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
120 should be used to
trip the breaker. With this logic, reliable tripping can be ensured as angle shift has been
detected in all the three phase voltages.
DELVSPVC
U3P*
BLOCK
START
STARTL1
STARTL2
STARTL3
STRISE
STRISEL1
STRISEL2
STRISEL3
STLOW
STLOWL1
STLOWL2
STLOWL3
DELMAGL1
DELMAGL2
DELMAGL3
AND
VectorShiftSTART
IEC18000903‐1‐en.vsdx
IEC18000903 V1 EN-US
Figure 120: DELVSPVC connection diagram
1MRK 505 370-UEN D Section 11
Secondary system supervision
Busbar protection REB670 245
Application manual
To Next Page
Loading...
