General Calculation; Secondary System Supervision - ABB REG670 Product Manual

Generator protection REG670 2.2 and Injection equipment REX060,
REX061, REX062
Product version: 2.2.1
Rotor earth fault protection using CVGAPC
The field winding, including the rotor winding and the non-
rotating excitation equipment, is always insulated from the
metallic parts of the rotor. The insulation resistance is high if
the rotor is cooled by air or by hydrogen. The insulation
resistance is much lower if the rotor winding is cooled by
water. This is true even if the insulation is intact. A fault in the
insulation of the field circuit will result in a conducting path
from the field winding to earth. This means that the fault has
caused a field earth fault.
The field circuit of a synchronous generator is normally
unearthed. Therefore, a single earth fault on the field winding
will cause only a very small fault current. Thus the earth fault
does not produce any damage in the generator. Furthermore,
it will not affect the operation of a generating unit in any way.
However, the existence of a single earth fault increases the
electric stress at other points in the field circuit. This means
that the risk for a second earth fault at another point on the
field winding has increased considerably. A second earth fault
will cause a field short-circuit with severe consequences.
The rotor earth fault protection is based on injection of an AC
voltage to the isolated field circuit. In non-faulted conditions
there will be no current flow associated to this injected
voltage. If a rotor earth fault occurs, this condition will be
detected by the rotor earth fault protection. Depending on the
generator owner philosophy this operational state will be
alarmed and/or the generator will be tripped. An injection unit
RXTTE4 and an optional protective resistor on plate are
required for correct rotor earth fault protection operation.

10. General calculation

Multipurpose filter SMAIHPAC
The multi-purpose filter function block (SMAIHPAC) is
arranged as a three-phase filter. It has very much the same
user interface (e.g. inputs and outputs) as the standard pre-
processing function block SMAI. However the main difference
is that it can be used to extract any frequency component
from the input signal. Thus it can, for example, be used to
build sub-synchronous resonance protection for synchronous
generator.

11. Secondary system supervision

Current circuit supervision CCSSPVC
Open or short circuited current transformer cores can cause
unwanted operation of many protection functions such as
differential, earth-fault current and negative-sequence current
functions.
Current circuit supervision (CCSSPVC) compares the residual
current from a three phase set of current transformer cores
with the neutral point current on a separate input taken from
another set of cores on the current transformer.
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A detection of a difference indicates a fault in the circuit and
is used as alarm or to block protection functions expected to
give inadvertent tripping.
Fuse failure supervision FUFSPVC
The aim of the fuse failure supervision function (FUFSPVC) is
to block voltage measuring functions at failures in the
secondary circuits between the voltage transformer and the
IED in order to avoid inadvertent operations that otherwise
might occur.
The fuse failure supervision function basically has three
different detection methods, negative sequence and zero
sequence based detection and an additional delta voltage
and delta current detection.
The negative sequence detection algorithm is recommended
for IEDs used in isolated or high-impedance earthed
networks. It is based on the negative-sequence quantities.
The zero sequence detection is recommended for IEDs used
in directly or low impedance earthed networks. It is based on
the zero sequence measuring quantities.
The selection of different operation modes is possible by a
setting parameter in order to take into account the particular
earthing of the network.
A criterion based on delta current and delta voltage
measurements can be added to the fuse failure supervision
function in order to detect a three phase fuse failure, which in
practice is more associated with voltage transformer
switching during station operations.
Fuse failure supervision VDSPVC
Different protection functions within the protection IED
operates on the basis of measured voltage at the relay point.
Some example of protection functions are:
• Distance protection function.
• Undervoltage function.
• Energisation function and voltage check for the weak
infeed logic.
These functions can operate unintentionally, if a fault occurs
in the secondary circuits between voltage instrument
transformers and the IED. These unintentional operations can
be prevented by fuse failure supervision (VDSPVC).
VDSPVC is designed to detect fuse failures or faults in voltage
measurement circuit, based on phase wise comparison of
voltages of main and pilot fused circuits. VDSPVC blocking
output can be configured to block functions that need to be
blocked in case of faults in the voltage circuit.
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