CONTROL SYSTEMS
HARDWARE IN THE LOOP TESTING

The RTDS Simulator is the ideal tool for testing of power system controls where real time closed-loop interaction between the power system simulation and the control hardware is essential.

The real time digital simulation environment provided by the RTDS allows the controls to be subjected to everything from steady state to rare emergency operating conditions. All conditions created can quickly and easily be repeated to investigate, understand and optimize the control behavior.

The RTDS Simulator has been used by many clients to test a wide variety of power system controls, including:

  • HVDC and UHVDC
  • VSC-based HVDC
  • SVC (network and industrial)
  • FACTS (including STATCOM, UPFC, SSSC, DVR, etc.)
  • TCSC series compensation
  • Exciter and voltage regulators for synchronous machines
  • Governors for synchronous machines
  • Power System Stabilizers (PSS)
  • Distributed Generation and Renewables (wind, solar, fuel cell, etc.)
  • Smart Grid (IEC 61850, RAS – remedial action schemes, SCADA interface, etc.

Since the simulation runs in real time, the physical control equipment can be connected in closed-loop with the power system model. The closed-loop interaction of the control system and the network model provides insight on both the performance of the control scheme as well as its effect on the power system.

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As illustrated, a model of the power system is implemented on the RTDS Simulator that includes the high voltage components (e.g. power electronics, generators, filters, lines, breakers, power transformers, etc.) plus the required protection and control functions not included in the equipment under test.

The controls are connected to the real time simulation using the input/output (I/O) components provided with the RTDS Simulator. Analogue output signals (representing various voltages, currents, etc.) sent from the simulator to the controls are typically provided by the GTAO card which operates over a maximum range of ± 10 Vpeak. The GTAO output is updated a minimum of once per timestep. If the simulation signal is sent from a standard timestep model, typically in the range of 50 µs, an option is provided to oversample the output through linear interpolation at a rate of 1 µs thus reducing the time between updates of the analogue output. If the simulation signal is generated by a small timestep subnetwork (typically running with a timestep in the range of 1.5 – 3 µs), the GTAO output will be updated once every small timestep.

Digital output signals (e.g. breaker status, tap position, etc) from the simulator to the controls are provided by the GTDO card and/or the dry contacts included on the 250 Vdc digital output interface panel. The GTDO outputs are updated every timestep whether standard or small timestep, but the 250 Vdc Digital Output Interface panel is only operated by standard timestep components (i.e. in the 50 µs range). The GTDO output voltage can be set up to provide any level from 5 to 24 Vdc by connecting the appropriate supply voltage to the card. The GTDO is split into two banks of 32 channels and can be configured to provide different output voltage levels from channels 1-32 and 33-64.

Analogue input signals are passed from the controls to the simulator with the GTAI card. Analogue input are sometimes used as control signals in the simulation, but in some instances are simply imported for data acquisition purposes. The GTAI input can be read into either the standard timestep area or into a small timestep subnetwork. However, a new value is read by the card a maximum of once every 6 µs.

Digital input signals from controls are typically brought in to the simulation through the GTDI cards. The GTDI input circuit includes an LED that turns on with a current of approximately 10 mA. Therefore any input voltage level can be accommodated through the use of an appropriately sized current limiting resistor. The GTDI input can be utilized in either the standard or small timestep areas and the status is passed into the simulation every timestep, large or small. However, if the GTDI input is a firing pulse for a line commutated device (i.e. thyristor), the Digital Input Time Stamp (DITS) function can be used to improve the firing pulse resolution. The DITS function works by having the GTDI sample the input every 125 ns. If the input is activated at any point during the timestep, the GTDI card timestamps the arrival of the firing pulse. The DITS information is passed to the valve group model and is used to provide an effective interpolation of the firing pulse instant and allows a continually variable firing instant to a resolution of approximately 1 µs. The DITS function is very important when modeling HVDC, SVC and TCSC.

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