Protective Elements
Delta phi - Vector Surge
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 reclosings for synchronous generators. The
mains voltage returning typically after 300 ms can hit the generator in asynchronous position. A very fast decoupling
is also necessary in case of long time mains failures.
Generally there are two different applications:
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.
Mains parallel operation and 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 required as an emergency set.
A very fast decoupling in case of mains failures for synchronous generators is very difficult. Voltage supervision
units cannot be used because the synchronous alternator as well as the consumer impedance support the
decreasing voltage.
In this situation the mains voltage drops only after some 100 ms below the pickup threshold of the voltage
supervision and therefore a safe detection of mains auto reclosings is not possible with voltage supervision only.
Frequency supervision is partially unsuitable because only a highly loaded generator decreases its speed within
100 ms. Current relays detect a fault only when short-circuit type currents exist, but cannot avoid their development.
Power relays are able to pickup within 200 ms, but they also cannot prevent the power rising to short-circuit values.
Since power changes are also caused by sudden loaded alternators, the use of power relays can be problematic.
Whereas the vector surge supervision of the device detects mains failures within 60 ms without the restrictions
described above because it is specially designed for applications where very fast decoupling from the mains is
required. Adding the typical operating time of a circuit breaker or contactor, the total disconnection time remains
below 150 ms.
Basic requirement for tripping of the generator/mains monitor is a change in load of more than 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 voltage 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.
To prevent a possible false tripping, the vector surge measuring is blocked at a low input voltage e.g. <15% Vn
(settable via parameter »
V Block f«). The undervoltage lockout acts faster then 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.
411
MRU4
DOK-HB-MRU4-2E
