tie arrangement
Two different approaches have been used in the bus protection of such arrangements:
the fully switched scheme
the paralleling switch scheme
They are both complicated (inserting switch contacts in the ct circuits) and / or imperfect in protection.
These schemes either require switching ct's and/or disabling the bus protection before any switching operation.
This is a period when the probability of a bus fault occurring is high and it is most desirable that the bus protection be in service.
A third scheme can be considered.
It is similar to the paralleling switched scheme except a check-zone relay is added as shown.
Two bus differential zones are provided, one for each bus, with each one overlapping the bus breaker.
Each primary circuit is normally switched to a specific bus, and relay input circuits and breaker control circuits are wired accordingly.
The additional check-zone device supervises the trip circuits.
If it becomes necessary to clear one of the buses, all the primary circuits may be switched to the opposite bus and it is needless to disable the bus protection before any switching operation.
However, this scheme still has two drawbacks when any one or all of the primary circuits is switched to the opposite bus:
(1) It will lose its selectivity, and
(2) it will reduce its sensitivity since the two relays are paralleled.
Partial differential relaying
This type of protection is also referred to as “bus over-load” or “selective backup” protection.
It is a variation of the differential principle in which currents in one or more of the circuits are not included in the phasor summation of the current to the relay.
In this scheme, only the source circuits are differentially connected, as shown in Figure, using a high-set over current relay with time delay.
The ct's protecting the feeders or circuits are not in the differential.
Essentially, this arrangement combines time-delay bus protection with feeder backup protection.
The sensitivity and speed of this scheme are not as good as with complete differential protection.
This method may be used as a backup to a complete differential scheme, as primary protection for a station with loads protected by fuses, or to provide local breaker failure protection for load breakers.
Some partial differential circuits use distance-type relays in the scheme.
The use of a distance relay for this scheme produces both faster and more sensitive operation than the over current scheme.
Directional comparison relaying
Occasionally, it is desirable to add bus protection to an older substation where additional ct's and control cable are too costly to install.
In this instance, the existing ct circuits used for line relaying can also be used for the directional comparison bus relaying protection.
As shown in Figure, the directional comparison relaying uses individual directional over current relays on all sources and instantaneous over current relays on all feeders.
Directional comparison relaying
The directional relays close contacts when fault power flows into the bus section. Back contacts on the over current relays open when the fault is external on the feeder.
All contacts are connected in series, and when the fault occurs on the bus, the trip circuit is energized through a timer.
A time delay of at least four cycles will allow all the relays to decide correctly the direction of the fault and to permit contact coordination.
In this scheme, the ct's in each circuit do not require the same ratio and can be used for other forms of relaying and metering.
The disadvantage of this scheme is the large number of contacts and complex connections required. There is also the remote possibility of the directional elements not operating on a solid three-phase bus fault as a result of 0 voltage.
Fault Bus (Ground-Fault Protection Only)
This method requires that all the bus supporting structure and associated equipment be interconnected and have only one connection to ground.
An over current relay is connected in this ground path.
Any ground fault to the supporting structure will cause fault current to flow through the relay circuit, tripping the bus through the multiple contact auxiliary tripping relay.
A fault detector, energized from the neutral of the grounded transformer or generator, prevents accidental tripping.
This scheme requires special construction measures and is expensive.

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