Key Takeaways & Executive Findings
- •• • The proposed method dynamically computes the power controllable range (PCR) under real-time rotor speed, ensuring rotor current remains below the allowable limit; this prevents overcurrent trips that would otherwise cause voltage support failure in weak grids with high renewable penetration. • • Coordinated active and reactive power control achieves the point of common coupling voltage target more effectively than reactive-only control; this is critical for grids where active power variations from renewables cause voltage fluctuations that reactive power alone cannot mitigate. • • Ignoring rotor speed variation leads to inaccurate power control capability assessment; the SC-HI-ES speed drops by 0.222 p.u. when delivering 1.5 p.u. active power, directly reducing available reactive power margin and risking voltage support inadequacy. • • The method enables SC-HI-ES to provide both inertia and voltage support without a prime mover, addressing the hollowing-out of synchronous generation in high-IBR systems; this reduces reliance on fossil-fueled synchronous condensers and supports renewable integration.
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Abstract
The high-inertia energy-storage synchronous condenser (SC-HI-ES) can provide reactive power support by adjusting excitation current and active power support by actively varying rotor speed. However, the absence of a prime mover makes its control capability difficult to quantify, preventing coordinated active and reactive power support for grid voltage. This paper analyzes the factors influencing SC-HI-ES power control capability, derives rotor current equations considering stator and rotor circuit constraints, and establishes the power controllable range (PCR) under rotor current constraints and speed variations. The relationship between the PCR and voltage support power demand is parsed, and a coordinated active-reactive power voltage support control method is proposed. Case studies validate the method's effectiveness. Results show that the proposed method enhances voltage support and avoids rotor current over-limit. Compared with methods ignoring speed variation or controlling only reactive power, the proposed method dynamically calculates PCR-based power demand under current speed, and through coordinated active and reactive power control, maximally satisfies the point of common coupling voltage target while ensuring rotor current does not exceed allowable values. This provides effective technical support for SC-HI-ES application in new-type power systems. Frequency control capability characterization and frequency-coordinated control are identified as future research directions.
1. Introduction
High-penetration renewable generation and HVDC infeed have eroded system inertia and dynamic reactive power reserves, causing voltage instability and insufficient short-circuit capacity. Traditional synchronous condensers provide fast reactive power but lack active power output and rotational inertia, limiting their effectiveness in weak grids where voltage is influenced by both active and reactive power. The SC-HI-ES, based on a doubly-fed induction machine with a flywheel, offers both high inertia and energy storage, enabling active power support through speed variation. However, its lack of a prime mover means rotor speed changes significantly with active power output, directly affecting power control margins. Existing control methods for DFIGs or conventional synchronous condensers cannot account for this speed-dependent limitation, leading to rotor current over-limit and inadequate voltage support.
This paper derives analytical expressions linking SC-HI-ES power, rotor speed, and rotor current, and establishes a power controllable range (PCR) that incorporates rotor current constraints. The PCR's dynamic variation with speed is characterized, and its relationship with voltage support power demand is analyzed. A coordinated active-reactive power control method is proposed, which dynamically allocates power commands within the PCR to satisfy voltage targets while preventing rotor current violations. Case studies demonstrate improved voltage support compared to methods that ignore speed variation or control only reactive power. The method provides a quantifiable framework for SC-HI-ES deployment in new-type power systems, with future work focusing on frequency control capability and coordinated frequency support.
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OUYANG Jinxin, YE Zhiqi, YAO Jun, LIN Yaowei (2026). Active and Reactive Power Coordinated Voltage Support Control Method for High-Inertia Energy-Storage Synchronous Condenser. Power Automation Equipment. https://doi.org/10.16081/j.epae.202605019
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Frequently Asked Questions
What is the maximum active power output of the SC-HI-ES before rotor speed drops below acceptable limits, and how does this affect voltage support duration?
The SC-HI-ES can deliver 1.5 p.u. active power, causing rotor speed to drop by 0.222 p.u. This speed reduction directly reduces the available reactive power margin and the PCR. For sustained voltage support, the control must dynamically adjust active and reactive power within the shrinking PCR to avoid rotor current over-limit. The method ensures that even under such speed drop, the voltage target is met without exceeding rotor current limits, but prolonged high active power output will deplete kinetic energy and require re-acceleration from the grid.
How does the proposed method prevent rotor current over-limit during transient voltage events when both active and reactive power demands are high?
The method continuously calculates the PCR based on real-time rotor speed and rotor current constraints. When voltage support requires power beyond the PCR, the control allocates the available margin between active and reactive power according to their relative effectiveness in supporting voltage. This prevents any single current component from exceeding the allowable limit. Simulation results show that compared to methods ignoring speed variation, the proposed method avoids rotor current over-limit while still maximizing voltage support.
What are the key differences in control capability between SC-HI-ES and conventional synchronous condensers that necessitate a new control method?
Conventional synchronous condensers have no active power output and constant speed, so their reactive power capability is limited only by field current. SC-HI-ES lacks a prime mover, so its speed varies with active power output, directly affecting the available reactive power margin and the PCR. The rotor current constraint couples active and reactive power, making traditional decoupled control ineffective. The proposed method explicitly models this coupling and dynamically coordinates active and reactive power to stay within the PCR.
What is the impact of ignoring rotor speed variation on voltage support performance, and how does the proposed method quantify this improvement?
Ignoring speed variation leads to overestimation of available reactive power and potential rotor current over-limit, resulting in failed voltage support or equipment tripping. The proposed method calculates the PCR at each speed, ensuring that power commands are feasible. Case studies show that the proposed method achieves the voltage target while keeping rotor current below the limit, whereas methods ignoring speed variation either violate current limits or fail to reach the voltage target. The improvement is quantified by the ability to maintain voltage within the target range under varying speed conditions.
What are the scalability and cost implications of implementing the proposed control method in existing SC-HI-ES installations?
The method requires real-time measurement of rotor speed and current, which are already available in SC-HI-ES control systems. The computational burden is low, as it involves solving analytical expressions for the PCR and allocating power commands. No additional hardware is needed. The cost is primarily in software updates and tuning, which is minimal compared to the benefits of avoiding rotor overcurrent and improving voltage support. This makes the method scalable to existing and future SC-HI-ES units.
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