Volume 10, Issue 10, Octoberber 2021

Impact Factor: 7.282

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DOI:10.15662/IJAREEIE.2021.1010027 |

Interlocking and Intertriping in MV (1kV to

35kV) Switchgears, as applied in a typical Oil
& Gas Refinery / Petrochemical Project
Sukhraj Singh
Department of Electrical Engineering, Fluor Daniel India Pvt Ltd, New Delhi, India

ABSTRACT: A typical oil &gas mega-refinery or a petrochemical project has a huge and complex power distribution
network. And, equally complex is the data network with thousands of signals flowing across in different directions. Of
these, the intertriping and interlocking signals have the highest priority to ensure safe & correct operation of the plant.
And, in this data network, switchgears act as a central node, receiving, processing and further distributing data/signals.
There are a number ofswitchgear internal safety interlocks as stated in IEC 62271 and IEEE C37.20.2, that are required
to be implemented by the manufacturer/supplier/vendor for the respective IEC/NEC compliant switchgear. While these
have been discussed in this paper, but emphasis is on the external interlocking & intertriping mechanisms and
interfaces that generally are designed/implemented as part of switchgear or the power system design, during project
execution. Examples have been included for various such mechanisms & schemes from oil &gas and petrochemical
industry projects, considering the best practices implemented from past and ongoing projects. Also elaborated are the
important aspects that are to be considered while designing, reviewing or approving the switchgear design during the
project execution.
This paper doesn’t include protection interlocking schemes, like the Zone Selective Interlocking or GOOSE based bus
differential protection. Each of these topics is a subject in itself and a brief discussion in this paper can do no justice.
The paper moreover is primarily for metal clad switchgears, although most examples and aspects discussed shall be
applicable for an outdoor MV switchyard as well. Also, the paper doesn’t elaborate on the procedure-based safety
mechanisms, like the site padlocking or LOTO procedures.

KEYWORDS:Switchgear, intertrip, interlock, IEC-61850.

I. INTRODUCTION
To ensure correct and safe plant operation, there’s extensive interlocking and intertriping circuitry and logics that are an
integral part of a switchgear and power system design. Well-designed intertriping and interlocking schemes ensure:
a. Fault isolation, like a faulty line section being isolated and blocked for closing by effective intertriping &
interlocking between upstream and downstream circuit breakers (CB) and isolators.
b. That the personnel do not access potentially dangerous areas without the adequate safety mechanisms being in
place, like the power supply being switched off and the earth switch being closed in order to access the
switchgear cable compartment in a metal-clad switchgear.
c. That the equipment is safely operated, and the risk of damage is greatly reduced, like interlocking to prevent
paralleling of power supplies in Ring Main Units (RMU).
Interlocking or intertriping schemes can be implemented either mechanically (like shutters, draw-out mechanisms, key
interlocks etc.), by control wiring (i.e. control circuitry using copper wiring & coils), using relay logics or by serial
communication, like the GOOSE messaging. A few of such mechanisms and schemes have been detailed in further
sections and explained with examples.
II. MECHANICAL INTERLOCKS
The simplest example of mechanical interlocking in a switchgear is the CB interlock with an earth switch (ES) and
isolator/disconnect switch (DS). In the fig., for a typical outgoing feeder, ES cannot be closed unless the DS is open
and CB is closed, and CB cannot be closed unless the ES is open. Also, the DS cannot be opened unless the CB is open,
and DS cannot be closed if CB is closed.

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Another example of a mechanical interlock is the trapped key arrangement used in many industry
applications. As an example, in case of a rod-mill, to power the main drive, key is removed from
inching drive and inserted in the lock mounted on main mill drive switchgear feeder. The main mill
drive feeder breaker can be racked in only when this key is inserted, and likewise, the key can be
removed only when the breaker is racked out. This is to ensure that the inching drive and the main
drive are never power fed at the same time. In application, there are many other permissivesalso
built into the above interlocking scheme to ensure safe and reliable operation, like the indication
light on control panel being ON or OFF (indicating “Inching Drive Ready/Not Ready”) depending
on if the key is respectively inserted in or not; drive “Run” command generated by PLC when the
“Main Drive Ready” signal is healthy from switchgear etc. These interlocks however are built-in
using control wiring or logics, as elaborated in the next section.

Another common interlocking

application is the interlocking
between incomer feeders (connected
to the same bus) to prevent parallel
feeding paths. As an example,
consider a RMU used for load bank
testing of 2 EDGs (Emergency Diesel
Generators). The interlock is
designed such that RMU-CB1/2 can
be closed only if DG-CB1/2 is
respectively closed. Also, out of
RMU-CB1 & RMU-CB2, only one can be closed at a time. And, RMU-CB3 can be closed only if either RMU-CB1 or
RMU-CB2 is closed.
This interlocking can be achieved through key locking arrangements (e.g. castell key or trapped key). The RMU CB-
1/2/3 interlocking may also be achieved through an internal mechanical interlock.
For the RMU earth switches (ES), in additional to the interlock with their respective CB, an interlock is provided with
upstream CB as well, i.e. DG-CB1/2 cannot be closed unless RMU-E1/2 are respectively open. Likewise, RMU-E1/2
cannot be closed unless DG-CB1/2 is respectively open.

The key interlocks may also be used between upstream and downstream switchgears to ensure safe ES operation, as
illustrated in below figure, with an additional interlock for transformer access.

a. ES1/2 can be closed only if CB2/1 are respectively withdrawn, i.e. keys K1 & K3 are free
from respective CBs when withdrawn and inserted into corresponding ES slots.
b. The trafo can be accessed only if both ES1 & ES2 are closed, i.e. keys K2 & K4 are free
from respective ESs and inserted into corresponding trafo locks (maybe located on trafo yard
gate or terminal box).
ES & CB also have an internal mechanical interlock, i.e. at a time, either ES1 or CB1 can be
closed. Same holds for ES2 & CB2.

This concept is also applied for interlocking between switchgears/RMUs connected in a ring mains
system. Especially in networks that are spread across many a buildings or facilities, like in the
campus area of a refinery complex, key interlocking can replace the complex network of cables. The key interlocks are
simple to implement, more reliable and also cost effective compared to (control or fibre optic) cables.
Other examples of mechanical interlocksare:
1. In a dual motor feeder scheme (as illustrated), castell key interlock is provided
between circuit breakers (CB-A and CB-B) and earth switches (ES-A & ES-B),
such that:
a. At a time, only one CB (i.e. either of CB-A or CB-B) can be closed, in a break-
before-make arrangement.
b. CB-A or CB-B can be closed when both ES-A and ES-B are open.

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c. ES-A or ES-B can be closed when both CB1 & CB2 are in withdrawn position.

2. Panel rear door interlock with earth switch, i.e. the panel rear door can be opened only after the earth switch is
closed. Likewise, in a 2-section switchgear,the bus-coupler panel rear door can be opened only when (both the
buses are dead and) earth switches of both the buses are closed. This interlock can be achieved either through
an internal mechanical interlock or trapped key mechanism as explained in examples above.
3. Interlocking between bus-couplers in a 3 X 50% scheme (illustrated below) such that only one of the bus-
couplers can close at a given time (i.e. break-before-make scheme).

A similar interlocking is also implemented in a 2-section switchgear, where 2-out-of-3 breakers can be closed
at a time, i.e. either both incomers are closed, or one of the incomers and bus-couplers are closed at a time.

The interlocking schemes/mechanisms discussed in S.No. 1, 2 & 3above can also be achieved using control circuitry or
IED logics. However, many a Clients/operators prefer either the mechanical interlocks (for reliability reasons;
especially for maintenance access) or dual interlocks (i.e. a combination of electrical/serial/mechanical interlocks). The
selection is determined by criticality of operation and also on the access restrictions/procedures. In some cases, a tool or
procedure based access restriction may do away with requirement of another level of interlocking, e.g. rear door access
in a switchgear being regulated by an effective LOTO procedure or requiring a special tool for opening the panel door.
Furthermore, many a safety or operational interlocks are an integral part of switchgear design. A few examples are:
1. CT terminals of a feeder not being accessible unless the CB (or Contactor) is withdrawn.
2. CB racking operation being possible only with the door closed, so that the IAC integrity is not compromised.
3. Shutter mechanism not allowing access to bus terminals when a CB is withdrawn. A padlock (implemented
through site access/LOTO procedures) may also be applicable for additional security.
4. Withdrawable CBs prevented by interlock from complete draw-out until their mechanism is discharged.
5. The CT secondary terminals being short-circuited as the relay is withdrawn (applicable for withdrawable
relays).
6. The feeder ES getting closed as the contactor/CB is withdrawn.
III. ELECTRICAL INTERLOCKS & INTERTRIPS
The electrical interlocking/intertriping maybe implemented using the switchgear control circuitry i.e. a combination of
coils, contacts and wires (hardwiring, typically copper core or paired wires/cables), or relay logic programming or
using serial/ethernet communication. The signal transmission to other feeders/IEDs is achieved either using control
circuitry or GOOSE/IEC-61850 communication via Ethernet LAN (i.e. Cat 5/5e etc.) or fiber-optic network.
Many of the interlocking examples discussed in above section (II. Mechanical Interlocks) can be implemented using
electrical interlocking as well, like the panel rear door interlock, ES interlock for RMU (in load bank testingdiscussed
above) etc. Some more examplesfrom practical applications are:
A. Switchgear internal interlocks/intertrips:
1. Tripping of CBs (and closinginhibited)on account of a bus fault (87B): Upon activation of the 87B protection,
trip signal is transmitted to all I/C & O/G CBs (& their closing blocked unless fault is cleared), thus isolating
the faulty bus. This can be achieved using control circuitry or GOOSE communication.
2. CB opening & closing being blocked on account of lower gas pressure in GIS bus chamber. This is achieved
through IED internal logics.

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This blocking is generally applicable to the specific panel witnessing low gas pressure or leakage. However, if
the blocking is to be extended to other panels in line-up, signals maybe transmitted using either the control
circuitry or GOOSE messaging.
Low gas pressure in outgoing feeder(s) may also warrant tripping of upstream CB, which can also be achieved
through electrical intertripping by implementing either the control circuitry or relay logics and GOOSE.
3. CB closing being blocked, if any of the PT MCBs (i.e. PT for incomer or the bus) is open. This interlocking is
generally achieved by IED internal logics itself.
4. CB closing blocked on account of activated TCS (trip circuit supervision) alarm (generally implemented using
a combination of control wiring and relay logics).
5. Incomer CB closing being blocked, if bus side voltage is higher than 10% of the switchgear rated voltage (e.g.
3.3kV for a 33kV switchgear).
6. Incomer CB tripping on under-voltage being blocked, if both incomers experience the undervoltage
simultaneously.
7. ATS blocked in case of a CB failure or a bus-fault.
The IEDs also contain a variety of safety and operational interlocks in the form of logic functions or Boolean equations,
that are a part of the switchgear design by vendor. A few of the examples are: CB closing being inhibited in case “CB
Not Ready” signal is high, CB closing from remote workstation/annunciator being inhibited unless the Local-Remote
Selector Switch is in “Remote” position etc.
B. Switchgear external interlocks/intertrips:
1. Intertriping/interlocking between upstream & downstream CBs:
a. Opening of upstream CB shall intertrip & block closing of downstream CB.
b. Tripping of downstream breaker will trip and block closing of upstream CB, to isolate the faulty
section/equipment.
c. Upstream CB cannot be closed with downstream ES closed. Downstream CB cannot close with upstream
ES closed or upstream CB open (ES is not applicable for LV).
d. Downstream ES cannot close unless upstream ES is closed or upstream breaker is open.
For implementing the above criterion,following signals are exchanged between upstream & downstream
switchgears:
a. Upstream feeder CB close/open status sent to downstream incomer IED
b. Upstream &downstream feeder ES contact status are exchanged
c. Downstream switchgear incomer CB trip status sent to upstream
This scheme can be implemented using the control circuitry (copper core/paired cables or pilot wires) or protocol
communication via FO cables. The fig. below shows 4 such cases for upstream-downstream communication
between relays/IEDs as implemented in different projects:
(i) The communication between relays is through inter-
connecting control cables, which typically may be
copper core / paired cables or pilot wires.
In certain specific or critical applications (especially in
case of grid interface), cores for intertrip signals are
wired onto the CB contacts to trip the CB directly, while
the status signals are communicated through the
IED/relay. This wiring is most cost effective for shorter
distances and thus can especially be utilized for
intertrip/interlocks within a substation or within a small
plant.
(ii) The IED-to-IED communication is through FOC.
(iii) & (iv) If either of (or both) the relays/IEDs is not capable of direct FOC communication or connection or if
there’s some inter-operability issue, the FOC interface can still be utilized by introducing an intermediate I/O
module that converts FO signals into hardwired inputs to relay / IED.

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The benefit of using FOC is that there are minimal signal distortion issues on account of long distances and a
multi-core/pair control cable (like 12C X 2.5 sq. mm) can be replaced by a 2-core fiber cable, thus reducing
trench space and also the installation cost of handling & installation. Especially on a large project, these
compounded savings do make a significant impact.
For protocol communication, an important aspect to be considered is the inter-operability of devices. In the
above example, suppose the considered IED has only 3 ethernet ports: 2 nos. compatible for IEC-61850 and
the third one compatible with vendor signaling protocol (e.g. Schneider InterMiCOM for P543 IEDs). So, if
the 2 nos. IEC-61850 ports are being used for IEC-61850 daisy chain (for ENMCS interface), only the third
one can be utilized for intertriping/interlocking with
upstream/downstream switchgear IED. So, while
there’s no likely issue if the same IED make devices
are being used at both the ends, but if one of the
IEDs is of a different make/type/model, there may be
challenges encountered. One of the solutions in such
a case would be to use an IO module (e.g. SEL-2505
etc.), as also discussed above.
(For simplicity, the relay/IED – CB interface and
other components like FDF/gateway etc. have been
omitted in adjacent fig.)

2. Switchgear intertrips and interlocks with other interfacing equipment. Some of the examples for this
category are:
a. The ATS (Automatic Transfer Switch) operation being blocked in case of an active transformer alarm in
the healthy section. In a 2-section switchboard (2 X 100% configuration), in case of an undervoltage on
one of the sections, ATS would be accomplished only if the healthy switchgear section doesn’t have any
active transformer alarm, examples of such trafoalarms being :winding or oil temperature high, fan failure
or any problem with its cooling system.
ATS may also be blocked during load-shedding.
b. The motor space heater being turned on as the contactor or CB is open (i.e. motor heater is interlocked
with position of switching device).
c. CB tripped and closing blocked on account of STOP command from process control (i.e. DCS/PLC) or
plant safety shutdown system (i.e. SIS/ESD).
This interface is implemented either through hardwiring i.e. using IRP (inter-posing relays) or serially i.e.
communication via FOC/ethernet over Profibus or Modbus communication protocol.
d. Interlocking is also used to ensure correct closing sequence, like in the following example of Emergency
Power System start-up:
In the fig., CB-BC can close only when CB-GA/B/C are closed
(i.e. after all 3 EDGs have synchronized), CB-INC is open (to
avoid paralleling of power supplies) and CB-ET is open (to
avoid multiple earthing paths). Also, to ensure stepped loading
on EDGs, the load feeder CBs may be sequentially closed, as
determined by EDG loading study.
This scheme can be implemented by a combination of control
wiring (especially if all equipment are a part of the same
substation) and IED logics. The interlocking between CB-
GA/B/C and CB-BC can be via control wiring while the
interlocking between CB-BC, CB-INC & CB-ET can be done
using IED logics itself since all these breakers are supposedly a
part of the same switchgear line-up.
IV. ELECTRICAL INTERFACE CONSIDERATIONS FOR SWITCHGEAR EXTERNAL INTERFACES
Switchgear external interfaces through IRP: For IRP application, it is important to align with IRP design team/vendor
on the relay requirements in terms of NO or NC contact application, use of dry or wet contacts, applicable voltage
levels (e.g. 24VDC or 110VDC), current ratings for coils and contacts, fail-safe circuits requirements etc.

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An example of IRP application is: NO contact of IRP is wired to the contactor, deenergizing to trip the motor. Now this
NO contact is closed in energized state and requires continuous power to stay in closed condition. To trip the motor,
power supply is withdrawn, the contact is back to NO state and motor trips. If there’s any problem with internal
circuitry or power interruption, the contact would open thus tripping the motor and providing a fail-safe operation.
In another example, wet NO contact from switchgear with 110VDC is provided to energize a relay in IRP cabinet.
Application may include Drive Run Status or Drive Available / Fault Status being signalled from switchgear to DCS
through the IRP.

Thus, the IRP & switchgear design is aligned and switchgear logics are accordingly designed. It is important to note
that this interface needs to be timely fixed during the project execution to contain any later modifications or delays.
Certain critical applications may also require SIL certified interposing relays to be used, as required per the project SIL
verification study and the selection has thus to be accordingly made.
While in general application, all trip commands/signals are assigned BI of IED and then through a series of logic gates
a common trip signal (i.e. BO) is transmitted to the CB trip coil, however, the SIL requirement may ask for directly
wiring the IRP trip signal to CB trip circuit, thus bypassing the IED.
SIL study sometimes also mandates redundant tripping, e.g. motor feeder VCB being provided with 2 tripping coils, i.e.
to comply with SIL 3 capability. In this case, ESD PLC would send out two trip signals via IRP to the CB. This may be
implemented such that the eachBI (i.e. trip from ESD) is converted to a respective BO to each of the CB tripping coil.
For the critical applications, some Clients/operators also mandate to utilize the relay/IED contacts (like forATS, trip
signals from ESD/SIS etc.) rather than contacts from multiplying relays or communication DI/DO cards installed with
IED to accommodate increased no. of signals.
These are some important aspects that the design engineer should be aware of, so that such a requirement can be timely
identified and implemented, without causing any delays or need for site modifications.
Protocol Communication: IEC-61850 peer-to-peer/horizontal communication (or GOOSE messaging) provides many
advantages over conventional hardwiring (like reduced copper wiring, ease of expansion etc.) and alsothe legacy
protocols.
For copper hardwiring, the wiring and space problem gets compounded in the longer line-ups. Imagine a 33kV GIS
line-up with say 90 verticals and the amount of copper wiring and panel/tray/duct space saved by replacing the copper
wiring with ethernet or fiber which allow multiple signals to flow along the same cable/cores at the same time. Due to
this reduced inter-panel wiring, the site erection time for a switchgear is also saved. Also eliminated are the other
hardware like contact multipliers, interposing relays, auxiliary coils etc. and thus also the time delays, leading to faster
signal transfers.
The GOOSE priority tagging for intertriping/interlocking signals (i.e. time critical GOOSE messages: Type 1/1A Class
P1/P2) can help achieve signal transfer times much lesser compared to conventional copper wiring. In terms of
flexibility, any later/future addition of signals (or, other interfaces) can be easily accommodated without any physical
modifications to the switchgear, thus saving on both time and cost.
There have been other serial protocols used in past and some are still in practice, but due to a variety of reasons
(especially the inter-operability and expandability), IEC-61850 is the most common communication protocol being
used in any new project for electrical protections and communication.
During the initial phase of a project, it is very important that the system architecture is explicitly defined. As a
minimum, this requires defining the redundancy configuration (i.e. RSTP, PRP, HSR etc.) and communication
protocols, application versions (like IEC-61850 Edition 1 or 2, OPC UA or DA etc.), base speed (like 1000Basefx,
100BaseT etc.) and cabling interfaces (i.e. Cat 5/6 or FOC etc.). From the switchgear perspective, this is very important
as it may affect the component selection (like for IEDs, Ethernet Switches, Data Concentrator (if applicable) and other
communication equipment like Gateways etc.), pricing and procurement in a significant way. Time stamping, if
applicable, also requires the time stamping protocol, wiring interface etc. to be defined in early stages of project, for
correct selection of communication equipment, especially the IEDs and also the time signal distribution equipmentlike
splitters, couplers etc., as applicable per the selected protocol.
Consider the following example: There are hardly any IMCS/MCU available in market that can support dual redundant
communication simultaneously for IEC-61850 as well as Modbus (or Profibus) for electrical (i.e. ENMCS) and process
(i.e. DCS/PLC) interfaces respectively. So, while one may have envisaged a system architecture with dual redundant

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ENMCS and DCScommunication, but its practical application may not be possible during the execution phase. Hence,
during the conceptual stage of a project, it needs to be checked whether the hardware supporting the envisaged
architecture is easily available in market. And, if the available devices are also compatible with other protection and
specification requirements of the project/Client.Moreover, what is the equipment (i.e. IED, MCU, IMCS, Data
Concentrator, Gateway etc. as applicable) size? Can it fit into the considered switchgear dimensions or is it leading to
increased panel line-up (and hence substation) size(s)? Is it even required to duplicate motor data in ENMCS (since this
is already monitored in DCS)? Such questions need addressing while designing a reliable, prudent and cost-effective
project system architecture.
While redundancy is preferred at every level, but in the above problem, a more prudent approach may be to use
Gateways and Ethernet Switches for protocol conversion while considering the applicable performance parameters
(signal criticality, response time etc.). Due to process criticality, the protocol convertors might not be allowed for DCS
interface, however, the same may be used for ENMCS interface as in this case ENMCS would have only the
monitoring functionalityfor the motor feeders, as is the general practice in oil &gas industry projects.
During the detailed engineering phase, there are other parameters and interfaces, like the MAC & IP addresses,
connector types (like LC, SC etc. for FOC interface; RJ11/45 for Cat 5/6 or RS-485 pin etc.) that need to be timely
considered and coordinated for. It also needs to be ensured that the inter-operability documents of interfacing
equipment are aligned, and that each equipment vendor is using KEMA certified IEDs to ensure inter-operability for
IEC-61850 implementation.
From the security perspective, virus/malware protections need to be considered and adequate firewalls, as required per
the project architecture, needs to be considered. At site, an efficient password access system needs to be implemented
giving access only to the designated personnel for system configuration and relay programming etc.
V. CONCLUSION
Switchgears have numerous safety&procedural interlocking and intertriping mechanisms/interfaces that are critical for
correct equipment operation and personal safety. During project execution, it is of utmost importance that each of these
interfaces are timely and wisely coordinated and designed for. Thispaper discusses some of these important interfaces,
design considerations and best practices generally implemented in oil &gas/petrochemical industry projects for a
reliable &safe system and a good engineering design.
ABBREVIATIONS USED

BI: Binary Input IED: Intelligent Electronic Device

BO: Binary Output I/C: Incomer or Incoming
CB: Circuit Breaker IMCS: Integrated Motor Control System
DS: Disconnect Switch / Isolator IRP: Interposing Relay(s)
ENMCS: Electrical Network Monitoring & Control LOTO: Lock Out Tag Out
System MCU: Motor Control Unit
ES: Earth Switch MV: Medium Voltage
ESD: Emergency Shutdown O/G: Outgoing
FDF: Fibre Distribution Frame PRP: Parallel Redundancy Protocol
GIS: Gas Insulated Switchgear RMU: Ring Main Unit
GOOSE: Generic Object-Oriented Substation Event RSTP: Rapid Spanning Tree Protocol
HSR: High-availability Seamless Redundancy SIS: Safety Instrumented System
IAC: Internal Arc Classification VCB: Vacuum Circuit Breaker

REFERENCES

[1] IEC 62271-1: High-Voltage Switchgear and Controlgear - Part 1

[2] IEC-62271-200: High-Voltage Switchgear and Controlgear - Part 200
[3] IEEE C37.20.2: IEEE Standard for Metal-Clad Switchgear

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