Alstom MCAG 34 - Section 2. COMMISSIONING

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Section 2. COMMISSIONING
2.1 Description of relay
This is a current operated relay having seven equally spaced settings. The setting
ranges are given below.
0.025 – 0.1A
0.05 – 0.2A
0.1 – 0.4A
0.2 – 0.8A
0.25 – 1.0A
0.5 – 2.0A
1.0 – 4.0A
The relays may be used for any type of high impedance circulating current
protection.
When used as a high impedance relay a stabilising resistor must be connected in
series with the current circuit.
2.1.1 List of abbreviations
I
E
= Current transformer exciting current at relay setting voltage, (referred to the
CT secondary current).
I
F
= Maximum value of primary through fault current for which protection must
remain stable.
I
FM
= Maximum value of primary fault current for internal fault.
I
P
= Primary current for operation of protection.
I
R
= Relay operating current.
N = Turns ratio of current transformer.
n = No. of current transformers in parallel with relay.
R
CT
= Secondary resistance of current transformer.
R
L
= Lead resistance between furthest current transformer and relay connection
point.
R
R
= Relay impedance.
R
ST
= Value of stabilising resistor.
V
A
= Relay burden at setting.
V
F
= The theoretical voltage which would be produced across the relay circuit
under internal fault conditions.
V
F
=
I
FM
N
(R
CT
+ 2R
L
+ R
ST
+ R
R
)
V
K
= Knee point voltage of current transformer.
V
P
= Peak voltage across relay circuit under maximum internal fault conditions.
V
S
= Setting voltage (calculated).
V
R
= Setting voltage measured across relay and stabilising resistor
(approximately equal to V
S
).