Author: BSEE Hugo E Reyes
INTRODUCTION
The goal of this publication is to apply everything we learned in paper called "FUNDAMENTALS OF NON-DIRECTIONAL OVERCURRENT PROTECTION RELAYS":
(https://www.electricaltopics.info/2021/01/fundamentals-of-non-directional.html).
OVERCURRENT RELAYS COORDINATION
The “IEEE STD 242
RECOMMENDED PRACTICE FOR PROTECTION AND COORDINATION OF INDUSTRIAL AND
COMMERCIAL POWER SYSTEMS” section 15.6 outline seven steps that should be
followed to make an overcurrent relays coordination.
In summary, these steps are:
a) Develop a detailed
One-Line-Diagram (OLD).
b) Determine the various normal,
temporary, and emergency operating configurations for the electrical power
system.
c) Determine the load flow in the
system.
d) Determine levels of
short-circuits in each location:
-
Maximum and minimum momentary single and
three-phase short-circuit currents (1/2 cycle network).
-
Maximum and minimum interrupting duty
three-phase short-circuit currents (1.1/2 to 4 cycle network).
-
Maximum and minimum ground fault currents.
e) Determine the characteristics
of protective devices and collects the Time-Current Characteristic (TCC)
curves.
f) Collect the thermal damage
limit curves from every electrical component, i.e. conductors, generators,
transformers, motors, etc.
g) Determine the existing
settings of the upstream and downstream overcurrent devices that have to be
coordinated.
When every step has been done, we
need to start the process at the loads, and select the branch circuit with the
largest current setting, then establishing coordination relays pairs and work
back to the power source.
Consider a radial power system with two buses 1 and 2 where
non-directional overcurrent relays are to be used (Figure 1).
-
The
maximum short-circuit current available at Bus 2 is 5 kA,
- The maximum load current at Bus 2 is 360 A
-
The minimum
short-circuit current available at Bus 1 is 2 kA
RELAY
1 SETTINGS:
-
IPK-51P1P-1 = 4.6 A secondary, this
is 276 A primary.
-
IPK-50P1P-1 = 45 A secondary, this
is 2700 A primary.
-
Time Dial Multiplier equal to 0.5 (TD1 = 0.5).
Using the data given in
Figure 1 we need to determine next Phase
Relay 3 Settings:
-
IPK-50P1P-3 =?
-
IPK-51P1P-3 =?
-
TD3 =?
SOLUTION
The purpose of the relay 1 is to provide primary protection to load 1,
and the purpose of the relay 3 is to provide primary protection to the power
line (266.8kcmil) and backup protection to the feeders that supplies load 1 and
2.
RELAY
3: RELEVANT DATA
-
Maximum Load =
360 A
-
Ampacity of the
conductor 266.8 kcmil ACSR: 460 A @ 75ºC.
-
Maximum
Short-circuit on the remote Bus 1: 3000 A (3kA).
-
The conductor damage current (I2. t) must
be larger than the conductor ampacity at 75ºC.
a)
INSTANTANEOUS
CURRENT PICKUP OF RELAY 3 (50P1P-3)
An instantaneous pickup current setting for backup
protection is 125% the maximum current fault of the next downstream overcurrent
relay 1.
IPK-50P1P-3 = 1.25 * IFMAX1-1 / CTR_3)
IPK-50P1P-3 = 1.25 * 3 000/100)
≈ 38 A
IPK-50P1P-3 = 38 A secondary (this is 3
800 A primary)
b)
CURRENT
PICKUP OF RELAY 3 (51P1P-3)
The pickup current for the phase
overcurrent relay 3 (51P1P-3) must to be set at a
value greater than the maximum load current (IML) and lower than the minimum fault current in its circuit
(IFMIN), so:
IML1
< IPK-51P1P-3 < IFMIN1
So,
360 A
< IPK-51P1P-3 < 3 000
A
The
Pickup Current Element (51-3) can be selected as:
IPK-51P1P-3 = 125% * IML = 1.25 * 360 A
IPK-51P1P-3 = 450 A (primary)
Here is better to select the conductor ampacity
at 75ºC (460 A) as the pickup current setting than 450 A (1.25 times the
maximum load (450 A).
Finally,
IPK-51P1P-3 = 460 A - primary
IPK-51P1P-3 = 4.6 A - secondary (value that the relay sees)
a)
TIME OPERATION OF RELAY 3 (51P1T-3)
Since Relay 1 does not have to maintain selectively
with any other relay, it can be made to operate the fastest.
In order to maintain selectively between
Relay 1 and Relay 3, the following coordination criterion have to be met:
TO-R3
= TO-R1 + CTI
Where:
CTI: is the coordinating time interval
The typical value of the coordinating time
interval (CTI) for Relay-Relay coordination is 0.30 seconds.
Now,
knowing the Time Dial (TD1) of the Relay 1 that is equal to 0.5 seconds we
can calculate the operating time of the Relay
1:
The
Multiple of Pick-up Current of relay 1 (M1)
would be:
M1 = IPK-50P1P-1 / IPK-51P1P-1
M1 = 45 / 4.6 = 9.8
Substituting these
values (TD1 and M1), we get:
Then, the operating time of the relay 3 is:
TO-R3
= 0.12 s + 0.30 s = 0.42 seconds
Also, the Multiple of Pick-up Current of relay 3 (M3) would
be:
M3 = IPK-50P1P-3 / IPK-51P1P-3
M1 = 38 / 4.6 = 8.3
Thus, we can now get the Time Dial of the
relay 3 (TD3):
This finishes the settings of both relays.
In Figure 2 is shown the Time-Curve Characteristics (TCC) of Relays 1 and 3.
Figure 2 TCCs of the Exercise 1.
SUMMARY OF RELAYS SETTINGS
RELAY 1:
-
IPK-51P1P-1 = 4.6 A secondary, this
is 276 A primary.
-
IPK-50P1P-1 = 45 A secondary, this
is 2 700 A primary.
-
TD1 = 0.5
-
TO-R1 = 0.12 s
RELAY 3:
-
IPK-51P1P-3 = 4.6 A secondary, this
is 460 A primary.
-
IPK-50P1P-3 = 38 A secondary, this
is 3 800 A primary.
-
TD1 = 1.55
- TO-R3 = 0.42 s
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