chapter 3 a new directional over current...

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CHAPTER 3

A NEW DIRECTIONAL OVER CURRENT RELAYING SCHEME FOR DISTRIBUTION FEEDERS IN THE PRESENCE OF DG

3.1 INTRODUCTION

In plain radial feeders, the non-directional relays are used as they operate when the CT

secondary current exceeds the threshold value of pickup setting in relays. This type of relay

operates irrespective of the direction of current flow.

The feeders other than plain radial feeders are not protected by the non-directional

overcurrent relays as they require the creation of zones. The protection of such parallel feeders

or double-end-fed feeders is protected by the directional relays. By introducing the directional

feature in relays, uninterrupted supply can be made possible at all load points connected in

parallel/ring system.

In plain radial feeder as shown in Figure 3.1, if the breaker 1 trips because of any

abnormalities in the section between bus A and bus B, it will interrupt the power supply at the

buses B, C, and D. Thus, because of the tripping of former breaker, the load connected to the

lateral buses will not receive power supply.

Figure 3.1 Single line diagram of a radial system

In case the same radial feeder is fed from both the ends (double-end fed) with

necessary modification in the protection scheme using directional feature at relay point R2, R3,

R4, and R5, as shown in Figure 3.2, zones are created for different sections between any two

buses.

Load Load

B C D A

R1 R3 R5

1 2 3

Load Load

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Figure 3.2 Double-end feed radial feeder

In the event of any abnormalities (fault) in section between bus A and bus B, the

breakers 1 and 2 will isolate the faulty section, without interrupting the supply to load the

connected at the buses A, B, C, and D. Hence, to discriminate the faulty section, the relay R2

should be direction sensitive so that it operates only in the direction indicated by the arrows as

shown in Figure 2. It can be concluded that the relays R2, R3, R4, and R5 should operate for a

current that flows away from the bus where the relay is located, and it restrains if the current

flows towards the bus.

3.1.1 Directional Relay Characteristics

The directional relay is a two input quantities relay that receives line current and bus

voltage. The relay compares the direction of the current flow with reference to the bus voltage

by measuring the phase angle between line current and bus voltage. The directional relay

operates on watt metric principle, where the voltage coils (VCs) receive voltage from the bus

potential transformer (PT) and the current coils (CC) receive the current from the line CT

secondary. A maximum positive torque is produced when the current and voltage supplied to

the CCs and VCs are in phase. Hence, the angle between current and voltage at which the

relay develop maximum torque is defined as the maximum torque angle (MTA).

In a directional relay, if V is the voltage given to the VC of the directional relay, then a

current IV lags the voltage V by very large angle θ (because of inductive nature of VC). The

flux produced by this current IV is ФV. I is the current given to the current coil of directional

relay, which setups a flux ФI because of current I. If the angle γ between these two fluxes ФV

and ФI is 90, the relay produces maximum torque. The vector diagram of these quantities for

a directional relay is shown in Figure 3.3.

6 5 4 3 2 1

C B A

R2 R1 R3

D

R4

1 1

R5 R6

1 Gen-1 Gen-2

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Figure 3.3 Vector diagram for directional relay

The operating torque (T) can be expressed as follows:

operating ФV × ФI × sin γ (3.1)

Since, ФI I and ФV V

operating V × I × sin γ (3.2)

From the vector diagram, it can be seen that

+ = 90

= 90 − and

γ + β = , γ = − β

operating sin( )V I

sin(90 )V I

sin 90 ( )V I

cos( )V I (3.3)

From the phasor, the maximum torque angle is given by = 90 −.

If the angle ( + β) is less than +90 and more than −90, the torque will be positive and

overturn results in negative torque. The directional relay measures the angle between the

voltage and current to identify the correct directional of current flow away from the bus. In

case of fault near the bus, the voltage available on PT secondary is not enough to produce an

operating torque in directional relay (electromagnet). This voltage value depends on the

location of the fault on line from the relaying point. The minimum fault distance from the

relay point for which the relay fails to operate is known as dead zone. Figure 3.4 shows the

Position of I for maximum torque

I

V γ

ФI

β = 90 −

0

ФV

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characteristic of directional relay with dead zone [16].

Figure 3.4 Directional relay characteristic

3.1.2 Polarizing Quantity

Directional relay is a two-quantity relay, and it compares the phase angle of the input

voltage and current quantities. The directional overcurrent relay operates only when the

magnitudes of current become higher than the set value of the threshold, and the current flows

in its correct operating direction (forward direction). The torque produced in the directional

overcurrent relay is maximum when = −β. During fault, the power factor angle is large i.e.

of the orders of 80 to 90 and it depends on the location of fault.Hence, the maximum torque

angle = 90 − should be of the same order to achieve maximum torque in the relay during

fault. The maximum torque angle can be set to 30, 60, and 90 by suitable connection of

CTs and PTs in the relaying circuit. Connections of 30 offer negative torque and

maloperation of the directional relaying scheme for certain types of faults. Connections of 60

produce low torque for certain types of faults. Hence, 30 and 60 connections are not widely

used for directional relaying scheme. In 90 connections, the polarizing voltage is fed to phase

element in such way that it produces maximum torque.

In case of R–B (L–L) fault, the voltage across the R element is VYB and across the B

element is VRY. Thus, the required maximum torque is produced by providing the polarizing

voltage of healthy phase to the voltage coil of active (faulted) phase. Figures 3.5(a) and 3.5(b)

show the 90connection and the vector diagram for R–B fault, respectively.

Blocking direction

Dead zone

Operating direction

Directional relay characteristic

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Figure 3.5 R–B fault (a) 90 connection (b) Vector diagram

In case of unity power factor, the position of faulted phase current (IR) leads the

polarizing voltage (VYB) by 90. In the event of high resistance fault, the directional

overcurrent relay with 90 connections produces less torque. Hence, to achieve maximum

torque, the maximum torque angle can be adjusted to any desired value by inserting a

resistance or capacitance in series with the voltage coil of the directional relay. Table 3.1

shows the various combinations of voltages and current fed to the directional relays for 30,

60, and 90 connections [16].

Table 3.1 Quantities fed to phase element of directional relay

Types of

connections

Fault involving

phase R

Fault involving

phase Y

Fault involving

phase B

Current Voltage Current Voltage Current Voltage

30 IR VRB IY VYR IB VBY

60 IR − IY VRB IY − IB VYR IB − IR VBY

90 IR VYB IY VBR IB VRY

3.1.3 Directional Ground-Fault Relays

A residual current and residual voltage are fed to the directional ground-fault relay. The

value of residual voltage (VR + VY + VB) is zero for normal operating condition as well as

during phase faults. During ground fault, the residual voltage of open delta PT secondary

operates the directional relay. Figure 3.6 shows the vector diagram for L–G (R–G) ground

fault.

(a)

V′B

V′RY

−Vy IR

V′R

VR

VB

V′B

IB = −IR VY

V′YB VYB

VRY VBR

IR

IY IB 90

(b)

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Figure 3.6 Vector diagram for L–G (R–G) ground fault

3.2 CURRENT STATE OF THE ART

In order to achieve coordination of overcurrent relays in ring-fed distribution network

containing DG, So et al. [26] presented time coordinate method based on evolutionary

programming. However, the prime limitation of this method is that it cannot handle many

fault current redistributions at a time. After that, Jager et al. [60] suggested a coordination

method for ring and radial distribution network containing DG. But the above scheme suffers

with the problem of sympathy trips depending upon the location and level of fault [28]. Brito

et al. [135] discussed the impact of the insertion of DG in the protection coordination using

simulation software. James et al. [64] presented an analysis of several protection coordination

problems such as fault detection ability, the characteristic contribution of fault current, effects

of increased short circuit capacities and islanding of DGs due to integration of DGs into the

electric distribution system. However, no malfunctioning of protection device has been

reported due to incorporation of DGs. Thereafter, Rifat [109] has given several considerations

for utility/cogeneration inter-tie protection scheme. But these considerations are mainly for

distance relays. Salman et al. [124] presented an investigation on the impact of the integration

of embedded synchronous generators and embedded induction generators on the settings of

protective devices installed on the distribution systems. Afterwards, several case studies

related to the impact of DG on protection system have been presented by different authors

[55], [23], [145]. It has been observed that the conventional distribution protection schemes

do not have sufficient capability to protect radial distribution network with all possible

configurations and operating conditions of DG [110].

VRes

IR

VR VR

VY

VY

VB

VB

VRes = VR + VY+ VB IRes = IR + IY + IB

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Tong and his co-worker [45] presented a concept of FCL (Fault Current Limiter) to

limit the effect of DG on the coordinated relay protection scheme in a radial system. But the

proposed concept can be applicable only up to certain tolerable levels with reference to

tripping time. Manjula et al. [79] proposed an inverse time admittance relay based on line

admittance measurement to protect a distribution network with converter interfaced DG.

However, relay operation may be restrained beyond predefined value of fault resistance.

Further, the cost of distance relays is comparatively higher than overcurrent relays. However,

none of these schemes completely solved the problem of miscoordination between relay in

radial distribution systems in the presence of DG.

In order to avoid most of the above drawbacks, an attempt has been made to

demonstrate the concept of directional relay for the protection of radial distribution network

containing DGs. The proposed work was supported with the developed prototype of 3-phase

radial distribution system in laboratory environment along with their comparative evaluation

with the results obtained using PSCAD/EMTDC software package.

3.3 THE PROPOSED DIRECTIONAL RELAYING SCHEME

Figure 3.7 shows a single line diagram of the proposed directional relaying scheme.

The relays nearest to the utility and DG2 (R1 and R4) are non-directional whereas the

remaining relays (R2 and R3) are directional in nature as the fault current at a particular bus

changes its direction (away and toward the bus).

Figure 3.7 Single line diagram of the proposed directional relaying scheme with DG

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The directional feature is gained by comparing the direction of current flow in the line with

reference to the bus voltage. Thus, the directional overcurrent relay operates only when the

current flowing through the relay is in its correct direction and more than its pickup setting.

With reference to Figure 3.7, for a fault in section-1, relays R1 and R2 operate and disconnect

the section-1 only, while the section-2 remains in healthy condition.

During the implementation of the proposed scheme in the laboratory environment and

also in the computer simulation, the following assumptions have been carried out.

1. In the development of laboratory prototype, authors have used class one CTs and PTs

whereas for computer simulation in PSCAD/EMTDC, available “Jiles-Atherton” CT

model has been used.

2. The effect of saturation of CT on the performance of the proposed scheme during

computer simulation in PSCAD/EMTDC software package has not been considered.

On the other hand, the effect of CT saturation, depending upon the magnitude of fault

current, will affect the performance of relays used in developed laboratory prototype.

3. Relays used in the developed laboratory prototype are electromechanical in nature

whereas in the computer simulation, modules of static relays having similar

characteristic to that of electromechanical relays used in developed laboratory

prototype are used.

4. In the proposed developed laboratory prototype, state electricity board supply has been

used as utility. On the other hand, three-phase synchronous generators having different

capacities have been used as DGs.

As some system structure is required to produce realistic data, a small portion of large

power distribution system has been used. Due to practical limitations, only two sections for

the implementation of the proposed scheme in the laboratory have been chosen. However, the

idea of the proposed directional protection scheme can be further extended to larger

distribution network.

3.4 DEVELOPED LABORATORY PROTOTYPE OF THE PROPOSED

DIRECTION RELAYING SCHEME

Figure 3.8 shows laboratory prototype of the proposed directional relaying scheme.

Synchronous generator has been used as DG which is connected at Bus-B and Bus-C whereas

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the utility supply is connected at Bus-A. Radial distribution network is simulated using two

impedances namely Z1 and Z2. Star connected load bank is used as 3-phase load at Bus-B and

Bus-C. Two 3-phase tap loads (1 and 2) are taken from the middle of the section-1 and

section-2. Faults are simulated using four toggle switches namely S1, S2, S3 and S4. Variable

rheostat Rh1 is used as a fault current limiter for practical purposes. All the four relays are

connected in secondary side of Current Transformers (CTs). CT shorting switch is used to

check the back-up protection of a particular section of radial distribution network. Circuit

breakers are simulated by contactors. Different loads of the distribution network are protected

by Miniature Circuit Breakers (MCBs) M1, M2, M3 and M4 respectively. In order to protect

DG against short-circuit, another MCB M5 for DG2 and M6 for DG1 are used. Power circuit

of DG1 is same as DG2. Moreover, relays, CTs and impedances are also connected in other

Figure 3.8 Power circuit of laboratory prototype of the proposed directional relaying scheme

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two phases. However, due to complexity, it has not shown in Figure 3.8. Detailed

specifications of the different components used in the said laboratory prototype are given in

Appendix-B. Figure 3.9 shows control circuit of the said laboratory prototype.

Looking to Figure 3.8 and Figure 3.9 together, by pressing the spring loaded push

button of section-1 (PB1), contactor (C1) energises. The hold on path is being provided by

auxiliary contact of contactor C1-2 and hence, C1 remains in energised condition. Thus,

contact C1-1 in Figure 3.8 remains in close condition until contactor C1 will de-energise. The

contactor (C1) is de- energised using stop push button of section-1 (PB2).

The magnitude of fault current is very high and depends on the location of fault,

capacity of DG and fault resistance. For a fault in section-1 (by closing switch S1), the fault

current referred to secondary of CT1 exceeds pickup setting of the relay R1, and hence, relay

R1 operates. Closing of relay contact R1-1 energises auxiliary relay A1 in the control circuit

(Figure 3.9) and finally, the contact of auxiliary relay (A1-1) opens out. Opening of A1-1 de-

energises the contactor (C1), and hence, disconnecting the section-1 from the utility supply

side due to opening of the contact C1-1 (Figure 3.8). When the relay contact R1-1 opens out,

coil of auxiliary relay A1 remains energized due to hold on path provided by contact A1-2

(Figure 3.9). The coil of auxiliary relay A1 de-energises when reset push button PB3 is

pressed. For the same fault in section-1, due to contribution in the fault current from DGs,

current in secondary of CT2 also exceeds pickup setting of the directional relay R2. Therefore,

relay R2 operates which closes the relay contact R2-1. This energises auxiliary relay A2 in the

control circuit (Figure 3.9) and hence, auxiliary relay contact A2-1 opens out. Opening of A2-

1 de-energises the contactor (C2), and hence, disconnecting the section-1 from DG side due to

opening of the contact C2-1 (Figure 3.8). When relay contact R2-1 opens out, coil of auxiliary

relay A2 remains energized due to hold on path provided by its own contact A2-2 (Figure

3.9). The coil of auxiliary relay A2 de-energises when reset push button PB6 is pressed.

Similarly, the same procedure is followed for a fault in section-2 (Figure 3.8). Figure 3.10

shows pictorial view of the developed laboratory prototype for three phase radial distribution

network having two sections.

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Figure 3.9 Control circuit of laboratory prototype of the proposed directional relaying

Scheme

Figure 3.10 Developed laboratory prototype of three phase radial distribution network of the

proposed directional relaying scheme

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3.5 SOFTWARE SELECTION FOR MODELING OF THE SYSTEM

The software selected for this purpose is PSCAD (Power System Computer Aided

Design) which uses EMTDC (Electromagnetic Transient in DC System) [106]. This software

has the capability of performing interpolation between minimum time steps. It uses

trapezoidal methods for solving numerical integrations and differential equations. It manifests

the continuous oscillation of the node voltage (branch current) with changing direction in

every time step, which is not a representative of any electrical behavior of the network. It

enables the user to schematically construct a circuit, run a simulation, analyse the results, and

manage the data in a completely integrated, graphical environment. Online plotting functions,

controls, and meters are also included so that the user can alter system parameters during a

simulation run and view the results directly. It comes with a library of pre-programmed and

tested models, ranging from simple passive elements and control functions to more complex

models, such as electric machines, FACTS (flexible AC transmission systems) devices,

transmission lines, relays, cables and many power system devices. If a particular model does

not exist, it provides the flexibility of building custom models, either by assembling those

graphically using existing models, or by utilizing an intuitively designed design editor.

3.6 MODELING AND SIMULATION OF RADIAL DISTRIBUTION SYSTEMS IN

PRESENCE OF DG USING PSCAD AND LABORATORY PROTOTYPE

A part of the radial distribution system, as shown in Figure 3.7, has been used to access

the problems associated with radial system containing DGs and also to validate the proposed

scheme.

Figure 3.11 Simulation model of radial feeder with DG

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The distribution line parameters and the generating station details are given in

Appendix-C. Test data for verifying the proposed scheme have been generated by modeling

the complete system of Figure 3.7 using PSCAD/EMTDC software package [106]. Figure

3.11 shows simulated model of radial feeder containing DG with two bidirectional relays (R1

and R4) and two directional relays (R2 and R3). The distribution feeder is represented using the

Bergeron line model. For modeling the whole system, some of the main library components in

PSCAD such as utility source, DG and circuit breakers etc. have been used. They are designed

according to the collected data and specifications. Further, some self-created components such

as directional relays have been developed in PSCAD. Further, the updating of system

parameters is achieved using Multi-Run block available in PSCAD to generate multiple

simulation cases in single shot. The relays, as shown in Figure 3.7, are located at each end of

the distribution feeder. PS and TMS of each relay are calculated using IEC standard relay

characteristics equation. Plug setting of each relay is based on full load current of the feeder

during normal/prefault condition. Time Multiplier Setting of relay R1, R2, R3 and R4 are

calculated based on fault calculations. We have achieved 0.2, 0.1, 0.1 and 0.2 TMS for relay

R1, R2, R3 and R4, respectively. However, in order to avoid practical limitations, we have set

0.7, 0.6, 0.6 and 0.7 TMS for relay R1, R2, R3 and R4, respectively [18]. The performance of

the proposed scheme has been evaluated for various types of faults in each section at different

fault locations.

3.7 RESULT OBTAINED FROM EXPERIMENTAL PROTOTYPE AS WELL AS

FROM PSCAD/ EMTDC SOFTWARE PACKAGE

3.7.1 Performance of the Proposed Scheme during Varying Load Condition

The settings of directional and non-directional overcurrent relays should be done

according to the maximum balance and unbalance loading conditions. If this factor is not

considered than there is a possibility of mal-operation of relays in normal conditions.

Therefore, it is necessary to investigate the effect of varying load conditions on the

performance of the protective devices. Table 3.2 shows the performance of the proposed

scheme during varying unbalance load conditions. It has been observed from the Table 3.2

that during the said situations no relay operation has been observed.

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3.7.2 Faults within Section

Various types of faults have been simulated in two different sections (1 and 2) at

different fault locations using fault selector switches (S1, S2, S3 and S4). Table 3.3 to Table

3.7 show results obtained from experimental prototype as well as from PSCAD/EMTDC

software package for various types of faults at different fault locations in two different

sections with zero fault resistance. In all the tables, F1 and F2 indicate close-in fault in

section-1 and section-2, respectively, whereas F1’ and F2’ indicate remote end fault in

section-1 and section-2, respectively.

It has been observed from Table 3.3 to Table 3.7 that for a close-in and remote end

fault in the respective section (section-1 and section-2), the respective relays (R1 & R2 for

section-1 and R3 & R4 for section-2) operate and disconnect the faulty section. The

maloperation of bidirectional relays due to incorporation of DG has been eliminated by

directional relays. The operation of relay sequence is perfectly matched with the selectivity of

the protection system. It has been observed that for all types of close-in fault in section-1, the

operating time (Top) of local relay (R1) is lower than the remote-end relay (R2) as contribution

in fault current from utility side is significant, hence R1 operates first compare to R2. On the

other hand, for all types of remote end fault in section-1, the remote end relay (R2) operates

first than the local relay (R1) as the contribution in fault current from both DGs are significant

than the utility. Moreover, for all types of close-in and remote-end faults in section-2, the

local relay (R3) operates first than remote-end relay (R4) as contribution in fault current from

the utility and DG1 is significant than DG2. It has been observed from Table 3.3 to Table 3.7

that the time of operation of all the relays using PSCAD/ EMTDC software package is almost

equal to the time of operation of relays obtained using laboratory prototype.

3.7.3 High Resistance Fault

When an overhead distribution phase conductor breaks and falls on a high impedance

surface or trees, high impedance fault occurs [14]. The conventional overcurrent relays at the

radial distribution network may not be able to detect this type of fault and hence, relay does

not operate. Table 3.8 and Table 3.9 show results obtained for single line to ground (A-g) fault

at different locations in all the two sections with a fault resistance of 10 Ω and 18 Ω,

respectively. Figure 3.12 shows directional relay responses, which has been obtained from

PSCAD simulation, for a single line- to-ground fault at F2’ in section-2 with RF = 10 Ω.

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Figure 3.12 Response of directional relay for single line-to-ground fault (A-G) in section-2

with RF =10 Ω.

It has been observed from Table 3.8 and Table 3.9 that the time of operation of all the

relays increases as fault resistance increases. The proposed scheme gives satisfactory result in

the said condition. However, the operating time of all the relays is higher with reference to

low resistance fault. In certain conditions, it has been observed that the relay may not be able

to pickup in case of a fault with a very high value of fault resistance. The best available

solution for this situation is the usage of digital relays.

3.7.4 Backup Protection

Backup protection is extremely important for any protection system if primary

protection system fails to clear the fault within its own zone of protection. Backup protection

feature is simulated for each section and the sample results have been shown in Table 3.8 and

Table 3.9.

3.8 RESULT OBTAINED FROM PSCAD/ EMTDC SOFTWARE PACKAGE FOR

ACTUAL TMS

Time Multiplier Setting of relay R1, R2, R3 and R4 are calculated based on fault

calculations. We have achieved 0.2, 0.1, 0.1 and 0.2 TMS for relay R1, R2, R3 and R4, respectively. Table 3.10 to Table 3.12 show results obtained from PSCAD/EMTDC software

package for single line-to-ground fault at different fault locations in two different sections

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with a fault resistance of 0 Ω, 10 Ω and 18 Ω, respectively. The performance of the proposed

scheme has been evaluated for various types of faults in each section at different fault

locations. It has been observed from Table 3.10 to Table 3.12 that the time of operation of all

the relays decreases as TMS of all relays are less compare to results obtained from laboratory

prototype with higher TMS.

Moreover, the proposed scheme also provides back-up protection if primary protection

system fails to operate for a fault within its own zone. Backup protection feature is simulated

for each section and the sample results have been shown in Table 3.10 to Table 3.12.

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Table 3.2 Performance of the proposed scheme during variation in loading conditions

Table 3.3 Result obtained for A-g faults in different sections with RF = 0 Ω

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Table 3.4 Result obtained for A-B faults in different sections with RF = 0 Ω

Plug Setting (PS) of R1=75%, R2=50% ,R3=50% and R4=75% of IR (Relay rated current), Time Multiplier Setting (TMS) of R1=0.7, R2=0.6, R3=0.6 and R4=0.7,System Voltage (L-L) = 300 V, Frequency = 49.69Hz, RF = 0 ohm

Fault Location

Current (A)

Time of Operation (Top (s))

Using Laboratory Prototype


Using PSCAD

IUtility

IDG1

IDG2 IFault

R1 R2 R3 R4 R1 R2 R3 R4 Prototype PSCAD Prototype PSCAD Prototype PSCAD Prototype PSCAD

Pre-fault condition 0.97 1.00 0.5 0.48 0.5 0.62 - - NO NO NO NO NO NO NO NO

F1 24.37 24.75 4.0 4.4 3.60 3.9 29.60 30 1.93 2.36 - - 1.7 2.08 - - F1’ 11.49 11.8 5.45 5.9 5.10 5.3 19.50 19.8 2.65 2.03 - - 2.35 1.75 - -

F2 11.55 11.7 5.30 5.9 5.20 5.34 19.45 19.65 - - 1.85 4.20 - - 1.53 3.93 F2’ 6.25 6.46 3.0 3.25 5.80 6.1 11.20 11.57 - - 2.30 3.75 - - 2.0 3.52

NO: No Operation

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Table 3.5 Result obtained for A-B-g faults in different sections with RF = 0 Ω


Fault Location

Current (A)




Using PSCAD

IUtility

IDG1 IDG2 IFault



F1 23 23.5 4.5 4.8 4.0 4.3 28 28.5 1.96 2.42 - - 1.73 2.18 - - F1’ 10.7 11 6.0 6.33 5.4 5.8 18.1 18.5 2.73 2.17 - - 2.46 1.95 - -

F2 10.8 10.9 5.9 6.3 5.2 5.7 17.8 18.3 - - 1.97 4.35 - - 1.73 4.20 F2’ 5.7 6.0 3.1 3.5 6.4 6.6 9.9 10.3 - - 2.50 3.96 - - 2.21 3.75

NO: No Operation

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Table 3.6 Result obtained for A-B-C faults in different sections with RF = 0 Ω


Fault Location

Current (A) Time of Operation (Top (s))


Time of Operation (Top (s)) Using PSCAD

IUtility

IDG1

IDG2 IFault



F1 27.90 28.27 4.60 5.0 4.20 4.53 34.70 35.06 1.84 2.23 - - 1.62 1.96 - - F1’ 13.0 13.4 6.35 6.7 5.85 6.03 22.85 23.3 2.28 1.90 - - 2.21 1.65 - - F2 12.70 13.22 6.35 6.68 5.95 6.05 22.60 23.07 - - 1.70 3.80 - - 1.45 3.57 F2’ 7.0 7.25 3.30 3.62 6.40 6.81 12.95 13.5 - - 2.10 3.45 - - 1.87 3.2

NO: No Operation

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Table 3.7 Result obtained for A-B-C-g faults in different sections with RF = 0 Ω


Fault Location

Current (A) Time of Operation (Top (s))



IUtility

IDG1

IDG2 IFault



F1 26.55 27.0 4.50 4.8 4.0 4.33 32.90 33.28 1.87 2.25 - - 1.65 2.0 - - F1’ 12.30 12.64 6.0 6.33 5.20 5.7 21.30 21.8 2.40 1.94 - - 2.28 1.70 - - F2 12.10 12.48 5.80 6.3 5.4 5.7 21.10 21.62 - - 1.75 3.95 - - 1.48 3.62 F2’ 6.60 7.0 3.10 3.5 6.10 6.6 12.30 12.98 - - 2.30 3.50 - - 1.90 3.25

NO: No Operation

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Table 3.8 Result obtained for A-g faults in different sections with RF =10 Ω

Plug Setting (PS) of R1=75%, R2=50% ,R3=50% and R4=75% of IR (Relay rated current), Time Multiplier Setting (TMS) of R1=0.7, R2=0.6, R3=0.6 and R4=0.7,System Voltage (L-L) = 300 V, Frequency = 49.69Hz, RF =10 ohm

Fault Location

Current (A)

Time of Operation (Top (s)) Using Laboratory

Prototype


IUtility

IDG1

IDG2 IFault



F1 11.05 11.4 1.93 2.18 1.75 1.96 12.85 13.2 2.78 3.85 - - 2.38 3.45 - - F1’ 6.05 6.5 2.90 3.22 2.60 3.0 9.63 10.11 3.73 2.82 - - 3.23 2.52 - - F2 5.70 6.46 2.60 3.22 2.90 3.03 9.40 10.07 - - 2.57 5.65 - - 2.01 5.15 F2’ 4.40 4.68 2.05 2.32 3.95 4.22 7.20 7.77 - - 2.85 4.72 - - 2.45 4.22

Backup Protection For First Section (F1’)

6.05 6.5 2.90 3.22 2.60 3.0 9.63 10.11 3.73

If Relay

R2 Fails

- 6.60 3.33

If Relay

R2 Fails

- 6.28

Backup Protection

For Second Section (F2’)

4.40 4.68 2.05 2.32 3.95 4.22 7.20 7.77 4.8 -

If Relay

R3 Fails

5.20 4.3 -

If Relay

R3 Fails

4.85

NO: No Operation

63

Table 3.9 Result obtained for A-g faults in different sections with RF = 18 Ω

64

Table 3.10 Simulation result obtained for A-g faults in different sections with RF = 0 Ω

Plug Setting (PS) of R1=75%, R2=50% ,R3=50% and R4=75% of IR (Relay rated current) Time Multiplier Setting (TMS) of R1=0.2, R2=0.1, R3=0.1 and R4=0.2 System Voltage (L-L) = 300 V, Frequency = 50 Hz, RF =0 Ω

Fault Location

Current (A) Time of Operation (Top (s)) Using Pscad

IUtility IDG1 IDG2 IFault R1 R2 R3 R4 Pre-fault condition 1.00 0.48 0.62 - NO NO NO NO

F1 26.9 4.8 4.3 33.2 0.48 0.35 - - F1’ 12.64 6.33 5.7 21.82 0.65 0.29 - - F2 12.48 6.3 5.73 21.62 - - 0.25 1.04 F2’ 7.01 3.5 6.57 12.97 - - 0.32 0.93


12.64 6.33 5.7 21.82 0.67 If Relay

R2 Fails

- 1.08

Backup Protection


7.01 3.5 6.57 12.97 0.93 - If Relay

R3 Fails

0.93

NO: No Operation

65



Fault Location


IUtility IDG1 IDG2 IFault R1 R2 R3 R4 Pre-fault condition 1.00 0.48 0.62 - NO NO NO NO

F1 11.4 2.18 1.96 13.2 0.7 0.74 - - F1’ 6.5 3.22 3.0 10.11 0.9 0.42 - - F2 6.46 3.22 3.03 10.07 - - 0.35 1.36 F2’ 4.68 2.32 4.22 7.77 - - 0.42 1.15


6.5 3.22 3.0 10.11 0.96 If Relay

R2 Fails

- 1.83

Backup Protection


4.68 2.32 4.22 7.77 1.23 - If Relay

R3 Fails

1.41

NO: No Operation

66



Fault Location


IUtility IDG1 IDG2 IFault R1 R2 R3 R4

Pre-fault condition 1.07

0.48

0.62

- NO NO NO NO

F1 7.91 1.59 1.44 8.77 0.84 1.07 - - F1’ 4.94 2.43 2.32 7.18 1.07 0.55 - - F2 4.91 2.43 2.34 7.15 - - 0.42 1.64 F2’ 3.84 1.90 3.38 5.94 - - 0.51 1.32


4.94 2.43 2.32 7.18 1.17 If Relay

R2 Fails

- 2.59

Backup Protection


3.84 1.90 3.38 5.94 1.51 -

If Relay

R3 Fails

1.84

NO: No Operation

67

3.9 MODELING AND SIMULATION OF A LARGE 11 KV RADIAL

DISTRIBUTION SYSTEMS IN THE PRESENCE OF DG USING PSCAD

3.9.1 System Description of a Large 11 kV Radial Distribution Systems in the Presence

of DG

A part of the Indian 11 kV radial distribution system, as shown in Figure 3.7 and

Figure 3.8, has been used to access the problems associated with radial system containing

DGs and also to validate the proposed scheme. The distribution line parameters and the

generating station details are given in Appendix-D. Test data for verifying the proposed

scheme have been generated by modeling the complete system of Figure 3.7 using the

PSCAD/EMTDC software package [106]. The performance of the proposed scheme has been

evaluated for various types of faults in each section at different fault locations. Relay

responses for some special cases such as high resistance fault and backup protection were also

investigated.

3.9.2 Modeling of Bidirectional Relays in PSCAD

Figure 3.13 shows simulated model of radial feeder containing DG with the

conventional (bidirectional) relays. Figure 3.14 shows the tripping logic of bidirectional

relays.

With reference to Figure 3.14, when a current through a bidirectional relay (51)

exceeds the pick-up setting of relay (details are given in Appendix-D), it generates a constant

signal (logic 1) through Hysteresis-buffer. The output of the respective phase relay is given to

OR gate. Depending upon the output (logic 1/logic 0) of any phase relay, OR gate generates a

trip signal which will be further given to the respective breakers of the respective sections

(B1, B2, B3). In order to check back up protection of each section, two state (0-off, 1-on)

selector switch is used which bypass the relay of a particular section of the respective phase.

Figure 3.13 Simulated model of bidirectional relay

68

Figure 3.14 Tripping logic of bidirectional relay

3.9.3 Modeling of Directional Relays in PSCAD

Figure 3.15 shows simulated model of radial feeder containing DG at Bus-2, Bus-3

and Bus-4 with two bidirectional relays (R1 and R6) and four directional relays (R2, R3, R4 and

R5). Figure 3.16 shows the tripping logic of directional relays.

In this logic, bidirectional relays are made directional through a logic circuit.

Generally, 90° connection scheme is used in two phase overcurrent and one earth fault

protection scheme. The directional feature is accomplished by comparing the phase angle

between the current of the faulted phase and line voltage of healthy phase in phase

comparator block. For example, for fault in phase-A in section-1, phase angle (P2A) of

current (IT2) in phase A is compared with phase angle (E2B) of line voltage (Eb2) of other

two lines (Figure 3.15 & 3.16).The FFT (Fast Fourier Transformations) block has been used

in order to obtain phase angle between the voltage and the current quantity of the respective

phase.

Figure 3.15 Simulated model of directional relay

69

Figure 3.16 Tripping logic and phase comparator block of directional relay

Line currents of each section are fed to the respective relay of the respective phase

through CTs. As shown in Figure 3.16, in order to determine the region of operation of

directional relay, the phase difference between current and voltage which is shifted by +90° is

fed to the trigonometric ‘COS’ function. If this phase difference is within the forward limit

than the output of AND gate of the respective phase (T2A) is made positive (high) which is

used to activate the two-state switch. With this situation of two-state switch, it allows the CT

secondary current to pass through the respective relay of that phase (phase A) otherwise two-

state switch remains inoperative. If fault current is in forward direction and line current fed to

the relay from respective CT through two-state switch exceeds the pick-up setting than the

respective directional relay generates a spike which will be made constant by Hysteresis-

Buffer. Thereafter, the output of each phase relay is given to OR gate. Depending upon the

output (logic 1) of the respective phase relay, OR gate generates a final tripping signal which

will be given to the respective breaker of the respective section (B2, B3, B4, B5).

To check the backup protection in a particular section, another ON- OFF switch

(R2A) is used in phase comparator block. Trip characteristic constants of each relay are set in

such a way that relay gives normal inverse characteristic. PS and TMS of each relay is set

such that relay R3 gives back up to relay R5 and relay R1 gives back up to relay R3 for

downstream faults. Similarly relay R4 gives back up to relay R2 and relay R6 gives back up to

relay R4 for upstream faults.

70

3.10 RESULTS AND DISCUSSION

3.10.1 Bidirectional Relay

Table 3.13 shows simulation results obtained in terms of fault currents and Time of

Operation (Top) of three bidirectional relays for faults at different locations in three different

sections with zero fault resistance. The radial feeder containing DG is protected with three

bidirectional over current relays namely R1, R2 and R3 as shown in Figure 3.13. The fault

locations F1, F3 and F5 indicate close-in fault whereas F2, F4 and F6 indicate remote end

fault in section-1, section-2 and section-3, respectively.

Table 3.13 Simulation results of bidirectional relay for L-G faults in different sections with

RF = 0 Ω

Plug Setting (PS) of R1=100% of IR , R2=75% of IR , R3=50% of IR (Relay Rated

Current = 1 A)

Time Multiplier Setting (TMS) of R1=0.15, R2=0.125, R3=0.1

Fault

Location

RF = 0 Ω Time of Operation

(Top (s))

IU (A) IDG (A) IF (A)

R1

R2

R3 A B C A B C A B C

Pre-fault

condition 152 152 152 31 31 31 - - - NO NO NO

F1 4019 217 286 201 87 34 4184 - - 0.1239 - 0.2035

F2 1414 185 235 207 64 12 1583 - - 0.1782 - 0.1902

F3 1408 186 234 206 64 12 1576 - - - 0.1370 0.1902

F4 785 177 225 230 62 11 968 - - - 0.1752 0.173

F5 784 177 225 230 62 11 966 - - - - 0.1034

F6 563 172 224 266 65 12 762 - - - - 0.1186

Backup

Protection

For Third

Section (F5)

784 177 225 230 62 11 - - - 0.1759

If Relay

R3

fails

NO: No Operation

71

It has been observed from Table 3.13 that for a close-in and remote end fault in

section-1 (F1 and F2), relay R1 operates first as the contribution of fault current from the

utility side (IU) is significant (strong source). On the other hand, relay R3 operates after some

time delay as the contribution of fault current from DG (weak source) is comparatively lower

than the utility. With the present structure of radial distribution network along with DG, relay

R1 and relay R2 has to trip for a fault in section-1. But, the breaker in section-3 trips

unnecessary due to operation of relay R3. This mal-operation occurs due to the presence of

DG on the other side, which is against the selectivity of the protection system.

Correspondingly, for a close in and remote end fault in section-2 (F3 & F4) relay R2 and relay

R3 operate whereas for a close in and remote end fault in section-3 (F5 & F6), relay R3 and

local protection of DG (Fuse/MCB) operate.

It is to be noted that as the relay is located near the DG, the contribution in fault

current from DG is higher. This would not create a problem in maintaining selectivity as one

can easily disconnect DG when its penetration is low. On the other hand, when penetration of

DG is high and more than one DG is connected at different location, the conventional relay is

not in a position to maintain the proper selectivity. This problem is rectified using the

proposed scheme which works perfectly even during different DG capacities and also during

more than one DG connected at various locations.

3.10.2 Directional Relay

Table 3.14 to Table 3.17 show the simulations results for L-G, L-L-G, L-L-L-G and

L-L faults at different fault locations in three different sections with zero fault resistance. The

radial feeder containing DG is protected with four directional over current relays R2, R3, R4,

R5 and two bidirectional relays R1 and R6 as shown in Figure 3.15.

It has been observed from Table 3.14 to Table 3.17 that for a fault in section-1,

section-2 and section-3 at any locations relay R1 & relay R2, relay R3 & relay R4, relay R5 &

relay R6 operate and disconnect the faulty section, respectively. The maloperation of

bidirectional relays due to incorporation of DG has been eliminated by the proposed scheme.

The operation of relay sequence is perfectly matched with the selectivity of the protection

system. Further, it has been observed from Table 3.14 to Table 3.17 that for all close-in and

remote-end faults in all the three sections, the local end relay (R1, R3 and R5 ) operates first

than the remote-end relay (R2, R4 and R6) as the contribution of fault current from the utility

(IU) is significant (strong source). Figure 3.17 shows directional relay responses, which has

been obtained from PSCAD simulation, for a single line- to-ground fault at F4 in section-2

with RF = 0 Ω.

72

Figure 3.17 Response of directional relay for single line-to-ground fault (A-G) in section-2

with RF = 0 Ω

3.10.3 Backup Protection

Backup protection is extremely important for any protection system if primary

protection system fails to clear the fault within its own zone of protection. Backup protection

feature is simulated for section-3 and the sample result has been shown in Table 3.14.

3.10.4 High Resistance Fault

When an overhead distribution phase conductor breaks and falls on a high impedance

surface or trees, high impedance fault occurs [14]. The conventional overcurrent relays at the

radial distribution network may not be able to detect this type of fault and hence, relay does

not operate. To analyze this condition, a case study has been set up and a single line-to-

ground fault with a fault resistance equal to 15 has been simulated. However, in practice,

this value may exceed 30 Ω. Table 3.18 shows the simulation results for L-G faults at

different locations in three different sections with a fault resistance of 15 Ω. It has been

observed from Table 3.18 that for a fault in any section, the primary protection relay of that

section operates first and clears the fault within its own zone. Though, a slower relay

performance is achieved, the proposed scheme operates successfully.

73

Table 3.14 Simulation results of directional relay for L-G faults in different sections with RF = 0 Ω

Plug Setting (PS) of R1=50% of IR, R3=75% of IR , R5=100% of IR , R2=100% of IR, R4=75% of IR , R6=50% of IR (Relay Rated Current = 1 A)

Time Multiplier Setting (TMS) of R1=0.1, R3=0.125, R5=0.15, R2=0.15, R4=0.125, R6=0.1

Fault

Location

RF = 0 Ω Time of Operation (Top (s))

IU (A) IDG 2 (A) IDG3 (A) IDG4 (A) IF (A) R1 R2 R3 R4 R5 R6

A B C A B C A B C A B C A B C

Pre - fault

condition 122 120 120 14 14 14 22 22 22 25 25 25 - - - NO NO NO NO NO NO

F1 4020 177 268 202 19 19 144 30 30 120 33 33 4410 - - 0.1266 0.2228 - - - -

F2 1244 182 310 290 19 19 200 30 30 163 33 33 1840 - - 0.1900 0.2042 - -

- -

F3 1235 185 308 288 19 19 200 30 30 163 33 33 1827 - - - - 0.1410 0.2162 - -

F4 649 154 270 133 19 19 275 30 30 220 33 33 1222 - - - - 0.1802 0.1838 - -

F5

648 156 268 132 19 19 274 30 30 220 220 33 33 - -

-

-

- - 0.094 0.2725

Back up

protection 0.1776 -

If R5

fails 0.2725

F6 419 137 231 77 19 19 157 30 30 272 272 33 33 - - - - - - 0.1132 0.2506

NO: No Operation

74

Table 3.15 Simulation results using directional relay for L-L-G faults in different sections with RF = 0 Ω



Fault

Location


IU (A) IDG2 (A) IDG3 (A) IDG4 (A) IF (A) R1 R2 R3 R4 R5 R6


Pre -

fault

condition

122 120 120 14 14 14 22 22 22 25 25 25 - - - NO NO NO NO NO NO

F1 4790 4838 237 186 186 20 146 146 30 129 129 33 5156 5156 - 0.1204 0.2223 - - - -

F2 1433 1405 279 252 252 20 186 186 30 160 160 33 1931 1931 - 0.1780 0.1769 - - - -

F3 1413 1400 279 250 250 20 186 186 30 160 160 33 1901 1901 - - - 0.1288 0.225 - -

F4 731 698 246 120 120 20 246 246 30 205 205 33 1211 1211 - - 0.1710 0.1972 - -

F5 728 698 246 120 120 20 245 245 30 205 205 33 1201 1201 - - - - - 0.095 0.2862

F6 479 444 213 72 72 20 144 144 30 244 244 33 828 828 - - - - 0.1122 0.2490

NO: No Operation

75

Table 3.16 Simulation results using directional relay for L-L-L-G faults in different sections with RF = 0 Ω



Fault

Location



A B C A B C A B C A B C

Pre -

fault

condition

122 120 120 14 14 14 22 22 22 25 25 25 - - - NO NO NO NO NO NO

F1 5236 5236 5236 173 173 173 144 144 144 130 130 130 5626 5626 5626 0.1186 0.2221 - - - -

F2 1532 1532 1532 212 212 212 173 173 173 155 155 155 2019 2019 2019 0.1741 0.1844 - - - -

F3 1517 1517 1517 211 211 211 173 173 173 155 155 155 2001 2001 2001 - - 0.1273 0.2283 - -

F4 767 767 767 105 105 105 212 212 212 188 188 188 1218 1218 1218 - - 0.1681 0.1968 - -

F5 765 765 765 105 105 105 211 211 211 188 188 188 1211 1211 1211 - - - - 0.0938 0.2993

F6 495 495 495 67 67 67 132 132 132 212 212 212 826 826 826 - - - - 0.1105 0.2745

NO: No Operation

76

Table 3.17 Simulation results using directional relay for L-L faults in different sections with RF = 0 Ω



Fault

Location




Pre -

fault

condition

122 120 120 14 14 14 22 22 22 25 25 25 - - - NO NO NO NO NO NO

F1 4593 4477 164 156 156 20 134 134 30 123 123 33 4873 4873 - 0.1207 0.2630 - - - -

F2 1381 1271 159 190 190 20 159 159 30 145 145 33 1749 1749 - 0.1824 0.2152 - - - -

F3 1365 1262 159 189 189 20 159 159 30 145 145 33 1733 1733 - - - 0.1313 0.2560 - -

F4 715 614 154 97 97 20 193 193 30 174 174 33 1055 1055 - - - 0.1757 0.2186 - -

F5 711 613 154 97 97 20 193 193 30 174 174 33 1049 1049 - - - - - 0.097 0.3233

F6 478 385 152 64 64 20 124 124 30 194 194 33 715 715 - - - - - 0.1158 0.2953

NO: No Operation

77

Table 3.18 Simulation results using directional relay for L-G faults in different sections with RF =15 Ω

Plug Setting (PS) of R1=50% of IR, R3=75% of IR , R5=100% of IR , R2=100% of IR, R4=75% of IR , R6=50% of IR (Relay Rated

Current = 1 A) Time Multiplier Setting (TMS) of R1=0.1, R3=0.125, R5=0.15, R2=0.15, R4=0.125, R6=0.1

Fault

Location

RF =15 Ω Time of Operation (Top (s))



Pre -

fault

condition

122 120 120 14 14 14 22 22 22 25 25 25 - - - NO NO NO NO NO NO

F1 475 129 120 31 31 31 33 33 33 33 33 33 409 - - 0.3607 0.7924 - - - -

F2 357 149 134 71 31 31 60 33 33 54 33 33 379 - - 0.5026 0.8624 - - - -

F3 354 149 134 71 31 31 60 33 33 55 33 33 376 - - - - 0.2817 0.5978 - -

F4 285 150 143 50 31 31 97 33 33 84 33 33 351 - - - - 0.3699 0.5312 - -

F5 284 150 143 50 31 31 97 33 33 84 33 33 349 - - - - - - 0.1526 0.4592

F6 252 148 147 41 31 31 78 33 33 125 33 33 326 - - - - - - 0.1703 0.3773

NO: No Operation

78

3.11 EFFECT OF CHANGE IN CAPACITY OF DG

In order to analyze the effect of penetration of DG on the proposed scheme, different

capacities of DG in terms of total load on the radial feeder have been considered. Hence, 21%

capacity of DG indicates that out of total load on the feeder, DG supplies 21% whereas 79%

of total load is supplied by the utility. The simulation has been carried out for four different

DG capacities viz., 21%, 33%, 50% and 59% of the total load on the feeder.

Figure 3.18 and Figure 3.19 show the simulation results in the form of time of

operation of relay R1 and relay R2 for the said four different DG capacities for L-G fault

(minimum fault current) and L-L-L-G fault (maximum fault current) at location F1 & F2.

The simulation results in the form of time of operation of relay R3 and relay R4 for the

said four different DG capacities for L-G fault (minimum fault current) and L-L-L-G fault

(maximum fault current) at location F3 & F4 is shown in Figure 3.20 and Figure 3.21,

respectively.


operation of relay R5 and relay R6 for the said four different DG capacities for L-G fault

(minimum fault current) and L-L-L-G fault (maximum fault current) at location F5 & F6.

It has been observed from Figure 3.18 to Figure 3.23 that, as the penetration of DG

increases, time of operation of relays R1, R3 and R5 increases whereas time of operation of

relays R2, R4 and R6 reduces. The proposed scheme gives satisfactory result in the said

condition.

Figure 3.18 Effect of change in capacity of DG on time of operation of relay R1 and relay R2

for L-G fault at F1 & F2

79

Figure 3.19 Effect of change in capacity of DG on time of operation of relay R1 and relay

R2 for L-L-L-G fault at F1 & F2

Figure 3.20 Effect of change in capacity of DG on time of operation of relay R3 and relay R4 for L-G fault at F3 & F4

80


R4 for L-L-L-G fault at F3 & F4


R6 for L-G fault at F5 & F6

81

Figure 3.23 Effect of change in capacity of DG on time of operation of relay R5 and relay R6

for L-L-L-G fault at F5 & F6

3.12 EFFECT OF CHANGE IN LOCATION OF DG

Different possible combinations of DGs with their appropriate capacities at various

buses have been considered. These are as under.

Case-I: At a time only one DG is connected to Bus 2, Bus 3 and Bus 4.

Case-II: Two DGs are connected at Bus 3 & Bus 4, Bus 2 & Bus 4 and Bus 2 & Bus 3,

respectively at a particular time.

Case-III: Three DGs are connected to Bus 2, Bus 3 and Bus 4 simultaneously.

The performance of the proposed scheme has been checked for all types of faults (L-

G, L-L, L-L-G, L-L-L and L-L-L-G) at different fault locations (close-in faults F1, F3, F5 and

remote-end faults F2, F4, F6) in three different sections with zero fault resistance. However,

as it is not feasible to include all of the simulation results due to space limitations, the relay

trajectories for only close-in faults in three different sections are given in graphical form.


operation of relay R1 and relay R2 for L-G fault (minimum fault current) and L-L-L-G fault

(maximum fault current) at location F1, respectively, considering all three different

combinations of DGs as mentioned above.

82

Furthermore, the simulation results in the form of time of operation of relay R3 and

relay R4 for L-G fault (minimum fault current) and L-L-L-G fault (maximum fault current) at

location F3, are shown in Figure 3.26 and Figure 3.27 respectively, considering all three

different combinations of DGs as mentioned above.


operation of relay R5 and relay R6 for L-G fault (minimum fault current) and L-L-L-G fault

(maximum fault current) at location F5, respectively, considering all three different

combinations of DGs as mentioned above.

It has been observed from Figure 3.24 to Figure 3.29 that, as location of DG/DGs is

away from the utility, the chances of mis-coordination of relay increases. However, in the

proposed scheme, though the time of operation of relays (R2, R4 and R6) which operates due

to current fed by DG/DGs is higher, proper coordination between relays have been maintained

for L-G and L-L-L-G faults. The proposed scheme gives satisfactory result in the said

condition.

Figure 3.24 Effect of change in location of DG on time of operation of relay R1 and relay R2

for L- G fault at F1

83

Figure 3.25 Effect of change in location of DG on time of operation of relay R1 and relay

R2 for L-L-L-G fault at F1


R4 for L- G fault at F3

84


R4 for L-L-L- G fault at F3

Figure 3.28 Effect of change in location of DG on time of operation of relay R5 and relay R6

for L- G fault at F5

85


R6 for L-L-L- G fault at F5

3.13 CONCLUSION

This chapter deals with the study of the integration of DG into radial distribution

networks. A new directional relaying scheme for radial distribution network in the presence of

DG which avoids miscoordination between relays has been presented. For different

configurations of relay-relay coordination, detailed analysis of the directional relaying scheme

is carried out taking into account the effects of various types of faults in different sections,

single line-to-ground fault at different fault locations with high resistance, capacities of DG

and location of DGs. The proposed scheme has also been verified using data that was

generated by modeling an existing power distribution system using PSCAD/EMTDC software

package. It has been observed that the time of operations of all the relays of radial distribution

network obtained from the developed laboratory prototype for different fault locations in

various sections have been found to be in close conformity with the with the simulation results

obtained using PSCAD. Further, the effect of high resistance fault on relay operation has been

analyzed for faults at different locations in different line sections. It has been observed that the

possibility of operation of IDMT overcurrent relays reduces as the value of fault resistance

86

increases. Moreover, the proposed scheme also provides back-up protection if primary

protection system fails to operate for a fault within its own zone.

The proposed scheme was also tested extensively by using realistic data that was

generated by modeling an existing Indian 11 kV radial distribution system using

PSCAD/EMTDC software packages. The effect of penetration of DG as well as various

locations of DG(s) has been analyzed with reference to coordination between relays. It has

been observed that as the penetration of DG increases, the local end relay (R1, R3 and R5 )

operates first than the remote-end relay (R2, R4 and R6) as the contribution of fault current

from the utility (IU) is significant (strong source). Further, it has been observed that as the

location of DG/DGs is away from the utility, the chances of mis-coordination of relay

increases. It is to be noted that the proposed directional relaying scheme for radial distribution

network has the ability to resolve the problem of miscoordination of the conventional

overcurrent relays of radial distribution system in the presence of DG.

chapter 3 a new directional over current...

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