ABB Relion 670 Series Applications Manual page 263

1MRK 505 370-UUS Rev. K
• Instantaneous sample based delta detection
• True RMS value based delta detection
• DFT magnitude based delta detection
• Vector shift protection
The Delta detection mode is selected on the basis of application requirements. For example,
Instantaneous sample based delta supervision is very fast; the delta is detected in less than a cycle
typically. Hence, instantaneous sample based delta supervision can be used for functions that are used
as protection enablers or fault detectors
All the other supervision modes like RMS/DFT Mag or Angle requires minimum one cycle for delta
detection and can be used for time delay functions.
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
breaker. With this logic, reliable tripping can be ensured as angle shift has been detected in all the three
phase voltages.
Busbar protection REB670
Application manual
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Secondary system supervision
122 should be used to trip the
Section 11
257