Understanding the GIS

Operation and Interlocking

schemes

By:

Omer Nawaz Malik

Understanding the GIS Interlocks – One & Half Breaker Scheme 0

 Preface
This manual is an effort to create an understanding for the logic behind the Operation
Interlocks of all types of switches i.e. Disconnectors, Earth Switches and Circuit
Breakers in One & Half Circuit Breaker and Double Bus Single Circuit Breaker Schemes.
It should be noted that this manual should not be used solely to design an interlocking
scheme or to compare an already designed interlocking scheme for any differences. The
reason being that it is quite possible that a given interlock can be achieved by more than
one method. And the methods discussed in this manual may differ from some schemes
.This manual shall only be used to develop and understanding of the necessary
conditions that govern the interlocks for switches.

Suggestions for improvement of this manual shall be addressed to:

omer.nawaz@hotmail.com

Understanding the GIS Interlocks – One & Half Breaker Scheme 1

 Table of Contents

CHAPTER-1 : Introduction to a Sub-Station ............................................................................................ 3

1.1 Familiarization with some general terms ........................................................................................ 4
1.1.1 What is a Sub-Station: ............................................................................................................ 4
1.1.2.What is a switch? .................................................................................................................... 4
1.1.3 What is Load? ......................................................................................................................... 4
1.1.4 What is On Load switching?..................................................................................................... 4
1.1.5 What is Off Load switching? .................................................................................................... 4
1.1.6 What is Arcing? ....................................................................................................................... 4
1.2 Understanding the single line diagram of a substation ................................................................... 5
1.2.1 Circuit breaker: ....................................................................................................................... 5
1.2.2 Disconnector: .......................................................................................................................... 5
1.2.3 Earthing switch: ...................................................................................................................... 5
1.2.4 What is an Interlock? .............................................................................................................. 6
1.2.5 Some general rules for interlocks ............................................................................................ 7
CHAPTER-2 : One and Half Breaker Scheme Interlocks ............................................................................ 8
2.1 What is On and Half Breaker Scheme? ........................................................................................ 10
2.2 Interlocks of one and half circuit breaker system ......................................................................... 10
2.2.1 Interlocks of Upper portion ................................................................................................... 11
2.2.2 Case Study- Upper Portion : (DS.1 Service Position Interlock) :.............................................. 12
2.2.3 Interlocks of Transformer Feeder .......................................................................................... 13
2.2.4 Case Study- Transformer Feeder : (DS.3 Service Position Interlock) :..................................... 14
2.2.5 Interlocks of Middle Portion .................................................................................................. 15
2.2.6 Interlocks of Line Feeder ....................................................................................................... 16
2.2.7 Case Study- Line Feeder : (ES.8 Interlock) : ........................................................................... 17
2.2.8 Interlocks of Lower portion ................................................................................................... 18
CHAPTER-3 : Double Bus Single Circuit Breaker Scheme Interlocks ........................................................ 19
3.1 What is Double Bus Single Breaker Scheme? ............................................................................... 21
3.2.1 Interlocks of Line Feeder ....................................................................................................... 22
3.2.2 Case Study- Line Feeder : (ES.3 Interlock) : ........................................................................... 23
3.2.3 Interlocks of Transformer Feeder .......................................................................................... 24
3.2.4 Interlocks of BusCoupler....................................................................................................... 25
3.2.5 Interlocks of Bus Section ...................................................................................................... 26

Understanding the GIS Interlocks – One & Half Breaker Scheme 2

 CHAPTER-1 : Introduction to a Sub-Station

Understanding the GIS Interlocks – One & Half Breaker Scheme 3

 1.1 Familiarization with some general terms

1.1.1 What is a Sub-Station:

A Substation is an entity housing a set of equipment that is used for generation, transmission or
distribution of electrical power.

1.1.2.What is a switch?
A switch is a device for making and breaking the connection in an electric circuit. A simple
example is the light switch in a room which can be used to turn the lights on or off. In a
substation, Disconnectors, Earthing Switches and Circuit Breakers are typical examples of
switches.

1.1.3 What is Load?

In electrical terms, Load is referred to any equipment of group of equipment’s that are drawing
some amount of current from the system. This current is called load current. Examples of load
are: transformer , a sub-station that is receiving current from a line feeder etc.

1.1.4 What is On Load switching?

The condition in which turning on a switch will allow current to pass through it is called on load
switching. On load switching should take place at very high speeds to avoid arcing. On load
switching is done through circuit breakers and that is why circuit breakers are categorized as on
load switches.

1.1.5 What is Off Load switching?

The condition in which turning on a switch will not allow current to pass through it is called off
load switching. The voltage , if present, will reach to the next point after the switch is closed. Off
load switching does not need to be at very high speeds. The typical closing and opening times of
off load switches are in seconds. Off load switching is performed by disconnectors and earthing
switches and hence they are categorized as off load switches.

1.1.6 What is Arcing?

Arcing is a high magnitude and highly luminous (ultra-bright) current discharge which is
produced when a high current leaps across a gap between two electrodes. In some industrial
applications, acring is useful and the apparatus through which arcing is expected to take place
are specially designed for that purpose. The arcing produced in these cases is used for
illumination and welding etc. In substations arcing is not desired and unwanted arcing can result
in damage to the moving or stationary components of the system.

Understanding the GIS Interlocks – One & Half Breaker Scheme 4

 1.2 Understanding the single line diagram of a substation
1.2.1 Circuit breaker:
A circuit breaker is a high speed switch used for on load switching in substations. The operating
time of 380kV Circuit breaker is typically 50ms to 80ms for closing and typically 33ms to 50ms
for opening but it may vary a little depending on voltage level and manufacturer. The ANSI
standard code for AC Circuit Breaker is ‘52’.Circuit breaker is sometimes referred to as ‘CB’.

Figure-1 : ANSI Symbol for a power circuit breaker

1.2.2 Disconnector:
A disconnector or disconnecting switch a switch used typically in HV and EHV switchgears to
perform off load operations which in turn make the system ready for on load switching. Off load
switching is always done before on-load switching when the system is dead. Disconnecting
switch is sometimes denoted as ‘DS’

Figure-2 : ANSI Symbol for a power motorized disconnector

For simplicity, the symbol used in this manual for DS is:

Figure-3 : DS symbol used in this manual for simplicity

1.2.3 Earthing switch:

An earthing switch is an offload switch used to connect an offload equipment to earth (or
ground). Offload equipment can be a bus, a cable, an overhead line, a transformer or any other
equipment belonging to the substation. Off load equipments are capable of bearing large
amounts of induced voltages from nearby on load equipment and to avoid this scenario,

Understanding the GIS Interlocks – One & Half Breaker Scheme 5

 earthing switch is closed to ensure that the equipment remains at zero potential during de
energized or maintenance conditions. For maintenance purpose, earth switches are available on
both sides of all DS and CBs in the system. Earthing switch is also known as grounding switch.
Earthing switch is sometimes denoted as ‘ES’ for simplicity.

Figure-4 : ANSI Symbol for a power motorized earthing switch

For simplicity, the symbol used in this manual for ES is:

Figure-5 : ES symbol used in this manual for simplicity

1.2.4 What is an Interlock?

An interlock is a condition or set of conditions that has to hold true for a switch operation to
take place. The switch operation (Opening or Closing) can take place only when the complete set
of conditions defined in its interlock holds true at the same time. Even if one of the conditions of
interlock stands false, the switch operation cannot take place.

For Example :

DS
Power Cable Feeder
Supply Load

CB
ES

Figure-6 : A simple illustration of a disconnector and earthing switchused in a power system.

Consider the following cases from Figure-6 shown above:

Understanding the GIS Interlocks – One & Half Breaker Scheme 6

 *Before turning on the power supply CB, The disconnector ‘DS’ must be closed (Off load
switching) so that the feeder is connected to the system. But if the earthing switch ‘ES’ is also
closed at the same time, then closing the Power supply circuit breaker will directly ground the
power supply which will be catastrophic.

*The DS must be closed before closing the CB because if the CB is closed and the DS is allowed to
close after that, it will be on load switching at a very slow speed which will damage the system
due to arching.

*If we want to close the earthing switch, The DS should be open. Otherwise if the CB is closed ,
the power supply will be grounded. Or if the CB is open , anyone can close it which will ground
the power supply.

These are few simple cases which illustrate that in order to safely perform a switching operation,
a certain set of conditions need to be satisfied to allow the operation. In the above cases:

 For CB operation, the DS must be closed. If DS is open, it should not allow CB to close.
 For DS operation , the ES and the CB must be open. If anyone is close, it should not allow the
DS to operate.
 For ES Operation, the DS must be open. If DS is closed, ES should not be allowed to operate.

So we can see that these are the 3 governing conditions or interlocks for ensuring safe switching
operation in the above 3 cases described. In other words, the first condition is the interlock for
the CB. The second condition is the interlock for the DS. And the third condition is the interlock
for the ES.

It should also be noted that interlock exists in both directions. i.e. For operation of disconnector,
the ES must be open. Subsequently , for operating of earth switch , disconnector must also be
open. Before closing of CB, DS must be closed. Subsequently , DS must only be allowed to
operate when CB is open.

All these conditions are included in the control scheme of the switches by wiring. If a bay control
unit relay is available for the bay, then these conditions can be included in soft logic also.

1.2.5 Some general rules for interlocks

A particular interlock can be obtained by different methods. But some general rules have to be
satisfied while designing the interlock.

 Interlock of earthing switch should always be with service position of the feeder CB.In
switchgears, CB in service means the CB is racked in. In GIS , CB in service means the
disconnectors of CB are closed. If So if DS is closed (or CB is racked in), ES should not be
allowed to close and vice virsa.
 Interlock of DS should be with CB and ES.

Understanding the GIS Interlocks – One & Half Breaker Scheme 7

 CHAPTER-2 : One and Half Breaker Scheme
Interlocks

Understanding the GIS Interlocks – One & Half Breaker Scheme 8

 REMOTE
380KV BUS.1
Bus Section
END LINE 380KV
REMOTE DS REMOTE
END END S/S
LINE ES Bus1 ES
DS.1

ES.1

Upper Portion
UPPER
Remote End SS CB

ES.2

DS.2

ES.3

DS.4

Transformer Feeder
Middle Portion
ES.5 DS.3
VT.DS.2
MIDDLE
ES.8
CB
ES.4
Line Feeder

ES.6 VT.DS.1
DS.6

DS.5

ES.7 380KV

DS.7

230KV 13.8KV
ES.9
230KV GIS
Lower Portion

LOWER
CB

13.8KV
ES.1
ES.10 230KV
TRAFO 13.8KV
ES.1 CB
DS.8

BUS2 ES
13.8KV
230KV ES.2
380KV BUS.2 TRAFO
230KV DS 230KV
CB2 CB1
13.8KV BUS

13.8KV SWITCH GEAR

230KV ES.2

Figure-7 : Single Line Diagram of a one and half breaker scheme bay .

Understanding the GIS Interlocks – One & Half Breaker Scheme 9

 2.1 What is On and Half Breaker Scheme?
Consider the Single Line Diagram arrangement shown in Figure-7 .As we can see that there are 3
circuit breakers. ‘Upper CB’ , ‘Middle CB’ and ‘Lower CB’. And using these breakers ,two feeders can
be connected to the Bus. One line feeder going to remote end station and one transformer feeder.

The Transformer feeder can be connected to the Bus by two methods:

1- Close ‘Upper CB’

2- Close ‘Lower CB and Middle CB’

Similarly the line feeder can also be connected to the Bus by two methods:

1- Close ‘Lower CB’

2- Close ‘Upper CB and Middle CB’

So we can see that three CBs are used to connect two feeders to the bus inside the substation. In
other words , one and half CB per feeder is required in this arrangement. Hence it is called one and
half circuit breaker scheme.

2.2 Interlocks of one and half circuit breaker system

In the last chapter we discussed the general rules for designing interlocks of different types of
switches. Here we will see the actual interlocks for each and every switch in the one and half circuit
breaker arrangement shown in figure-7.All switches are labelled for the same purpose , e.g. DS.1 ,
ES.3 .The interlocks are shown in equation for easy understanding.

It is to be noted that the general conditions for CB that are not related to switches e.g. All Lock out
relays reset , Synchro check OK , CB spring charged , SF6 pressure normal,have not been mentioned
here. It is assumed that the reader is familiar with these conditions.

For easy understanding, we will break down the single line diagram into 6 portions:

1- The upper CB and its related disconnectors and earth switches or the ‘Upper portion’.
2- The Transformer feeder.
3- The middle CB and its related switches or the ‘Middle portion’.
4- The line feeder.
5- The lower CB and its related disconnectors and earth switches or the ‘lower portion’.
6- Bus Section.

Understanding the GIS Interlocks – One & Half Breaker Scheme 10

 2.2.1 Interlocks of Upper portion

DS.1

TEST=BUS1 ES CLOSE+ES1 CLOSE

SERVICE= BUS1 ES OPEN+ES1 OPEN+ES2 OPEN+UPPER CB OPEN

ES.1

DS1 OPEN+DS2 OPEN

Upper CB

TEST=DS1 OPEN+DS2 OPEN

SERVICE=DS1 CLOSE+DS2 CLOSE+SYNCH CHECK OK+CLOSE BLOCK OK+CLOSE AUTH OK

DS.2

TEST=ES2 CLOSE+ES3 CLOSE

SERVICE= ES3 OPEN +ES1 OPEN+ES2 OPEN+UPPER CB OPEN

ES.2

DS1 OPEN+DS2 OPEN

Understanding the GIS Interlocks – One & Half Breaker Scheme 11

 2.2.2 Case Study- Upper Portion : (DS.1 Service Position Interlock) :

As it is described in the previous section , the interlock of DS is with ES and CB. So logically the
interlock of DS.1 should exist with all the earth switches and Circuit Breakers that are directly
connected to DS.1.

In this case, The service condition of DS.1 Should be:

SERVICE=BUS1 ES OPEN+ES1 OPEN +UPPER CB OPEN

But the actual service condition is:

SERVICE=BUS1 ES OPEN+ES1 OPEN +ES 2 OPEN + UPPER CB OPEN

So the question arises that what is the reason to include ‘ ES.2 OPEN’ status in the interlock when
the remaining statuses were enough to fulfill the interlock condition. The reason is that , every
switch is provided with several auxiliary contacts (Normally open and Normally Closed). These
contacts are used wherever the status of the switch is required in the scheme. An interlock circuit is
one example. The switch and its auxiliary contacts are tested during the commissioning of the
substation. However, the possibility cannot be over ruled that with the passage of time, there can be
a slight chance of any one of the contacts of one switch to malfunction. The possibility is very less
but it cannot be established that it will never happen.

Now let’s consider that in the given case, one (52-B) auxiliary contact of Upper CB gets stuck in
closed position. This means that its interpreting that the CB is Open. Now even if the CB is closed ,
this particular contact is giving the status that the CB is open. So if the same faulty auxiliary contact
is used in this DS.1 interlock, and the general rule is used to design interlock . i.e.

SERVICE=BUS1 ES OPEN+ES1 OPEN +UPPER CB OPEN

Then DS.1 will be allowed to close only if ‘Bus ES’ and ‘ES.1 ‘are open regardless of CB status. It
means that if CB is closed, and ES.2 is also closed , the system will think that CB is open and hence it
will allow DS.1 to close the connect the live bus bar to earth. To cope with this kind of situation, the
interlock is made more efficient by including ES.2 open status.

Hence the DS.1 service condition interlock is:

SERVICE=BUS1 ES OPEN+ES1 OPEN +ES 2 OPEN + UPPER CB OPEN

Understanding the GIS Interlocks – One & Half Breaker Scheme 12

 2.2.3 Interlocks of Transformer Feeder

DS.3

TEST=ES3 CLOSE+ES4 CLOSE

SERVICE=UPPER CB OPEN+MIDDLE CB OPEN+ES3 OPEN+ES4 OPEN+230 CB1 OPEN+230 CB2

OPEN+230 TRAFO ES1+230 TIE ES2+13.8 ES1+13.8 CB OPEN+13.8ES2

ES.3=

DS2 OPEN+DS3 OPEN+DS4 OPEN

ES.4=

DS3 OPEN+VT DS CLOSE+VT FUSE CLOSED+UV OPERATED+TRAFO DS OPEN+13.8 CB IN TEST

Understanding the GIS Interlocks – One & Half Breaker Scheme 13

 2.2.4 Case Study- Transformer Feeder : (DS.3 Service Position Interlock) :

As we can see that the service condition interlock for DS.3 is including switches from 380kV GIS as
well as 230kV GIS and 13.8kV switchgear. Lets break down the interlock into portions

DS.3 SERVICE=

(380kV)

UPPER CB OPEN+MIDDLE CB OPEN+ES3 OPEN+ES4 OPEN

+
(230kV)
230 CB1 OPEN+230 CB2 OPEN+230 TRAFO ES1 OPEN+230 TIE ES2 OPEN
+
(13.8kV)
13.8 ES1+13.8 CB OPEN+13.8ES2

There are 2 reasons for including the 230kV side and 13.8kV side in the interlocks.

1- If the disconnector on the HV side of transformer (DS.3) is closing , the interlock should not
only exist with the HV side earth switches but also the LV and tertiary (if tertiary winding is
available) side earth switches.
2- To avoid back energization. Or in other words , to define the direction of the load flow
through the transformer.

Now let’s consider the 230kV portion of the interlock:

230 CB1 OPEN+230 CB2 OPEN+230 TRAFO ES1 OPEN+230 TIE ES2 OPEN

(230 TRAFO ES1 OPEN + 230 TRAFO ES2 OPEN) is used for condition #1 described above. It is making
sure that if the Transformer DS is going to close on the upper side, the earth switches on lower side
are open.

(230 CB1 OPEN+230 CB2 OPEN) is used for condition #2 described above. It is making sure that if the
Transformer DS is going close on the upper side , the lower side is still open and hence the flow of
load shall only be from upper side to lower side. If this condition was not included , there might be a
possibility that if 230kV Bus Bar is already energized and the respective CB is closed. It can back
energize the transformer.

It should be noted that instead of using (230 CB1 OPEN+230 CB2 OPEN) , (230 TRAFO DS OPEN)
could also be used to fulfill condition #2 for 230kV and avoid back energization. Hence we can see
that a particular interlock condition can be achieved in more than one way.

The 13.8kV Earth switches and CB are also included for the same purpose as 230kV.

Understanding the GIS Interlocks – One & Half Breaker Scheme 14

 2.2.5 Interlocks of Middle Portion

DS.4

TEST=ES3 CLOSE+ES5 CLOSE

SERVICE=ES3 OPEN+ES5 OPEN+MIDDLE CB OPEN+ES6 OPEN

ES.5=

DS4 OPEN+DS5 OPEN

MIDDLE CB

TEST=DS4 OPEN+DS5 OPEN

SERVICE=DS4 CLOSE+ DS5 CLOSE +DS3 OPEN OR CLOSE+DS OPEN OR CLOSE +SYNCH CHECK
OK+CLOSE BLOCK OK +CLOSE AUTH OK

DS.5

TEST=ES5 CLOSE+ES7 CLOSE

SERVICE=ES5 OPEN+ES6 OPEN+ES7 OPEN+MIDDLE CB OPEN

ES.6=

DS4 OPEN+DS5 OPEN

Understanding the GIS Interlocks – One & Half Breaker Scheme 15

 2.2.6 Interlocks of Line Feeder

ES.7=

DS5 OPEN+DS6 OPEN +DS7 OPEN

DS.6

TEST=ES8 CLOSE+ES7 CLOSE

SERVICE=ES7 OPEN+ES8 OPEN+LOWER CB OPEN+MIDDLE CB OPEN

ES.8=

DS6 OPEN+VT DS CLOSE+VT FUSE CLOSE+UV OPERATED

Understanding the GIS Interlocks – One & Half Breaker Scheme 16

 2.2.7 Case Study- Line Feeder : (ES.8 Interlock) :

As we can see that the interlock of ES.8 is having some conditions that are different from the
conditions of all other switches.This holds true for line earth switches and transformer earth
switches. The interlock of Transformer feeder earth switch ES.4 in section 2.2.3 can be seen for
reference.

Now let us analyze the condition for ES.8 earth switch operation.

ES.8 = DS6 OPEN+VT DS CLOSE+VT FUSE CLOSE+UV OPERATED

This interlock condition can be broken down in to 2 portions.

ES.8 = (DS.6 OPEN ) + (VT DS CLOSE + VT FUSE CLOSE + UV OPERATED )

‘DS.6 OPEN’ is the normal condition which is making sure that the Line ES should not be allowed to
close if the Line DS is already close.But this condition is not enough to guarantee safe operation for
ES.8. The reason is that if the Line is energized from the remote end side the ES should not be
allowed to close in that case also as it will ground an energized line.For this purpose,An Under
Voltage relay is provided in the control panel of every feeder and the second portion of the interlock
is used to make sure that if the earth switch is going to close, it is a 100% safe operation to perform.

Now let us take a deeper look into the second part of the interlock equation.

(VT DS CLOSE + VT FUSE CLOSE + UV OPERATED )

We can see that it can further be divided into 3 portions. We will discuss the purpose of each
portion.

‘VT DS CLOSE’ :

A safe operation of the line earthing switch is performed by making sure that the line is already de-
energized or dead.For this purpose, on the local side we have already made sure that our line
disconnector is open by including it in the interlock condition. But the remote end line disconnector
is not available to us for using in the interlock.But if the line disconnector of the remote end is
closed, It will supply voltage to our station which can be read by the Line VT of my Feeder as the VT
is present after the Eathing switch.This voltage can then be used in the interlock.But in order for the
line VT to sense the voltage, we have to make sure that the VT disconnector is already closed.And
hence this condition has been added as the first portion of the interlock.

Understanding the GIS Interlocks – One & Half Breaker Scheme 17

 2.2.8 Interlocks of Lower portion

DS.7

TEST=ES7 CLOSE+ES9 CLOSE

SERVICE=ES7 OPEN+ES9 OPEN+LOWER CB OPEN+ES10 OPEN

ES.9=

DS7 OPEN+DS8 OPEN

LOWER CB

TEST=DS7 OPEN+DS8 OPEN

SERVICE=DS7 CLOSE+DS8 CLOSE+SYNCH CHECK OK+CLOSE BLOCK OK+CLOSE AUTH OK

DS.8

TEST=ES10 CLOSE+BUS2 ES CLOSE

SERVICE=BUS2 ES OPEN+ES10 OPEN+LOWER CB OPEN+ES9 OPEN

ES.10=

DS7 OPEN+DS8 OPEN

Understanding the GIS Interlocks – One & Half Breaker Scheme 18

 CHAPTER-3 : Double Bus Single Circuit Breaker
Scheme Interlocks

Understanding the GIS Interlocks – One & Half Breaker Scheme 19

 BUS SECTION
BUS
LINE FEEDER BUS TRANSFORMER FEEDER
COUPLER-1
COUPLER-2

ES.11 ES.12 ES.13 DS.14

DS.11 DS.12
BUS-1A BUS-1B
BS CB

DS.101 VT-1A VT-1B DS.201

ES.101 ES.201
DS DS
BC-1 BC-2
CB CB
ES.21 ES.22 ES.23 ES.24 ES.202
ES.102
DS.102 DS.202
DS.21 DS.22 BUS-2B
BUS-2A

VT-1B VT-2B
DS.1 DS.2 DS DS.1 DS.2
DS

ES.2 ES.2
ES.1 ES.1

LINE TRAFO
CB CB

ES.3 ES.3

DS.3 DS.3

VT DS
VT DS
ES.4 ES.4

115 KV

13.8 KV
REMOTE END
LINE ES

REMOTE END
LINE DS
13.8KV ES1

13.8KV
CB

13.8KV ES2

13.8KV BUS

Understanding the GIS Interlocks – One & Half Breaker Scheme 20

 Figure-8 : Single line diagram of Double Bus –Single Breaker arrangement

3.1 What is Double Bus Single Breaker Scheme?

Consider the Single Line Diagram arrangement shown in Figure-8 .As we can see that for every
feeder (line or transformer) there is one circuit breaker.Each feeder can be energized through Bus-1
or Bus-2 by connecting the respective disconnector of that feeder.Hence heach feeder has only one
breaker and it can be energized by two buses.So it is called double bus signle breaker scheme.

3.2 Interlocks of Double Bus Single breaker system

Consider ‘Figure-8’ above showing the single line diagram representation of a typical double bus
single breaker system. All switches have been labelled for easy understanding. The same general
rules for interlocking will be applied here also as it is in one and half breaker scheme.

Like the one and half breaker scheme , the general conditions for CB that are not related to switches
e.g. All Lock out relays reset , Synchro check OK , CB spring charged , SF6 pressure normal, have not
been mentioned here also and it is assumed that the reader is familiar with these conditions.
Mechanical interlocks are also not mentioned although one example is given in Line Feeder
interlocks (ES.3) for understanding how mechanical interlocks work.

As we can see that in double bus single breaker scheme , the bus section is also having a circuit
breaker unline the one and half breaker scheme.Also for coupling of the two buses (for load transfer
or load sharing purpose) , two bus couplers are available.

For easy understanding, we will break down the single line diagram into 5 portions:

1- Line Feeder
2- Transformer feeder.
3- Bus Coupler -1
4- Bus Section
5- Bus Coupler-2

The interlocks of each portion are discussed in the coming segments.

Understanding the GIS Interlocks – One & Half Breaker Scheme 21

 3.2.1 Interlocks of Line Feeder
CB:

TEST = DS.1 OPEN + DS.2 OPEN + DS.3 OPEN + ALL LOR RESET

MAINTENANCE =(ES.1 CLOSE OR ES.2 CLOSE) + ES.3 CLOSE

SERVICE = (DS.1 CLOSE OR DS.2 CLOSE) + DS.3 CLOSE

DS1:

SERVICE=ES.1 OPEN + ES.2 OPEN + ES.3 OPEN + ES.11 OPEN + DS.2 OPEN + CB OPEN

BUS TRANSFER: CB CLOSE + DS.2 CLOSE + { BC1 SERVICE CLOSE* OR (BC2 SERVICE CLOSE*+ BS
SERVICE CLOSE) } * Service close means Circuit breaker along with disconnectors on its both sides.

MAINTENANCE : SELECTOR SW IN EMERGENCY + ES.11 CLOSE + ES.2 CLOSE + CB OPEN

DS2:

SERVICE=ES.2 OPEN + ES.1 OPEN + ES.3 OPEN + ES.21 OPEN + DS.1 OPEN + CB OPEN

BUS TRANSFER: CB CLOSE + DS.1 CLOSE + { BC1 SERVICE CLOSE* OR (BC2 SERVICE CLOSE*+ BS
SERVICE CLOSE) } * Service close means Circuit breaker along with disconnectors on its both sides.

MAINTENANCE : SELECTOR SW IN EMERGENCY + ES.21 CLOSE + ES.1 CLOSE + CB OPEN

(LINE) DS.3 :

SERVICE = ES.1 OPEN + ES.2 OPEN + ES.3 OPEN + ES.4 OPEN + CB OPEN

MAINTENANCE = (ES.1 CLOSE OR ES.2 CLOSE ) + ES.4 CLOSE

ES.1 & ES.2

DS.1 OPEN + DS.2 OPEN + MAINTENANCE ES KEY ENABLED*

 Maintainence ES key enable : This condition may be taken by some other method depending
on the manufacturer of GIS.
ES.3

DS.1 OPEN + DS.2 OPEN

ES.4:

DS.3 OPEN+ ALL 3 PHASES UV RELAY PICKED UP + LINE PT DS & VT BOX MCB CLOSED + VT MCB FOR
UV RELAY IN LCC PANEL ON

LINE VT DS:

ES.4 CLOSE * VT DS is not motorized and is closed mechanically

Understanding the GIS Interlocks – One & Half Breaker Scheme 22

 3.2.2 Case Study- Line Feeder : (ES.3 Interlock) :
Consider the interlock equation mentioned for ES.3 Interlock in the above section.

ES.3 = DS.1 OPEN + DS.2 OPEN

As it has already been discussed that the interlock of an earth switch exists with service position of
CB. Now let us consider Figure-9 to analyze the portion of line feeder . To put the CB in service
position for this feeder , we need to close ‘DS.1 OR DS.2’ on the upper side of CB and DS.3 on the
lower side of CB.

DS.1
DS.2

ES.1
ES.2

CB

ES.3

DS.3

ES.4 VT DS

Figure-9 : Single line diagram of Line Feeder in Double Bus –Single Breaker arrangement

Hence according to the rules discussed above, the interlock of ES.3 should be as followes.

ES.3 = DS.1 OPEN + DS.2 OPEN + DS.3 OPEN.

But the actual interlock equation mentioned here is:
ES.3 = DS.1 OPEN + DS.2 OPEN.

The reason for not mentioning DS.3 is that sometimes in GIS , the disconnector and earth switch
come as one unit that are mechanically interlocked with each other. In this case , the ES can not be
closed unless the DS of the same DS/ES unit is open. Hence the interlock of ES.3 in this particular
case exists with DS.3 also but as it is a mechanical interlock so it is not available in the electrical
interlocks.And hence not mentioned in Interlock equation also on purpose.

It is to be mentioned here that when reviewing a GIS interlocking scheme , you may face a similar
situation where you may not be able to find a necessary interlock in the electrical ciruit.In this case it
has to be verified that the mechanical interlock exists before establishing that the scheme is having
an error.

It should also be considered that , depending on the GIS model and manufacturer, the same DS.3
and ES.3 may not be present as a signle unit in some GIS and in that case the electrical interlock for
DS.3 will be included for ES.3

Understanding the GIS Interlocks – One & Half Breaker Scheme 23

 3.2.3 Interlocks of Transformer Feeder
CB:

TEST = DS.1 OPEN + DS.2 OPEN + DS.3 OPEN + ALL LOR RESET

MAINTENANCE =(ES.1 CLOSE OR ES.2 CLOSE) + ES.3 CLOSE

SERVICE = (DS.1 CLOSE OR DS.2 CLOSE) + DS.3 CLOSE+ LV SIDE CB OPEN

DS1:

SERVICE= ES.1 OPEN + ES.2 OPEN + ES.3 OPEN + ES.12 OPEN + DS.2 OPEN + CB OPEN

BUS TRANSFER: CB CLOSE + DS.2 CLOSE + { BC2 SERVICE CLOSE OR (BC1 SERVICE CLOSE+ BS SERVICE
CLOSE) }

MAINTENANCE : SELECTOR SW IN EMERGENCY + ES.12 CLOSE + ES.2 CLOSE + CB OPEN

DS2:

SERVICE= ES.2 OPEN + ES.1 OPEN + ES.3 OPEN + ES.22 OPEN + DS.1 OPEN + CB OPEN

BUS TRANSFER: CB CLOSE + DS.1 CLOSE + { BC2 SERVICE CLOSE OR (BC1 SERVICE CLOSE+ BS SERVICE
CLOSE) }

MAINTENANCE : SELECTOR SW IN EMERGENCY + ES.22 CLOSE + ES.1 CLOSE + CB OPEN

DS.3 :

SERVICE = ES.1 OPEN + ES.2 OPEN + ES.3 OPEN + ES.4 OPEN + CB OPEN+ LV SIDE ES OPEN

MAINTENANCE = (ES.1 CLOSE OR ES.2 CLOSE ) + ES.4 CLOSE

ES.1 & ES.2:

DS.1 OPEN + DS.2 OPEN + MAINTENANCE ES KEY ENABLED

ES.3:

DS.1 OPEN + DS.2 OPEN

ES.4:

DS.3 OPEN+ ALL 3 PHASES UV RELAY PICKED UP + LINE PT DS & VT BOX MCB CLOSED + VT MCB FOR
UV RELAY IN LCC PANEL ON

TRAFO VT DS:

FEEDER DS OPEN + LV SIDE ES CLOSE

Understanding the GIS Interlocks – One & Half Breaker Scheme 24

 3.2.4 Interlocks of BusCoupler
CB:

TEST = BOTH DS OPEN

MAINTENANCE = BOTH ES CLOSE

SERVICE = BOTH DS CLOSE

DS.1:

SERVICE = ES.1 OPEN + ES.2 OPEN + CB OPEN + BUS ES OPEN

MAINTENANCE = BUS ES CLOSE + OTHER ES CLOSE + CB OPEN

ES.1 & ES.2

DS.1 OPEN + DS.2 OPEN + CB OPEN

BUS.1 ES OPEN

DS.1 OPEN + BUS SEC DS.1 OPEN + ALL 3 PHASE UV + BUS PT DS CLOSE + ALL BUS-1 FEEDERS DS
OPEN + ES KEY ENABLE + PT BOX MCB CLOSE + LCC MCB GOING TO UV RELAY CLOSE.

Understanding the GIS Interlocks – One & Half Breaker Scheme 25

 3.2.5 Interlocks of Bus Section
CB:

TEST = DS.1 OPEN + DS.2 OPEN + LOR RESET

MAINTAINENCE= ES.1 CLOSE + ES.2 CLOSE + LOR RESET

SERVICE = DS.1 CLOSE + DS.2 CLOSE + LOR RESET

DS.1:

SERVICE = BUS ES OPEN + ES.1 OPEN + ES.2 OPEN + CB OPEN

MAINTENANCE = BUS ES CLOSE + ES.2 CLOSE + CB OPEN

ES.1 & ES.2

SERVICE = DS.1 OPEN + DS.2 OPEN + CB OPEN

OTHER BUS SECTION DS.1

SERVICE = INNER ES.1 OPEN + INNER ES.2 OPEN + BUS ES OPEN +

a- ALL DS OF THAT BUS OPEN

OR
b- ALL DS OF OTHER BUS OPEN
OR
c- OTHER DS OPEN

MAINTENANCE= OTHER INNER ES CLOSE + BUS ES CLOSE

INNER ES.1

OTHER DS OPEN

Understanding the GIS Interlocks – One & Half Breaker Scheme 26

 Disclaimer
All the text in this manual has been written by the author himself. All images used
in this manual have been either made, or captured by ‘print screen’ method from
the authors own computer. No text passage and no image has been copied from
anywhere.

Understanding the GIS Interlocks – One & Half Breaker Scheme 27