ABB RELION REX640, Техническое руководство, Страница: 911 / 1960

larger utility main grid is no longer available after the opening of a circuit-breaker.
Islanding is also referred as Loss of Mains ( LOM) or Loss of Grid ( LOG). When
LOM occurs, neither the voltage or the frequency is controlled by the utility supply.
These distributed generators are not equipped with voltage and frequency control;
therefore, the voltage magnitude of an islanded network may not be kept within
the desired limits which causes undefined voltage magnitudes during islanding
situations and frequency instability. Uncontrolled frequency represents a high risk
for drives and other machines. Islanding can occur as a consequence of a fault in
the network, due to circuit breaker maloperation or due to circuit breaker opening
during maintenance. If the distributed generator continues its operation after the
utility supply is disconnected, faults do not clear under certain conditions as the
arc is charged by the distributed generators. Moreover, the distributed generators
are incompatible with the current reclosing practices. During the reclosing sequence
dead time, the generators in the network tend to drift out of synchronism with
the grid and reconnecting them without synchronizing may damage the generators
introducing high currents and voltages in the neighboring network.
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 heavy loads in weak networks 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.
1MRS759142 F Protection functions
REX640
Technical Manual
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