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Modelling and Simulation of Reverse Power Relay for

Loss of Mains Protection of Distributed Generation in

Microgrids

C. Buque1, S. Chowdhury

2, S.P. Chowdhury

3

Department of Electrical EngineeringUniversity of Cape Town

Cape Town, South Africa

[email protected], [email protected]

2, [email protected]

3

Abstract Integration of distributed generation (DG) into theutility grid has led to a renewed emphasis on looking into novel

power system control and protection issues pertaining to DG

units. This paper focuses on loss of mains (LOM) detection and

protection for DG. Commonly used methods of detection fail to

effectively detect the loss of mains scenario when the local areanetwork load and generation are closely matched. The proposed

method of detection and protection is highly efficient when the

demand and supply are similar. For this reason it can be used

together with the present methods to provide a complete

solution to LOM protection.

Index TermsReverse Power Relaying, Loss of Mains,

Distributed Generation, Point of Common Coupling.

I. INTRODUCTION

Loss of mains occurs when the microgrid, formed by the

distributed generator and local load, become disconnected to

the utility source of electrical power but remain connected topart of the utility load [1][9]. Loss of mains is less apparent in

terms of system stability when the DG is large enough to

support the utility load to which it is connected, however it

can become a big challenge to system operators if the DG

does not have enough capacity to support the utility load.

Consequences related to LOM include frequency instability

and voltage dips.

There is no industry standard method for LOM detection

and protection, various literatures present different methods

of detection ranging from active methods, passive methods

and remote techniques. In this paper a passive method

(reverse power relaying) is explored in order to provide a

single and complete solution for loss of mains protection for

microgrids. This relaying system demonstrates the benefits of

active methods including speed of operation as well as

avoiding the difficulty presented in passive methods when

establishing detection thresholds.

The developed relay is installed on the utility side of thepoint of common coupling (PCC). This enables it to detect theLOM occurrence and signal the static switch (SS), which is apower electronics device at the PCC. Once the SS is open themicrogrid can safely operate in isolated mode. In this mode

the DG has enough capacity to support the local load withminimal negative effect on voltage levels and frequencystability.

II. LOSS OF MAINS

Loss of mains can occur due to faults in the utility grid,

maintenance in the system or even circuit breaker nuisance

trip. This event may have many negative effects on the

distributed generator and microgrid.

The real challenge faced by the system operators comes in

when the microgrid has a small distributed generator. This

can cause voltage dips and system instability. As a systems

engineer it is important to determine at design stage whether

the DG is suitable to support the utility load or not. In some

areas for example, the UK, the DG is by no means allowed to

support the utility load under a loss of mains scenario [2].

This is due to the lack of the reliability of the grid when

operating with a DG as its only source and the difficulties

encountered when attempting to reconnect the system. Loss

of mains, for whatever reason it may occur is a severe

situation which design engineers must take into account,

especially in terms of system control and protection during

the occurrence.

A. Loss of Mains Detection and ProtectionOver the years there have been many detection techniques

developed, each of these have their own advantages anddisadvantages.

Local detection means that long distance communication

is not required. The detection device is located close to the

The authors are grateful to the authority of the Electrical EngineeringDepartment, University of Cape Town for providing the support and

infrastructure necessary for carrying out this research.

978-1-4799-1303-9/13/$31.00 2013 IEEE

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switch. There are two types of local detection methods:

Active and Passive method.

In the active method the detection device functions by

directly interacting with the system under consideration.

Perturbations are purposely injected to the system. The

systems response to the perturbations determines whether

loss of mains has occurred or not. Examples of active

techniques are [3]: Reactive power export error detection

Impedance measurement method

Slip-mode frequency shift method

Active Frequency Drift method

Automatic phase-shift methodActive methods of detection have the disadvantage of

directly affecting the system to which they are applied. If the

injected signal is not adjusted correctly it may have a

significant effect on the magnitude of the frequency of the

voltage, current and power output of the DG, it can break the

power balance between the DG and the local loads. However,

it has a significant advantage in that it is cheaper than passive

means of detection.In passive techniques there is no added signal to the

system. This technique relies on the detection of certain

distinct patterns at the DG output when islanding occurs. A

difficulty faced when dealing with passive techniques is the

proper selection of detection thresholds. This technique

depends vastly on load condition. When there are local

balanced loads LOM detection becomes difficult. Examples

of these techniques are [3]:

Rate of change of output power

Rate of change of frequency (ROCOF)

Rate of change of frequency over power

Harmonics Detection

Reverse Power RelayingLastly there are remote techniques for LOM detection.These are based on communication systems between the

utility and the DG. These are expensive to implement,

especially when small DGs are involved. Examples of remote

detection include:

Power Line carrier communications

Supervisory Control and Data Acquisition systems

When implementing Passive techniques of detection, the

DG protection against LOM is provided for by the relays

used to detect LOM. Once they detect an occurrence they

send a signal to open the appropriate Circuit Breaker (CB)

and from those pre-determined actions, the DG andMicrogrids are guaranteed protection.

When implementing remote techniques, the

communication links can be used to communicate with the

PCC or the Static Switch to disconnect the microgrid from

the utility grid when LOM occurs. The communications

systems are usually reliable enough to provide effective

signaling.

However, when active detection techniques are

implemented depending on the infrastructure, not always are

appropriate protection mechanisms activated. Detectors will

merely warn that LOM has occurred, they will not necessarily

open any breakers or disconnect the microgrid at the point of

common coupling. An effective way of making use of the

information provided by the detector is to send it to relays

located at the microgrid. Based on pre-determined settings,

the relays can activate the SS at the PCC to open and leave

the microgrid to operate in isolated mode.

When the generation and load in a network area are closelymatched, it becomes difficult to detect a loss of grid supply at

the generator [4].

III. POWER SYSTEM MODEL

The power system model, as shown in Fig. 1, consists of a

utility source with impedance representing the estimated

impedance for a utility grid of that capacity. There are two

busbars with a reverse power relay between them. Once the

relay detects an abnormality the Static Switch is opened and

the microgrid becomes isolated. A normal installation would

have different types of relays triggering the various circuit

breakers due to certain events. For the purposes of this paper

only 1 relay is used so that its capabilities can be clearlydemonstrated. The System characteristics are shown in Table

1.

Due to the configuration of the power system, the sizes of

the generators and loads, it is expected that active power will

only flow from busbar A to busbar B. If active power flows

from busbar B to busbar A it is considered abnormal.

Fig. 1. Power System Model

TABLE 1

POWER SYSTEM CHARACTERISTICSSystem Voltage (ph-ph) 132kV

Nominal Frequency 50 Hz

Utility Source Capacity 100 MVA

DG Capacity 10MVA/ 10KVA

Utility Load 100MVA

Local Load 8MVA

IV. REVERSE POWER RELAY

Reverse power protection is generally applied to preventdamage to mechanical plant items in the event of failure of

the prime mover. Common generator damage includes

gearbox damage and mechanical damage to shafts.

Power relays currently used in industry are capable of

measuring system voltage and current. They also measure the

angle between these two signals, the angle . With this

information they can calculate real power which is as shown

in Equation 1.

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P = V I cos (1)

Under normal operation, real power flow is in a determined

forward direction and -90 < < 90, in the case of reverse

power flow 90 < < 270. The relay allows for

unidirectional flow of active power from A to B.

V. REVERSE POWER RELAY MODELThe reverse element of the relay is the most important in

identifying an abnormal condition, therefore it is the most

important in modelling the relay. In this paper the directional

component of the relay is adopted from [6], where a similar

relay is used for generator protection.

In this model, as depicted in Fig. 2, voltage and current

signals are modified to square waves, with maximum and

minimum values of 1 and -1. When the signal is positive, the

value is 1 and when the signal is negative it is represented by

-1. The two signals are then multiplied to give an output of 1

when the signals overlap and -1 when they dont. The product

is then integrated from 0 to L. The upper limit of the

integrator is 0 so that under normal power flow conditions theintegral remains less than 0. However, under reversed power

flow conditions the integral output tends to fall until it

reaches a threshold value of L. The value of L varies

according to the allowable reverse power, the higher the

value of L, the higher the amount of reverse power. Once

there is a reverse power flow in the location where the relay

is situated the relay identifies the abnormal condition and can

immediately react to it.

A time delay component is incorporated in the relay. This

ensures that the circuit breaker only trips if a prolonged fault

or abnormal event occurs. The relays shouldnt trip for

transient power swings.

The last 4 elements in the model are used to ensure a logic0 is sent to the CB when an abnormality is detected so that

the CB can open.

A.Definition of Reverse Power LevelWhen reverse power relaying is used to protect a generator

a relay setting is chosen based on the type of generator it is.

An example of this is protection of a Diesel Engine the

allowable motoring power is 5 to 25% of the rated generation

capacity. The relay setting is chosen based on Equation 2 [8]:

ratioratio VTxCT

(MVA)CapacityGeneratingxPower(%)MotoringSetting = (2)

For Loss of mains detection the method of determining

allowable reverse power flow is different because it is not the

generator that is primarily being protected. In this case -0.01

was chosen as the lower limit because this is the lowest

integral value during normal system operation. For other

systems this value would have to be chosen based on

modelling results as it depends on the size of the generation

capacity and the system loads.

B.Definition of Trip TimeReverse power relays with either built in timers or external

timers must be used to avoid spurious isolation under

transient reversal of power, which may arise following

synchronisation or in the event of a power transmission

system disturbance. The bigger the generating capacity, the

lower the time delay should be.

For loss of mains detection the delay time will essentially

be defined by breaker reclosing times. When using high

speed auto-reclosing, it is important to know the time for

which the line must be de-energised in order to allow

complete de-ionization of the arc, so that it will not strike

when the voltage is re-applied. The de-ionization time

depends on various factors, of these factors circuit voltage isthe most important. Utility reclosers usually reclose after

0.17s for a 132kV system as shown in Table 2. For this

reason loss of mains relays must activate before the circuit

breakers attempt to reclose and reconnect the utility power

source, this is to avoid reclosing on to unsynchronised

systems.TABLE2

ARC DE-ENERGISATION TIMES FOR DIFFERENT VOLTAGE LEVELS [8]

Transmission

Line Voltage (kV)

Minimum de-

energisation time

66 0.1110 0.15132 0.17220 0.28275 0.3

The trip signal time delay has to be long enough to not trip

for transient cases but fast enough to open the circuit breaker

before the recloser activates an unsynchronised circuit to be

reconnected. It is assumed that [7], at medium level voltage

it takes a relay 2 cycles to pick up the fault and the circuit

breaker another 3 to 5 cycles to open. Therefore, in addition

to the set trip time, it takes from 5 to 7 cycles (0.1s to 0.14s in

a 50Hz system) from the time the fault occurs to the time the

fault is cleared.

Based on the preceding argument the trip time delay

should be: 0.1s < t < 0.17s. The value chosen for t in this

paper is 0.14s, which corresponds to 7 cycles in a 50Hzsystem.

Trip1

VariableTime Delay

ToSwitch1

Switch

Product Integrator

1s

If ActionSubsystem

if { }In1 Out1

If

u1if(u1 < -0.01)

else

Gain

-1

Constant6

1

Constant5

0.06

Constant4

1

Constant3

-1

Constant2

1

Constant1

-1

Constant

1

Add1V_in

2

I_in1

Fig. 2. Reverse Power Relay Matlab Model

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VI. TEST CASES

A. Test Case 1: Loss of Mains due to open utility switch,large DG Capacity.

In Test Case 1, CB1 is opened after 2 seconds. This

represents a scenario where there is maloperation of a switch

or nuisance tripping. It causes reverse power flow into the

utility load from the 10MVA microgrid generator. It is

important that the reverse power relay detects this scenario

because it will prevent unsynchronized reconnection of the

utility power source with the microgrid. In this case the

reverse power relay is expected to cause the static switch at

the point of common coupling to open.

B. Test Case 2: Loss of Mains due to a 3-phase Fault,withlarge DG Capacity.

3-phase faults are the most severe faults in terms of the

levels of fault current. It is important to provide a protection

system that can handle such high currents. In this case the

Utility Source is lost due to a 3-phase fault between CB1 andbusbar A. After the fault has occurred, power will flow from

the DG towards the fault. This power in the reverse direction

should be detected by the reverse power relay and the fault is

expected to be cleared by the relay and switch combination in

between busbar A and busbar B.

C. Test Case 3: Loss of Mains due to open utility switch, withSmall DG Capacity.

This case is similar to test Case 1 but here it is expected

that there will be the least active power flow which might

make it more difficult for the reverse power relay to detect

the abnormality. This test is included in order to test the

sensitivity of the reverse power relay. Again the switch CB1will be opened at 2 seconds. This case will demonstrate the

impact on the size of the DG with regards to reverse power

relaying for loss of mains.

D. Test Case 4: Loss of Mains due to a 3-phase Fault, withSmall DG Capacity.

When it comes to reverse power flow with small DG,

detection is the main issue and sensitivity of the relay model is

of paramount importance. The reverse current is so small that

it makes it difficult to detect the reverse active power. In this

case the three phase fault will cause a relatively high reverse

power flow and the relay is expected to detect it.

In all of the test cases the relay setting is the same. The

relay plays the vital role of detecting reverse active power. For

all the tests the setting is as described in Section V.

VII. RESULTS

A. Test Case 1The results for this case indicate the relay detected the

reverse power at 2 s. The SS was signalled to open at 2.14 s,

after the 0.14 s time delay, as shown in Fig. 3. This was as

expected. The reverse current was 10A which was detected

by the relay, as shown in Fig. 4. The reverse active power can

be equated using the values in Table I, Equation 1 and the

measured reverse current.

Fig. 3. Relay status during Test Case 1

Fig. 4. Power System Current During Test Case 1

B. Test Case 2The results for Test Case 2 indicate that the reverse current

was 50A as shown by Fig. 6. This current was easily detected

by the relay. The SS was opened at 2.14 s, as can be seen in

Fig. 5.


Fig. 6. Power System Current during Test Case 2

C. Test Case 3The lowest active reverse power was experienced in test

case as shown in Fig. 8. For this reason the relay did not

detect the reverse active power and the SS did not open, as

suggested by the Relay Status signal in Fig. 7. This result

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shows that reverse power relaying is ineffective when the

installed DG capacity is small. For this reason it cannot be

used as the only means of protection for loss of mains.



D. Test Case 4In Test Case 4, where loss of mains occurs because of a

fault, the reverse power flow is slightly higher than when loss

of mains occurs due to a switch opening. The reverse current

witnessed by the relay is 3A, this can be seen in Fig. 10. The

relay detected this reverse in power flow and the SS was

opened after a 0.14 s delay, as displayed in Fig. 9.

Fig. 9. Relay Status during Test Case 4


VIII. CONCLUSIONS

It can be concluded from the test cases and results that

reverse power relaying is most effective in detecting loss of

mains when the DG capacity is large. This is useful because

most methods of loss of mains protection being used at the

moment are least effective when the DG capacity is large.

This reverse active power relaying method can be used to

compliment or as back-up to more traditional loss of mains

protection methods, such as ROCOF relaying, to provide a

complete solution which will detect all of the abnormalities

and protect the microgrid from all of the harms of Loss of

Mains. This scheme is useful when other methods such asROCOF and frequency drop relays are not sensitive enough

to detect an abnormal scenario. It is also better than active

systems because it is not intrusive and better than most

passive systems because it is reliable.

REFERENCES

[1] P. O'Kane and B. Fox, "Loss of mains detection for embeddedgeneration by system impedance monitoring," in Developments in

Power System Protection, Sixth International Conference on (Conf.

Publ. No. 434), 1997, pp. 95-98.

[2] P.D. Hopewell, N. Jenkins, A.D.Cross, Loss-of-mains detection forsmall generators, IEE Proc. Electr. Power Appl., Vol. 143, No. 3,May 1996.

[3] J. Yin, L.Chang and C. Diduch, Recent Developments in Islanding

Detection for Distributed Power Generation, Power Engineering,2004. LESCOPE-04. 2004 Large Engineering systems Conference, July

2004, pp.124 128.

[4] P. Crolla, A.J. Roscoe, A Dysko and G.M. Burt, Methodology fortesting loss of mains detection algorithms for microgrids and

distributed generation using real-time power hardware-in-the-loop

based technique, 8thInternational Conference on Power Electronics ECCE Asia, May 2011.

[5] Areva,Network Protection & Automation Guide, First Edition, July2001, pp.

[6] M.M. Aman, G.B. Jasmon, Q.A. Khan, A.H.B. Abu Bakar, J.J. Jamian,Modeling and Simularion of Reverse Power Relay for Generator

Protection, 2012 IEEE International Power Engineering and

Optimization Conference (PEOCO2012),Malaysia, June 2012.[7] S.Chowdhury, S.P.Chowdhury and P.Crossley, Microgrids and Active

Distribution Networks,UK: The IET(UK), 2009.[8] ABB,ABB Switchgear Manual, 10thEdition, 2001.[9] S.Chowdhury, S.P.Chowdhury and P.Crossley. Microgrids and Active

Distribution Networks, The IET(UK), July 2009.

IX. BIOGRAPHIES

C. Buque received his BSc in Electrical Engineering in 2011 and is currentlypursuing his MSc in Electrical Engineering at the University of Cape Town,

South Africa. Email: [email protected]

S.Chowdhury is currently the Senior Lecturer in the Electrical Engineering

Department of The University of Cape Town, South Africa. She became

Member of IEEE in 2003 and Senior member of IEEE in 2011. She haspublished three books and over 150 papers in power systems modeling and

simulation, power system protection, distributed generation and renewable

energy systems. She is Member of SAIEE, Member of the IET (UK) andIE(India) and Senior Member of IEEE(USA). Email:

[email protected]

S.P. Chowdhury is currently Associate Professor in Electrical Engineering

Department of the University of Cape Town, South Africa. He became

Member of IEEE in 2003 and Senior Member of IEEE in 2011. He has

published three books and over 200 papers in power systems, distributed

generation and renewable energy systems. He is a Member of SAIEE, Fellow

of the IET (UK) with C.Eng. IE (India) and the IETE (India) and SeniorMember of IEEE (USA). Email: [email protected]

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