Yipeng Dong

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.

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.

A STATCOM control strategy in support of direct on line starting of large induction motor in offshore oilfield power systems

The offshore oilfield power system is a independently operated weak AC system, composed of gas-turbine generators, cables and large loads. One of its serious problems is that the direct on line starting of a large induction motor (such as the water-injection pump) results in sharp increase of reactive power requirement and thus causes deep drop of the bus voltage, which in turn lead to overload of generator and drop-off of load at the same bus. To deal with the problems, an improved dynamic var control strategy based on STATCOM is presented in this paper. The strategy is an improvement on the conventional var/voltage regulation. Two novel control functions, i.e., the motor-starting var compensation and the motor-starting switch control, are designed and combined with the var/voltage regulation. Thus the motor and STATCOM are controlled to go through the states of pre-starting, start-preparation, motor-starting and start-completion successively and smoothly. The control strategy is applied to an offshore oil rig power system on Southern China Sea. The simulation results show that it is very effective. By providing sufficient dynamic var support for the direct on line starting of large induction motors (water-injection pump) with a STATCOM of appropriate capacity, the bus voltage is maintained, the safe operation of the gas-turbine generator and the loads is guaranteed.

Improving AGC Performance of Coal-Fueled Thermal Generators Using Multi-MW Scale BESS: A Practical Application

With the increasing penetration of renewables in power system, frequency regulation is becoming a big challenge for conventional coal-fueled thermal generator units. To address this issue, more stringent criteria of automatic generation control (AGC) has been established by grid operators to regulate the generators. However, old-fashioned units can hardly meet the requirements and thus are subject to penalty. In this paper, an approach of using battery energy storage systems (BESS) for coordinated frequency regulation is proposed to improve the AGC performance of such generators. A demonstration project with a 2 MW BESS has been commissioned at Shijingshan Thermal Power Plant, Beijing. The basic principles, coordinated control strategy, technical performance as well as economic benefits have been presented and validated with field test results. As a pioneer of its kind in China, this project offers an excellent demo of utilizing BESS for improving AGC performance under the context of high renewable integration.

Improving AGC performance of a coal-fueled generators with MW-level BESS

With the increasing penetration of renewables in power system, secondary frequency regulation (SFR) is becoming a big challenge for conventional coal-fueled thermal generator units. To address this issue, a stricter AGC criteria to regulate the generators has been but forward. However, old-fashioned units can hardly meet the requirements and thus are subject to penalty. In this paper, an approach of using BESS for coordinated SFR is proposed to improve the AGC performance of such generators. A demonstration project with a 2 MW BESS has been commissioned at Shijingshan Thermal Power Plant, Beijing. The basic principles, coordinated control strategy, technical performance as well as economic benefits were validated from field measurement. This project, as a pioneer in this domain in China, offers an excellent demonstration of utilizing BESS for improving the performance of SFR under the enviroment of high renewable integration.

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 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.