NPU, SW Version 1.8,
©
ComAp, September 2003
NPU-1.8r2.PDF 12
Vector Shift Protection
The vector shift protection protects synchronous generator in parallel operation in case of mains failure by
very fast opening the coupling breaker. Automatic mains reclosing for synchronous generator is very
dangerous. The mains voltage returning after 300 ms can meet the generator in asynchronous mode.
There are two main applications for vector protection:
1. parallel mode only (no island mode)
The vector protection controls the generator circuit breaker.
2. parallel and island mode
The vector protection controls the mains circuit breaker.
The NPU detects mains failure within 30 ms. The total time of disconnection is shorter than 100 ms
including disconnecting time of the circuit breaker.
The requirement for generator disconnection comes on power change of more than 15% - 20%. Slow
changes of frequency (e.g. governor commands) don’t activate the protection.
Measuring principle
When a synchronous alternator is loaded, the rotor displacement angle
ϑ
is build between the terminal
voltage (mains voltage) g and the synchronous electromotive force e. Therefore a voltage difference
∆
U
is built between e and g. The rotor displacement angle
ϑ
between stator and rotor is depending on
mechanical moving torque of the generator shaft. The mechanical shaft power is balanced with the
electrical feeder mains power and therefore the synchronous speed keeps constant.
In
a
oad. The rotor
isplacement angle is decreased repeatedly and the voltage vector
Mains
U
e
U
g
Z
l
1
l
2
∆
U
Generator
Mains/Load
ϑ
U
e
U
g
In parallel with the mains
c se of mains failure or auto reclosing the generator suddenly feeds a very high consumer l
g changes its direction to
d
g’.
Mains
U
e
U ´
g
Z
l ´
1
∆
U´
Generator
Load
∆ϑ
ϑ
U
e
U
g
U ´
g
At mains failure
