12
TD_MRN3-3_08.06_GB
4.5 Measuring of frequency gradient
Electrical generators running in parallel with the mains,
e.g. industrial internal power supply plants, should be
separated from the mains when failure in the intrasys-
tem occurs for the following reasons:
•
It must be prevented that the electrical generators are
damaged in case of asynchronous mains voltage
recovering, e.g. after a short interruption.
•
The industrial internal power supply must be main-
tained.
A reliable criterion of detecting mains failure is the
measurement of the rate of change of frequency df/dt.
Precondition for this is a load flow via the mains cou-
pling point. At mains failure the load flow changing
then spontaneously leads to an increasing or decreas-
ing frequency. At active power deficit of the internal
power station a linear drop of the frequency occurs
and a linear increase occurs at power excess. Typical
frequency gradients during application of "mains de-
coupling" are in the range of 0.5 Hz/s up to over 2
Hz/s. The
MRN3-3
detects the instantaneous fre-
quency gradient df/dt of each mains voltage period
in an interval of one half period each. Through multi-
ple evaluation of the frequency gradient in sequence
the continuity of the directional change (sign of the fre-
quency gradient) is determined. Because of this spe-
cial measuring procedure a high safety in tripping and
thus a high stability against transient processes, e.g.
switching procedure are reached. The total switching
off time at mains failure is between 60 ms and 80 ms
depending on the setting.
4.6 Vector surge supervision
The vector surge supervision protects synchronous gen-
erators in mains parallel operation due to very fast de-
coupling in case of mains failure. Very dangerous are
mains auto reclosings for synchronous generators. The
mains voltage returning after 300 ms can hit the gen-
erator in asynchronous position. A very fast decoupling
is also necessary in case of long time mains failures.
Generally there are two different applications:
a)
Only mains parallel operation no single
operation:
In this application the vector surge supervision
protects the generator by tripping the generator
circuit breaker in case of mains failure.
b) Mains parallel operation and isolated single
operation:
For this application the vector surge supervision
trips the mains circuit breaker. Here it is insured
that the gen.-set is not blocked when it is just
required to operate as the emergency set.
A very fast decoupling in case of mains failures for
synchronous generators is known as very difficult. Volt-
age supervision units cannot be used because the syn-
chronous alternator as well as the consumer imped-
ance support the decreasing voltage.
For this reason the mains voltage drops only after
some 100 ms below the pickup threshold of voltage
supervision relays and therefore a safe detection of
mains auto reclosings is not possible with this kind of
relay.
Frequency relays, as well, are partially unsuitable be-
cause only a highly loaded generator measurably de-
creases its speed within 100 ms. Current relays detect
a fault only when short-circuit type currents exist, but
cannot avoid their development. Rate of change of
power supervision are able to pickup within 200 ms, but
they cannot prevent power to rise to short-circuit values
too. Since power changes are also caused by sudden
loaded alternators, the use of rate of change of power
supervision can be problematic.
The
MRN3-3
detects mains failures within 60 ms
without the restrictions described above because it
was specially designed for applications where very
fast decoupling from the mains is required.
Adding the operating time of a circuit breaker or con-
tactor, the total disconnection time remains below
150 ms. Basic requirement for tripping of the genera-
tor/mains supervision unit is a change in load of at
least 15 - 20% of the rated load. Slow changes of the
system frequency, for instance at regulating processes
(adjustment of speed regulator) do not cause the relay
to trip.
Trippings can also be caused by short-circuits within
the grid, because a voltage vector surge higher than
the preset value can occur. The magnitude of the volt-
age vector surge depends on the distance between
the short-circuit and the generator. This function is also
of advantage to the Power Utility Company because
the mains short-circuit capacity and consequently the
energy feeding the short-circuit is limited.
