Operation of STATCOM connected to a weak grid

The purpose of this thesis was to investigate the operational possibility of MMC STATCOM in weak grid conditions. Weak grid is a power system which has low short circuit capacity, high equivalent grid impedance, high dV/dQ sensitivity and higher volatility to voltage instability. Hence integration of power electronics based equipment due to their fast response is a big challenge in weak grid. Interaction with high grid impedance of weak grid leads to loss of synchronization and consequently unstable operation of VSC connected to weak grid. In this thesis first, the effects of high grid impedance on the PLL synchronization was investigated. It was observed that high grid impedance has high impact on PLL dynamics and introduces self-synchronization. However, it was noticed that the PLL remains stable if there is no power exchange between grid and VSC. Therefore, STATCOM performance is not deteriorated by the weak grid conditions. STATCOM model connected to weak grid was simulated in Matlab/Simulink environment to study the impact of the weak grid on STATCOM control system. Initially STATCOM was simulated as a constant current source to investigate the factors that impacts STATCOM stability. It was found that STATCOM operation in weak and very weak grid conditions is limited due to some factors that affects STATCOM stability. In capacitive mode the amount of the DC link voltage is main limiting factor and insufficient amount of DC link voltage results in the harmonic injection by STATCOM. In inductive mode high reactive power absorption results in high frequency oscillations in grid voltage which leads to loss of synchronization. Making PLL slower improves the synchronization with the grid; however, this modification deteriorates the DC link voltage performance which requires DC link voltage controller retuning. Second, STATCOM was simulated in the voltage regulation mode and it was noticed that STATCOM operation introduces high frequency ripple to grid voltage. The ripple frequency changes with the grid strengths and at very low short circuit levels system becomes unstable. To improve the system stability a notch filter tuned to the ripple frequency was added. Notch filter significantly improved STATCOM performance and extended the operational limits of STATCOM. However, it was noticed that at some short circuit levels resonance happens and in inductive mode high inductive current absorption makes system unstable. Further elaborations showed that interaction with the HF filter cause the system instability and reduction of the HF filter rating improves the system stability. Finally, STATCOM performance was tested under symmetrical and asymmetrical fault conditions. In case of asymmetrical fault due to unbalanced grid voltages part of the current is used to balance DC link voltage waveforms and the output current of the STATCOM was reduced

Operation of STATCOM connected to a weak grid

The purpose of this thesis was to investigate the operational possibility of MMC STATCOM in weak grid conditions. Weak grid is a power system which has low short circuit capacity, high equivalent grid impedance, high dV/dQ sensitivity and higher volatility to voltage instability. Hence integration of power electronics based equipment due to their fast response is a big challenge in weak grid. Interaction with high grid impedance of weak grid leads to loss of synchronization and consequently unstable operation of VSC connected to weak grid. In this thesis first, the effects of high grid impedance on the PLL synchronization was investigated. It was observed that high grid impedance has high impact on PLL dynamics and introduces self-synchronization. However, it was noticed that the PLL remains stable if there is no power exchange between grid and VSC. Therefore, STATCOM performance is not deteriorated by the weak grid conditions. STATCOM model connected to weak grid was simulated in Matlab/Simulink environment to study the impact of the weak grid on STATCOM control system. Initially STATCOM was simulated as a constant current source to investigate the factors that impacts STATCOM stability. It was found that STATCOM operation in weak and very weak grid conditions is limited due to some factors that affects STATCOM stability. In capacitive mode the amount of the DC link voltage is main limiting factor and insufficient amount of DC link voltage results in the harmonic injection by STATCOM. In inductive mode high reactive power absorption results in high frequency oscillations in grid voltage which leads to loss of synchronization. Making PLL slower improves the synchronization with the grid; however, this modification deteriorates the DC link voltage performance which requires DC link voltage controller retuning. Second, STATCOM was simulated in the voltage regulation mode and it was noticed that STATCOM operation introduces high frequency ripple to grid voltage. The ripple frequency changes with the grid strengths and at very low short circuit levels system becomes unstable. To improve the system stability a notch filter tuned to the ripple frequency was added. Notch filter significantly improved STATCOM performance and extended the operational limits of STATCOM. However, it was noticed that at some short circuit levels resonance happens and in inductive mode high inductive current absorption makes system unstable. Further elaborations showed that interaction with the HF filter cause the system instability and reduction of the HF filter rating improves the system stability. Finally, STATCOM performance was tested under symmetrical and asymmetrical fault conditions. In case of asymmetrical fault due to unbalanced grid voltages part of the current is used to balance DC link voltage waveforms and the output current of the STATCOM was reduced

Low capacitance cascaded H-bridge StatCom

The application of Cascaded H-Bridge (CHB) multilevel converter StatCom is well established in the industry. In a conventional CHB StatCom, low frequency ripple on the dc side is limited to 10% by utilizing large capacitance. Having a smaller capaci-tance is advantageous because the system will be smaller, cheaper, and more reliable. However, reducing the capacitors will increase the ripple’s magnitude and causes problems such as, control and filtering issues, and high voltage stress on the semicon-ductors. In this thesis, the next generation of CHB StatCom i.e. Low Capacitance StatCom (LC-StatCom) is developed which is able to operate with ripple magnitudes close to the theoretical limit (smallest capacitors’ size). A feed-forward filtering scheme is developed that is able to effectively filter out large ripples without imposing any delay to the control loop and facilitates design of higher bandwidth capacitors’ voltage controllers. A decoupling theory which outlines requirements for completely decoupling the individual capacitors’ voltage controller from the rest of the control system is introduced. The decoupled control system has a linear cluster capacitors’ voltage controller which is essential for operation of the LC-StatCom The proposed LC-StatCom utilizes this linearity to have a variable capacitors’ voltage reference in order to limit the maximum voltage stress on the semiconductors. The proposed LC-StatCom and innovative solutions are evaluated by simulation and experimental case studies. It is shown that the LC-StatCom can achieve 80% reduction in the capacitors’ size, improve current quality, and reduce the maximum voltage stress on the semiconductors com-pared to a conventional StatCom. The LC-StatCom has a reduced operating area in the inductive region compared to conventional StatComs. In this thesis, to overcome this drawback, a hybrid LC-StatCom that utilizes a series thyristor bypassed inductor is developed. The compensated LC-StatCom developed in this thesis, provides approxi-mately 75% reduction in overall energy storage capacity of passive components

Power Converters in Power Electronics

In recent years, power converters have played an important role in power electronics technology for different applications, such as renewable energy systems, electric vehicles, pulsed power generation, and biomedical sciences. Power converters, in the realm of power electronics, are becoming essential for generating electrical power energy in various ways. This Special Issue focuses on the development of novel power converter topologies in power electronics. The topics of interest include, but are not limited to: Z-source converters; multilevel power converter topologies; switched-capacitor-based power converters; power converters for battery management systems; power converters in wireless power transfer techniques; the reliability of power conversion systems; and modulation techniques for advanced power converters

Grid-forming wind power plants

With growing concerns over climate change, the power system is witnessing an unprecedented growth in electricity generation from intermittent renewable energy sources (RES) such as wind and solar, which are commonly interfaced to the grid by power-electronic converters. However, increasing the penetration level of converter-interfaced generation units reduces the number of synchronous generators (SGs) in the grid that provide system services to support voltage and frequency, either inherently or through mandatory requirements and market products. This brings several challenges for the grid operators, which include increasing risk of harmonic interactions, decreasing system inertia and reduction in the short-circuit power of the grid, which all together might jeopardize the security and availability of the power systems. As a countermeasure, it is necessary that the power-electronic-based generation units not only provide grid support services that are originally provided by the SGs, but also operate in harmony with other generation units in all kinds of grid conditions. As a result, the concept of grid-forming (GFM) control, which mimics the beneficial properties of the SGs in converter systems, has emerged as a viable solution to allow effective and secured operation of power systems with increased penetration of converter-based resources.\ua0\ua0 This thesis investigates the application of GFM control strategies in wind power plants (WPPs). In particular, the focus of the work will be on developing an effective GFM control strategy for the energy storage systems (ESS) in WPPs that not only supports the operation of the WPP in various grid conditions, but also offers a certain degree of GFM properties to the overall WPP. To start with, the selection of the most suitable GFM control strategy for wind power applications is made by evaluating and comparing various control strategies available in the literature. The comparison is based on their influence on the frequency characteristics of the converter and robustness of the controller in varying grid strength. To address the transient stability problem of GFM converters during current limitation, a novel strategy based on the limitation of converter\u27s internal voltage vector is developed, which effectively limits the converter current to a desired value and retains the GFM properties of the converter at all times. An experimental setup is used to validate the effectiveness of the proposed limitation strategy in case of various grid disturbances. By implementing the proposed GFM control strategy for the ESS in a test WPP model, it is shown using detailed time-domain simulation results that the GFM behaviour can be offered to the overall WPP. The Network Frequency Perturbation (NFP) plots are used to verify the GFM behaviour of the considered WPP. Furthermore, an overview of various energy storage technologies (ESTs) suitable for providing ancillary services from WPPs is presented. With a focus on the two most suitable ESTs, i.e., batteries and supercapacitors, recommendations are given for design and sizing of the ESS for a given application. Finally, a coordinated control strategy between the WPP and SGs is developed, which facilitates the provision of frequency support from the WPP and at the same time reduces the energy storage requirements for the converter system

POWER QUALITY CONTROL AND COMMON-MODE NOISE MITIGATION FOR INVERTERS IN ELECTRIC VEHICLES

Inverters are widely utilized in electric vehicle (EV) applications as a major voltage/current source for onboard battery chargers (OBC) and motor drive systems. The inverter performance is critical to the efficiency of EV system energy conversion and electronics system electro-magnetic interference (EMI) design. However, for AC systems, the bandwidth requirement is usually low compared with DC systems, and the control impact on the inverter differential-mode (DM) and common-mode (CM) performance are not well investigated. With the wide-band gap (WBG) device era, the switching capability of power electronics devices drastically improved. The DM/CM impact that was brought by the WBG device-based inverter becomes more serious and has not been completely understood. This thesis provides an in-depth analysis of on-board inverter control strategies and the corresponding DM/CM impact on the EV system. The OBC inverter control under vehicle-to-load (V2L) mode will be documented first. A virtual resistance damping method minimizes the nonlinear load harmonics, and a neutral balancing method regulates the unbalanced load impact through the fourth leg. In the motor drive system, a generalized CM voltage analytical model and a current ripple prediction model are built for understanding the system CM and DM stress with respect to different modulation methods, covering both 2-level and 3-level topologies. A novel CM EMI damping modulation scheme is proposed for 6-phase inverter applications. The performance comparison between the proposed methods and the conventional solution is carried out. Each topic is supported by the corresponding hardware platform and experimental validation

Delta STATCOM with partially rated energy storage for intended provision of ancillary services

This thesis presents research on two distinct areas, where the work carried out in the first half highlights the challenges posed by the declining system inertia in the future power systems and the potential capability of the energy storage systems in bridging the gap, supporting a safe and reliable operation. A comparison of various energy storage technologies based on their specific energy, specific power, response time, life-cycle, efficiency, cost and further correlating these characteristics to the timescale requirements of frequency and RoCoF services showed that supercapacitors (SC) and Li-ion batteries present the most suitable candidates. Results of a network stability study showed that for a power system rated at 2940 MVA with a high RES contribution of 1688 MVA, equating to 57% of the energy mix, during a power imbalance of 200 MW, an ESS designed to provide emulated inertia response (EIR) in isolation required a power and energy rating of 39.54 MW and 0.0365 MWh respectively. Similarly, providing primary frequency response (PFR) on its own required a power and energy rating of 114.52 MW and 2.14 MWh respectively. ESS providing these services in isolation was not able to maintain all the frequency operating limits and similar results were also seen in the case of the recently introduced Dynamic Containment service. However, with the introduction of a combined response capability, a significantly improved performance, comparable to that of the synchronous generators was observed. In order to maintain the RoCoF and the statutory frequency limit of 0.5 Hz/s and ±0.5 Hz respectively, an ESS must be able to respond with a delay time of no more than 0.2 seconds and be able to ramp up to full response within 0.3 seconds (0.5 seconds from the start of contingency) for a frequency deviation of ±0.5 Hz. The second half of the thesis focused on investigating the current state-of-the-art power conversion system topologies, with the objective of identifying a suitable topology for interfacing ESSs to the grid at MV level. A delta-connected Modular Multilevel STATCOM with partially rated storage (PRS-STATCOM) is proposed, capable of providing both reactive and active power support. The purpose is to provide short-term energy storage enabled grid support services such as inertial and frequency response, either alongside or temporarily instead of standard STATCOM voltage support. The topology proposed here contains two types of sub-modules (SM) in each phase-leg: standard sub-modules (STD-SMs) and energy storage element sub-modules (ESE-SMs) with a DC-DC interface converter between the SM capacitor and the ESE. A control structure has been developed that allows energy transfer between the SM capacitor and the ESE, resulting in an active power exchange between the converter and the grid. A 3rd harmonic current injection into the converter waveforms was used to increase the amount of power that can be extracted from the ESE-SMs and so reduce the required ESE-SMs fraction in each phase-leg. Simulation results demonstrate that for three selected active power ratings, 1 pu, 2/3 pu, & 1/3 pu, the fraction of SMs that need to be converted to ESE-SMs are only 69%, 59% & 38%. Thus, the proposed topology is effective in adding real power capability to a STATCOM without a large increase in equipment cost. Furthermore, modifying the initially proposed topology with the use of Silicon Carbide (SiC) switching devices and interleaved DC-DC interface converter with inverse coupled inductors resulted in similar efficiencies when operated in STATCOM mode.Open Acces

AC Voltage Control of a Future Large Offshore Wind Farm Network Connected by HVDC

The offshore wind resource around the seas of the UK is a very large renewable energy resource. Future offshore wind farm sites leased by the Crown Estate for Round 3 development will need high power capacity grid connection, but their remote location presents a challenge for the electrical connection to the grid. Long distance AC cable transmission is not practical due to the large cable capacitance which leads to reactive power loss. This thesis considers the voltage source converter and high voltage direct current (VSC-HVDC) technology as the future grid connection for the offshore wind farm network, which is more controllable and has better transmission efficiencies for long distance and high power cable transmission applications. The offshore AC network is weak with very little inertia and has limited rating at the HVDC converter substation. The dynamics in key variables in the offshore wind farm AC network and how they affect certain components in the system were studied. Without proper control, the offshore voltage and the frequency will be sensitive to change. The capacitor of the AC filter at the offshore VSC-HVDC station was found to be vulnerable to over-voltage, therefore a closed loop AC voltage controller was proposed here to maintain a constant capacitor voltage and to prevent tripping or over-voltage damage. The tuning of the control gains were optimised with a pole placement design method and small signal analysis for observing the system eigenvalue damping. The control parameters were then tuned for a fast and well damped controller. The AC voltage controller was evaluated and compared to an open loop system. The controller was able to limit the resonance in the LC filter that can be triggered by large and fast disturbances in the current, voltage and frequency. Current saturation could be implemented within the control structure for device protection from over-currents. Insight on how the wind turbine fully rated frequency converters and controllers may interact with the VSC-HVDC converter station is also discussed