Application Notes P54x/EN AP/La4
MiCOM P543, P544, P545 & P546
(AP) 6-11
AP
Relay sensitivity when |bias| > Is2:
For |bias| > s2, the relay would operate if
|diff| > k2 |bias| - (k2 - k1) s2 + s1
or |F| > k2 (|L| + 0.5 |F|) - (k2 - k1) s2 + s1
or (1 - 0.5 k2) |F| > (k2 |L| - (k2 - k1) s2 + s1)
or |F| > (k2 |L| - (k2 - k1) s2 + s1) / (1 - 0.5 k2)
For s1 = 0.2 pu, k1 = 30%, s2 = 2.0 pu and k2 = 100%, then,
1. for |L| = 2.0 pu, the relay would operate if |F| > 1.6 pu
2. for |L| = 2.5 pu, the relay would operate if |F| > 2.6 pu
Fault resistance coverage:
Assuming the fault resistance, RF, is much higher than the line impedance and source
impedance, then for a 33 kV system and 400/1 CT:-
|F| = (Vph-n /RF) * (1/CT ratio) pu
= (33000 /
3 )/RF)/400 pu
= 47.63/RF pu
Based on the above analysis, the relay will detect a fault current in excess of 0.59 pu with a
load current of 1 pu flowing. The fault resistance would have to be less than 47.63/0.59 =
81 in this case.
With a short time overload current of 2.0 pu, the relay will be able to detect a fault resistance
of 47.63/1.6 = 30 or lower.
2.1.3 CT ratio correction (all models)
Ideally, the compen
sated current values should be arranged to be as close as possible to
relay rated current to provide optimum relay sensitivity.
If there is not mismatch between the CTs, the CT correction factor should be set to 1:1.
2.1.4 Transformers in zone applications (P543 & P545 models)
In applyin
g the well established principles of differential protection to transformers, a variety
of considerations have to be taken into account. These include compensation for any phase
shift across the transformer, possible unbalance of signals from current transformers either
side of windings, and the effects of the variety of earthing and winding arrangements. In
addition to these factors, which can be compensated for by correct application of the relay,
the effects of normal system conditions on relay operation must also be considered. The
differential element must restrain for system conditions which could result in maloperation of
the relay, such as high levels of magnetizing current during inrush conditions.
In traditional transformer feeder differential schemes, the requirements for phase and ratio
correction were met by correct selection of line current transformers. In the P543 and P545,
software interposing CTs (ICTs) are provided which can give the required compensation.
The advantage of having replica interposing CTs is that it gives the P54x relays the flexibility
to cater for line CTs connected in either star or delta, as well as being able to compensate
for a variety of system earthing arrangements. The P543 and P545 relays also include a
magnetizing inrush restraint facility.
Note: The P544 and P546 relays do not include any of the above features,
except CT ratio mismatch compensation, and as such would not be
suitable for the protection of in-zone transformer feeders.
On P543 or P545 relays where capacitive charging current compensation is available, there
is a setting to select if capacitive charging current compensation is used or if interposing CTs
are used.
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