Dynamical modelling of power systems with power electronic controllers using individual channel analysis and design

Global demand for electrical energy is at an all time high. In industrialised societies consumers have come to expect an interruption-free, high-quality electricity supply and environmentally aware consumers and pressure groups have been very successful in encouraging the electricity supply industry towards incorporating, as part of the generation mix, sources of electricity that are benign to the environment. In some European countries great progress has been made in the integration of wind generation and photo-voltaics. Moreover, the industry has gone through major privatisation and deregulation programmes worldwide; and there is the notion in some quarters that deregulation and widespread cross-border interconnections may have exacerbated the incidence of wide-area break-downs in electricity supply. The challenges facing today’s electricity supply industry are many, and the technology to deliver the necessary grid control is still underdeveloped. A major research thrust is required to make a power network flexible, resilient and responsive to the consumer’s wishes of being supplied with environmentally sound electricity. Renewable generation such as wind and marine turbines and photo-voltaic cells need power electronics and effective controllers if they are to be successfully integrated into the electricity grid without reduction of supply quality. The dynamical interaction of multi-machine networks, power electronics and large penetration of intermittent generation are highly complex phenomena and a better understanding of their dynamical behaviour is mandatory before larger increases of intermittent generation are added to it, to avoid widespread black-outs and thwarted energy transactions. The impact of successful integration of FACTS equipment into power systems networks worldwide is affecting all sectors of the market: power generation, transmission, distribution, utilisation and equipment manufacturers. However, further progress requires investigating further the dynamic performance of the FACTS technology in order to continue acquiring leading-edge, relevant knowledge. Devices used to enhance the stability of power systems such as the Static VAr Compensator (SVC) and the Thyristor-Controlled Series Compensator (TCSC) are prime candidates for investigation owing to their popularity. Both FACTS controllers are comprehensively investigated in this research. The main aims of this research project are to develop and evaluate dynamic high-order multi-machine models, dynamic models of FACTS devices, such as the SVC and the TCSC, with particular emphasis in their electromechanical oscillations damping capabilities; to carry out fundamental analyses and control system designs of synchronous generators and FACTS controllers; and to investigate their dynamic effects and interactions with the power network. Individual Channel Analysis and Design (ICAD), a classical oriented multivariable control systems framework is used in this research project. ICAD has shown its suitability for carrying out small-signal stability assessments, with which it has been possible to evaluate the potential robustness and performance of the control system design, affording physical insight

Performance of pitch and stall regulated tidal stream turbines

Controllers for a pitch and a stall regulated horizontal axial flow, variable-speed tidal stream turbine are developed, and a performance comparison is carried out. Below rated flow speed, both turbines are operated in variable-speed mode so that the optimum tip-speed ratio is maintained. One of the turbines has variable pitch blades, which above rated speed are pitched to feather in order to regulate power. The other turbine has fixed pitch blades and uses speed-assisted stall to regulate power. The control system design behind both strategies is examined in MATLAB, with the performance under turbulent flows, loading and energy yield analysis being evaluated in GH Tidal Bladed. Both strategies provide a satisfactory performance, but the out-of-plane loads on the stall regulated turbine were higher over the entire range of operation. In addition, the dynamic characteristics of the stall regulated turbine require a more complex control design. The results suggest that the pitch regulated turbine would be a more attractive solution for turbine developers

Fast frequency support control in the GB power system using VSC-HVDC technology

A fast frequency support control scheme for voltage source converter based high-voltage direct-current (HVDC) links has been designed, simulated and experimentally implemented and validated. The effectiveness of the proposed scheme has been tested on simplified GB power system models with both averaged and switched converter models. System performance has been initially assessed using different software simulation platforms (PSCAD and MATLAB/Simulink). System validation has been carried out using an experimental test-rig. It is shown that simulation and experimental results agree on well when the fast frequency support provision is enabled. For completeness, the effectiveness of the control scheme has been tested for two contingency scenarios: (i) when a high-voltage alternating-current interlink in parallel with the HVDC link is disconnected, and (ii) for a substantial increase in system load

Criterion for the electrical resonance stability of offshore wind power plants connected through HVDC links

Electrical resonances may compromise the stability of HVDC-connected Offshore Wind Power Plants (OWPPs). In particular, an offshore HVDC converter can reduce the damping of an OWPP at low frequency series resonances, leading to system instability. The interaction between offshore HVDC converter control and electrical resonances of offshore grids is analyzed in this paper. An impedance-based representation of an OWPP is used to analyze the effect that offshore converters have on the resonant frequency of the offshore grid and on system stability. The positive-net-damping criterion, originally proposed for subsynchronous analysis, has been adapted to determine the stability of the HVDC-connected OWPP. The reformulated criterion enables the net-damping of the electrical series resonance to be evaluated and establishes a clear relationship between electrical resonances of the HVDC-connected OWPPs and stability. The criterion is theoretically justified, with analytical expressions for low frequency series resonances being obtained and stability conditions defined based on the total damping of the OWPP. Examples are used to show the influence that HVDC converter control parameters and the OWPP configuration have on stability. A root locus analysis and time-domain simulations in PSCAD/EMTDC are presented to verify the stability conditions

Progressive fault isolation and grid restoration strategy for MTDC networks

A multi-terminal dc (MTDC) grid has a number of advantages over traditional ac transmission. However, dc protection is still one of the main technical issues holding back the expansion of point-to-point dc links to MTDC networks. Most dc protection strategies are based on dc circuit breakers (DCCBs); however, DCCBs are still under development and their arrival to the market will come at an unclear time and cost. Conversely, ac circuit breakers (ACCBs) are readily available and represent a more economic alternative to protect dc networks. Following this line, a protection strategy for MTDC grids is proposed in this paper. This uses ACCBs for dc fault current clearing and fast dc disconnectors for fault isolation. The faulty link is correctly discriminated and isolated while communication links are not required. This strategy contributes to a reduced network outage period as the non-faulty links are out of operation for a relatively short period of time and are restored in a progressive manner. The effectiveness of the proposed strategy is tested in PSCAD/EMTDC for pole-to-ground and pole-to-pole faults

Input-output signal selection for damping of power system oscillations using wind power plants

AbstractDuring the last years wind power has emerged as one of the most important sources in the power generation share. Due to stringent Grid Code requirements, wind power plants (WPPs) should provide ancillary services such as fault ride-through and damping of power system oscillations to resemble conventional generation. Through an adequate selection of input–output signal pairs, WPPs can be effectively used to provide electromechanical oscillations damping. In this paper, different analysis techniques considering both controllability and observability measures and input–output interactions are compared and critically examined. Recommendations are drawn to select the best signal pairs available from WPPs to contribute to power oscillations damping. Control system design approaches including single-input single-output and multivariable control are considered. The recommendation of analysis techniques is justified through the tools usage in a test system including a WPP

Protection strategy for multi-terminal DC networks with fault current blocking capability of converters

High voltage dc networks are a promising technology to flexibly transmit power over long distances. However, dc grid protection is still a major challenge. DC fault clearance can be mainly achieved with three devices. These are ac circuit breakers (ACCBs), dc circuit breakers (DCCBs) and converters with fault current blocking (FB) capability. In spite of their great operational advantages, FB converters have attracted less attention than ACCBs or DCCBs in dc protection research. To bridge this gap, this paper investigates a protection strategy for a multi-terminal dc (MTDC) network equipped with FB converters and fast dc disconnectors. A novel minimum opening protection approach fully based on local data is proposed. Digital simulations are carried out using PSCAD/EMTDC. Simulation results show that only the two fast dc disconnectors placed in a faulty link operate following a dc fault. These results have verified proposed ideas for the protection of MTDC networks

Coordination of MMCs with hybrid DC circuit breakers for HVDC grid protection

A high-voltage direct-current (HVDC) grid protection strategy to suppress dc fault currents and prevent overcurrent in the arms of modular multi-level converters (MMCs) is proposed in this paper. The strategy is based on the coordination of half-bridge (HB) MMCs and hybrid dc circuit breakers (DCCBs). This is achieved by allowing MMC submodules (SMs) to be temporarily bypassed prior to the opening of the DCCBs. Once the fault is isolated by the DCCBs, the MMCs will restore to normal operation. The performance of the proposed method is assessed and compared to when MMCs are blocked and when no corrective action is taken. To achieve this, an algorithm for fault detection and discrimination is used and its impact on MMC bypassing is discussed. To assess its effectiveness, the proposed algorithm is demonstrated in PSCAD/EMTDC using a four-terminal HVDC system. Simulation results show that the coordination of MMCs and DCCBs can significantly reduce dc fault current and the absorbed current energy by more than 70 and 90% respectively, while keeping MMC arm currents small