Wideband Series Harmonic Voltage Compensator Using Trajectory Prediction Control for Network Stability Enhancement and Condition Monitoring


Student thesis: Doctoral Thesis

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Award date22 May 2023


Nowadays, people are moving toward reducing greenhouse gas emissions, and the government has increased the use of renewable energy and reduced the use of fossil fuels. In the microgrid system, multiple renewable energy sources such as wind energy, hydroelectric power, and solar energy are connected to the utility grid via grid-connected inverters (GCIs). Renewable energy sources have become more popular. As more and more GCIs connect to the utility network, as well as being connected close to each other, system stability becomes a major concern, with multiple stability issues that can arise. In general, the stability challenge is a dynamic interaction, parallel GCIs current circulation, and self-oscillation due to the filter resonance interaction with the grid side impedance. Traditional inverters can generally be operated independently according to their design claims. Paralleled GCIs cause additional stability challenges when connected to the grid system due to the parallel resonance interacting with the GCIs. Due to this fact, the active damping (AD) technique is a cost-effective and easily-implemented solution to the problem of high-order (inductive-capacitive-inductive LCL) output filter resonances. By using different feedback controls, the virtual damping resistor can be connected in parallel or series with the output filter.AD is one of the solutions that can improve the low-frequency power quality and suppress the LCL resonant peak with a smooth phase change. As the resonance frequency varies between 1kHz and 6kHz, the design must take into account the sample-to-update time delay. It is possible that the controller cannot introduce a virtual resistance function if the resonance frequency is close to the switching frequency and that the control becomes unstable.

In recent years, the digital controller has become more powerful and user-friendly. The converter is not limited to conventional control methods such as proportional-resonant (PR) and proportional-integral (PI). There are numerous types of control methods available for modulating, such as non-linear digital control, sliding mode control, and model predictive control. In the current day and age, wide bandgap semiconductors can increase the switching frequency with a small delay time. Thus, using a wide bandgap semiconductor and a powerful digital controller, modern control theory can further enhance the performance such as dynamic response. Even though the modern control method can ensure the stable operation of the GCI itself, different transfer characteristic drives can be connected to the grid. Therefore, in the microgrid system, GCI should take into consideration both parallel and series resonances interacting with the system components.

Typically, Nyquist stability criteria has been widely used to study the system stability of a network with multiple devices. In order to satisfy the criteria, explicit tuning the output and grid impedances of an inverter can reduce multi-device resonance. A central damping approach has been proposed. It is similar to the active damping approach realized by using a resistive active power filter (R-APF), solid-state transformer, series active power filter (SAPF), and dynamic voltage restorer. However, many challenges are identified.
1. For a long distribution feeder, voltage harmonics can be mitigated at the point with R-APF, but for other buses without the filters, harmonics can be amplified
2. Operating bandwidth is critical as the equipment requires handling high-frequency resonances. For example, the resonant frequency of the high-order output filter can be higher than 6kHz. However, the operating bandwidth of the equipment is sometimes limited by the control method, switching frequency, and rating of switching devices.
3. Output filters in the devices could cause resonance.
4. Small values of equivalent damping resistance will lead to increased high-frequency resonance current.

Hence, this thesis presents a wideband series harmonic voltage compensator (WSHVC) for mitigating the adverse effects of unknown grid impedance and load conditions on the stability of microgrids with multiple grid-connected inverters (GCIs). A predictive control scheme that utilizes a nonlinear switching surface is proposed. It can achieve a fast dynamic response. Detailed analysis of the small-signal characteristics has been performed.

The concept of the WSHVC is based on extending the theory of a series active power filter. A wideband series voltage source inverter is used to compensate for the high-frequency harmonic voltage caused by the impedance at the point of common coupling (PCC), thereby creating virtually zero high-frequency impedance at the output of the GCIs. Under any operating condition, the stability of the system is assured. By using the fixed frequency prediction scheme, it can realize the bandwidth of the WSHVC ranges from the second harmonic of the grid frequency to 8kHz, which is sufficiently higher than the cut-off frequency of typical GCIs. Since the WSHVC handles harmonic power only, its volt-ampere rating is lower than that of the entire system. A 500 VA prototype for a 6.5 kVA testbed with three commercial GCIs, non-linear load, and adjustable grid impedance has been evaluated. The power dissipation of the WSHVC is less than 1% of the VA rating of the testbed. The effectiveness of the WSHVC on improving system stability is studied.

Based on the concept of WSHVC, an enhanced predictive control is proposed to further minimize the computation resource and implemented with an operating bandwidth of 10kHz for microgrids with a plurality of grid-connected inverters (GCIs). It not only generates a voltage to compensate for high-frequency harmonic voltage at PCC, resulting in virtually zero high-frequency impedance at the aggregate output of the GCIs and thus improving system stability. The harmonic voltage is generated by predicting the state trajectory of the WSHVC after a switching period. WSHVC switches are controlled during every half-switching cycle to determine the changes from one state to another. The PCC impedance and the equivalent impedance of the GCI network are extracted by analyzing the system response after injecting the entire system with particle-swarm-optimization (PSO)-optimized multi-sine power disturbances. Therefore, the stability of the entire system is predicted. The condition of passive components in the WSHVC is also monitored by estimating their values from the sampled state variables. A 500VA, 1MHz, Field Programmable Gate Array (FPGA)-controlled, Gallium Nitride (GaN)-based prototype is evaluated on a 6.5 kVA testbed that consists of three commercial GCIs, non-linear load, and adjustable grid impedance. A comparative study of system performance with and without the WSHVC is conducted. The experimental results are consistent with theoretical predictions.

    Research areas

  • Grid impedance, harmonic filters, microgrids, predictive control, series filter, weak grid, Fast dynamic response, cancelator, harmonic compensator, impedance-based stability criterion, inductive-capacitive-inductive (LCL) filter, power harmonic filters, single-phase inverters