1

Overall workshop Objectives

At the end of the course, participants should be able to;

1 Determine fault level of a Distribution Network

2 Select the correct type of CT for Distribution Network

protection scheme(s)
Identify the right protection schemes for the Distribution networks

3 Describe Differential protection and its applications in

Distribution Network

4 Coordinate Protection systems considering different zones in

Distribution protection system
5
Target Group : Distribution Companies
Executives
Duration : 6.50 hour 2
2
Workshop Content

Relay Configuration,
Brief Introduction to 1 Coordination and Calibration
Protection

Fault level calculation in 2 5 Zones of Protection

simple Distribution network

Basic Protection schemes in

3 4
Instrument Transformers Distribution Network

3
3
 AGENDA
Time Activities Remarks

10.00-13.00hrs 1. Opening/Self introduction – 30 minutes Presentation,

Demonstration,
2. Brief Introduction to Protection – 30 minutes
Discussion and case
3. Fault Level Calculation – 1 hour study methods

4. Instrument Transformers – 1 hour

METHODOLOGY
1. Presentation/Demonstration
2. Collaborative Discussion/Questioning
3. Case Study
4. Simulation 13.00-14.00hrs Break time for Prayers and Lunch -

LEARNING STYLE 14.00-16.00hrs 5. Protection Schemes in Distribution – 1.5hrs Case study, Simulation
and Demonstration
Virtual Learning via Zoom or Google Meet 6. Relay Configuration, Coordination and
methods
Calibration – 0.5hrs
synchronous platforms.

16.00-16.30hrs Closing – 0.5hrs -

4
 NATIONAL POWER TRAINING INSTITUTE OF NIGERIA
………………….power trainer with a difference
Power System Protection Workshop for Distribution Companies Executives

N
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S T
LA
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EL L

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V AU

LC
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5
Did you Know that a detailed fault study Can;

a v e y ou
S of
li o n s
mil
o l la rs?
D

Prevent accident
at work place?

Let us take you through how to realize that.

 Learning Outcomes
By the end of this workshop, you should be able to:

2.
Identify the causes of faults in the
distribution Network

1. 3.
Identify and describe the Perform Fault Calculation
various types of fault in the for different types of faults
distribution network

Target Group : Distribution Companies

Executives
Duration : 1 hour 7
 Course Content

03
01
Characteristics of faults
Introduction
Causes of Fault,
Effect of fault.
04
02
Necessity of fault calculation
Types of

faults

05
Case study on fault
current calculation
with simulations on
Powerfactory
8
 Introduction
.
Distribution substations as a target of study, consists of some elements
like Lines, Bus Bars, Transformers, Outgoing Feeders, and Bus Couplers.
Regardless of the design and the systematic preventive maintenance
procedures instituted, failures due to abnormal or fault conditions do
occur.
Therefore, the system must be protected against flow of heavy short
circuit currents, which can cause permanent damage to major equipment,
by disconnecting the faulty section of system by means of circuit breaker
and protective relaying.
Hence, the need for a reliable detailed fault studies and calculations to
enable proper choice and selection of the protective switchgears.
9
 Introduction: Causes of Power System Faults
• Healthy insulation in the equipment is subjected to either
transient over voltages of small time duration due to
switching and lightning strokes, direct or indirect. Failure of
Overvoltages insulation may happen, resulting in very high fault current.
This current may be more than 10 times the rated or nominal
current of the equipment.

• Aging of power equipment may cause breakdown of its

Insulation Aging insulation even at normal power frequency voltage.
• External object such as bird, kite, or tree branch are
considered as external cause of fault. These objects may span
External Causes one conductor and ground causing single line to ground fault
(phase-earth) or span two conductors causing phase-phase
fault

• Wrong sequence of operation such as closing breaker with

Operator’s Mistake the grounding leads in position after a maintenance work can
lead to a fault.
10
 Effect of Fault
.

Due to overheating and the mechanical forces developed by

faults, electrical equipment such as bus bars, generators,
transformers will be damaged
Source: Mumbai mirror
Negative sequence current arising from unsymmetrical faults
will lead to overheating.

Voltage profiles may be reduced to unacceptable limits as a

result of faults and frequency drops due to faults may lead to
instability

Source: EC&M
Broad Categorization of Faults Types
Symmetrical or Asymmetrical or
balanced faults unbalanced
and faults

Unbalanced Faults may be classified into:

These rarely occurs as majority of Faults are a. Shunt Faults(short-circuit)
Unbalanced. E.g b. Series Faults
Line - Line - Line (5%) i.e. 3Ø; Ia + Ib + Ic = 0
and Va = Vb = Vc
The causes of 3Ø faults are: a. Shunt (Short circuit) Faults:
a. System energization with maintenance Line to Ground
Earthing clamps still connected. Line to Line
b. 1Ø Faults developing into 3Ø Faults Line to Line to Ground
b. Series or Open Circuit Faults:
Single Phase Open Circuit or
Double Phase Open Circuit
12
Characteristics of Faults
 A fault is characterized by:

1. Magnitude of the fault 2. Power factor or phase angle

current of the fault current
• The capacity and magnitude of the generating • For phase faults - the nature of the source
sources feeding into the fault. and connected circuits up to the fault
• The system impedance up to the point of fault or location
source impedance behind the fault
• Type of fault
• For ground faults - the type of system
• System grounding, number and size of overhead
ground wires. grounding in addition to above
• Fault resistance or resistance of the earth in the
case of ground faults and
• Arc resistance in the case of both phase and
ground faults
13
 Necessity for Fault Calculation
 Fault calculations are done primarily due to the following reasons:
5.
To co-ordinate the relay
4. settings in the overall
protection scheme of
To select the appropriate the system
3. relay settings of the
protection scheme
To determine the type of
2. protection scheme to be
deployed
To select the type of
1. circuit breaker
depending upon the
To determine the nature and type of fault.
maximum fault current at
the point of installation of
a circuit breaker and to
choose a standard rating
for the circuit breaker The calculation is not only limited to present system requirements but also to meet:
(rupturing) The future expansion schemes of the system such as addition of new generating units.
Construction of new transmission lines to evacuate power.
Construction of new lines to meet the load growth and or construction of interconnecting tie lines.
14
Methods of Fault Calculation

Per unit /Percenrage reactance

Actual reactance or
or Impedance method
Impedance method

3. VRE Integration Symmetrical

Components

5. Electricity
Promotion
Markets

15
 Per Unit Impedance system
 Power system quantities such as voltage, current and impedance are often expressed in
per unit or percent of specified values.
 Per unit quantities are calculated as: Actual Value
Per Unit Value 
Base Value

S I V Z
Per Unit Values S pu  I pu  Vpu  Z pu 
S base I base Vbase Z base

Conversion of Per Unit Values

2
Z Sbase Vbase
Z pu   2 Z ..(1) Z  Z base Z pu  Z pu ..(2)
Z base Vbase Sbase
16
 Percentage Impedance system

 Usually, the nominal apparent power (S) and nominal voltage (V) are taken as the base
values; for power (Sbase) and voltage (Vbase).
 The base values for the current (Ibase) and impedance (Zbase) can be calculated based on the
first two base values expressed in percentage

Z actual
Z%   100%..(3)
Z base
Example: Given that a synchronous generator has its nominal voltage as 13.8 kV , instead of
saying the actual voltage is 12.42 kV, we say the voltage is 0.9 p.u. or 90% of the nominal
value. 17
 Per unit system 3Ø Circuit
 Usually, the 3-phase SB or MVAB and line-to-line VB or kVB are selected
 IB and ZB dependent on SB and VB
SB  3VB I B , VB  3I B Z B

IB 
SB
, ZB 
VB / 3

VB 
2

3VB IB SB

Change of Base
 When pieces of equipment with
various different ratings are
connected to a system, it is 2
MVAbase ( new) KVbase ( old )
necessary to convert their Z pu ( new)  Z pu ( old ) * * 2
..(4)
impedances to a per unit value MVAbase ( old ) KVbase ( new)
expressed on the same base 18
 PROCEDURE FOR CALCULATING MAXIMUM
FAULT CURRENT (SHORT CIRCUIT CACULATIONS)

Draw a single-line Collect detailed impedance Use the per-unit method

01 diagram of the
power system
02 data for all the components
of the power system i.e.
03 where all of the
impedance are referred
Resistance R and Reactance to an arbitrarily chosen
X. common BASE MVA.

For low-voltage distribution

Convert all of the various
04 impedances to per-unit
values with a common
05
Find the total impedance
from the source to the 06 systems where there is a
significant motor load, the
point of fault motor contribution to the fault
base MVA. can be approximated as:

Symmetrical contribution = 4 * Motor full load current

Asymmetrical contribution = 5 * Motor full load current
 Key Formulas for Per Unit System
1 For Generators and Transformers –Per unit impedance with respect to the rating is given as:

2 Feeders and Lines – actual impedance/phase

3 Reactors – voltage drop at rated current

20
 Key Formulas for Per Unit System
4 Source Impedance Zp.u when given short-circuit MVA

5 Fault MVA for 3Ø or short-circuit MVA

6 Fault Current

21
Case Study 1: Fault Calculation
Consider the distribution network shown in figure 1.0, meant to service a community in New Town, through a 240mm2,11kV
ACSR three lines conductor with the following parameters:

Figure 1.0 : New Town Network

22
Case Study 1: Fault level Calculation Solution(Case1)

23
Case Study 1: Fault level Calculation Solution(1)

24
Case Study 1: Fault level Calculation Solution(Case2)

25
Case Study 1: Fault level Calculation Solution(2) Power Factory

26
Summary
• Power Distribution networks are subject to failures arising from fault
which cannot be prevented.
• Power system faults tends to damage network equipment
• Fault level studies is necessary in order to determine the magnitude
of fault current for appropriate choice of protection equipment.

27
28
 NATIONAL POWER TRAINING INSTITUTE OF NIGERIA
Power System Protection Workshop for Distribution Companies Executives

Instrument Transformers

29
………………….power trainer with a difference
Course Content
1
Introduction

2 Types of Instrument Transformers

3
Current Transformers

4 Voltage Transformer

30
 Introduction .

Instrument Transformers also known as transducers are used to

transform the power system currents and voltages to lower magnitudes
and to provide isolation between the high-voltage power system and the
relays and other measuring instruments (meters) connected to the
secondary windings of the transducers.
In order to achieve a degree of interchangeability among different
manufacturers of relays and meters, the ratings of the secondary
windings of the transducers are standardized.
The current and voltage ratings of the protective relays and meters are
same as the current and voltage ratings of the secondary windings of
the CTs an VTs respectively.
The transducers should be able to provide current and voltage signals to
the relays and meters which are faithful reproductions of the
corresponding primary quantities.

Figure 1.0 substation instrument Transformers

31
Types Of Instrument Transformers
4p
Ste

Current Transformer
3p
Ste

Voltage or Potential Transformer

1p
Ste

32
32
 OPTICAL CURRENT AND VOLTAGE TRANSDUCERS
 These are non-Conventional Instrument Transformers
(NCIT) now being used by some utilities in different
parts of the world.
 There are multiple parts roughly divided into
conventional and low power instrument transformers.
Benefits :
• Reduced cabling
• Reduced engineering time
• Reduced commissioning time
• Increased reliability
Figure 2.0: Integrated switchgear showing conventional and non-conventional instrument
• Improved safety transformers 33
 Current Transformer
The main tasks of current transformers are:

1. To transform currents,
from a high value to a value
easy to handle for relays and
instruments,
2. To insulate the metering
circuit from the primary high
voltage,
3. To provide possibilities of
a standardizing rated
currents of meters and
relays

The equation for an ideal CT:

. 34
Classification Of Current Transformer

Current
Transformers
(CTs)

Based on Based on Based on

Technology Application Location

Electromagnetic Opto-electronic
Rogowski Coil Metering CTs Protective CTs Indoor CTs Outdoor CTs
CTs CTs

35
 Classification of CTs(Based On Application)

Protection/Disturbance
Measuring CTs
recording CTS

Intended to transmit Intended to transmit

an information an information signal
signal to measuring to protective and
instruments and control devices.
meters.

lower accuracy is required but also a high

Required to give high accuracy for all capability to transform
load currents up to 125% of the
rated current. high fault currents and to allow protection relays
to measure and disconnect the fault.
Accuracy classes for metering cores
are 0.1 (laboratory), 0.2, 0.5 and 1 as Protection classes are 5P and 10P according to
described in IEC 61869-2 STD
IEC 61869-2 STD 36
 Current Transformer Cores
 CTs can contain three to six cores normally and the cores are then one or two for measuring purposes, and
two to four for protection purposes.

Metering Core Protection Core

• To protect instruments and meters from high fault
currents, the metering cores must saturate for 10-40
times the rated current depending on the type of • Lower accuracy than for measuring transformers.
burden. • High saturation voltage.
• Normally the energy meters have the lowest • Little or no turn correction at all.
withstand capability. Typical values are 12-20 times • Reference should be made to the Accuracy Limiting
the rated current. Factor.
• The instrument security factor FS, indicates the
overcurrent as a multiple of rated current at which
the metering core will saturate based on the rated
burden.

37
 PROTECTIVE CURRENT TRANSFORMERS AS PER IEC
IEC61869-2
 Defining a Current Transformers to meet the Composite Error Requirements of a Short Circuit Current under Symmetrical
Steady State Conditions:
These are Protective current transformers without remnant flux limit for which the saturation behaviour in
Class P: the case of asymmetrical short circuit is specified. The standard accuracy limit factors (ALF) are: 5-10-15-
20-30. 5P and 10P are commonly used.

These are Protective current transformers with remnant flux limit for which the saturation behaviour in
Class PR: the case of a symmetrical short circuit is specified.
 Defining a Current Transformers by Specifying its Magnetization Characteristic:

These are Protective current transformers of low leakage reactance without remnant flux limit for which
Class PX: knowledge of the excitation characteristic and of the secondary winding resistance, secondary burden
resistance and turns ratio is sufficient to assess its performance in relation to the protective relay system
with which it is to be used.

These are Protective current transformers with remnant flux limit for which knowledge of the excitation
Class PXR: characteristic and of the secondary winding resistance, secondary burden resistance and turns ratio, is
sufficient to assess its performance in relation to the protective relay system with which it is to be used.
38
PROTECTIVE CURRENT TRANSFORMERS AS PER INDIAN
STANDARDS IS 2705

Class P: Identical to class P CTs defined in IEC61869-2 standard

Class PS: The class PS current transformers are of low reactance and their performance is specified in terms of the
Turns ratio, Minimum Knee point voltage (VK) and Maximum Exciting Current.

Vk= K.IS(Rct + Rb )

Where VK is the minimum knee point voltage in volts,

K is a parameter to be specified by the purchaser which depends on the system fault level and the
characteristics of the relay intended to be used,
IS is the rated secondary current of the current transformer,

Rct is the resistance of the secondary winding corrected to 75°C

Rbis the impedance of the secondary circuit 39

 Modern Trends in CT Design

High-voltage CTs are the oil-filled type. At a system voltage of 400 KV and above
there is a severe insulation problem. CTs for this range of system voltage become extremely expensive.

Their performance is also limited due to the large dimensional separation of the secondary winding from
the primary winding. These problems are overcome using SF6(gas) and clophen (liquid) as insulation, thus
reducing the size and cost of CTs

A new trend is to use opto-electronic CTs and Rogowski coil current sensors to tackle this problem occurring in
Extra High Voltage (EHV) and Ultra High Voltage (UHV) systems. A Rogowski coil and a linear coupler encircle the
EHV conductor.

A signal proportional to the secondary current is generated and transmitted via the communication channel. Light
beem, laser beam and radio frequency are being used to transmit this signal.

40
 SELECTION OF CT CORES
Some general guidelines for selecting current transformer cores, for metering and protection purposes, are
given below.

The Rated Current Burden Factors ISF and ALF

• Select the primary rated • Do not use core with rated • Select the correct Security
current of the CT to be 10- burden more than necessary. factor Fs and Accuracy Limit
40% higher than the rated A too high rated burden Factors ALF, depending on
current of the equipment. compared to actual burden the type of equipment
• This gives a high resolution of can lead to damage of connected
the metering equipment. metering equipment as the • Always check the overcurrent
• For the protection cores it Security factor FS factor is capability of instruments and
can be of interest to have valid at rated burden. meters and the VA
highest possible ratio; this • Value are 2.5,5.0,10, 15 & requirement of the meters
gives a smaller core size. 30VA(IEC 185) connected
• The secondary rated current • The manufacturer tunes the
can be 1 or 5A, preferable 1A CT for accuracy at rated
Burden. At 25% of the rated
burden, the accuracy is
guaranteed between 25% &
100% burden
41
 SELECTION OF CT CORES
Some general guidelines for selecting current transformer cores, for metering and protection purposes, are
given below.

Accuracy Over Rule of

Class dimensioning thumb
• The accuracy of any CT is • In practice all current • RCT ≤ 0.2 – 0.5 Ω per 100
determined essentially by transformer cores should be turns.
how accurately the CT repro- specially adapted for their • Bigger values for big cores
duces the primary current in application in each station. and small values for small
the secondary. cores.
• Do not specify higher
requirements than is
necessary.
• For Protection Class 5P and
10P

42
Case Study On CT Sizing

Figure 3.0: Application of CTs

43
Case Study On CT Sizing (Solution)

44
Case Study On CT Sizing (Solution)

Note: The performance of a protection function will depend on the quality of the measured current signal.
 The protection IEDs normally are designed to permit heavy CT saturation with maintained correct operation.

45
Voltage Transformer .

Figure 4.0 substation Voltage Transformers

46
Types Of Voltage Transformers
4p
Ste

Magnetic Voltage Transformers (VT)

3 Most economical for voltages up to about 145 kV
p
Ste

Capacitive Voltage Transformers (CVT).

Most economical for voltages above 145 kV
1p
Ste

47
47
 VT Errors
 The voltage transformer is normally loaded by an impedance consisting of relays, instruments and, perhaps
most important, the cables.
 The induced emf required to achieve the secondary current through the total burden, requires a magnetizing
current which is taken from theprimary side voltage. This magnetizing current introduces errors in the
voltage transformer.
Ratio Error (Voltage Error)

Phase Angle Error The phase difference between the primary voltage and the reversed secondary phasors.
In order to keep the overall error within the specified limits of accuracy, the winding must be
designed to have:
(i) the internal resistance and reactance to an appropriate magnitude, and
(ii) mininum magnetizing and loss components of the exciting current required by the core.
48
 SELECTION OF VTs
Some general guidelines for selecting Voltage transformer cores, for metering and protection purposes, are
given according to IEC 61869-3
Accuracy Class
Rated Output Accuracy Class (Protection)
(Metering)
• With the exception of
• The standard values of rated • The standard accuracy class residual voltage windings,
output at a power factor of for single-phase inductive PVTs shall be assigned same
0.8 lagging are: measuring VT are: measuring accuracy class.
• 10, 25, 50, 100VA 0.1,0.2,0.5, 1.0,3.0 • The standard accuracy
classes for protective voltage
transformers are 3P and 6P.
• The accuracy class for a
Limits of voltage error and phase displacement for measuring voltage transformers residual voltage winding shall
be 6P or better
Limits of voltage error and phase displacement for protective voltage transformers

49
 SELECTION OF CVTs
Some general guidelines for selecting Voltage transformer cores, for metering and protection purposes, are
given according to IEC 61869-3

Accuracy Class
Rated Output Accuracy Class (Protection)
• The standard values of rated
(Metering)
• With the exception of
output at a power factor of • The standard accuracy class residual voltage windings,
0.8 lagging are: for single-phase inductive PVTs shall be assigned same
• 10, 25, 50, 100VA measuring VT are: 0.2,0.5, measuring accuracy class.
• Where accuracy is specified 1.0,3.0 • The standard accuracy
from 25% to 100% of rated classes for protective voltage
burden. transformers are 3P and 6P.
• The accuracy class for a
residual voltage winding shall
Limits of voltage error and phase displacement for measuring capacitive voltage transformers
be 3P or 6P

50
 VOLTAGE TRANSFORMERS WITH SEVERAL
SECONDARY WINDINGS

Where the transformer has one secondary winding

which is intended to serve a dual purpose, that is,
both for measurement as well as protection, it shall
comply with the requirements of both measuring as
well as protective voltage transformer

Voltage Transformer with 2-secondary 51

 VOLTAGE TRANSFORMERS WITH SEVERAL
SECONDARY WINDINGS
 Where the transformer has one secondary winding which is intended to serve a dual purpose,
that is, both for measurement as well as protection, it shall comply with the requirements of
both measuring as well as protective voltage transformer.

 Where the transformer has two or more separate secondary windings, one of measurement
and the others for protection, having the same or different transformation ratios, they shall
respectively comply requirements as laid out for respective type of voltage transformer in the
standard.

52
 FERRO-RESONANCE

In • This is an oscillation between the inductance of the VT and the

capacitance of the network characterize by voltage fluctuation.
Magnetic • The damping of ferro-resonance is normally done with a 27-60Ω,200W
resistor connected across the open delta winding.
VTs
In • The phenomena is started by a sudden voltage change .
Capacitive • capacitor voltage transformer shall be designed and constructed to
VTs prevent sustained ferro-resonance oscillation

53
54
Basic Protection Scheme In distribution Network

55
Distribution System .

Figure 1.0 Electrical Distribution Network

56
Types of
Feeders Radial Parallel

One feeder Two or more

from source feeder connected
to load in in parallel each
with capacity to
one supply the required
direction power

Ring Main

A loop formation
for power supply
and most reliable
.

57
Protection Schemes in Distribution Network

1 Fuse Protection

2 Overcurrent Protection(Directional & Non-Direction)

Earth fault Protection

3

Differential Protection
4

5 Over-volatge Protection

58
 Fuse Protection
.

 A fuse is an overcurrent protective device with a circuit-

opening fusible element that is heated and severed by the
flow of overcurrent through it.
 A fuse is used to protect the circuits and equipment
against overload and short circuits.
The basic properties/characteristics of a fuse element are
low melting point & high conductivity.
Fuse operation is based on the heating effect of the current
which flows through the element
Figure 2.0 Fuses
Fuse has inverse time current characteristics.
Its rating can start from few mA to several kA.
59
Types of Electrical Fuse

Fuses

Low Voltage High Voltage

Fuses Fuses

Semi-enclosed Totally
Drop-out
or rewireable Enclosed or
Fuse
type Cartridge type
60
Important Terms
• Is the nominal rated current in Amps
marked on the fuse body that the fuse
Current rating will carry continuously without
deteriorating

• Is defined as the minimum value of

current at which the fuse element or
Fusing current fuse wire melts. Its value will be more
than the current rating of the element.
61
 Important Terms(Fusing Current)
 Fusing current depends upon various factors such as:
i. Types of material used
ii. The cross sectional area
iii. Length
iv. Diameter of wire
v. Types of enclosure employed

 The approximate value for fusing current of a round wire is given as:

where : I = k √d3
I = fusing current
k = Preece constant depending upon the material of the wire(K for copper is 10244)
d = diameter of the wire in inches.

62
 Important Terms

 For semi enclosed or rewire-able fuse which employs copper wire as the fuse element, fusing
factor is equal to 1.9 - 2.0
 For the standard duty cartridge fuses, the fusing factor is equal to 1.2 -1.45.

 Breaking capacity: the breaking capacity is the maximum current that can be safely
interrupted by the fuse. Some fuse are designated as high rupture capacity (HRC).
 Voltage rating: this indicates the maximum circuit voltage in which the fuse can be used.

63
 Case Study
Q1. What will be the fuse rating if a 500KVA, 11/0.415KV transformer is to be fused with HRC fuse & D-fuse respectively?

64
Case Study(Solution)

65
Case Study 2

66
67
 Overcurrent Protection with Relays
.

 An overcurrent relay is a protective relay which operates when

the load current exceeds a preset value called the pick-up
current
 Overcurrent relays offer the cheapest and simplest form of
protection for distribution network.
 An overcurrent protection scheme may include one or more
overcurrent relays.
 They can be directional(67) or non-directional(50/51)

Figure 3.0 Application of OC relays

68
 TIME-CURRENT CHARACTERISTICS
 A definite-time overcurrent relay operates
 Definite-time Overcurrent after a predetermined time when the current
Relay(51) exceeds its pick-up value.
 The operating time is constant, irrespective of
the magnitude of the current above the pick-
up value. See Figure 4a.

 Instantaneous Overcurrent  An instantaneous relay operates in a definite Figure 4a.: Definite time characteristics
Relay(50) time when the current exceeds its pick-up
value.
 The operating time is constant, irrespective of
the magnitude of the current.
 There is no intentional time-delay. It operates
in less than 0.1s. See figure 4b.

69
Figure 4b.: Instanteneous time characteristics
 TIME-CURRENT CHARACTERISTICS
 An inverse-time overcurrent relay operates
 Inverse-time Over-current when the current exceeds its pick-up value.
Relay(51)  The operating time depends on the magnitude
of the operating current.
 The operating time decreases as the current
increases. See figure 4c.
Figure 4c.: inverse time characteristics

Inverse De inite Minimum Time The relay gives an inverse-

Overcurrent (I.D.M.T) time current characteristic
Relay(50/51) at lower values of the fault
current and definite-time
characteristic at higher
values of the fault current.
 See figure 4d.

70
Important Terms In Relay Setting

Current Setting • The current above which an overcurrent relay should operate.
• The plug-setting (current-setting) can either be given directly in amperes or
(Plug setting) indirectly as percentages of the rated current.

Plug setting • The actual r.m.s. current flowing in the relay expressed as a multiple of the
setting current (pickup current)
Multiplier(PSM)

• Steps of time settings.

Time Multiplier • The adjustment of travelling distance of an electromechanical relay.

setting(TMS)
71
 Non-directional and Directional Application of OC Relays
 Radial Feeder
 Radial feeders are easily protected from overcurrent fault by non-directional definite time relays or inverse time relays.
 Time grading is used such that the relay nearest to the end of the line has the minimum time setting while the time setting
of other relays increases successively toward the source has shown in the figure 5.0

Figure 5.0: Radial feeder protection

 Parallel Feeder
 In an event of a fault on a parallel feeder, only the faulted feeder is isolated leaving the healthy feeder. This is achieved with
directional over current relay(67) with a grading of the time setting for selective tripping.
 Unlike non-directional OC relays, directional OC relays requires a voltage input for polarization.

Clockwise: B – P- A coordinated
Anti-clockwise: A- Q-B coordinated

Figure 6.0: Parallel Feeder Protection

72
 Non-directional and Directional Application of OC Relays
 Ring Main System Feeder
 Is a system where sources and series of substations are connected through alternate route path forming a loop. Protection
is achieved with the help of both non-directional and directional OC relays.

Clockwise: A – B – C – D- A
Anti-Clockwise: A – D – C – B - A

Figure 5.0: Radial feeder protection

Figure 7.0: Ring Main Feeder Protection

73
Earth Fault Protection
 This relays are used to provide protection against earth fault in generators, transformers, lines and feeders. Their
current setting is usually lower that the Phase overcurrent relay. The setting range is between 10 - 40% of the rated
current( e.g 1A).
 A definite time delay is usually introduced to prevent unwanted tripping due to transient unbalance phase fault.
 The setting is usually above the residual current that may be present under normal load condition

50/51ABC-N

C IA=IF
IA T
A A
IB
B B
IC IB=0
C C

IN IC=0
In Ia
50/51 N C B A

Figure 8.0: External Ground fault protection

75
Transformer Differential Protection

 Basic principle of Differential Protection is based on Kirchoff’s

current law (Sum of currents flowing through a node is zero),

i,e the currents into the Transformer is equal to the currents
flowing out of the Transformer.
 Differential protection is applicable to all parts of the power
system such as: Generator, Transformers, Motors, Buses,
Lines and feeders, Reactors and capacitors I1 BIAS BIAS I
2

 Overall differential protection may be justified for larger I1 - I2

transformers (generally > 5MVA). OPERATE

Figure 9.0: Differential Relay Principle 76

 Types of Differential Protection
Current Balance • Here, two current transformers are fitted on either side of the
equipment to be protected. The secondary circuit of the CTs are
Differential connected in series in such a way that they carry current in same
protection. direction. Under normal or external fault condition no operation.

(Low Impedance)

Normal condition Internal fault condition condition 77

Types of Differential Protection

Voltage Balance • The CTs are connected on either side of the

Differential equipment in such a manner that emf induced in
Protection the secondary of the current transformer will
oppose each other hence, ino current flow in
(High Impedance)
the relay operating coil (Vs1-Vs2 = 0).

During internal fault Vs1-Vs2 ≠ 0 78

Figure 10.0: Voltage Balance
 Transformer Differential Protection
Factors to consider in transformer differential protection
2.The effects of the
1.Correction for possible 3. Correction for possible
variety of earthing and
phase shift across the unbalance of signals from
winding arrangements.
transformer winding. CT’s on either side of the
(filter of zero sequence
(phase correction) winding. (ratio correction)
currents)

5. 4. The effect of
6.
The possible occurrence magnetizing inrush during
CT saturation initial energization
of overfluxing.

7. Tap-changing. CT polarity
79
 Percentage Differential Relay
 To avoid unwanted relays operation under the above conditions a
"Percentage Bias" differential relays is used.
 Percentage differential relay has an operating coil (No) and two
restraining coil (Nr).
 In the scheme, the relay operates when the magnitude of the
secondary operate current, ĪOP = Ī1 + Ī2, is larger than a given
proportion of the secondary restraint current, IRT.
 For this particular scheme, the restraint current is chosen to be k.IRT
= k.(|Ī1| + |Ī2|)/2.
 The proportionality constant, k (sometimes called the slope), may
be adjustable and have typical values from 0.1 to 0.8 (or 10 percent
to 80 percent).
 Bias is a fixed "percentage“ of the ratio of the differential
operating current to the average restraining current
 In other words, % bias can be define as the ratio between the
number of turns of the restrain coil (Nr) to the number of turns of
the operating coil (NO) (i.e. %bias = Nr/No = K)
80
Operating Characteristic of a Percentage Differential
relay
Differential
Current

Positive torque region

I1 - I 2 OPERATE

negative torque region

RESTRAIN

(I1+I2)/2 Restraint Current

81
 Operating Characteristic of a Percentage Differential
relay(Cont’d)
 The normal maximum setting for slope 1 is the cumulative rated error of the CTs. For instance, if two
CTs have a maximum error of 10% each, the slope would be set at 20%.
 Any value below the 20% slope, is in the restraint region while any value above the 20% slope, is in
the operating region.
 To provide greater stability under large external fault conditions, differential element utilizes a steeper
slope beyond the breakpoint, resulting in a dual slope percentage differential characteristic.
 However, if the fault current will generate a voltage that is less than or equal to half of the knee point
voltage of the CTs, the common practice is to reduce the CTs rated error by half.
 Therefore, the CT maximum error would be 5% each for a 10% rated error CTs and the slope setting
will now be 10%. This practice provides greater sensitivity for low-level faults.
 The K1 slope setting is further complicated by the existence of On-load tap changer of the power
transformer which creates an imbalance in the secondary outputs of the differential zone CTs. 82
 Operating Characteristic of a Percentage Differential
relay(Cont’d)
 This imbalance that can occur from tap changer operation needs to be determined for maximum
stability of the differential scheme.
 Thus, since CT ratios are fixed and there is known CT error, the spill current which will be equivalent
to the maximum current on the HV side can be determined.
 Therefore, the ratio of maximum spill current to the restraint current for the maximum tap changer
deviation gives the required slope setting. K1 = 20% slope of CT error
K2 = 80 - 100% slope of CT saturation
Differential
Differential
Current
Current

K2

I1 - I 2
I1 - I 2 20% slope of CT error
K1 Breakpoint

(I1+I2)/2 Restraint Current

(I1+I2)/2 Restraint Current
Maximum overload current
83
 Operating Characteristic of a Percentage Differential
relay(Cont’d)

 Consequently, there are two (2) options or methods that can be used to set K1 slope of a
% differential protection:
1. Cumulative maximum error method.
2. The ratio of maximum spill current to the restraint current for the maximum tap
changer deviation.

84
 Case Study
A 30/40MVA, 132/33KV transformer in Benin T/S has the following data available on the name plate:
VHV = 112200V @ tap 17
CTRHV = 200/1; 5P20
CTRLV = 1200/1; 5P20

Determine the expected K1 and K2 settings of your slope for the %bias differential protection of the transformer ?
Solution Where :
Given parameters VMax = Maximum voltage @ HV side of the transformer
Vmax = 112200V @tap 17 VNom = Nominal voltage @ LV side of the transformer
Vnom = 33000V @ nominal tap of LV side CTRHV = CT Ratio @ HV side of the transformer
CTRHV = 200/1A; 5P20 CTRLV = CT Ratio @ LV side of the transformer
CTRLV = 1200/1; 5P20 CTeHV = CT error @ HV side of the transformer
CTeHV = 5% CTeLV = CT error @ LV side of the transformer
CTeLV = 5% IeHV = Spill current @ HV side of the transformer
IOp = Operating current
IeLV = Spill current @ LV side of the transformer
85
IRes = Restraint current
Case Study Cont’d

86
 Overvoltage Relay Protection(59)
 This is a protection against system Over-voltages on the feeder, lines ,Transformer and generators.
This might be as a result of sudden load throw off, AVR malfunctioning, Power transformer tap failure,
lightning strike etc.
 It will have 2 stages:
 Stage-I:
 Setting: 110%
 Time delay: 5 Sec.

 Stage –II
 Setting: 140%
 Time delay: Instantaneous.
87
IMPORTANT TRENDS IN PROTECTION SYSTEMS
• Many manufacturers of numerical protection relays have provided the
ability to implement different relay settings on the same relay and to be
selected by remote means.
Use of Nested Settings • This functionality is little used and yet provides the ability to re-protect the
affected network after a circuit reconfiguration or in different operating
and Custom Curves conditions of distributed Energy sources (DERs) like wind speed or solar
insolation.

• This involves the use of real-time adaptive protection curves that could
adapt to different network conditions. E.g. to manage wind farms in a
micro-grid application.
Adaptive Protection • To keep generation connected even during a fault condition would require
protection to “hold-off” while fault-ride through and other network
actions are tried before disconnection of the generation.

88
 IMPORTANT TRENDS IN PROTECTION SYSTEMS
• This involves the use of real time thermal rating of transmission lines
to avoid unnecessary costly network reinforcement and add extra
capacity to the DER project especially during installation of a new
Dynamic Thermal DER .
• The line thermal rating can be calculated dynamically in real time from
Protection the local weather measurements such as wind speed and ambient
temperature to co-ordinate allowed generation automatically.

• Growing DER penetration levels of all sizes and connection voltages

causes an increased need for high performance anti-islanding
Anti-islanding protection protection.
• This is because a compromise between sensitivity for islanding
detection and stability under external disturbances is required

89
 DC AUXILIARY SUPPLY SYSTEMS
 All protection schemes designed to prevent or (Tripping Units)
minimize damage to equipment requires an
actuating signal. This signal can be an alternating
current(A.C) or a direct current (D.C.) signal.
 Battery bank with a matching charger (rectifier) are
the inseparable pair which are installed for a
healthy d.c. system performance.
 The charger provides d.c. to the standing load and
at the same time provides trickle charge or boost
charge to the battery bank depending on the state-
of-charge of the batteries in the bank.
 The battery bank supplies the load during
emergency or loss of output from the charger.
 It is safer to shut down the station if the battery
bank output is not available because without d.c.
there is no protection for the bulk of the
equipment/apparatus in service in the station.
90
 DC AUXILIARY SUPPLY SYSTEMS
(Tripping Units)
 Auxiliary D.C. supply has standard voltage ratings of 24V, 30V, 36V, 48V, 50V, 60V, 72V, 110V, 220V and
250V.
 The ampere ratings of the charges are usually 3A, 6A, 10A, 20A, 30A, 40A , 50A etc.

 In most 11KV, 33KV and 132KV substations, voltage rating of 110V DC (2.2V per cell) are installed and is
used for:
 Tripping and closing coil of circuit breakers
 Station Control Board (SCB). e.g. the Relay master tripping board.

 A ground fault relay is installed to protect the D. C. circuits from ground fault, which usually flags whenever there is a ground
fault within any of the poles of the D.C circuits.

91
Factors To Consider in Sizing of DC Supply unit

The size and

capacity of the The standing
generating load
station and or
substations

The bus bar

switching The distance
arrangement, from the control
which decides room to the
the number of controlled
circuit breakers apparatus.
and isolators.

92
 Battery Capacity
 The capacity (Cn ) of a battery refers to the amount of amperage that can be drawn until the battery is fully
discharged for a given period of time at a giving initial cell temperature while maintaining voltage above a given
minimum level. The unit is ampere-hour(Ah)

Cn is determined by the geometry and number of cells

Cn varies depending on the temperature, the discharge voltage
and most importantly, the discharge
current In

93
 THE CHARGER
 Battery chargers in use at all distribution stations convert AC to
DC at a suitable voltage for battery charging.
 Station batteries primarily have a stand-by function. They are
“floated” across the battery charger which will supply the
normal dc load. The battery must supply all of the load
whenever the charger is out of service.
 The dc output of the Charger, the battery and the load are all
connected in parallel.
 The charger should maintain a voltage of 2.037 volts per cell,
for example 110 volts in the case of 54 cell battery

94
TYPES OF CHARGER

Automatic constant
Two-rate charger
Potential Charger

This is the type of charger that is The simplest type is a trickle charger
practically specified for all substation with two rates (low and high rates).

It has a built-in control circuit which A series resistor is partially shorted out
detects any variation in the charging to switch from the low rate to higher
voltage due to varying load rate

As result of this circuit the output of the

The charging current is alternated
charger will change automatically to
between the high and low rate.
maintain the voltage across the battery.

Supplies substantially constant voltage The transfer from high to low rate is
to the battery for maximum cell life usually controlled by a voltage relay
95
TYPES OF BATTERY CHARGE

BOOST FLOAT EQUALIZATION

CHARGE CHARGE CHARGE
• Boost charging is used to • Float charging is situation • Is a corrective type of
quickly restore the capacity of whereby the charger supplies charge meant to bring the
the battery, usually following a a trickle charge of 2.037 volts battery cell’s voltages to
heavy demand across the battery for a 2 volts
• When boost charging is battery, to avoid self-discharge
the same level.
or standing loss which tend to • This is because of a number of
required the charger operates
in the constant voltage mode drain the battery. factors which may cause the
but with a raised voltage • The charger operates in an charge on the cells of a
which brought about high almost constant voltage battery to become unequal in
boost (charging) current manner with its voltage specific gravity or state-of-
• As the battery becomes normally just above the charge.
charged the boost current falls battery voltage but when a
then the control circuit sudden demand of current
switches the charge back to occurs the battery and the
float charge charger will share the current

96
Classification of Batteries(Based on Technology)

Lithium-ion Lead Acid Nickel

Batteries Batteries Cadmium
-used in high-end electronics -Deep Circle & Starter
such Batteries. Rechargeable battery using
as laptops and mobile phones -Flooded(Liquid Electrolyte) nickel oxide hydroxide and
and Valve(semi solid metallic cadmium as
-Moderate discharge current electrolyte) Regulated electrodes.
-High manufacturing costs

-High energy density

Absorbed glass mat (AGM)
-Low self-discharge Energy density about double
battery()
- High efficiency that of LA.
Gel battery
- High cell voltage

97
 Case Study Battery Sizing
 A switchboard consists of 20 circuit breakers. Each circuit breaker has two indicator lamps each taking 1 amp
continuously, a tripping solenoid taking 5 amps for one second, and a spring charging motor for reclosing
which takes 3 amps for 30 seconds. The battery needs to supply current for 4 hours when a mains failure
occurs.
Solution

98
 Case Study Battery Sizing Solution

Cell choice: 2.2V, 400Ah

All connected in series

Charger

99
 CHARGER MAINTENANCE

The following maintenance procedures are to be carried out on a D.C unit (i.e. the charger):
1. Periodic check of d.c. voltage output;
2. Panel cleaning and removal of dirt/cobwebs;
3. Check for Charge retention ability;
4. Replace ruptured fuses.

100
 SPECIFIC GRAVITY & SOC OF A BATTERY
Specific Gravity Values
 The specific gravity of the electrolyte to a large extent
determines the status of the cells.
 The values of the specific gravity when the cell is fully
charged is 1.22± 0.1 at a temperature of 77 F and
State-of –Charge Values
1.18 when discharged.
 The specific gravity is measured with a hydrometer.
During charging, the density of the electrolyte
increases due to evaporation of water.

101
FACTORS AFFECTING BATTERY LIFE
• Gas formation which will tend to scrub the active materials from the
plates
OVERCHARGING • lower the water level
Leads to: • plate buckling and warping which may result in damage to the
separators
• Corrosion of the positive plates

• Sulphation of the plates will occur. This will result in

UNDERCHARGING buckling of the plates, drop in specific gravity, formation
Leads to: of metallic lead in separators.
• Inability to deliver full energy.

• local action, loss of active material, electrolytic action,

Other factors
low water level, freezing and entrance of impurities.

102
 Maintenance of Battery Bank
 Checks of battery casing for cracks.
 The most important aspect of battery maintenance is the addition of distilled water to correct electrolyte
level.
 Scheduled checks(daily) of electrolyte’s specific gravity, voltage levels, cell voltage, etc. should be noted and
recorded in the battery maintenance report sheet.
 Battery cleaning(with sand paper), greasing of terminals and connections should be done every month.
 Battery room should always be kept clean and properly ventilated.
 Bring old batteries to recycling; never dispose of them with normal waste or by leaving them outside
(in nature).

103
104
 NATIONAL POWER TRAINING INSTITUTE OF NIGERIA
………………….power trainer with a difference
Power System Protection Workshop for Distribution Companies Executives

Relay Coordination

105
 What Can relay coordination do for you?

Relay Coordination can ensure continuity of

service thereby reducing losses in revenues;

Relay Coordination can reduce the volume

of maintenance work and maintain system
stability.

Let us know more about the subject…………………

106
 Learning Outcomes
By the end of this workshop, you should be able to:

CONCEPT

1. 2. 3.
Explain relay coordination Design and Implement relay
PROPOSAL
Coordination

Identify and describe the Types of relay

coordination

Target Group : Distribution Companies Executives

Duration : 0.5 hour

 Course Content

1 2 3
Procedure of
Why relay Types of Relay relay
Coordination? Coordination Coordination

4 5

Criteria of setting Design & Implementation of

Instantaneous units RC(case study)

108
Relay Coordination

This is the process of designing and

configuring the protection settings of
relays to operate in an orderly manner in
order to avoid indiscriminate tripping in
an event of a fault at a certain location or
zone of protection.
3

Power Transformer
Figure 1.0 Relay co-ordination network
109
 Why co-ordination?

3. Prevent 4. Protect other

undesirable tripping healthy circuits and
2. Prevent tripping of of other healthy apparatus in the
1. To Isolate only the healthy circuits or circuits or apparatus
apparatus adjoining adjoining system
faulty circuit or
the elsewhere in the when a faulted
apparatus from the
system when a fault circuit or apparatus
system. faulted circuit or occurs somewhere is not cleared by its
apparatus. else in the own protection
system. system

110
When is Coordination Required?
Relay setting Coordination is require under the following Circumstances:

Commissioning of new circuit or network

Network changes of reconfiguration

After the incident of uncoordinated or unselective tripping

111
 3. A combination
2. Time graded of time and
protection current grading
protection.

4. Time in
relation to
1.Current graded
thermal
protection
Relay Co- capability of the
equipment.
ordination
Methods

112
 Current Graded Systems 630A
600A
(E q uiva le nt
H V C urre nts )
F a ult M a x . 1 3 1 0 0 A 8800A 2900A 1200A
c urre nt M in. 6 8 5 0 A 5400A 2400A 1100A E
MV
630A (Equivalent L o a d
 This principle is based on the fact R e la y a t 'A ' s e t to Fault Max. 13100A
F1
8800A
F2
2900A 1200A 600A HV
F
Currents)
o p e ra te fo r m a x . fa ult 8 8 0 0 A
that the fault current varies with c urre nt a t re m o te ecurrent
nd Min. 6850A 5400A 2400A 1100A E
MV
the position of the fault because of A R A D IA L
Relay at 'A' setDtoIS T R IB U T IO N
BF1 C D Load

F2 F
the difference in impedance values operate for m ax. fault
current at rem ote end
SYST EM
8800A

between the source and the fault. D C B A

A RADIAL B C D
 The relays are set to pick up at AIM - P rotection co-ordinated
disconnected
to ensure m inim
DISTRIBUTION
um unfaulted load is t

progressively higher currents U nreliable S chem e SYSTEM

towards the source. C urrents F1 and F2 m ay be sim ilar - loss of discrim ination
For m inim um infeed A - B m ay be unprotected

 This current grading is achieved by N O TEFigure 2a,2 b.0 Current grading method D C B A
: M ax.AIM
fault- Protection
at S /S tn.co-ordinated to ensure
E < m in. fault minimum
current betwunfaulted
een D -E load is t
Amps
disconnected
high set instanteneous over current
Challenge: F1 and F2 may have the same
relays. Unreliable Scheme
valueCurrents
leading toF2loss
F1 and ofsimilar
may be discrimination
- loss of discrimination
 Since their selectivity is based solely For minimum infeed A - B may be unprotected
on the magnitude of the current,
NOTE: Max. fault at S/Stn. E < min. fault current between D-E
there must be a substantial Amps

difference (preferably a ratio of 3:1)

113
 Time Graded Systems Infeed E
MV
Load
F
1.4s 1.0s E0.6s 0.2s
 In this method, selectivity is Infeed
MV
achieved by introducing time Load
A B C F D
intervals for the relays. 1.4s 1.0s 0.6s 0.2s
 The operating time of the relay is
increased from the farthest side A B C D t
towards the generating source. This Relays nearer to power source are set to 1.4s A
is achieved with the help of definite operate in progressively longer times
Operating
t 1.0s B
time delay over current relays. characteristic:
Figure 3a, 3b.0DISADVANTAGE
Relays nearer to power source
Time grading
are setclearance
to
method Definite time
0.6s C
Longest 1.4s
time for faults nearest A
delay
operate in progressively longer times
to source Operating
1.0s 0.2s
B D
Challenge:
DISADVANTAGE
Clearance time increases
characteristic:
Definite time Amps
toward
Longest thetimesource
clearance for faults nearest 0.6s C
delay
to source
0.2s D
Amps

114
Time and Current Graded Systems
 In this relays the time of operation
is inversely proportional to the fault

Operating Time
current level and the actual T i m e M u l ti p l i e r = 1 .0
characteristics is a function of both
time and current settings.
 The most widely used is the IDMT 10s

characteristic where grading is

possible over a wide range of 3s
2s
currents and the relay can be set to
any value of definite minimum time
required.
 The characteristics are classified
2x 10x 30x
based on IEC 60255(SI/NI,VI, EI
M u l ti p l e o f C u r r e n t S e tti n g
&LTI) and IEEE (MI, VI, EI, US CO8
Inverse, US CO2 STI) Figure 4.0: Inverse Definite Minimum Time Characteristics

115
Inverse Definite Minimum Time Characteristics

Specification of IDMT Curves

0.14
t 0.02
 T .M .S
PSM  I 
  1
 IS 
116
 Co-ordination Procedure
• A single line diagram of the power system,
• The impedance of transformers, feeders, motors etc.
in ohms, or in p.u. or % ohms,
• The maximum peak load current in feeders and full
load current of transformers etc, with permissible
Information required overloads,
to achieve proper co-
ordination are: • The maximum and minimum values of short circuit
currents that are expected to flow,
• The type and rating of the protective devices and
their associated protective transformers,
• Performance curves or characteristic curves of relays
and associated protective transformers.
117
Protection Co-ordination Principles
1. Whenever and wherever possible, use relays with the same
characteristics in series with each other.

2. Set the relay farthest from the source at the minimum current settings.

3. For succeeding relays approaching the source, increase the current setting or retain
the same current setting. (i.e that is the primary current required to operate the relay
in front is always equal to or less than the primary current required to operate the relay
behind it. )
4.Instantaneous units should be set so they do not trip for fault
levels equal or lower to those at busbars or elements protected by
downstream instantaneous relays.

5. Time-delay units should be set to clear faults in a selective and

reliable way, ensuring the proper coverage of the thermal limits of
the elements protected
118
Criteria for setting instantaneous units
Lines between
Distribution lines Transformer units
substation
• Between 6 and 10 • 125% to 150% of the • 125% to 150% of the
times the maximum short circuit current short circuit current
circuit rating or existing on the next existing on the low
• 50% of the substation voltage side the
maximum short units at the low
circuit at the point voltage side are
of connection of the overridden unless
relay there is
communication with
the relays protecting
the feeders
119
 Challenges /Solutions of Relay Coordination

• Collaboration between protection Engineers and system planners;

Inadequate network data • Liaising with OEMs

Sympathetic tripping • Adjustment of operating Time of relays

Different Relay OEMs • Standardization will enhance interoperability e.g IEC61850

Decrease in the level of short circuit • Oversizing the inverter of the PV system to increase the fault current
current resulting from Integration of • Performing network studies and reconfiguration of relays on the
variable renewable energy sources network.
120
Case Study 1.0: Relay Coordination

Figure 5.0: Distribution Network

Question
The figure 5.0 above is a network having indiscriminate tripping at different stations. As a protection Engineer,
design a coordination scheme for the network;
1. Determine the relay settings at various stations. Assume standard inverse characteristics of IDMT numerical
relays with coordination time interval of 0.4s.
2. Validate your results using DigSilent Power Factory PSAT software 121
Case Study 1.0: Relay Coordination(Solution)

122
Case Study 1.0: Relay Coordination(Solution)

123
Case Study 1.0: Relay Coordination(Solution)

124
Case Study 1.0: Relay Coordination(Solution)

For a fault at station B the operating time will be:

125
Case Study 1.0: Relay Coordination(Solution)

126
Case Study 1.0: Relay Coordination(Solution PF)
Table 1.1: Bus Data of the six power system network Table 1.2: External Grid data
Bus name Bus Voltage (KV) System Type Phase Technology Description Voltage(KV Bus Type Active Voltage set-point in p.u
Bus SL 33 AC ABC ) Power(MW)
Bus A 11 AC ABC External Grid 33 PV 2 1.0
Bus B 11 AC ABC
Bus C 11 AC ABC Table 1.3: Transformer Data
Bus D 11 AC ABC Description Voltage(KV Short circuit Copper losses(kW) Rated Power(MVA)
Bus F 0.415 AC ABC ) Voltage(%)
Transformer 1 33/11 7 7.5 10
Transformer 2 33/11 7 7.5 10
Table 1.4a Transmission Line Data Transformer 3 11/0.4 4 3 1
Description Rated Rated Line Type Resistance(2 Reactance
voltage(k Current length(km) 0o) (20o) Table 1.5 load Data
V) (kA) (ohms/km) Description Voltage Technology Reactive Power Mvar Power Factor
(ohms/km) (pu)
Line 1 11 0.37 10 OHL 0.3621 0.471 General load 1.0 3PH-PH-E 0.7 0.9
Line 2 11 0.37 10 OHL 0.3621 0.471 Table 1.6: instrument transformer settings of the six bus power system
Line 3 11 0.37 10 OHL 0.3621 0.471 Protection Location Branch Manufacturer Model CT Slot Ratio
Device [pri.A/sec.A]
Table 1.4b Transmission Line Data(same for line 1 to 3)
Descriptio Susceptance Susceptan Conductor Maximum
n B uS/km ce B0 B Material Operating 1 Relay A Bus A Line 1 Areva P12x CT1 Ct-3P 400A/5A
uS/km Temp. deg C Ct-
Line 1 0.3621 1.817 3.582 1.324 Aluminium 80 CT1 E/N 400A/5A
2 Relay B Bus B Line2 Areva P12x CT2 Ct-3P 300A/5A
Ct-
CT2 E/N 300A/5A
3 Relay C Bus C Line3 Areva P12x CT3 Ct-3P 200A/5A
Ct-
CT3 E/N 200A/5A
4 Relay D Bus D TF3 Areva P12x CT4 Ct-3P 100A/5A
Ct- 127
CT4 E/N 100A/5A
Table 1.7: The Micom Areva overcurrent relay settings of the six bus power system(P123)
Protection Location Branch Manu Model Stage Current Current Current Time Charac Direc
Device facturer (Phase) [pri.A] [sec.A] [p.u.] teristic tional

1 Fuse Bus F TF3 A4BY 1000 Fuse 1000.00 Fuse

IEC Standard
2 Relay A Bus A Line 1 Areva P12x I> 390.40 4.88 4.88 0.17 Inverse None
Definite time
I>> 1600.00 20.00 20.00 0.03 (51) None

IEC Standard
3 Relay B Bus B Line2 Areva P12x I> 225.00 3.75 3.75 0.17 Inverse None
Definite time
I>> 900.00 15.00 15.00 0.03 (51) None

IEC Standard
4 Relay C Bus C Line3 Areva P12x I> 130.00 3.25 3.25 0.22 Inverse None
Definite time
I>> 520.00 13.00 13.00 0.03 (51) None

IEC Standard
5 Relay D Bus D TF3 Areva P12x I> 50.00 2.50 2.50 0.35 Inverse None
Definite time
I>> 200.00 10.00 10.00 0.03 (51) None
3-phase Min., Fault Fuse Relay Tripping Time(s)
balanced Current(A) Tripping
fault Time(s)
location
Tripping Times F1(t) RD (t>)/(t>>) RC(t>)/(t>>) RB(t>)/(t>>) RA(t>)/(t>>)

obtained
Bus F 7330 1.352 1.439/0.035 2.177/9999 7.224/9999 9999/9999
128
Bus D 320 99999 9999/9999 1.732/9999 3.465/9999 9999/9999
Time overcurrent Plot

129
130
 References
1. Instruction Manual for Schweitzer SEL-787-0 “Current Differential and Overcurrent Relay” (Manual P/N: SEL-
787-0 dated 20050919).
2. ANSI / IEEEC37.91, “Guide for Protective Relay Applications for Power Transformers”
3. Tracy Toups and Prashanna Bhattarai, “Microprocessor Based Differential Relaying ” Lab procedure, 2014.
4. Chapman, S. J. (2012). “Electric Machine Fundamentals Fifth Edition”. New York, NY: McGraw Hill.
5. Edmund O. Schweitzer, I. a. (2010). “Modern Solutions for Protection, Control, and Monitoring of Electric Power
Systems”. Pullman, WA 99163: Schweitzer Engineering Laboratories, INC.
6. Turan Gonen, “Electrical Power Distribution System Engineering”, (Second Edition), CRC Press, Taylor & Francis
Group, Boca Raton, FL, 2008.
7. Central Board of IrrIgatIon & Power research report(2018). Manual on Power System Protection. Malcha Marg,
Chanakyapuri, New Delhi – 110021

131