Micom P721, Technical Manual

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Application Guide P72x/EN AP/C21
MiCOM P721/P723 Page 9/52
2.6 High Impedance Differential Protection
The application of the MiCOM P72x to the protection of machines, power transformers and
busbar installations is based on the high impedance differential principle. The P72x offers
stability for any type of fault occurring outside the protected zone and satisfactory operation
for faults within the zone.
Although the P72x only has current inputs to be used in a high impedance scheme, the Input
Selection
setting under CONFIGURATION/CONNECTION may be Voltage or Current.
When set to Current, the Threshold I Diff parameter should be set. When the Input
Selection is Voltage, the Threshold V Diff parameter should be set in addition to the
Stabilizing Resistor . This is because the relay calculates the Threshold I Diff parameter
as Threshold V Diff/Stabilizing Resistor.
In a high impedance differential scheme, the currents entering and leaving the protected
zone are compared continuously. The CTs are also loaded with a high impedance to force
the error differential current (due to CT saturation and CT errors) through the CTs instead of
the current input in the P72x. Stability should be maintained during external faults with only
one CT heavily saturated (this is the worst condition). In a high impedance relay the voltage
setting is greater than the calculated maximum voltage that can appear across the high
impedance path under the assigned maximum through fault current condition multiplied by a
K factor. This K factor is indicated in the CT requirement tests results. The objective is to
provide fast operation at a low fault setting on internal faults yet retain stability up to the
highest possible value of short circuit current on through faults.
FIGURE 8 shows the high impedance differential principle. Current transformers on each
side of the protected zone are connected in parallel which will produce a resultant current to
operate a relay for internal faults only. Theoretically such a system is unaffected by through
faults, but in practice the associated current transformer may not behave ideally when the
current exceeds a certain value. Errors in transformation due to saturation of the current
transformer cores may be sufficient to cause maloperation if special precautions are not
taken. Therefore, to ensure stability for external faults the current through the relay is limited
by the insertion of an external resistor in series with the relay. This resistor is known as a
stabilizing resistor. The main application consideration with a high impedance scheme is
stability for heavy through faults. The stability is determined by the CTs and relay settings
which in turn affect the sensitivity of the scheme.
The stability limit of a busbar protection scheme is based on the maximum through fault
current. In general this takes the value of the associated switchgear rating irrespective of the
existing or anticipated fault levels. The differential relay and stabilizing resistor are
connected across the paralleled CTs.
Consider the simple two CT differential circuit shown in FIGURE 8. During an external fault
the through fault current should circulate between the current transformer secondaries. The
only current that can flow through the relay circuit is that due to any difference in the current
transformer outputs for the same primary current. On the other hand, if the right hand side
CT fully saturates, Z
M2
appears as a short circuit. It is considered that CT1 is not saturated
and it is delivering its full current output. As a result, the current from CT1 is divided
between the high impedance path (relay plus stabilizing resistor) and the saturated CT2.
The current flowing through the high impedance path is as follows:
) R 2 R R R (
) R 2 R (
I I
L CT ST r
L CT
S
+ + +
+
=
Where:
I
S
= current flowing in the relay
I = current output from CT1
R
CT
= Resistance of the current transformer secondary winding.
R
L
= Resistance of a single wire from the relay to the CT.
R
r
= Relay resistance, which is neglected in the P72x
R
ST
= Stabilizing resistor