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Calibrating the
Impedance
Protection
Note on Testing
7UM62 Manual
C53000-G1176-C149-3
Set impedance protection (address 3301) to IMPEDANCE PROT. = Block relay:
With the primary plant voltage-free and earthed, install a three-pole short-circuit bridge
which is capable of carrying rated current (e.g. earthing isolator) to the primary side of
the unit transformer.
DANGER!
Primary measurements must only be carried out with the machine at stand–still
on disconnected and earthed equipment of the power system.
Start-up machine and slowly excite to 20 % of rated machine current.
A test with about 20 % of the rated generator current is sufficient for checking the
transformer connections and the operational measured values. If the relative short-
circuit voltage of the transformer is small, the voltage values measured are very low,
so that it may be necessary to increase the generator current somewhat. A test with
the full rated generator current is only required for the quantitative calibration of the
impedance protection (e.g. for calibrating the transformer u
The relay calculates from the currents and voltages the impedance between the point
of installation (voltage transformers) and the short-circuit bridge, i.e. normally the
short-circuit impedance of the unit transformer. Read out the reactance and resistance
values in the measured values. Note that the secondary values depend on the rated
relay current, 1 A or 5 A. In the case of the transformer impedance, the following
results:
Primary transformer impedance:
U
⋅
----------
Z
=
u
T prim
sc
S
with
u
- percent impedance voltage of transformer
sc
U
- rated voltage of transformer
N
S
- rated apparent power of transformer
N
In secondary values:
Z
Z
=
T sec
T prim
with
N
- current transformer ratio
CT
N
- voltage transformer ratio
VT
If substantial deviations or wrong sign occur, then the voltage transformer connections
are incorrect.
After shutdown and de-excitation of the generator, and removal of the short-circuit
bride, the short-circuit tests are completed. No further tests are required for
unbalanced load protection, overcurrent time protection, thermal overload
protection, impedance protection and out-of-step protection.
Switch the overcurrent time protection and the impedance protection operative
(address 1201: O/C I> = ON resp. address 1401 O/C Ip = ON, address 3301:
IMPEDANCE PROT. = ON). They serves from now on as short-circuit protection. If
used, the I>> stage (address 1301 O/C I>> = ON), the thermal overload protection
(address 1601: Ther. OVER LOAD = ON), the unbalanced load protection (address
1701: UNBALANCE LOAD = ON) and the out-of-step protection (address 3501: OUT-
OF-STEP = ON) can be switched to be operative. Otherwise, they are set to OFF.
3.4 Primary Commissioning Tests with the Generator
2
N
N
2
N
U
N
CT
N
CT
⋅
⋅
⋅
---------- -
u
----------
---------- -
=
sc
N
S
N
VT
N
VT
).
K
365
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