Introduction of LVAC Auto Change Over Scheme in DEWA Transmission System

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Introduction of LVAC Auto Change Over Scheme in DEWA
Transmission System
Owen Raymond Glynn, Jayasingh Balakrishnan, Amod Madhukar Khirwadkar, Muthusamy
Veerabahu, Pownrajan Chinnadurai, Raghavendra Upadhya Heggodlu-
Transmission Protection Department, Dubai Electricity & Water Authority.
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Abstract- Dubai’s transmission power system plays a vital
role in delivering optimum reliability and uninterrupted
power. We therefore need to maintain the highest levels of
technology, operational and best design practices for all
digital systems.
This paper aims to provide the conceptual design overview
and benefits of the innovative design of LVAC Auto
Changeover Scheme in DEWA Transmission Substations
(132kV).
The existing design of LVAC switchboards (400V AC supply
for substation equipment’s) used in 132/11 KV substations
under DEWA transmission network have three buses
powered through three earthing transformers and has two
bus sections. During normal operation, all three buses in
LVAC switchboard are independently supplied by each
earthing transformer (incomer) respectively and bus sections
are in open condition. To maintain uninterruptable LVAC
supply to substation equipment’s, a PLC is used and
programmed to execute automatic switching operation of
concerned bus section / incomer breakers in case of any of
the incomer supply fails. LVAC switchboard has an Auto/
Manual switch and PLC is operational if the switch is on Auto
mode. The existing scheme has mainly two issues as below:
A) In case of outage required on any of the IDT’s, operator
intervention is required to execute switching by shifting
Auto/ Manual switch in Manual mode. Manual switching
results in loss of LVAC supply as well as generation of
several alarms through SCMS.
B) In case of switching off any IDT during LVAC in Auto
mode, switching through PLC will affect only with a time
delay results in momentary interruptions to downstream
LV circuits which generates large number of undesired
alarms in SCMS.
With an objective to eliminate momentary supply
interruptions and unwanted alarms due to that, the team has
developed new logic diagrams to facilitate high speed
Automatic changeover of LVAC supplies with minimum
allowable paralleling time for the IDT’s after study on the
risks involved during momentary paralleling. In place of PLC,
Bay Control Units (BCU) from approved OEM’s are utilized
which are in use at higher voltage levels in DEWA system.
BCU’s are having major advantages over PLC in terms of ease
and flexibility in logic buildings, programming of the device
and speed of operation.
This paper aims to provide the conceptual overview and
benefits of LVAC High speed Auto Change Over scheme
developed in house by DEWA specialists in conjunction with
specialists from the original equipment manufacturers. The
new scheme involves use of Bay Control Units in place of
PLC’s in LVAC switchgear design. The BCU’s and overall
scheme is engineered, configured and extensively laboratory
tested to ensure perfect functionality of High-Speed Auto
Change Overs in LVAC board. Utilizing the capabilities of
modern IED’s to build large numbers of user defined logic
and implementing these in innovative new ways has
produced new unique LVAC changeover scheme that enhance
functionality and reliability by eliminating discreet
components and circuitry whilst maximizing overall
efficiency by reducing supply interruptions and unwarranted
alarms during supply changeover.
This innovative initiative will ensure that DEWA will have
arguably the most advanced LVAC switchboards in its
transmission systems both regionally and globally by
introducing the latest digital technologies, optimized designs
in line with the industry leading best practices.
I. INTRODUCTION
Dubai Electricity & Water Authority (DEWA) was
established in 1992 by His Excellency Sheikh Maktoum bin
Rashid Al Maktoum with the objective to provide reliable
power to residents of the Emirate of Dubai. Since its
inception, DEWA has continuously invested in maintaining
and upgrading the power delivery infrastructure to meet
the increasing demands of the growing Emirate of Dubai.
International Research Journal of Engineering and Technology (IRJET) e-ISSN: 2395-0056
Volume: 10 Issue: 12 | Dec 2023 www.irjet.net p-ISSN: 2395-0072

Dubai Electricity & Water Authority (DEWA) Power System
includes Generation, Transmission and Distribution
networks. The transmission network operates at 400kV and
132kV voltage levels with over 350 transmission
substations consisting of 400/132kV, 132/33kV and
132/11kV sites. LVAC switchgears are installed in all types
of transmission substations and are used to supply station
loads (station auxiliaries, air conditioning, heating, and
lighting etc.). LVAC supplies to various equipment’s and
related systems are continuously monitored by connecting
the alarms through SCMS to DEWA control center. As per
existing design, during supply changeover in LVAC, many
unwanted alarms are received. Moreover, if the LVAC
operational switch is kept in manual mode, manual
intervention is required to execute supply changeovers.
In order to overcome the above-mentioned constraints,
DEWA team has developed a high speed LVAC auto
changeover scheme which allows momentary paralleling of
transformers without any side effect thus to avoid supply
interruption. The innovative LVAC scheme under
discussions is approved for implementation in DEWA’s all
future 132/11 KV substations.
The major concerns while finalizing the scheme were as
follows:
1) 1) During any one of the IDT Switching, paralleling of
secondary side may involve circulating currents. In
order to overcome the same it is ensured that the
primary of the concerned IDT’s are already parallel and
maximum paralleling time is set as 500 msec and to
eliminate any adverse effect such as IDT circulating
currents. Moreover the LVAC busbar fault levels are
within the limits for paralleling the two LVAC incomers
(LVAC fault levels are shown in figure 1).
Figure-1: LVAC Fault Levels
2) The complications of switching LVAC breakers from
control center would have been challenging with
existing PLC, however use of Bay Control Units having
IEC 61850 communication facility found as a suitable
solution to achieve remote switching.
3) High speed changeover in case of tripping of any one
IDT was considered as a risk to other healthy IDT’s for
momentary paralleling. Hence a hybrid solution was
adapted. High speed paralleling was configured for
planned switching and low speed LVAC changeover
with 10 sec time delay was maintained for IDT
protection tripping scenarios without paralleling the
IDT’s.
II.PROBLEMS WITH EXISTING LVAC SCHEME
Existing LVAC Single Line Diagram
The salient features specified for the existing LVAC scheme
related to supply changeover scheme are as below:
1) The substation shall be provided with three auxiliary
supplies derived from the three earthing transformers
2) Normally three sections (Bus 1, 2, 3) of the main LVAC
switchboard shall be supplied via the associated main
incoming circuit breakers with both the bus section
breakers in open condition.
3) Electrical interlock shall be provided such that it shall not
be possible to parallel any two transformers both on
Auto and Manual mode of operations.
4) Only one Auto/Manual switch common to all 5 ACBs
shall be provided, and operation logic shall be submitted
for DEWA approval.
5) If a fault occurs on a section of the busbars the relevant
circuit breaker shall open to isolate the faulty section and
any auto-changeover on to a faulty section shall be
inhibited.
6) When selected on manual mode no automatic
changeover shall occur.

7) SCADA alarms are hardwired since the existing PLC
devices are not capable to send alarms directly to SCMS
system.
It is evident from the above-mentioned specification clauses
that paralleling of transformers was not permitted and
manual operations are required for supply changeover
during planned outages on any one of the IDT’s.
Moreover, momentary supply interruption will results in
several unwanted alarms from associated LVAC circuits to
SCMS.
III. High Speed Auto Changeover Scheme
The high-speed auto changeover (ACO) scheme is
implemented by redesigning the LVAC control system as
follows:
1. Auto Changeover (ACO) can be enabled or disabled
using the existing auto/ manual switch.
2. The ACO is completely autonomous requiring no action
from TCC or HMI.
3. Detection of IDT1 switching out: (the same is applicable
for IDT2 & IDT3)
a) Detect IDT1 132kV CB Open command (switching
out IDT1).
b) Initiate auto changeover of LVAC Bus 1.
c) Close BS1 and Open Incomer 1 (Aux contact of BS1
Opens IC1 to achieve coordinated high speed
switching).
4. Detection of IDT1 switching back into service:
a) 132kV CB Closed and LVAC supply healthy for 5
seconds.
b) Initiates restore of LVAC Bus 1 to normal supply.
c) Close Incomer 1 and then open BS1 (Aux contact of
IC1 opens BS1 to achieve coordinated high-speed
switching).
5. Duration of paralleling is kept to minimum
(milliseconds) therefore no overload concerns.
IDT1 Switching – ACO Scenario is shown in Figure-2.
Figure-2: IDT1 Switching
A. ACO Logic
Sample logic – LVAC Incomer CB close
Sample logic – LVAC Incomer CB Open
B. Methodology: The steps undertaken for
implementing the LVAC changeover scheme are as
explained below:

1) Existing LVAC logic and schematic diagrams are
thoroughly studied by the team and based on it,
new logics are prepared to incorporate hybrid
changeover scheme. One high speed logic for
routing switching of IDT’s and other slow logic
for IDT tripping cases.
2) PLC was proposed to be replaced with modern
and sophisticated BCU’s.
3) The proposal was presented to DEWA
Transmission Power Technical Committee and
approval was received.
4) BCU input /output requirements were finalized
and accordingly, Generic wiring diagrams were
prepared.
5) DEWA approved OEM’s (SEL, Hitachi, GE and
Siemens) were approached to finalize suitable
BCU device.
6) M/s Hitachi REC 670 and SEL451 BCU’s found
most suitable for this application.
7) Extensive laboratory testing was performed on
these BCU’s considering all network conditions
in presence of DEWA stakeholder departments
and BCU settings and configurations were
finalized.
8) Based on successful evaluation of the BCU’s,
tender documents were updated to include High
Speed Automatic changeover.
C. Advantages of the High-Speed Auto Changeover
Scheme
The new innovative LVAC design provides significant benefits
to DEWA as a utility. The new design for 132/11kV
substations includes the following advantages:
 Eliminate momentary interruption of LVAC supply
during IDT outage or switching
 The Scheme is completely autonomous.
 Fast switching avoids exposure to circulating
currents.
 It is a cost-effective design improvement due to less
number of components and wiring.
 Eliminate unwanted alarms during IDT outage.
 Increased digitization due to elimination of hard
wiring (BCU option). BCU is directly connected to
SCADA system and all LVAC related alarms are send
to SCADA via IEC61850 protocol.
 BCU’s are having major advantages over PLC in terms
of ease and flexibility in logic buildings, programming
of the device and speed of operation.
IV. CONCLUSION
Implementing a high-speed auto changeover (ACO) will
avoid interruption of LVAC supply in the event of switching
IDT’s in and out of service. This will in turn avoid
unnecessary supply interruption, associated alarms and
improve reliability of equipment.
The high speed ACO avoids the need for operator
intervention and avoids exposure to overload since
paralleling is extremely short in duration.
The proposed ACO scheme was configured and tested with
different OEM BCU. All the test scenario was simulated and
passed. Hence the same concept can be implemented
wherever applicable.
BCU is the preferred option rather than the PLC.
V. REFERENCES
[1] Aster A. Roy, Gesa F. Newton & Aondoakaa I. Solomon:
“Design and Implementation of a 3-Phase Automatic Power
Change-Over Switch” published in American Journal of
Engineering Research (AJER) Vol-3, Issue-9, pp-07-14
[2] Scott Manson and Saurabh Shah “Automated Power
Management Systems for Power Consumers with On-site
Generation” Available on SEL website, SEL Inc. Pullman
[3] Scott M. Manson, Ashish Upreti and Michael Thompson
Available on SEL website, SEL Inc. Pullman
[4] Stefan Meier, Thomas Werner, “Performance
considerations in digital substation applications” white
paper presented at PAC World UK 2015.
[5] Greg Rzepka, Scott Wenke, Sarah Walling, “Choose
Simplicity for a Better Digital Substation Design”, 70th
Annual Conference for Protective Relay Engineers, Texas,
April-2017.

Introduction of LVAC Auto Change Over Scheme in DEWA Transmission System

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Introduction of LVAC Auto Change Over Scheme in DEWA Transmission System