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Electrical Power and Energy Systems 55 (2014) 581–591

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Electrical Power and Energy Systems

journal homepage: www.elsevier .com/locate / i jepes

Directional overcurrent and earth-fault protections for a biomassmicrogrid system in Malaysia

0142-0615/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.ijepes.2013.10.004

⇑ Corresponding author.E-mail address: [email protected] (A.H.A. Bakar).

A.H.A. Bakar a,⇑, BanJuan Ooi b, P. Govindasamy b, ChiaKwang Tan a, H.A. Illias c, H. Mokhlis c

a University of Malaya Power Energy Dedicated Advanced Center (UMPEDAC), Level 4, Wisma R&D, Jalan Pantai Baharu, University of Malaya, 59990 Kuala Lumpur, Malaysiab ABB Malaysia Sdn Bhd, Block B, Level 3, Lot 608, Jalan SS 13/1K, 47500 Subang Jaya, Selangor, Malaysiac Department of Electrical Engineering, Faculty of Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia

a r t i c l e i n f o

Article history:Received 24 May 2012Received in revised form 24 September2013Accepted 9 October 2013

Keywords:Directional overcurrent relayDirectional earth-fault relayMicro grid

a b s t r a c t

Over-current protection is principally intended to counteract excessive current in power systems. In dis-tribution systems in Malaysia, non-directional over-current protection is adopted because of the radialnature of the power system used. Relay typically used in distribution network are designed to caterfor current flow in one direction, i.e., from transmission network to load. However, with the forecastedincrease in generation from renewable sources, it is important that adequate codes are in place withregards to their integration to sub-transmission/distribution network. Distribution network dynamicallychanges from ‘‘passive’’ to ‘‘active’’ network. With distributed generation connected to distribution net-work, power flows bi-directionally. Hence, directional over-current protection is adopted along the linebetween the transmission grid and the distributed generation. The bi-directional flow of power also com-plicates the earth fault protection. This is due to the presence of the distributed generation that will causethe line near the delta side of the transformer to be still energized after the operation of earth fault relayduring single-phase-to-ground-fault. This paper investigates the directional over-current and earth faultprotections used to protect the microgrid (biomass generator) in Malaysia. In this study, under-voltagerelays are adopted at the delta side of the transformer to fully clear the single-line-to-ground fault, whichcannot be cleared by earth fault relay. Three-phase-balanced fault and single-line-to-ground-fault at allpossible locations in the network have been simulated. Simulation shows good coordination and discrim-ination between over-current relays.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

One of the main objectives of most utilities is to provide secureand reliable supply to their customers. However, the occurrence ofshort-circuit fault affects the reliability and quality of power sup-ply [1]. Radial system is the most common configuration in distri-bution systems. In this type of configurations, only one sourcefeeds a downstream network [2,3]. Most protection systems fordistribution networks assume power flows from the grid supplypointing to the downstream low voltage network [4]. Normally,protection is done using overcurrent relays with settings selectedto ensure discrimination between upstream and downstream re-lays [5]. In the event of electricity outage due to fault, fast isolationand restoration are required to minimize customers lost.

With the presence of distributed generators (DG) in distributionnetwork, the complexity of protection relay coordination, controland maintenance of power distribution systems increases. Withthe connection of DG, in case of fault, the system can lose the radial

configuration since the DG sources contribute to the fault. There-fore, the system coordination could be lost. High penetration ofDGs will have unfavourable impact on the traditional protectionmethods because the distribution system is no longer radial in nat-ure and is not supplied by a single main power source [6]. Powerflows bi-directionally and ordinary non-directional over-currentrelay will not be able to fully clear the fault. When DG is presentin the system, an additional power flow appears from the load sideto the source side and vice versa. Hence, the opening of the mainfeeder breaker does not assure that the fault is cleared. The shortcircuit rating of the power system also changes with the installa-tion of DG. If generation is embedded into the distribution system,the fault current seen by the relay may increase or decrease,depending on the location of the relay, the fault and the distributedgenerators [7]. In a fault situation, distributed generators modifythe current contribution to the fault and therefore, it influencesthe behavior of the network protection.

Distributed generations have posed some problems to protec-tion, which are false tripping of feeders (sympathetic tripping),nuisance tripping of production units, blinding of protection, in-creased or decreased fault levels, unwanted islanding, prohibition

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582 A.H.A. Bakar et al. / Electrical Power and Energy Systems 55 (2014) 581–591

of automatic reclosing and unsynchronized reclosing [8]. Earthfault happening at the delta side of the transformer will not be fullycleared by the earth fault relay with the presence of DG becausethere are more than one power source in the network. When themain utility service provider is the only sole power source providerto the customer load, the earth fault that happens between theutility source substation and delta-wye transformer can be fullycleared or completely isolated by the tripping of the utility-sourceline breaker alone. Hence, when there is another generation source(probably a distributed generator) being added in parallel with theutility power system at the customer load site, beyond a delta-wyetransformer, the transformer becomes a source of fault current forfaults on the utility source line [9].

This paper examined the use of directional over-current relaysalong the line connecting the two power sources in order to fullyclear the fault. Characteristics graph of relays were plotted andthe tripping time of the primary protection and back-up protectionwere recorded. The study also investigated the use of under-volt-age relay at the delta side of the transformer to fully extinguishthe earth fault.

2. Over-current protection

Two types of over-current protections are discussed here, thephase over-current and the earth fault over-current protections.It should be noted that the earth fault protection should not oper-ate for any phase to phase fault because there is no zero sequencecurrent flowing [10]. However, the phase over-current protectionmay operate when earth fault happens. Hence, coordination and

TX DG

50N/51N (Non-Directional E/F Relay M(E))

27 (Undervoltage Relay L)

67N (Directional E/F Relay De)

67N (Directional E/F Relay Ke)

67N (Directional E/F Relay Ee)

50N/51N (Non

50/51 (Non-Directional O/C Relay M)

67 (Directional O/C Relay L)

67 (Directional O/C Relay K)

50/51 (Non-Directional O/C

50/51 (Non-Dir

50/51 (Non-Dir

67 (Directional O/C Relay E)

67 (Directional O/C Relay D)

Biomass mechanical load(1)/410.5041.21421.5930.0000.000

Biomass substation/11kV13.4811.226-6.9080.0000.000

Substation(1)/33kV 40.2261.21921.2410.0000.000

Substation(5)/0.43kV 0.4921.18549.9790.0000.000

Substation(4)/33kV 39.5721.19921.0560.0000.000

Substation(2)/33kV 39.9301.21021.0790.0000.000

2.91 MW

4.452.76

0.00

Essential and Other load for Gener(1)

2.720.95

0.00

11/0

.43k

V

-2.72-0.9567.940.00

2.721.0367.940.00

G~Biomass Generator

11.772.6880.490.00

33/1

1kV(

2)

-9.05-1.3550.020.00

9.051.6550.020.00

Line

(3)

9.051.3544.140.00

-8.99-2.4444.140.00

33/0

.43

kV(1

)

8.905.7544.190.00

-8.90-5.5144.190.00

2.93.44 MW

0.000.00

0.00

2.47 MW

0.000.00

0.00

Line

(5)

8.984.6950.700.00

-8.90-5.7550.700.00

Fig. 1. Netwo

discrimination should be done properly to ensure no nuisance trip-ping of relays.

2.1. Need of directional element in over-current relay

Directional over-current protection is used against fault currentthat could circulate in both directions through a system elementand when non-directional over-current protection could produceunnecessary of disconnection of circuits [11–13]. This situationcan happen in a ring circuit and a circuit with a number of infeedpoints. When distributed generator (DG) is present, there are mul-tiple power sources and the opening of the utility breaker onlydoes not guarantee that the fault will be cleared. Hence, the natureof distribution network changes with multiple DG units and direc-tional relays are needed in the network [6]. Directional relaysshould be placed along the line that links the main grid and theDG. For the branch that does not link the two power sources,non-directional relays could be used.

2.2. Relay characteristics

2.2.1. Definite timeThis characteristic makes use of time delay element to provide

means of discrimination [14]. The relay, which is installed at thefurthest substation away from the source, is tripped in the shortesttime. The remaining relays are tripped in sequence having longertime delays, moving back in the direction of the source. For thepurpose of this study, definite time characteristic is used for direc-tional earth fault protection. This characteristic has advantage

FAULT

WYE

DELTA TX UTILITY

27 (Undervoltage Relay G)

50N/51N (Non-Directional E/F Relay H(E))

67N (Directional E/F Relay Je)

67N (Directional E/F Relay Ie)

67N (Directional E/F Relay Fe)

-Directional E/F Relay B(E))

67 (Directional O/C Relay J)

67 (Directional O/C Relay I)

50/51 (Non-Directional O/C Relay H)

67 (Directional O/C Relay G)

Relay C)

ectional O/C Relay A)

ectional O/C Relay B)

67 (Directional O/C Relay F)

5V

Existing System/11kV0.0000.0000.0000.0000.000

Substation(3)/33kV39.9721.21120.9860.0000.000

33/1

1kV(

3)

-0.00-0.000.000.00

0.00-0.000.000.00

3.49 MW

0.000.00

0.00

1 MW(1)

4.452.76

0.00

Existing 11kV System

0.00-0.000.000.00

Line

(4)

-0.000.0010.650.00

0.00-2.2510.650.00

DIg

SILE

NT

rk model.

Fig. 2. Voltage phasor diagram for (a) unfaulted system, (b) faulted system during parallel operation and (c) faulted system during islanded operation.

A.H.A. Bakar et al. / Electrical Power and Energy Systems 55 (2014) 581–591 583

when being applied in the earth fault protection because earthfault relay is usually set at current setting of 20–40% of the loadingcurrent. Therefore, earth fault relay with definite time characteris-tic has faster operation time for back-up protection when the pri-mary protection fails to operate.

2.2.2. Inverse currentThese relays are classified based on their characteristic curves,

which define the speed of operation as inverse, very inverse or ex-tremely inverse. IEC 60255 defines a number of standard charac-teristics as follows [14]:

Standard Inverse ðSIÞ : t ¼ TMS � 0:14=½ðIf=ISÞ0:02 � 1� ð16Þ

Very Inverse ðVIÞ : t ¼ TMS � 13:5=½ðIf=ISÞ � 1� ð17Þ

Extreme Inverse ðEIÞ : t ¼ TMS � 80=½ðIf=ISÞ2 � 1� ð18Þ

Long Time Standard Earth Fault : t ¼ TMS � 120=½ðIf=ISÞ � 1�ð19Þ

2.2.3. Combined Inverse Definite Minimum Time (IDMT) and high setinstantaneous over-current relays

These relays combine both the characteristic of the inverse cur-rent and definite time to improve the overall system grading byallowing the discriminating curves behind the high set instanta-neous element to be lowered. The main advantage of relays withthis characteristic is the high-speed protection over a large sectionof the protected circuit. In this study, this characteristic is used inthe phase over-current relays.

3. Current setting

3.1. Phase over-current

In general, for phase over-current protection, the current settingis selected to be above the maximum short time rated current ofthe circuit involved. The pick-up values of phase over-current re-lays are normally set at 30% above the maximum load current, pro-vided that sufficient short circuit current is available [15].

3.2. Earth fault over-current

Protection against earth faults can be obtained by using a relaythat responds only to the residual current of the system, since aresidual component only exists when fault current flows to earth.The typical setting for earth fault relays are 30–40% of the full loadcurrent or minimum earth fault current on the part of the systembeing protected.

4. Unexpected delta sources protection

Conventionally, delta-wye transformers are used to connecttransmission and sub-transmission systems to distribution sys-tems to improve a load balance and block zero sequence currentflow. However, when distributed generation is present, delta-wyetransformers become the sources of fault current that can be diffi-cult to be detected and isolated. In other words, directional over-current or distance protection at the interconnection pointbetween the utility system and source of distributed generationcan detect faults on the utility source line but cannot detect groundfaults on the high side of a delta-wye transformer when the utilitysource breaker is open [9]. Referring to Fig. 1 which shows a microgrid system in Malaysia, when ground fault occurs at busbar of‘‘existing system/11 kV’’ (Delta side of the utility transformer),the directional earth fault relay Je (Wye side of the utility trans-former) is not able to detect the fault after the breaker of RelayH(E) is opened.

When earth fault occurs at the delta side of the transformer, theearth fault relay located between the fault point and the delta sideof the transformer will not be able to operate because there is nozero sequence current being detected. When earth fault happensat the delta side of the transformer, before the utility source break-er is opened, the faulted phase voltage collapses. The unfaultedphase voltages are held close to their nominal by the effectivelygrounded utility source. However, when the utility source breakertrips before the distributed generation trips, the faulted phase volt-age remains zero in case of a bolted fault while the unfaulted phasevoltages are 1.73 times the nominal when supplied from a deltasource [9]. This is illustrated in Fig. 2.

Under and/or overvoltage detection on the high side of the deltatransformer winding supplements islanding protection and low-side fault detection by providing ground fault detection beforeand after the utility source breaker opens. Hence, voltage relayscan be used to solve this issue. Voltage relays that can be used in-clude under voltage relays, overvoltage relays and zero sequencevoltage relay. The under voltage element can be set at some frac-tion of nominal voltage, 50% and it picks up if the fault is the samephase as the VT connection [9]. The overvoltage element can be setabove nominal voltage, 130% and it picks up if the fault is on one ofthe other two phases [9].

5. Case studies

Simulation involves short circuit to obtain the fault current datahas been performed in order to study the over-current protection.The obtained fault current data was then used to calculate thegrading of the over-current relays. The relay settings were appliedinto the program that has been developed to verify the operation of

Table 1ASetting of phase over-current relays.

Relay Time-overcurrent (TOC) Instantaneous-overcurrent (IOC)

TMS Current setting (A) Current setting (A) Time setting(s)

Secondary Primary Secondary Primary

A 0.05 5.53 442 18.75 1500 0.02B 0.11 5.36 429 26.25 2100 0.02C 0.06 3.6 143 62.5 2500 0.02D 0.10 3.9 156 23.75 950 0.02E 0.17 3.9 156 32.5 1300 0.22F 0.17 5.36 429 18 1440 0.23G 0.26 6.44 1287 25 5000 0.235H 0.34 6.5 1300 50 10,000 0.24I 0.19 6.5 260 17.5 700 0.02J 0.17 4.77 286 13 780 0.2K 0.16 5.69 455 10 800 0.21L 0.20 5.74 1387 11 2640 0.215M 0.24 6.34 1521 12 2880 0.37

Table 1BSetting of earth fault over-current relays.

Relay Instantaneous-overcurrent (IOC)

Current setting (A) Time setting (s)

Secondary Primary

Be 27.5 2 200 0.02Ce 1.5 60 0.02De 10 400 0.52Ee 1 40 0.02Fe 10 800 0.42He 1 200 0.02Ie 10 400 0.52Je 1 60 0.02Ke 2 160 0.42Me 0.8 192 0.02

Table 2ATripping time of relays for three-phase balanced fault (bolted).

Fault location Primary protection Back-up protection

Relay Tripping t (s) Relay Tripping t (s)

Substation (5)/0.43 kV A 0.02 B 0.5070Substation (4)/33 kV B 0.02 F 0.4257

K 0.2101Line(5) B 0.02 F 0.2301

K 0.2101Substation (2)/33 kV F 0.2301 G 0.5191

K 0.2101 L 0.2151Line(4) F 0.2301 G 0.2351

J 0.2025 K 0.2101Substation (3)/33 kV G 0.2351 G (U<) 0.5139

J 0.2025 K 0.2101Existing system/11 kV H 0.0224 – –

I 0.2401 G (U<) 0.5163Line (3) E 0.2229 F 0.2301

K 0.2101 L 0.2151Substation (1)/33 kV E 0.2229 F 0.2301

L 0.2151 L (U<) 0.5154Biomass substation/11 kV D 0.3702 L (U<) 0.5151

M 0.0229 – –

Table 2BTripping time of relays for sungle-phase-ground-fault (bolted).

Fault location Primary protection Back-up protection

Relay Tripping t (s) Relay Tripping t (s)

Substation (4)/33 kV Be 0.02 Fe 0.4127Ke 0.2027

Line(5) Be 0.02 Fe 0.5128Ke 0.4123

Substation (2)/33 kV Fe 0.4128 Ie 0.5128Ke 0.4128 De 0.5128

Line(4) Fe 0.4130 Ie 0.5130Je 0.2000 Ke 0.4197

Substation (3)/33 kV Ie 0.5128 G (U<) 0.5163Je 0.2001 Ke 0.4128

Existing system/11 kV He 0.2000 – –G (U<) 0.5190 – –

Line (3) Ee 0.2000 Fe 0.4126Ke 0.4126 De 0.5126

Substation (1)/33 kV De 0.5124 L (U<) 0.5290Ee 0.2000 Fe 0.4124

Biomass substation/11 kV Me 0.02 – –L (U<) 0.518 – –

Table 3AFault current detected by phase overcurrent relays A, B, F, G and H.

Relay Fault location Fault current/A

A Substation (5)/0.43 kV 1663B Substation (4)/33 kV 2334F Substation (2)/33 kV 1636G Substation (3)/33 kV 5707H Existing system/11 kV 12,000

Table 3BFault current detected by phase overcurrent relays A, B, K, L and M.


A Substation (5)/0.43 kV 1663B Substation (4)/33 kV 2334K Substation (2)/33 kV 994L Substation (1)/33 kV 3104M Biomass substation/11 kV 4341

Table 3CFault current detected by phase overcurrent relays C, D, E, F, G and H.


C Biomass mechanical load (1)/415 V 2554D Biomass Substation/11 kV 1077E Substation (1)/33 kV 1516F Substation (2)/33 kV 1636G Substation (3)/33 kV 5707H Existing system/11 kV 12,000

Table 3DFault current detected by phase overcurrent relays I, J, K, L and M.


I Existing system/11 kV 732J Substation (3)/33 kV 920K Substation (2)/33 kV 994L Substation (1)/33 kV 3104M Biomass substation/11 kV 4341


the directional over-current (DOC) and directional earth-fault(DEF) relays.

A power system software, DIgSILENT PowerFactory developedby DIgSILENT GmbH was chosen to simulate the load flow andshort circuit fault on the 33/11 kV network. The chosen test systemis 33/11 kV micro grid system in Malaysia and the distributed gen-erator is the biomass generator. The snap shot of the studied net-

work is depicted in Fig. 1. After performing the grading on theover-current relays, time-over-current plots between different

Table 4AFault current detected by earth fault overcurrent relays Be, Fe and Ie.


Be Substation (4)/33 kV 2364Fe Substation (2)/33 kV 1704Ie Substation (3)/33 kV 2824

Table 4BFault current detected by earth fault overcurrent relays Be, Ke and De.


Be Substation (4)/33 kV 2364Ke Substation (2)/33 kV 1486De Substation (1)/33 kV 1969

Table 4CFault current detected by earth fault overcurrent relays Ee, Fe and Ie.


Ee Substation (1)/33 kV 1452Fe Substation (2)/33 kV 1704Ie Substation (3)/33 kV 2824

Table 4DFault current detected by earth fault overcurrent relays Je, Ke and De.


Je Substation (3)/33 kV 920Ke Substation (2)/33 kV 1486De Substation (1)/33 kV 1969


combinations of relays at different locations are plotted to verifythe coordination and discrimination.

For phase fault protection, coordination and discriminationamong relays need to be obeyed as follows: (a) from load up tothe 11 kV existing grid, i.e., between relays A, B, F, G and H, (b) frombiomass distributed generation up to 11 kV existing grid, i.e., be-tween relays C, D, E, F, G and H, (c) from 11 kV existing grid up

Fig. 3A. Relays A,

to biomass distributed generation, i.e., between relays I, J, K, Land M and (d) from load up to biomass distributed generation,i.e., between relays A, B, K, L and M.

For earth fault protection, coordination and discriminationamong relays need to be obeyed as follows: (a) from load up tothe 11 kV existing grid, i.e., between relays Be, Fe and Ie, (b) frombiomass distributed generation up to 11 kV existing grid, i.e.,between relays Ee, Fe and Ie, (c) from 11 kV existing grid up tobiomass distributed generation, i.e., between relays Je, Ke and Deand (d) from load up to biomass distributed generation, i.e.,between relays Be, Ke and De.

Short circuit calculation simulation was carried out using thesimulation software to obtain the fault current at all possible loca-tions of the fault. In order to grade the over-current relays forphase fault protection, three phase balanced fault (LLLF) simulationwas selected. Network representation used was balanced, positivesequence for LLLF simulation and the maximum short circuit cur-rent was calculated. To grade the over-current relay for earth faultprotection, single line to ground fault (SLGF) simulation was se-lected because this type of fault is the most common earth faultand contributes to almost 80% of total number of fault occurrence.For SLGF simulation, network representation used was unbalanced,3-phase (ABC) and the maximum short circuit current wascalculated.

6. Simulation results

The results obtained were classified into two groups, the time-over-current (TOC) – instantaneous over-current (IOC) plot forphase over-current relays and the instantaneous over-current(IOC) plot for earth over-current relays.

The settings for the Inverse Definite Minimum Time (IDMT)phase over-current relays are based on the fault currents obtainedfrom a three phase balanced fault. The values of these fault cur-rents are presented in Tables 3A–D. The setting for IDMT relaysare presented in Table 1A. From the simulation, it is found thatin primary protection, the shortest tripping time for phase over-current fault takes only 0.02 s while the longest tripping time is0.3702 s. The tripping time for relays at different locations of faultis presented in Table 2A.

B, F, G and H.

Fig. 3B. Relays A, B, K, L and M.

Fig. 3C. Relays C, D, E, F, G and H.


The settings for the earth fault over-current relays arebased on the fault currents obtained from single line to groundfault. The values of these fault currents are presented in Tables4A–D. The settings for these relays are presented in Table 1B.From the simulation, it is found that in primary protection, theshortest tripping time for over-current earth fault takes only0.02 s while the longest tripping time is 0.519 s. The trippingtime for relays at different locations of fault is presented inTable 2B.

6.1. Phase over-current relay coordination (TOC-high set IOC plot)

6.1.1. Coordination between relays A, B, F, G and HFig. 3A shows the time-over-current (TOC) – instantaneous-

over-current (IOC) plot for coordination between relays A, B, F,G and H. These relays are relays from the load side up to the

relays at the existing 11 kV system. In normal radial distributionsystem, the over-current characteristic curves should discrimi-nate each other by not overlapping with each other. However,as shown in Fig. 3A, the characteristic curves for relay A and Bare actually overlapping with that from relays F, G and H butcoordination is still achieved. This is because in normal radialdistribution system (without the distributed generation (DG)),the fault current increases from downstream towards upstream,i.e., fault current nearer to the source is bigger. As shown inTable 3A, the fault current detected by relay downstream, i.e.,relays A and B (1663 A and 2334 A respectively) are bigger ascompared to fault current detected by relay upstream, i.e., relayF (1636 A). This scenario is due to the fault current detected byrelays A and B are the total contribution from both existing11 kV system (slack bus) and biomass generator (PV bus).Another key point noted is that relays A, B and H are

Fig. 3D. Relays I, J, K, L and M.

Fig. 4A. Relays Be, Fe and Ie.


non-directional phase over-current relay while relays F and Gare directional phase over-current relays. The use of directionalelement for relays F and G is needed because these relays areplaced along the line between the two power sources, that isexisting 11 kV system and the biomass generator. Along this line,power flows bi-directionally.

6.1.2. Coordination between relays A, B, K, L and MFig. 3B shows the time-over-current (TOC) – instantaneous-

over-current (IOC) plot for coordination between relays A, B, K, Land M. These relays are relays from the load side up to the relaysat the biomass generator. As shown in Fig. 3B, the characteristiccurves for relay A, B and K are overlapping with that from relaysL and M but coordination is still achieved. As shown in Table 3B,the fault current detected by relay downstream, i.e., relays A andB are 1663 A and 2334 A respectively while the fault current de-

tected by relay upstream relay L is 3104 A. Although the fault cur-rent at relay L, 3104 A is bigger than 1663 A, fault current detectedby relay A. However, the 3104 A detected at relay L is after the 33/11 kV step down transformer (current is stepped up). Therefore,the equivalent fault current before passing through the trans-former should be 3104/3, or 1034 A. Equivalently, this fault valueat upstream is still smaller than the fault downstream at relays Aand B. The relays A, B and M are non-directional phase over-cur-rent relay while relays K and L are directional phase over-currentrelays.

6.1.3. Coordination between relays C, D, E, F, G and HFig. 3C shows the time-over-current (TOC) – instantaneous-

over-current (IOC) plot for coordination between relays C, D, E, F,G and H. These relays are relays from the DG up to the relays atthe existing 11 kV grid. As shown in Fig. 3C, the characteristic

Fig. 4B. Relays Be, Ke and De.

Fig. 4C. Relays Ee, Fe and Ie.


curves are not overlapping with each other and show good coordi-nation. As can be seen from Table 3C, the fault currents valueincreases from downstream to upstream. Relays D, E, F and G aredirectional relays because they are placed along the line betweenthe two power sources.

6.1.4. Coordination between relays I, J, K, L and MFig. 3D shows the time-over-current (TOC) – instantaneous-

over-current (IOC) plot for coordination between relays I, J, K, Land M. These relays are relays from the existing 11 kV grid up tothe relays at the DG. As shown in Fig. 3D, the characteristic curvesdo not overlap each other and show good coordination. Table 3Dshows the fault currents value increases from downstream to up-stream. Relays I, J, K and L are directional relays.

7. Earth fault over-current relay coordination (TOC-high set IOCplot)

Fig. 4A shows the instantaneous-over-current (IOC) plot forcoordination between relays Be, Fe and Ie. These relays are fromthe load side up to the Y-side of the 33/11 kV transformer nearthe existing system/11 kV busbar. Coordination of relay is doneup to the Y-side of transformer only because it is the source of zerosequence current flow. The relay Be used is non-directional earthfault relay and it is an IDMT relay. Relays Fe and Ie used are direc-tional earth fault relay which uses only the definite time character-istic which enhances the speed of operation during earth faultoccurrence. For relay with definite time characteristic, the time de-lay setting is the most crucial part. The current setting of earth

Fig. 4D. Relays Je, Ke and De.

Fig. 5A. Zero sequence voltage at existing system/11 kV (without undervoltage relay).


fault relay is normally taken to be 20–40% of loading current be-cause it only operates for earth fault detection and not for ordinaryphase over-current condition. It can be seen that from Fig. 4A, thecurrent setting of the relay Ie is smaller than Fe. This is because thelarge setting of relay Ie will cause the tripping of under-voltage re-lay G instead of relay Ie when there is earth fault happening at bus-bar Substation(3)/33 kV.

Fig. 4B shows the instantaneous-over-current (IOC) plot forcoordination between relays Be, Ke and De. These relays are fromthe load side up to the Y-side of the 33/11 kV transformer nearthe biomass substation/11 kV busbar. The relay Be used is non-directional IDMT earth fault relay whereas relays Ke and De usedare directional earth fault relay with definite time characteristic.Fig. 4C shows the instantaneous-over-current (IOC) plot for coordi-nation between relays Ee, Fe and Ie. All relays Ee, Fe and Ie used aredirectional earth fault relay with definite time characteristic

because they are located along the line between two powersources. Fig. 4D shows the instantaneous-over-current (IOC) plotfor coordination between relays Je, Ke and De. All relays Je, Keand De used are directional earth fault relay with definite timecharacteristic.

8. Case study: earth fault at the delta side of the transformer

When single phase to ground fault occurs at busbar on theexisting system/11 kV system, that is at the delta side of the trans-former TX UTILITY, earth fault relay H(E) will trip to isolate theearth fault from the transmission grid. The tripping mechanism issafe when the system is normal radial without embedded genera-tion. With the installation of biomass generator, power flows bi-directionally and tripping relay H(E) will not fully clear the earth

Fig. 5B. Three phase voltage at existing system/11 kV (without undervoltage relay).

Fig. 6A. Zero sequence voltage at existing system/11 kV (with undervoltage relay).


fault. As shown in Fig. 5A, by just tripping the earth fault relay H(E)when earth fault occurs at busbar on the existing system/11 kV, theearth fault is not fully cleared. The zero sequence voltage at busbaron the existing system/11 kV does not fall to zero after the opera-tion of relay H(E). This shows that the line is still energized andposes certain degree of danger. Fig. 5B shows phase A, B and C volt-ages at busbar on the existing system/11 kV, which when singlephase to ground fault occurs at phase A, voltage at phase A dropsbut voltage at phases B and C increases.

Therefore, in order to solve the unexpected delta source prob-lem, under voltage relay G is used to detect the single phase toground fault. As shown in Fig. 6A, after under voltage relay G isused, the zero sequence voltage at busbar on the existing sys-tem/11 kV falls to zero, which indicates the complete clearanceof the fault. Fig. 6B shows the phase A, B and C voltage at busbaron the existing system/11 kV before, during and after the fault.After the fault is cleared, voltage of all three phases falls to zero,which indicates that the line is totally de-energized.

Fig. 6B. Three phase voltage at existing system/11 kV (with undervoltage relay).


The same phenomena happen when earth fault occurs at busbaron the biomass substation/11 kV. Under voltage relay L is used tofully clear the earth fault.

9. Conclusion

The simulation results obtained in this study shows that direc-tional over-current and directional earth-fault relays should beused along the line connecting the two power sources becausepower flows bi-directionally. For the load side that branches outfrom the source, non-directional relays may be used. In a microgrid system, earth fault that happens at the delta side of the trans-former cannot be fully cleared by the use of earth fault relay. This isbecause the relay at the delta side of the transformer will not beable to detect the zero sequence current flow. Therefore, properoperation has to be carried out by using an under voltage relayto detect the collapse of phase voltage for the earth faultoccurrence.

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