Yixin Ni

Nonlinear modal interaction in HVDC/AC power systems with DC power modulation

In this paper investigation of nonlinear modal interaction using the normal form of vector fields technique is extended to HVDC/AC power systems with DC power modulation. The AC-DC interface equations are solved to form a state space model with second order approximation. Using the normal form technique, the systems nonlinear dynamic characteristics are obtained. The proposed approach is applied to a 4-generator HVDC/AC test power system, and compared with the time domain solution.

Discussion of "Application of unified power flow controller in interconnected power systems-modeling, interface, control strategy, and case study" [and reply]

E. Uzunovic et al. comment on the paper by Z. Huang et al. (see ibid., vol.15, no.2, p.817-24, 2000). They point out the importance of adequately representing inverter losses in the model. The original authors reply to the comments.

Study on power system vulnerability assessment based on cascading failure model

The scale-up of power grid arouses wide concerns on the security and stability of power system operation from industry and academia. How to defend power grid against cascading failures and large-scale blackout accidents by identifying weak areas has become one of key issues to be solved in power system research. To study power system vulnerability assessment, a kind of uncertainty analysis method based on cascading failure model is proposed in this paper. According to the cascading failure model, taking the randomness of line power flow and cascading failure spreading mode into consideration, the influence of line outage failure on system vulnerability is analyzed via risk and probability theory. Moreover, the positive correlation between probability of line outage failure and the extent of limit violation of power flow is studied. Finally, simulation results from IEEE 118-bus test system demonstrate the effectiveness and validity of the proposed model and indices.

An Analytical Solution for Voltage Stability Studies Incorporating Wind Power

Voltage stability is one of the most critical security issues which has not yet been well resolved to date. In this paper, an analytical method called PQ plane analysis with consideration of the reactive power capability of wind turbine generator and the wake effect of wind farm is proposed for voltage stability study. Two voltage stability indices based on the proposed PQ plane analysis method incorporating the uncertainties of load-increasing direction and wind generation are designed and implemented. Cases studies are conducted to investigate the impacts of wind power incorporation with different control modes. Simulation results demonstrate that the constant voltage control based on reactive power capability significantly enhances voltage stability in comparison of the conventional constant power factor control. Some meaningful conclusions are obtained.

A novel hysteresis current controller for active power filter with constant switching frequency

A novel hysteresis current controller for active power filter (APF) is suggested which is based on optimal voltage vectors and in the meantime with constant switching frequency. In this controller, the location region of the reference voltage vector is detected quickly by a set of hysteresis comparators through one trial-and-error process. Two appropriate switches are then selected to control the corresponding two line to line currents independently with constant switching frequency. The new method has the advantages of quick response, good current tracking accuracy, and constant switching frequency. Therefore it is very efficient and secure in operation. Computer simulation results show that the new current control method can improve APF performance noticeably.

Incorporating UPFC model into the power system toolbox of the MATLAB for transient stability study

In this paper, power frequency model for unified power flow controller (UPFC) is presented with its DC link capacitor dynamics included. A novel interface of UPFC to AC network for transient stability study is then suggested and realised using Power System Toolbox (PST) of MATLAB. The control strategies for UPFC shunt and series elements are also discussed. Computer results on a 4-generator interconnected test power system show that the convergence and accuracy of the suggested interface are challenging and the suggested interface makes it very easy to incorporate the UPFC model into the conventional transient stability programme. The results also show that the UPFC control strategy had strong impacts on the performance of UPFC.

A multi-agent based autonomous decentralized framework for power system restoration

This paper presents a multi-agent based power system restoration framework, of which the key characteristics can be summarized as autonomous decentralized control and centralized coordination. The power system restoration issue is generally divided into three sub-problems for research purpose, namely, units start-up, network reconfiguration, and load recovery. However, it is far from satisfaction since they are not in line with the factual practice. The proposed framework bears the advantage of taking all these three sub-problems into account instead of emphasizing specific ones. At the same time, a multi-agent based power system restoration model is built with the idea of coordinative optimization. In addition, the framework design can meet the requirements of smart grid development in the future. The IEEE 9-bus test system is used as benchmark to further illustrate the implementation of this framework.

Sub-synchronous resonance analysis of grid-connected DFIG- based wind farms

This paper aims to research the sub-synchronous resonance (SSR) in DFIG-based wind farms connected to an infinite bus via a series-compensated transmission line. The detailed mathematical models and state matrix involving the dynamics of mechanical shaft, generator, transmission line, and the reduced-capacity converters are derived. The PI parameters of the control systems of the DFIG-based wind farm are optimized based on an improved evolutionary programming algorithm, in which all the real parts of the eigenvalues of the test system can be kept negative. Based on the IEEE first benchmark model as example, the eigen-analysis along with participation factors is performed to study the impacts of series compensation level, parameters of PI controllers, and wind speed on SSR.

Small signal stability analysis with high penetration of grid-connected wind farm of PMSG type considering the wake effect

This paper aims to discuss the impacts of high penetration of grid-connected wind farm of permanent magnet synchronous generator (PMSG) type on the small signal stability of power systems with wake effect taken into account. Firstly, the modeling of PMSG as well as its converters is described. The modeling of wind farm wake effect and the corresponding wind farm equivalent model are given in detail. Then, the process of deducing the system state matrix is introduced. Finally, the simulations with multiple scenarios consideration are carried out on the IEEE 3-generator-9-bus test system as benchmark to reveal the impacts of wake effect on small signal stability.

A practical simulation model for event chains search in power system cascading failure

So far, a lot of studies have indicated that the cascading failure is one of the main reasons of causing blackout events took place around the world. Event chain search is one of research methods pertinent to cascading failure. However, the existing strategies are difficult to reveal actual situation. In this paper, the development process of blackout is mainly studied, and a practical simulation model of event chains search is proposed. In the proposed model, which is based on AC power flow solution to search event chains, the solution strategies are trying to mimic the response and failure handling process of the actual power grid. There are mainly not restricting failure numbers in the same layer of event chains, considering the importance of load during load shedding, allowing overload lines to operate in a period of time, the probability of hidden failure related to system parameters, and so on. The simulation results and comparisons are conducted based on a test power system to demonstrate the validity and effectiveness of the proposed model and method. At the same time, some meaningful conclusions are obtained.