Parallel-connected STATCOMs in high-voltage transmission system
Raja-Aho, Jussi (2024)
2024
Sähkötekniikan DI-ohjelma - Master's Programme in Electrical Engineering
Informaatioteknologian ja viestinnän tiedekunta - Faculty of Information Technology and Communication Sciences
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Hyväksymispäivämäärä
2024-08-19
Julkaisun pysyvä osoite on
https://urn.fi/URN:NBN:fi:tuni-202408098009
https://urn.fi/URN:NBN:fi:tuni-202408098009
Tiivistelmä
In this thesis, possible interactions between multiple STATCOMs (Static Synchronous Compensator) connected parallel in high-voltage transmission network was studied. These possible interactions are related to uneven power-sharing between devices, called hunting behavior, where STATCOMs compensate each others reactive power outputs, and possible effect to dynamic performance and controller response times. Because of the constantly increasing amount of high-power renewable energy generation units, the need for more reactive power compensation also increases. Eventually, one individual compensation device will not be able to provide enough reactive power output, and due to restrictions in power electronic equipments' power ratings and physical footprint of the compensation units, parallel connection of several compensation units is becoming a solution.
The objective of this thesis is to investigate possible interactions between parallel connected STATCOMs with traditional grid-following STATCOM, and also for grid-forming STATCOM, and to find possible solutions for mitigating these interactions. This topic was investigated through simulations in MATLAB Simulink and downscaled laboratory experiments. In the simulation, up to 10 parallel STATCOMs were connected in parallel in various configurations, and in experiments two inverters were configured to act as parallel STATCOMs.
The simulation results showed that in steady network with voltage control mode, the reactive power outputs of parallel STATCOMs started to diverge from the 0 MVAr point to opposite directions, and eventually saturated in power limits and thus, hunting occurs. When droop control was applied to the voltage controller, the divergence of reactive power outputs was limited to certain values well below the power limits, but still uneven capacitive-inductive power-sharing was seen. Possible solution with this control mode was to apply a slow cascade reactive power controller with the voltage controller. This way the controller drove the diverging reactive power outputs back to the steady-state value and hunting was mitigated. With reactive power control mode, where STATCOMs control the reactive power on the point of common coupling with common reactive power measurement, reactive power outputs of STATCOMs also diverged and hunting occurred. With grid-forming STATCOM, the hunting behavior was not visible, and parallel connection did not affect other parallel STATCOMs. The laboratory experiment showed, that the dynamic response time of one inverter was affected by approximately 50 \% of the parallel connected inverter. Also, the parallel connection caused some noise to the measurement of the inverter, and the cause to that phenomenon remained unclear.
Possible solutions to mitigate possible interactions between parallel STATCOMs is to have a communication link between the STATCOMs, so the parallel devices will know when the compensation is needed, and the hunting can be mitigated. Also, the control topologies of the controllers needs to remain similar, and voltage references for parallel STATCOMs needs to be equal.
The objective of this thesis is to investigate possible interactions between parallel connected STATCOMs with traditional grid-following STATCOM, and also for grid-forming STATCOM, and to find possible solutions for mitigating these interactions. This topic was investigated through simulations in MATLAB Simulink and downscaled laboratory experiments. In the simulation, up to 10 parallel STATCOMs were connected in parallel in various configurations, and in experiments two inverters were configured to act as parallel STATCOMs.
The simulation results showed that in steady network with voltage control mode, the reactive power outputs of parallel STATCOMs started to diverge from the 0 MVAr point to opposite directions, and eventually saturated in power limits and thus, hunting occurs. When droop control was applied to the voltage controller, the divergence of reactive power outputs was limited to certain values well below the power limits, but still uneven capacitive-inductive power-sharing was seen. Possible solution with this control mode was to apply a slow cascade reactive power controller with the voltage controller. This way the controller drove the diverging reactive power outputs back to the steady-state value and hunting was mitigated. With reactive power control mode, where STATCOMs control the reactive power on the point of common coupling with common reactive power measurement, reactive power outputs of STATCOMs also diverged and hunting occurred. With grid-forming STATCOM, the hunting behavior was not visible, and parallel connection did not affect other parallel STATCOMs. The laboratory experiment showed, that the dynamic response time of one inverter was affected by approximately 50 \% of the parallel connected inverter. Also, the parallel connection caused some noise to the measurement of the inverter, and the cause to that phenomenon remained unclear.
Possible solutions to mitigate possible interactions between parallel STATCOMs is to have a communication link between the STATCOMs, so the parallel devices will know when the compensation is needed, and the hunting can be mitigated. Also, the control topologies of the controllers needs to remain similar, and voltage references for parallel STATCOMs needs to be equal.