Manual MRG3
SEG Electronics GmbH
DOK-TD-MRG3, Rev. D
25
4.5.2 Vector surge supervision
The vector surge supervision protects synchronous generators in mains parallel operation due to
very fast decoupling in case of mains failure. Very dangerous are mains auto reclosing for
synchronous generators. The mains voltage returning after 300 ms can hit the generator in asyn-
chronous position. A very fast de-coupling 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 single operation
:
For this application the vector surge supervision trips the mains circuit breaker.
Here it is ensured that the gen.-set is not blocked when it is required as the
emergency set.
A very fast detection of mains failures for synchronous generators running in parallel to the mains
is known as very difficult. Voltage supervision units cannot be used because the synchronous
alternator as well as the consumer impedance support the decreasing voltage.
For this the mains voltage only drops after some 100 ms below the pickup threshold of voltage su-
pervision relays and therefore a safe detection of mains auto reclosing is not possible with this kind
of relay.
To some extent, frequency relays are partial unsuitable because only a highly loaded generator
decreases its speed within 100 ms. Current relays detect a fault only when short-circuit type cur-
rents exist, but cannot avoid their development. Power relays are able to pick up 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 power relays can be problematic.
Whereas the MRG3 detects mains failures within 60 ms without the restrictions described above
because they are specially designed for applications where very fast decoupling from the mains is
required.
When adding the operating time of a circuit breaker or the break time of a contractor, the total dis-
connection time remains below 150 ms. Basic requirement for tripping of the generator/mains
monitor is a change in load by at least 15 - 20% of the rated load. Slow changes of the system fre-
quencies, for instance caused by 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 high-
er than the preset value can occur. Short-circuits within the grid may also lead to trippings, though
here, too the vector surge could be higher than the preset value. The magnitude of the voltage vec-
tor 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 conse-
quently the energy feeding the short circuit is not unnecessarily increased.
To prevent a possible false tripping the vector surge measuring can be blocked at a very low input
voltage (refer to 5.9.1). The undervoltage lockout acts faster than the vector surge measurement.
Vector surge tripping is blocked by a phase loss so that a VT fault (e.g. faulty VTs fuse) does not
cause false tripping. When switching on the aux. voltage or measuring voltage , the vector surge
supervision is blocked for 5 s (refer to chapter 4.8).
