ABB Relion 670 Series Applications Manual page 273

1MRK 511 401-UUS F
Angle shift mode
Use of distributed generation (DG) units is increasing due to liberalized markets (deregulation)
and the global trend to use more renewable sources of energy. They generate power in the range
of 10 kW to 10 MW and most of them are interconnected to the distribution network. 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
the breaker. With this logic, reliable tripping can be ensured as angle shift has been detected in all
the three phase voltages.
Bay control REC670
Application manual
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Section 13
Secondary system supervision
108 should be used to trip
267