Connecting Decentralized Renewable Energy Sources

Author: Oliver Janke, OMICRON electronics, Germany

However, if the generators stay connected, it must be ensured that they are not receiving reactive power, as this would lead to a collapse of the grid.
In Germany, a number of legal requirements and grid codes are regulating the connection of such generators.
These documents also stipulate the use of Directional Reactive Power Undervoltage Protection (Q->& U< protection) that would disconnect such energy sources if they received reactive power during faults on the network.
This article gives an overview of the legal documents and an introduction to the protection function mentioned above. The basic principle of the Q->& U< protection is explained by means of its requirement specifications.
Furthermore, the article will show up standardized test methods evaluating the Q->& U< protection. It will describe how to test the releasing functions, the reactive power direction determination and also all binary inputs and outputs that are necessary for the protection function.
Decentralized and renewable energy sources are contributing an increasing amount to the electrical power mix in many regions of the world. Figure1 shows the growth of installed power from Wind Energy stations worldwide. During the past five years there has been an enormous rise in many countries while Germany observed a slight increase from a relatively high value. In many cases, this development is driven by society and subsidized by the state.
Those small or medium sized generators are often connected to the medium voltage network. Figure2 shows a typical example of a wind farm with multiple generators that are connected to a 20kV medium voltage busbar.
In the past, it was sufficient to disconnect such decentralized energy sources during a fault on the electrical network. This ensured that the fault was not feed from the distribution network and could be cleared by tripping the corresponding feeder.
With growing numbers, it is now important to use those suppliers to support the stabilization of the system voltage after short voltage dropouts. Otherwise, the short circuit power would decrease suddenly which would cause problems for many protection devices (such as decreasing the fault current). Loss of decentralized in-fed power should be avoided as the remaining conventional or nuclear power plants might not be sufficient to supply the whole network. A decrease of the network voltage followed by load shedding and blackouts would be the consequence.
With the high percentage of decentralized regenerative energy sources, the German grid operators had to find solutions to avoid these problems. A number of grid codes specify the requirements for connecting those power plants to the network and are also describing a protection function called Directional Reactive Power Undervoltage Protection (Q->& U<). This function disconnects the decentralized generators in the event that they receive reactive power during or after a network fault . As long as they are supplying reactive power to the network (overexcited operation or capacitive behavior) they stay connected to stabilize the network voltage.
The German regulations and the Q->& U< Protection could serve as an example for other countries or regions.

Legal Regulations

According to the Electricity Feeding Act the network operators in Germany were bound to connect renewable energy sources to their grid since 1st January, 1991. This law also stipulates minimum remunerations for the injected energy. It was replaced by the Renewable Energies Act in 2000 which was last revised in 2009.
The current version determines the rates for injected electrical energy into the grid in dependency on several factors such as the type of energy source, the date of connection to the grid, or the nominal power. These rates are guaranteed for a specified period. Therefore, the guaranteed rates for plants connected in earlier years are higher than for those connected later. This takes into account the higher investment cost for the new technologies and supports further research and development (for example in regards to efficiency, reliability etc.). It also specifies a system service bonus of 0.5 cent/kWh for wind energy power plants that are put into service before the 1st of January 2014, and are meeting a number of requirements mentioned in the Ordinance on System Services by Wind Energy Plants.
According to this regulation, Directional Reactive Power Undervoltage Protection (Q->& U<) must be installed.
As a guide for developers of such protection devices the FFN (committee for grid technologies and operations within the VDE, Federation of Electro Technology Electronics Information Technology) developed detailed requirement specifications.
The Q->& U< protection's setting  values can be derived from the Technical Guideline for Generating Plants of the Medium-Voltage Power Grid.
This document specifies the requirements according to safety and reliability for connecting electrical energy sources to the medium voltage network.  One requirement is the dynamic grid support during voltage dips within the high voltage network. The electrical energy source must be able to:

  • Stay connected to the grid during network faults
  • Support the system voltage by injecting a reactive current during the fault
  • Receive the same amount of reactive energy or less from the network after the fault was cleared

This must be proven to the network operator with a certificate for each single unit and another certificate for the whole plant.
In summary, the legal regulations state that renewable energy sources connected to the medium voltage network must support the system voltage during network faults (verified by certificate) and should they not do so, for any reason, must be disconnected from the grid by a Q-->& U< protection.

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

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.  

Biographies

Biography:
Oliver Janke was born in Nuremberg in Germany on June 20, 1981. After attending school in Höchstadt and obtaining the Abitur (university entrance examination) he studied electrical engineering from 2002 to 2008 at the Friedrich-Alexander-Universität Erlangen-Nürnberg and graduated as an engineer.
He joined the Software Test Automation department of OMICRON electronics in 2008 where he developed automated test plans for protection testing as a project engineer.

Currently he is working as a product trainer and responsible for the training portfolio for secondary assets.
Oliver is a member of the VDE (Federation of Electro Technology Electronics Information Technology) and the DPG (German Physical Society).

 

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