Baorong Zhou

An Emergency-Demand-Response Based Under Speed Load Shedding Scheme to Improve Short-Term Voltage Stability

The dynamics of load, especially induction motors, are the driving force for short-term voltage stability (STVS) problems. In this paper, the equivalent rotation speed of motors is identified online and its recovery time is estimated next to realize an emergency-demand-response (EDR) based under speed load shedding (USLS) scheme to improve STVS. The proposed scheme consists of an EDR program and two regular stages (RSs). In the EDR program, contracted load is used as a fast-response resource rather than the last defense. The estimated recovery time (ERT) is used as the triggering signal for the EDR program. In the RSs, the amount of load to be shed at each bus is determined according to the assigned weights based on ERTs. Case studies on a practical power system in China Southern Power Grid have validated the performance of the proposed USLS scheme under various contingency scenarios. The utilization of EDR resources and the adaptive distribution of shedding amount in RSs guarantee faster voltage recovery. Therefore, USLS offers a new and more effective approach compared with existing under voltage load shedding to improve STVS.

Continuous-Mass-Model-Based Mechanical and Electrical Co-Simulation of SSR and Its Application to a Practical Shaft Failure Event

To accurately evaluate the damage caused by subsynchronous resonance (SSR) to a turbo-generator shaft, we have developed a novel mechanical and electrical co-simulation method, wherein the continuous-mass-model-based mechanical system is simulated in a close-loop or coupled way with the electrical power system so that the detailed torque profile along the shaft can be obtained for further analysis of fatigue loss of life. Nonlinear mechanical damping is also modelled to reflect the torsional inter action more precisely. The co-simulation is implemented using commercial electromagnetic transient program and finite element analysis software. A special dynamic data exchange mechanism is designed to make it proceed very efficiently. As a case study, the proposed method has been applied for the root-cause analysis of a recent shaft failure incident in an India power plant. The extensive simulation results have fully demonstrated its effectiveness in acquiring fine-grained torques and its advantages over previous methods. Finally the direct cause of the shaft failure is identified to be the sustained torsional interaction (one type of SSR) between the turbo-generator and its connected series-compensated power system.

Demand-Response-Based Distributed Preventive Control to Improve Short-Term Voltage Stability

Short-term voltage stability (STVS) issues pose a significant threat to modern power systems. Based on extensive observations of STVS events, their primary predictors can be summarized as summer peak load levels with a high percentage of air conditioners, and occurrence of faults caused by thunderstorms or other emergencies. The predictors can be used as the triggering signals for preventive voltage control (PVC). Previous design of PVC dominantly focuses on the improvement of long-term voltage stability, and demand response (DR) resources are not fully utilized. In this paper, a DR-based PVC method is proposed to improve STVS if the day-ahead forecast indicates that the system will experience high STVS risk. We formulate PVC as an optimal power flow problem and solve it in a distributed manner, where the central controller (CC) and the local controllers jointly compute an optimal schedule. Thus, the computational burden of the CC is reduced compared with the existing centralized algorithms, and customer privacy is well protected. The simulation results not only verify the advantage of the improved index, but also demonstrate the fast convergence and effectiveness of the distributed PVC, which, therefore, offers an alternative approach to improve STVS.

Influence of HVDC control on HVDC reactive power overshoot after large disturbance

This paper presents a study of HVDC reactive power overshoot which presents unfavorable reactive load disturbance to AC power system and may result in transient voltage instability. The key idea of this paper is HVDC control has a strong effect on HVDC reactive power recovery characteristic. Based on an actual HVDC project and the PSCAD/EMTDC simulation platform, this paper systematically studies the influence mechanism and pathway of HVDC control on HVDC reactive power overshoot after the AC bus in inverter station is subjected to faults, and also refines related influence rules of control parameters. By keeping firing angle unchanged, the inherent HVDC reactive power recovery characteristic is analyzed, and the key control links that affect reactive power overshoot are excavated sequentially, containing constant extinction angle control (CEA), voltage dependent current order limiter (VDCOL) and transient current control function. According to the simulation results, the influence rules of parameters of the above-mentioned key control links on the overshoot magnitude are summarized, which contributes to HVDC control strategy optimization of enhancing the transient voltage stability.

Voltage-sag-severity-index based size planning of shunt capacitor banks to improve short-term voltage stability

Short-term voltage stability problem (STVSP) is a serious threat to receiving-end power systems which are short of reactive power reserves. This paper discusses how to plan the size of shunt capacitor banks (SCB) in a receiving-end power system to achieve a maximum voltage stability margin. The local generators and SCB are two important reactive power resources in receiving-end power systems. The size of SCB affects the reactive power output of generators in steady state. Furthermore, it impacts the voltage stability margin. It is necessary to obtain the best size of SCB, because an over-design or an under-design is not capable to eliminate the STVSP. We propose a voltage sag severity index to determine the size of SCB in this work. The proposed method has been tested on a typical receiving-end power system to work out the best size of SCB and then to verify its effectiveness through time-domain simulation.

A method of evaluating the transfer capacity for power system considering of the power injection and the power grid topology

The transfer capacity is a varying value for a certain power system, which may be changed with the evolution of the system. Although the topology of a power system is usually designed in an optimizing way to maximize the transfer capability, the capability itself will be inevitably weaken because of a time lag between the increasing load and the enhancement of grid topology. Due to the lack of qualification tools to deal with the varying transfer capability, related topics are seldom to be discussed. This paper proposes a new method to evaluate the transfer capacity, in which the mechanism describing the mismatch can be boiled down to a new index named Spatial Characteristic Length Ratio (SCLR) considering of both the power injection and the power grid topology. By calculating the index SCLR, the transfer capability as well as the factors affecting it can be analyzed, which has an important meaning for power grid planning personnel. The method is used to analyze the transfer capacity of IEEE 4-node system, IEEE 9-node system and IEEE 162-node system, each under different modes of operation. Calculation results has proved the validity of this concept, and the related method is effective.

A study on HVDC user-defined modeling in PSS/E

Power System Simulator for Engineering (PSS/E) is one of the most extensively used power system simulation software in the globe. In this paper, by modeling a high-voltage direct-current (HVDC) transmission system, an in-depth study of its user-defined function in PSS/E is illustrated. Furthermore, comparing the simulation curves of user-defined HVDC in PSS/E with those simulated by PSCAD/EMTDC and PSD-BPA, the correctness of the HVDC user-defined model, the feasibility as well as the practicability of PSS/Es user-defined function are validated. Similarly, the modeling method proposed in this paper provides guidance for modeling of other complex two-terminal and multiport components applied to power systems. The experimental results prove the accuracy of the HVDC user-defined model as well as the practicability of the method, by which the feasibility of two-terminal component modeling in PSS/E is also verified.

A wide-area var-voltage control method for generators to improve short-term voltage stability

The short-term voltage stability of receiving-end power systems is attracting considerable attention. One of effective solutions to this issue is to provide adequate dynamic reactive power for the system. This paper puts forward a reactive power-voltage (var-voltage) control method based on the wide area measurement system (WAMS). The method can regulate the voltage reference value of generator excitation system in real time with the WAMS measurement results, so generators can output more dynamic reactive power to support transient voltage after large disturbance. The proposed control method involves a continuous closed-loop control in consideration of data transmission and processing time delay. Effectiveness of this method is verified by simulation of the actual power system.

Investigating the influence of types and parameters of excitation systems on the dynamic reactive power reserve of synchronous generators

It is of great interest to fully utilize the dynamic reactive power reserve of synchronous generator, which is generally high in capacity and cheap in cost, to meet the demand of reactive power during the serious faults. The Guangdong power grid is used as the target system to evaluate the influence of the types and parameters of excitation controllers on the dynamic reactive power reserve of a generator. Two typical excitation systems, i.e., the static self-parallel excitation and the AC rotating excitation, are investigated in both frequency and time domain. Sensitivity analysis has been conducted on the linearized system model to obtain quantitative results and some key parameters (gains and time constants of AVR) affecting the dynamic reactive power reserve are identified, laying a foundation for optimizing excitation parameters of generators.

An integrated high side var/voltage control for improvement of transient voltage stability

Transient voltage stability is becoming one of the urgent issues of larger-scale receiving-end power systems. The root cause of voltage instability after a fault occurs is that the reactive power supply cannot meet the demand. The reactive power reserve of synchronous generators has relatively large capacity while much lower cost. To take full advantage of the dynamic reactive power reserve, this paper propose an integrated high side var/voltage control (IHSV2C) for power plants in receiving-end power systems. The IHSV2C can regulate the voltage at a point beyond the generator terminals by providing terminal voltage reference value for the excitation system, meanwhile, it can make the whole plant offer the largest dynamic var support in a coordinated way. Firstly, The IHSV2C control equation is introduced in detail. Secondly, a test receiving-end system is established in PSCAD / EMTDC according to the typical parameters of China Southern Power Grid. Thirdly, the effect of IHSV2C is verified in simulation. In addition, the start time and end time of IHSV2C is discussed.