Wide-area monitoring and control of future smart grids

Application of wide-area monitoring and control for future smart grids with substantial wind penetration and advanced network control options through FACTS and HVDC (both point-to-point and multi-terminal) is the subject matter of this thesis. For wide-area monitoring, a novel technique is proposed to characterize the system dynamic response in near real-time in terms of not only damping and frequency but also mode-shape, the latter being critical for corrective control action. Real-time simulation in Opal-RT is carried out to illustrate the effectiveness and practical feasibility of the proposed approach. Potential problem with wide-area closed-loop continuous control using FACTS devices due to continuously time-varying latency is addressed through the proposed modification of the traditional phasor POD concept introduced by ABB. Adverse impact of limited bandwidth availability due to networked communication is established and a solution using an observer at the PMU location has been demonstrated. Impact of wind penetration on the system dynamic performance has been analyzed along with effectiveness of damping control through proper coordination of wind farms and HVDC links. For multi-terminal HVDC (MTDC) grids the critical issue of autonomous power sharing among the converter stations following a contingency (e.g. converter outage) is addressed. Use of a power-voltage droop in the DC link voltage control loops using remote voltage feedback is shown to yield proper distribution of power mismatch according to the converter ratings while use of local voltages turns out to be unsatisfactory. A novel scheme for adapting the droop coefficients to share the burden according to the available headroom of each converter station is also studied. The effectiveness of the proposed approaches is illustrated through detailed frequency domain analysis and extensive time-domain simulation results on different test systems

Control and protection of HVDC grids

The decarbonisation of Europe’s energy sector is a key driver for the development of integrated HVDC networks or DC grids. A multi-terminal HVDC grid will enable a more reliable power transfer from offshore wind farms and will facilitate the cross-border exchange of energy between different countries. However, the widespread deployment of DC grids is prevented by technical challenges, including the control and protection of DC grids. In order to close the gap, this thesis aims to contribute to three aspects (1): developing a control method for DC grids operation; (2): developing a method for optimising wind power delivery using DC grids; (3): developing a protection method for fast DC fault current interruption. The control of a DC grid demands the regulation of DC voltage and hence keeps the power into and out from the DC grid balanced. It is also important to keep the accuracy of regulating the converter DC current. In this thesis, the Autonomous Converter Control (ACC) is developed to meet this requirement. With this method, alternative droop control characteristics can be used for individual converters to share the responsibility of regulation of DC voltage while precisely controlling the converter DC current. The control algorithms of alterative droop characteristics are developed and interactions of different control characteristics are analysed. Furthermore, the potential risk of having multiple cross-over in control characteristics is uncovered. The method for designing droop characteristics is provided to avoid the multiple cross-over. The ACC is demonstrated on different simulation platforms including the PSCAD/EMTDC and a real-time hardware 4-terminal HVDC test rig. It is found that the proper use of alternative droop characteristics can achieve better current control performance. The adverse impact of having multiple cross-over in control characteristics is also studied using both simulation platforms. The effect of the control of both converters and DC power flow controllers (DC-PFCs) on DC power flow in steady state is also investigated. A method for re-dispatching control orders to optimise the wind power delivery is developed. Case studies are undertaken and it is found that both the DC line power loss and wind power curtailment can be reduced by redispatching the control orders of converters and DC-PFCs. The protection of a DC grid demands a very fast speed for fault current interruption. Conventional methods proposed for HVDC grid protection take delays of several milliseconds to discriminate a faulted circuit to healthy circuits and then allow the DC circuit breakers (DC-CBs) to open at the faulted circuits. The fault current will keep rising during Control and Protection of HVDC Grids iv the delayed time caused by fault discrimination. The Open Grid protection method is thus developed to interrupt fault current before fault discrimination. With this method, multiple DC-CBs open to interrupt the fault current based on local measurements of voltage (and current) and the DC-CBs on healthy circuits will reclose to achieve discrimination afterwards. This will reduce the delay for fault current interruption and hence the fault current can be interrupted with a much smaller magnitude. The developed Open Grid method is tested via simulation models developed in PSCAD/EMTDC. The results show that the Open Grid can detect very quickly and discriminate various faults under different fault conditions in a meshed HVDC grid

Operation and control of voltage source converters in transmission networks for AC system stability enhancement

The rapid expansion in power transmission for the integration of large-scale renewables is foreseen in the future. This will be complemented by infrastructure reinforcements in the form of series compensation and high-voltage direct current (HVDC) links. These changes will bring forth new operability challenges to grid operators. The stability issues pertained to such reinforcements: potential threat of subsynchronous resonance (SSR) and frequency regulation will be investigated in this thesis. Utilising the existing and future voltage source converters (VSC) based HVDC links to support the AC system by proving ancillary services will be of significant importance in the coming decades. The research work presented in this thesis is aimed to address these challenges, in particular, the technical barriers associated with AC/DC interaction and to propose measures to avoid any potential instability. The main contributions of this research work comprise of four parts, namely, (1) analysis of interactions in-terms of SSR in AC/DC grids, (2) design of SSR damping (SSRD) controllers, (3) experimental demonstration of SSRD schemes, and (4) assessment and improvement of frequency regulation in a wind-thermal bundled AC/DC grid. An VSC-HVDC connected series-compensated AC system resembling the Great Britain (GB) power system has been used as the test network to evaluate the operability challenges pertained to the reinforcements. A state-space representation has been formulated and an eigenvalue analysis has been performed to assess the impact of VSC-HVDC on the torsional modes of nearby connected thermal generation plants. This is followed by damping torque investigation for SSR screening with the results compared against time-domain simulations for testing the accuracy of the small-signal models for SSR studies. A series of SSRD schemes is presented which have been integrated with the VSC-HVDC to damp SSR in the series-compensated GB power system. In addition, this thesis proposes an adaptive SSRD method based on the real-time estimation of the subsynchronous frequency v Abstract component present in series-compensated transmission lines–key information for the optimal design of HVDC subsynchronous damping controllers. Furthermore, the combined AC/DC GB network has been implemented in a real-time digital simulator and connected to a VSCHVDC scaled-down test-rig to performhardware-in-the-loop tests. The efficacy and operational performance of the AC/DC network while providing SSR damping is tested through a series of experiments. In order to provide frequency support in a wind-thermal bundled AC/DC system a dualdroop controlmethod is presented. The scheme binds the system frequency with the DC voltage of an HVDC network. For completeness, the performance of the proposed method is compared to conventional frequency regulation schemes. Sensitivity studies and eigenvalue analyses are conducted to assess the impact that wind penetration and changes in the dual-droop coefficient have on grid stability. Experimental validation is performed using a real-time hardware-inthe- loop test-rig, with simulation and experimental results showing a good agreement and evidencing the superior performance of the proposed frequency support scheme

Smart electric vehicle charging strategy in direct current microgrid

This thesis proposes novel electric vehicle (EV) charging strategies in DC microgrid (DCMG) for integrating network loads, EV charging/discharging and dispatchable generators (DGs) using droop control within DCMG. A novel two-stage optimization framework is deployed, which optimizes power flow in the network using droop control within DCMG and solves charging tasks with a modified Djistra algorithm. Charging tasks here are modeled as the shortest path problem considering system losses and battery degradation from the distribution system operator (DSO) and electric vehicles aggregator (EVA) respectively. Furthermore, a probabilistic distribution model is proposed to investigate the EV stochastic behaviours for a charging station including time-of-arrival (TOA), time-of-departure(TOD) and energy-to-be-charged (ETC) as well as the coupling characteristic between these parameters. Markov Chain Monte Carlo (MCMC) method is employed to establish a multi-dimension probability distribution for those load profiles and further tests show the scheme is suitable for decentralized computing of its low burn-in request, fast convergent and good parallel acceleration performance. Following this, a three-stage stochastic EV charging strategy is designed to plug the probabilistic distribution model into the optimization framework, which becomes the first stage of the framework. Subsequently, an optimal power flow (OPF) model in the DCMG is deployed where the previous deterministic model is deployed in the second stage which stage one and stage two are combined as a chance-constrained problem in stage three and solved as a random walk problem. Finally, this thesis investigates the value of EV integration in the DCMG. The results obtained show that with smart control of EV charging/discharging, not only EV charging requests can be satisfied, but also network performance like peak valley difference can be improved by ancillary services. Meanwhile, both system loss and battery degradation from DSO and EVA can be minimized.Open Acces