ABB RELION REX640 Technical Manual page 834

Section 4
Protection functions
828
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 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/undervoltage,
over/underfrequency, 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
1MRS759142 C
REX640
Technical Manual