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i.e. power delivered to the mains is close to zero, it may be difficult to sense loss of one phase further in the
system. The only change seen in such situation may be movement of the failed phase by a certain angle with
small or no voltage drop in the absolute values. This may not be detected by undervoltage or unbalance
protection stage. Symmetrical components provide very good and sensitive method to detect such a situation
and trip the generator in case of this situation.
Typical setting of the V< pos varies from 0,65 to 0,85 of the rated voltage value. The exact values are
provided by the DNO or may be set-up during commissioning after experimental verification of the protection
stage sensitivity to the single-phase failures in equity state of the generator (e.g. by opening one fuse on the
mains transformer).
Note: If a rapid voltage drop from 230V to e.g. 50V occurs in the phase L1, tripping times might be longer and
can reach 100ms.
8.5 ANSI 78 Vector shift
The vector shift is one of the fast "Loss of Mains" protection stages. The principle is based on the fact that if a
generator works into an islanded area of the electricity network, its voltage and frequency depend strongly on
the load size, remaining in the islanded area. Decrease of the generator speed due to overload may not be
fast enough to assure e.g. trip by underfrequency stage. The mains may be equipped with auto-reclosing
mechanisms and in case that the generator is not disconnected within the auto-reclosing delay, the area may
be reconnected back to the grid by this mechanism. This reconnection may meet the generator in
asynchronous state, imposing risk of severe damage to the generator, its feeder equipment as well as
equipment in the actual grid. Vector shift provides fast protective function for this situation.
8.5.1 Measuring principle
When synchronous alternator is loaded, the rotor displacement angle
(mains voltage)
g and the synchronous electromotive force
between
e and
g. The rotor displacement angle
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 case of mains failure or auto reclosing the generator suddenly feeds a very high consumer load. The rotor
displacement angle is decreased repeatedly and the voltage vector
MainsPro G99TT 1.0.0 Global Guide
between stator and rotor is depending on mechanical
Image 8.6 In parallel with the mains
is built between the terminal voltage
e. Therefore a voltage difference ΔU is built
g changes its direction to
g'.
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