Connecting Decentralized Renewable Energy Sources

Author: Oliver Janke, OMICRON electronics, Germany

Required Specifications of the Q -> & U < Protection

The Q->& U<  protection must trip the generators after 0.5 s if the voltage at the network connection point drops and remains below 85% of its nominal value in addition to the plant drawing reactive power from the network (under-exited operation).
For the supervision of the reactive power direction, generally two different variants are allowed (shown in Figures 4 and 5).  In both cases, the positive sequence power S1 according to (1) is used. For all graphics, the consumer meter arrow system is used.

S1 = P1 +  jQ1                               (1)

In variant 1, a minimum value for the positive sequence current I1 and the angle are defined to prevent over function of the protection function. If variant 2 is used, only a minimum reactive power threshold is necessary. A minimum current value may be optionally implemented.
Figure3 shows the logic diagram of the entire protection function. The top two blocks show the calculation of the positive sequence power S1 and the assessment of the reactive power direction. The block below describes the evaluation of the minimum current, which is mandatory for variant 1 and optional for variant 2.  Further down the three blocks of the undervoltage protection can be seen.  Each line to line voltage is evaluated to detect faults on the network.

The connected AND (&) logic activates two timers if the following conditions are met:

  • The positive sequence power S1 is within the trip area
  • The positive sequence current I1 is above its minimum value
  • All three line to line voltages are below their set minimum value

The first of the two timers trips the generating units after typically 0.5 s. If this trip should fail for any reason, the second timer will trip the grid connection point (GCP, see figure 2) and thereby disconnect the whole plant from the network.
Table 1 specifies the necessary ranges and default values for the setting values of the protection function. The network operator defines the values to be used for the specific device.
The Q->& U< shall be blocked, if the VTs miniature circuit breaker is tripped. Blocking should also be possible for testing purposes.
Furthermore, it is required that the binary output signals of the protection function must be freely routable to binary outputs of the device and to LEDs. If the Q->& U< protection function is integrated into a multifunctional numerical protection relay, which is also used for protecting the feeder, separate binary outputs shall be used for the feeder protection and for the network protection.

The Q->&U< protection function should not trip during high inrush currents. Therefore the fundamentals of the physical units (currents and voltages) should be used for all calculations. Another solution is to use an inrush detection based on harmonics to block the Q->& U< protection function.
The protection device must be connected on the same voltage level as the GCP. Normally it should be connected to the CTs and VTs directly at the GCP.
The Q->& U< protection function can be realized as an independent function within the protection device. Alternatively, logical blocks and function modules can be used for the implementation.
Some sources recommend a delay of up to a few minutes after a trip of the network protection during a fault on the network before reclosing. Therefore,  a release signal according to Figure 6 for reclosing,  is necessary and specified for power plants connected to the medium voltage network

The reclosure is enabled if:

  • A specified time has passed after the trip
  • The network frequency is within a given range
  • The three line to line voltages of the network are above 95% of their nominal value
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