Xiaoxin Zhou

Modelling of TCSC dynamics for control and analysis of power system stability

A Thyristor Controlled Series Compensator (TCSC) exhibits great nonlinearity and performs in a complex dynamic process. The aim of this paper is to propose a method that can accurately simulate this process when carrying out power system stability analysis, so that the impact of TCSC on power system stability can be more reasonably evaluated. A method that can incorporate the analysis of the electromagnetic transient process of TCSC into the power system stability analysis is described. Simulation studies using different modelling methods of TCSC are presented. The simulation results show the advantages of using the utilisation of the modelling method in power system control and stability analysis.

Power flow control devices in DC grids

A HVDC grid based on voltage source converters (VSC) is an attractive solution for the grid-integration of offshore renewable energy, interconnection of AC grids, facilitation of power markets, and energy emergency support. However, the power flow within the DC grids is determined by the line resistance, and cannot be fully controlled by the VSCs only. Thus, Power flow control of DC grids are needed not only to realize the economic operation and power markets, but also to prevent overload of DC lines in post-fault conditions. Three types of control devices, DC transformer, the variable resistor and series voltage source, are presented to achieve the power flow control. The topologies and modeling of these control devices are presented. Their control performance is evaluated through simulations using a 5-terminal DC grid. Their effectiveness, feasibility, and applications are discussed. Through the preliminary comparison, the series voltage source is considered as the best solution in terms of the rating, power losses, and control performance.

Observer-based nonlinear control of synchronous generators with perturbation estimation

This paper presents a new observer-based nonlinear controller (ONC) for excitation control of synchronous generators. The observer-based nonlinear control is basically an input/output feedback linearizing control of nonlinear systems but it employs output feedback and perturbation estimation. A sliding mode observer is used to estimate system states and a fictitious state which is introduced to represent the combinatorial effect of system uncertainties and nonlinearities. The successful estimation of states and perturbation allows the input/output linearization of the nonlinear system without requiring the accurate model. This constitutes a real-time compensation mechanism against the perturbations. The new ONC has main advantages of simple structure and robust performance. Simulations studies based on a single-machine quasi-infinite bus power system are undertaken to evaluate the proposed approach. The simulation results show that the ONC can provide superior performance in comparison with that obtained using the conventional model based state feedback linearizing controller.

Influence of the transient process of TCSC and MOV on power system stability

This paper presents a method of analyzing power system dynamic performance taking into account the transient processes of TCSC and metal oxidized varistor (MOV). Simulation studies of the TCSC in a simple power system are undertaken in three cases: (1) incorporating only the dynamics of the TCSC into the power system model, (2) the transient processes of both the TCSC and MOV are included, and (3) the transient process of the TCSC is ignored. The simulation results show that the influence of the transient processes of the TCSC and MOV on power system dynamic performance should be considered in studies of the TCSC control and power system stability analysis.

Improved ADC Model of Voltage-Source Converters in DC Grids

Due to a large number of converters in dc grids, the simulation speed using traditional electromagnetic simulation tools becomes very slow. An associated discrete circuit (ADC) switch model can improve the simulation efficiency greatly by avoiding the modification of system matrix during switching. However, existing ADC switches induce virtual power losses due to simulation errors during switching transients. The mechanism of the virtual power loss is analyzed, and a power loss model is established. An improved ADC switch model is designed by adding compensation sources to mitigate the simulation errors. Theoretical analyses are carried out to prove this improvement. A fast algorithm to obtain the initial values of the compensation sources is proposed by utilizing the complementary operation of IGBTs. The improved ADC switch provides fast simulation speed and high accuracy. The modeling is particularly suitable for investigating long term system dynamics of dc grids with multiple converters and fast converter transients at the same time.

Coordination of TCSC and SVC for improvement of power system performance with NN-based parameter adaptation

A nonlinear design technique, DFL (direct feedback linearizing), is used to deduce the control law for the TCSC (thyristor controlled series compensator) and SVC (static VAR compensator). The coordination between the two pieces of equipment is also designed in the paper with the SVC treated as the supplement of the TCSC. When operation of the TCSC is constrained by the inherent limitation of equipment, the adjustable SVC can supply the auxiliary support to improve the overall performance. In order to adapt to the changes of the operating mode and active power of generators, a neural network (NN) is applied to determine the control parameters of the equipment. Analysis and simulation of a case study have proved the effectiveness of the nonlinear control strategy.

Revenue Reconciliation for Spot Pricing: Implementation and Implication

The high capital investment in a power transmission system compared to its variable operating costs is an impediment to the balancing of revenues and costs when the system operates in a spot pricing marketplace. This paper presents a revenue reconciliation method that recognizes the “use of system” by means of power flow tracing. To maximize social welfare, the proposal is that the spot prices remain unchanged but subject to a surcharge, which reflects the location of the node and thus may be considered to be an economic signal for long-term development of the system. The main advantage of the proposal is that it is fair and equitable to every market participant. It is also shown that the power flow tracing concept can also be applied to wheeling transactions for use of the system and that it can handle multiple wheeling transactions simultaneously. The proposed method is illustrated by reference to the IEEE 30-bus test system. The simulation results show its feasibility and potential for electricity market applications.

Online decision-making and control of power system stability based on super-real-time simulation

During power grid cascading failure, the operating point changes frequently, resulting in a mismatch or inaccuracy of online pre-decision making emergency control strategy. To solve this problem, an online power system stability decisionmaking and control method based on super-real-time simulation is presented in this paper. A decision-making and control system is developed in which the three main operation modes, offline pre-decision-making, online pre-decision-making, and real-time control, are all coordinated. To verify the method, ADPSS (advanced digital power system simulator) is used to simulate a digital power grid with about 20,000 buses. The closed-loop operation of the digital power grid and the online decisionmaking and control system are implemented. Test results show that when online pre-decision making control strategies are mismatched, aided real-time control works well and can prevent power grid stability loss.

Co-ordinated nonlinear control of TCSC and SVC in power systems

This paper presents a co-ordinated control strategy, based on the disturbances auto-rejection control (DARC) theory, for the control of thyristor controlled series compensators (TCSCs) and static VAr compensators (SVCs) to improve the power angle and voltage stability of power systems. This novel co-ordinated nonlinear controller possesses a simple structure and performs as an adaptive, robust control scheme without requiring precise information of the power system. The controller can be easily designed and practically implemented and it automatically rejects system disturbances under a wide range of operation conditions. A simulation study based on nine-machine and forty-one-bus power system shows that the co-ordinated control can provide satisfactory control performance and improve power system stability greatly.

Fuzzy logic control of dynamic quadrature booster using reinforcement learning

This paper is concerned with the investigation of a learning fuzzy logic control of dynamic quadrature booster (DQB) to enhance power system stability. A fuzzy logic control strategy is proposed for DQB control and a reinforcement learning technique is employed to optimise the parameters of the fuzzy logic controller according to a given performance index. The parameter optimisation is carried out on-line in real time. Simulation results show a satisfactory learning and control performance provided by this strategy.