For its operation the function either measures the true RMS
current of the excitation transformer or calculates the DC
current in the rotor winding. The rotor winding DC current can
be calculated from the AC currents measured on either high
voltage side (HV) or low voltage side (LV) side of the
excitation transformer. For the HV side measurement ratings
of the excitation transformer shall be given. The use of the DC
current is default (i.e. recommended) measurement for
generators with static excitation system. When the DC current
is used, the function can provide a DC current ripple alarm,
due to possible problem with the static excitation equipment.
The rotor DC current can be also sent to the plant supervisory
system via communication channel or displayed on the IED
built-in HMI.
The function can also detect undercurrent condition in the
rotor winding which indicates either under-excitation or loss
of excitation condition of the generator.
The function is designed to work on 50/60 Hz systems.
6. Voltage protection
Two step undervoltage protection UV2PTUV
M13789-3 v11
Undervoltages can occur in the power system during faults or
abnormal conditions. Two step undervoltage protection
(UV2PTUV) function can be used to open circuit breakers to
prepare for system restoration at power outages or as long-
time delayed back-up to primary protection.
UV2PTUV has two voltage steps, each with inverse or definite
time delay.
UV2PTUV has a high reset ratio to allow settings close to
system service voltage.
Two step overvoltage protection OV2PTOV
M13798-3 v14
Overvoltages may occur in the power system during abnormal
conditions such as sudden power loss, tap changer
regulating failures, and open line ends on long lines.
OV2PTOV has two voltage steps, each of them with inverse
or definite time delayed.
OV2PTOV has a high reset ratio to allow settings close to
system service voltage.
Two step residual overvoltage protection ROV2PTOV
M13808-3 v10
Residual voltages may occur in the power system during
earth faults.
Two step residual overvoltage protection ROV2PTOV function
calculates the residual voltage from the three-phase voltage
input transformers or measures it from a single voltage input
transformer fed from an open delta or neutral point voltage
transformer.
ROV2PTOV has two voltage steps, each with inverse or
definite time delay.
Reset delay ensures operation for intermittent earth faults.
Overexcitation protection OEXPVPH
M13319-3 v9
When the laminated core of a power transformer or generator
is subjected to a magnetic flux density beyond its design
limits, stray flux will flow into non-laminated components that
are not designed to carry flux. This will cause eddy currents
to flow. These eddy currents can cause excessive heating
and severe damage to insulation and adjacent parts in a
relatively short time. The function has settable inverse
operating curves and independent alarm stages.
Voltage differential protection VDCPTOV
SEMOD153862-5 v7
A voltage differential monitoring function is available. It
compares the voltages from two three phase sets of voltage
transformers and has one sensitive alarm step and one trip
step.
95% and 100% Stator earth fault protection based on 3rd
harmonic STEFPHIZ
SEMOD143258-4 v9
Stator earth fault is a fault type having relatively high fault
rate. The generator systems normally have high impedance
earthing, that is, earthing via a neutral point resistor. This
resistor is normally dimensioned to give an earth fault current
in the range 3 – 15 A at a solid earth-fault directly at the
generator high voltage terminal. The relatively small earth fault
currents give much less thermal and mechanical stress on the
generator, compared to the short circuit case, which is
between conductors of two phases. Anyhow, the earth faults
in the generator have to be detected and the generator has to
be tripped, even if longer fault time compared to internal short
circuits, can be allowed.
In normal non-faulted operation of the generating unit the
neutral point voltage is close to zero, and there is no zero
sequence current flow in the generator. When a phase-to-
earth fault occurs the neutral point voltage will increase and
there will be a current flow through the neutral point resistor.
To detect an earth fault on the windings of a generating unit
one may use a neutral point overvoltage protection, a neutral
point overcurrent protection, a zero sequence overvoltage
protection or a residual differential protection. These
protections are simple and have served well during many
years. However, at best these simple schemes protect only
95% of the stator winding. They leave 5% close to the neutral
end unprotected. Under unfavorable conditions the blind zone
may extend up to 20% from the neutral.
The 95% stator earth fault protection measures the
fundamental frequency voltage component in the generator
star point and it operates when the fundamental frequency
voltage exceeds the preset value. By applying this principle
approximately 95% of the stator winding can be protected. In
order to protect the last 5% of the stator winding close to the
neutral end the 3rd harmonic voltage measurement can be
performed. In 100% Stator E/F 3rd harmonic protection either
the 3rd harmonic voltage differential principle, the neutral
1MRK 502 068-BEN D
Generator protection REG670 2.1 IEC
Product version: 2.1
24
ABB
