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DMRC/Elect/TR/T-28/B1 Dated:21.08.2020
Technical Instructionno.26
Sub: - Protection scheme for 33 kV Auxiliary Power Supply network for
Phase-IV and future projects of DMRC.
A. Purpose: -
The purpose of this Technical Instruction is to introduce a technically sound,
dependable, selective, secure and reliable protection scheme for 33kV Auxiliary
Power Supply Network of DMRC to be commissioned under Phase-IV and future
projects.
B. Reference Document/History:-
DMRC had last issued protection philosophy for 33kV Auxiliary Power Supply during
the year 2014. As per the existing philosophy, differential protection was the primary
protection to isolate defective 33kV cable section and Directional Over Current
protection, using PT at each Outgoing/Incoming Panel, as backup protection.
There have been few incidences of failure of 33 kV PT in the HT Panels
commissioned in DMRC in the past.A committee, consisting of Project and
O&M/Traction officials, was nominated to look into these failures and review the
existing protection scheme. The committee has submitted their recommendations for
change of 33kV Auxiliary Power Supply Protection scheme.
C. Revised Protection Scheme is as follows:-
1. 33kV Cable Ring Feeder Protection:Pilot wire protection as primary protection
and non-directional over current protection as back up protection shall be
provided on the ring feeder breakers of both UG and Elevated sections for
protection of 33kV Cable faults. Salient features of this protection are asbelow:
i. The Primary Protection for the cable faults between two stations shall be
pilot wire differential protection (87L). This will trip the respective MVCB on
either end of the faulty section of the cable.
Advantage of keeping the pilot wire protection is that it does not require any
current or time grading as it works on the differential of currents in CTs
provided at either end of the cable to feed respective pilot wire relays. Set
of Differential Relays communicate with each other through the optic fiber
laid between two stations for the purpose.
ii. Non-Directional overcurrent/earth fault protection (50,50G,51,51G) is
provided as a backup to pilot wire relay in the ring feeder breakers which
has two definite time over-current stages I> and I>>. By keeping the current
setting same, the tripping times can still be differed. Trip time of I> for
example is set to 150 ms, while Trip Time of T I >> is set to only 30ms.
With these functions enabled, a software configured logic can be developed
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to achieve the same advantages of Directional over current protection,
used in Phase-III.
2. Transformer feeder breaker Protection: The Transformer feeder breakers
Protection shall be the same, as was provided under Phase-3 and will have the
following protections:
i. Transformer differential protection (87T) (For UG only)
ii. Restricted earth Fault protection (64REF) (For UG only)
iii. Over Current Protection and earth Fault Protection
(50/51/50N/51N)
iv. Mechanical Door Limit Switch (Castle Key Arrangement) (33)
v. Winding Temperature indicator (49T).
vi. Breaker Failure (50BF).
vii. Standby Earth Fault Protection (For UG only)
3. Different fault scenarios and the behavior of the pilot wire protection and backup
non-directional overcurrent protection for ring feeder circuits are discussed below
for which logic can be developed and implemented:
I. Scenario having Fault in Cable Feeder:
a) Pilot Wire Protection is Healthy all along the ring in the direction of
power flow: -
PW4 issues trip command to MVCB12/22 and simultaneous all healthy PW
elements of upstream breakers inhibits I> Overcurrent element which
eliminates unwanted tripping in upstream breakers in the direction of power
flow.
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b) Pilot Wire Protection is Healthy in the faulty feeder butfaulty in any of
upstream feeders in the ring at that instance, in the direction of power
flow: -
As depicted in the below figure, healthy PW4 protection shall act
instantaneously and issues tripping command to the respective breaker,
however tripping in any of the upstream breakers where PW protection is
faulty shall be taken care by I> element with the suitable time gradation, for
example 150ms.
c) Unhealthy Pilot Wire Protection in the faulty feeder:-
In case of failure in Pilot wire differential protection relay of the faulty feeder
i.e. PW4, I> 4 element of the over current protection gets activated and also
simultaneously gets the input of pilot wire protection PW4 failure, thereby I> 4
element will issue trip command to the ring feeder breaker to trip in set time
delay (150ms) as shown in yellow in the figure below.
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II. With the scenario having Fault in BusBar:
As I> over current element is getting blocked by the healthiness status of pilot
wire relay protection element, it does not protect against bus-bar faults, which
will be taken care of I>> over current element of the same relay.
For all faults in the feeders either it be a transformer feeder, cable feeder and
bus bar, this element gets activated but differentiation can be achieved
between bus-bar faults and other faults by a reverse interlocking logic i.e., in
the event of a fault on any feeder, the protection device in that particular
feeder and of the ring main feeder in the direction of power supply will pick up.
By means of the over-current pick-up signal of the device in the
transformer/cable feeder, the I>> stage in the protection device of ring main
feeder in the direction of power supply, that has a time delay of 30 ms is
blocked via external wiring and a binary input along with a developedlogic.
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Therefore, during Bus-bar Faults only I>> overcurrent elements gets activated,
as healthiness of pilot wire relay already inhibited I> overcurrent element,
otherwise sufficient time grading already available between Trip Time of I>
(150ms) and Trip Time of I>> (30ms). At the same time, pickup of I>> element
of outgoing cable feeder or Transformer protection relay inhibits I>>
overcurrent element of incoming feeder panel, hence only incoming breaker of
the panel with the bus bar fault will get tripped.
D. New protection scheme being issued under this TI shall work without any
requirement of PTs in 33 kV ring feeder panels. Since reliability of PTs is relatively
poor and new protection scheme is equally efficient, secure and safe, reliability of
overall protection scheme shall improve and cost of implementation shall reduce.
Under new protection scheme, two different Protection Relays shall be installed, one
for Non-Directional Over Current protection and the other for Pilot wire Differential
Protection in each ring feeder panel.
E. Special Tools/Material Required: -NA.
F. Safety Precautions: -NA.
G. Implementation Strategy: -
It shall be implemented in all future projects.
H. Installation Drawings: -NA.
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I. Enclosures: -NA.
J. Applicability: - 33 kV Auxiliary Network.
K. Action by: -
DMRC Project team.
L. Distribution: -To Project and O&M team
ED/UG-TR.
GM/E/RC-1 GM/E/Technical GM/E/Plng.
GM/TR/O&M GM/E/Mumbai
GM/E/Jaipur
GM/E/Patna GM/E/UG-1 GM/E/UG-2
Copy to:
DE
For kind information please
Adv/Elect.
