ABB Relion 670 Series Applications Manual page 314

Section 14
Secondary system supervision
They can supply power into the network as well as to the local loads. It is not common to
connect generators directly to the distribution networks and thus the distributed generation
can cause some challenges for the protection of distribution networks. From the protection
point of view, one of the most challenging issue is islanding.
Islanding is defined as a condition in which a distributed generation unit continues to supply
power to a certain part of the distribution network when power from the larger utility main
grid is no longer available after 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 nor the frequency is controlled by the utility supply. Also, 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 resulting into
undefined voltage magnitudes during islanding situations and frequency instability. Further,
uncontrolled frequency represents a high risk for drives and other machines.
Islanding can occur as a consequence of :
• a fault in the network
• circuit-breaker maloperation
• 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 usually 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 neighbouring network.
Due to the technical difficulties mentioned above, protection should be provided, which
disconnects 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 feature
of voltage supervision focuses on voltage vector shift.
For islanding based on vector shift protection, the logic shown in Figure
trip the breaker. With this logic, reliable tripping can be ensured as angle shift has been
detected in all the three phase voltages.
U3P*
BLOCK
IEC18000903 V1 EN-US
Figure 137: DELVSPVC connection diagram
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
308
DELVSPVC
START
STARTL1
STARTL2
STARTL3
STRISE
STRISEL1
STRISEL2
STRISEL3
STLOW
STLOWL1
STLOWL2
STLOWL3
DELMAGL1
DELMAGL2
DELMAGL3
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VectorShiftSTART
AND
IEC18000903‐1‐en.vsdx
1MRK 511 401-UEN Rev. K
137 should be used to
Bay control REC670
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