ComAp MainsPro - Vector Shift Protection (ANSI 78)

MainsPro, SW version 1.4, ©ComAp – January 2013
MainsPro Application Guide
2-7
Picture 2: decomposition of a generic 3-phase voltage to symmetrical components
MainsPro provides positive and negative sequence voltage evaluation and compares the measured
values with V> neg and V< pos thresholds. In the perfectly symmetrical arrangement, negative
sequence voltage is zero and positive sequence voltage equals to the measured voltage. If the
asymmetry situation occurs, non-zero negative sequence voltage is calculated and positive sequence
voltage drops. When any of the preset limits is over/underreached, the appropriate LED signal is
issued by LED U and the output dU Sig moves to fault-indicating position immediately. If the
calculated values keep out of limits for the delay dU del, TRIP is issued. As the calculated values of
voltage asymmetry return back within limits, the LED and dU Sig output stop to signal the fault state
immediately, regardless of whether TRIP was issued or not or Fault reset was performed or not.
Some utilities strictly require in their regulations that symmetrical components are evaluated in the
mains-decupling relay and appropriate trip is provided. However, the method may also be used in the
areas, where no such requirement is in place, to minimize non-detection zones of detection of 1-phase
mains failures. In case that the generator, connected to the mains is operated close to equity-state, 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 vary from 0,65 to 0,85 of the rate voltage value. The exact values are
delivered by the mains operator 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 the phase L1 occurs a rapid voltage drop from 230V to e.g. 50V, tripping times might be longer and
can reach 100ms.
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 severe risk of damage to the
generator, its feeder equipment as well as equipment of the mains operator. Vector shift provides fast
protective function for this situation.
Measuring principle
When synchronous alternator is loaded, the rotor displacement angle 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 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.
U
A
U
B
U
C
=
U
A1
U
B1
U
C1
U
B2
U
C2
U
A2
U
B0
U
A0
U
C0
+
+
positive sequence
voltage
negative sequence
voltage
zero sequence
voltage