To avoid these technical challenges, protection is needed to disconnect the
distributed generation once it is electrically isolated from the main grid supply.
Various techniques are used for detecting Loss of Mains. However, the present
function focuses on voltage vector shift.
The vector shift detection guarantees fast and reliable detection of mains failure in
almost all operational conditions when a distributed generation unit is running in
parallel with the mains supply, but in certain cases this may fail.
If the active and reactive power generated by the distributed generation units is
nearly balanced (for example, if the power mismatch or unbalance is less than
5...10%) with the active and reactive power consumed by loads, a large enough
voltage phase shift may not occur which can be detected by the vector shift
algorithm. This means that the vector shift algorithm has a small non-detection-
zone (NDZ) which is also dependent on the type of generators, loads, network
and start or operate value of the vector shift algorithm. Other network events like
capacitor switching, switching of very large loads in weak network or connection
of parallel transformer at HV/MV substation, in which the voltage magnitude is not
changed considerably (unlike in faults) can potentially cause maloperation of vector
shift algorithm, if very sensitive settings are used.
The vector shift detection also protects synchronous generators from damaging
due to islanding or loss-of-mains. To detect loss-of-mains with vector shift
function, the generator should aim to export or import at least 5...10% of the
generated power to the grid, in order to guarantee detectable change in loading
after islanding or loss-of-mains.
Multicriteria Loss of Mains
Apart from vector shift, there are other passive techniques which are used for
detecting Loss of Mains. Some of these passive techniques are over/under voltage,
over/under frequency, rate of change of frequency, voltage unbalance, rate of
change of power and so on. These passive methods use voltage and frequency to
identify Loss of Mains. The performance of these methods depends on the power
mismatch between local generation and load. The advantage of all these methods
is that, they are simple and cost effective, but each method has a non detectable
zone. To overcome this problem, it is recommended to combine different criteria
for detecting Loss of Mains.
Two or more protection functions run in parallel to detect Loss of Mains. When all
criteria are fulfilled to indicate Loss of Mains, an alarm or a trip can be generated.
Vector shift and rate of change of frequency are two parallel criteria typically used
for detection of Loss of Mains.
Chosen protection criteria can be included in the Application Configuration tool to
create multicriteria loss of mains alarm or trip.
Protection functions
1MRS757644 H
698
620 series
Technical Manual
