Mains pro 1.6.1 Global Guide
37
Image 8.5 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, A.V> neg [V] (page 57)
and
V< pos, A.V< pos [V] (page 56)
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
moves to fault-indicating position immediately. If
the calculated values keep out of limits for the delay dU del,
is issued. As the calculated values
of voltage asymmetry return back within limits, the LED and
output stop to signal the fault
state immediately, regardless of whether
was issued or not or
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.
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 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.
