Connecting Decentralized Renewable Energy Sources

Author: Oliver Janke, OMICRON electronics, Germany

Testing the Q->& U< Protection

The correct operation of the Q->& U< protection function must be tested independent of its realization. This could be:

  • As dedicated Q->& U< protection device
  • Integrated into the feeder protection device
  • Integrated into the control device of the generating unit

With clear specification of the protection function requirements, a test procedure can be developed,  which is independent of the current realization and can be used for protection devices from multiple vendors. An example using test plan software can be seen in Figure 7. This template includes an XRIO converter with all necessary protection settings. All test modules are automated and are adapting to changed settings to support the test person. As the XRIO converter already holds the recommended default settings, the preparation time for the test is further minimized.

The basis for testing is the schematic diagram (Figure 3.) To ensure the correct operation of the Q->&U< protection all logical blocks are tested. Therefore test modules for testing the following functions are included in the test template:

  • Wiring check
  • Test if consumer meter arrow system is used
  • Pickup test for the minimum current or if not used, the minimum reactive power
  • Pickup of the undervoltage function
  • Logic test of the undervoltage protection (multiple test -Table II)
  • Test of the power direction determination (see Figure 8, a and b)
  • Blocking via binary input of the protection device
  • Blocking by inrush detection (if existing)
  • Trip time test for GCP and generating units
  • Test with GCP circuit breaker (CB) and the generating units' CB

For testing, if customer meter arrow system is used, negative values for the active power and reactive power are simulated, as if the generating units are working under normal conditions. The measured values at the protection device must be compared with the injected values. (Additionally the correct polarity and wiring of the connected CTs should be tested during commissioning.)
The pickup tests are performed with ramping modules. For the minimum current or the minimum power test, the injected value is increased in small steps until the protection device picks up. All other release criteria are fulfilled during this test. The trip signal of the generating units is used as trigger. During the pickup test of the undervoltage function, all three line to line voltages are decreased until reaching the pickup at Vmin. Again, all other release criteria are fulfilled. For both tests, the drop off value is tested by ramping the specific value into the opposite direction.
A State Sequencer Module is used for testing the logic of the undervoltage function. Therefore,  several different network faults (three phase, two phase and single phase) for all phases are simulated (see Table 2,) while all other release criteria are fulfilled. Between each simulated fault, a prefault with nominal voltages is simulated. As Figure 3 shows, the Q->& U< protection may only trip if all three line to line voltages are below the set threshold Vmin.
This only applies to the last simulated fault (3ph (L1L2L3) < Vmin), thus the protection function may only trip during this state.

The test of the power direction determination depends on which variant (mentioned in chapter IV) is implemented within the protection device. For variant 1,  the OMICRON Ramping module is used to vary the angle between voltages and currents, while their magnitude is fixed. This results in circular lines within the P-Q-plane as shown in Figure 8a. One of the trip signals is used as trigger as with all other tests. The angle  is tested in this way. It must be ensured that all other release criteria (e.g. undervoltage, minimum current etc.) are given during this test.
If variant 2 is implemented within the protection device to determine the direction of the power flow, a state sequencer module can be used. Along the characteristic, four shots are placed within the trip area and an additional four within the blocking area (Figure 8b). The shots must be placed slightly outside the tolerance band to gain reliable test results. With these test shots the characteristic is tested sufficiently.
An automated test procedure like this can ideally be used to generate the test report which is necessary for acquiring the certificates.  Depending on the software used, the layout and the content of the report can be adapted after the test without losing the test results.


Conclusion 
The progressive integration of decentralized energy sources into the electrical grid is one of today's challenges within the field of electrical power supply. The German regulations and grid codes can serve as an example for other regions on how to regulate this development. It is also necessary that small and medium sized generators are contributing to the system's stability. The integration of Q->& U< protection is a perfect solution to guarantee that those generators will not receive reactive power during network faults and therefore will stabilize the network voltage. Thus, the reliability of the network is improved and blackouts can be prevented.  

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