Guo Chunlin

Simulation and study on mitigation measures of frequent subsynchronous oscillation with low amplitude at multi-power plants

Subsynchronous oscillation (SSO) with low amplitude that exceeds cumulative fatigue threshold of the generator shaft frequently could significantly reduce the shaft’s service life, which is a new SSO problem that emerges in recent years. According to the real recording oscillograph, the basic reason for frequently over-threshold SSO with low amplitude at multi-power plants was analyzed based on Hulunbuir League system. The sensitivities of the electrical damping to the main electrical parameters in the contributing loop of subsynchronous torsional interaction were calculated. Based on the sensitivities, a simulation method was presented, which was used to excite the same oscillation as the actual case by exerting disturbance on the firing angle. The limitation of wide-band and narrow-band supplementary subsynchronous damping controller (SSDC) for mitigating this kind of SSO was analyzed based on the electromagnetic transient simulation model of Hulunbuir League system. The difference of supplementary excitation damping controller (SEDC) and parallel-form FACTS connected to the generator terminal was compared from the aspects of response time and the ability of damping torque supplying. The analysis indicates that their response time is similar but FACTS has stronger ability of damping torque supplying than SEDC. Time-domain simulation method was used to compare the mitigation effects of SEDC, static var compensator (SVC) and static synchronous compensator (STATCOM). Considering the mitigation effect, the floor space limit of the power plant and so on, STATCOM was considered as the best mitigation measure. A control strategy of cascaded STATCOM for engineering application was presented and the capacity for SSO mitigation as well as output characteristics was analyzed. The analysis indicates that STATCOM using the proposed control strategy has better mitigation effect and output characteristics with smaller capacity.

A new analysis method for compensation strategy of DVR and minimum energy control

The compensation strategy of the dynamic voltage restorer (DVR) has significant impact on the compensation result. A new analysis method which is based on pre-sag load voltage is proposed in the paper. The new method chooses the pre-sag load voltage phasor as the reference phasor and put the centre of the limit compensation voltage circle at the terminal of the system voltage phasor. It not only has definite physical concept but also is convenient to determine the compensation range. More importantly, it could be applied to unbalanced load. The basic theory and process of the new method was explained and compensation strategy of the three-phase DVR was analyzed. Using the new method, the minimum energy compensation strategy was realized through the simulation when unbalanced voltage sag happened in the case of the unbalanced load. The conditions for zero active power compensation was also analyzed which could prolong the compensation time of DVR. The simulation result shows the correctness of the theoretical analysis.

Impact of electric vehicle charging on power grid

This paper analyzes the prospect and the associated impact on power grid of large-scale applications of electric vehicles, and the countermeasures to reduce such kind of impact. Electric public bus, electric mini-car and electric passenger car have good prospects, so that they will became a great charging load. It also will produce a variety of disadvantage effects to the distribution network under the natural charging mode. These effects can be reduced by rebuild and expansion of the distribution network, governance of technology and orderly utilization. Orderly utilization can improve acceptance ability of power grid greatly without adding new facilities, so it should be a prior method in use.

A Stages Compensation and Control Strategy for Series Power-Quality Regulator

A novel stages compensation and control strategy suitable for a series power-quality regulator (SPQR) based on a voltage-source converter (VSC) inverter is proposed in which steady operation stage, overrun operation stage, and recovery operation stage are included. The presag voltage compensation, minimum energy compensation, and maximum energy compensation strategies are adopted to the three stages, respectively, based on the characteristics of the three stages. The double closed-loop control with feedbacks from load voltage and filter capacitor current is used in each stage. Moreover, the dc capacitor voltage regulation control for SPQR is applied in the steady operation stage. To avoid switching disorder during the stage change, a criterion based on voltage magnitude of the dc capacitor and source side is proposed. Simulation results in EMTDC/PSCAD show that voltage disturbances of the steady and transient state can be compensated by using the proposed compensation and control strategy and, at the same time, the dc capacitor voltage can be stabilized. The proposed strategy makes it possible to use the dc capacitor as the energy source of SPQR with the merit of a simple main circuit topology.

Transient stability control of TCSC

This paper studies the strategy of thyristor controlled series compensation (TCSC) to damp power swing in interconnected systems. In interconnected network, TCSC must operate in response to inter-area main swing model, and its capacitive impedance must switch to the largest when the rotated speed deviation between equivalent centers of inertia is positive, and to the smallest when negative, so as that the oscillations will be calmed down fastest.

The multi-objective voltage stability coordinated control strategy of FACTS

Flexible Alternate Current Transmission System (FACTS) is an important way to improve the utilization of the electrical equipments and system voltage stability. The paper mainly controls the voltage stability in adjacent areas of power system, analyzes the characteristics of different voltage stability problems, and consider the characteristics and control laws of medium and long-term voltage stability and transient voltage stability, then puts forward a multi-objective voltage stability coordinated control strategy that based on SVC ( Static Var Compensator) and TCSC (Thyristor Controlled Series Compensation). Besides, the paper designs appropriate structure controllers of SVC and TCSC, and verifies the control strategy which can be an effective solution to controlling needs and achieve coordination of different voltage control functions in simulation platform.

The study of complex torque coefficient approach's realization based on PSCAD/EMTDC

The method, complex torque coefficient approach, which is used to analyze the sub synchronous oscillation (SSO) problems with the help of PSCAD/EMTDC are recommended. While how to achieve the complex torque coefficient approach by time-domain simulation method effectively has not been specified. With the simulation study of different systems like HVDC system, AC system with series compensation device, etc, amplitude, injection time and additivity of injected oscillating torques are discussed. A set of conclusion to the method is drawn for reference.

Research of analysis method of power system real-time digital simulation error

Simulation accuracy has great effects on power system analysis and control. In this paper, in order to analyze the power system real-time digital simulation error, prony algorithm is used to extract the features of signal. Based on the analysis results of this algorithm, residual similar index was used to characterize the global error of simulation, and then frequency, damping and amplitude similar indexes were employed to represent features error of transient signals respectively. A calculation program for power system real-time digital simulation accuracy evaluation was employed to analyze the measured waveforms and results of fault simulations on RTDS. Simulation results have proved the validity of the proposed analysis method of power system real time digital simulation error.

Minimum Energy Compensation Strategy and Characteristic Analysis for Dynamic Voltage Restorer

The minimum energy compensation strategy and its characteristic for Dynamic Voltage Restorer (DVR) considering the equipment’s voltage limitation are discussed. The strategy when the injection voltage is under or especially above the voltage limitation is proposed. And the detailed expressions of the strategy which show that the compensation effect is influenced by three factors -- magnitude of the system voltage sag (or swell), power factor of load and limitation value of the compensation voltage were given. The compensation characteristic that DVR could operate in zero, minimum or voltage limitation minimum active power exchange state if the three factors satisfy certain relations was analyzed. Last, the quantitative curves of the compensation characteristic were given. The analysis results will be helpful to compensation strategy design and engineering practice for DVR.

The study of principle and applicability of complex torque coefficient approach

The paper analyzes the basic characteristics of the stable response of electromagnetic torque of synchronous generator, deduces the main calculation procedure of complex torque coefficient (CTC), and validates them by simulation. The study shows, the apply condition of CTC approach is the linearity and stability of the system. Under a stable system, the CTC can be calculated by the transfer function, and there is not a rigid CTC under an unstable system. The CTC based quasi-stable state model is an approximation in low frequency rang of the CTC discussed former, so they are different coefficients. The model of linear component does not influence the application of CTC approach, but will lead to error.