Shenghu Li

Reliability Analysis of Converter Valves for VSC-HVDC Power Transmission System

In this paper reliability model of the converter valves in VSC-HVDC system is analyzed. The internal structure and functions of converter valve are presented. Taking the StakPak IGBT from ABB Semiconductors for example, the mathematical reliability model for converter valve and its sub-module is established. By means of calculation and analysis, the reliability indices of converter valve under various voltage classes and redundancy designs are obtained, and then optimal redundant scheme is chosen.

Improved Parametric Estimation of Logistic Model for Saturated Load Forecast

The load saturation estimation helps to quantify the final load consumption of a given area, and avoid unnecessary investment to the transmission and distribution facilities. There are generally two methods to estimate the saturated load, based on the load curve and the spacial load distribution respectively. With the historical load instead of load classification data, the Logistic curve, i.e. the S curve, is more suitable to extrapolate the load curve, and forecast the saturated load consumption. In the existing literatures, the parametric estimation of the Logistic curve is based on randomly selected 3 or 4 load data with equal intervals, and can not avoid abnormal or ill data. In this paper, improved parametric estimation methods are proposed. With the average value or the largest correlation index is applied to find the parameters of Logistic curve. The numerical results among the proposed and existing methods are presented, and the forecast feasibility for different load increase stages are discussed.

Reliability evaluation of distribution system with wind farms based on network-equivalent method

One of the most important issues in the development and utilization of wind-based distribution generation is to quantify the impact of wind power on distribution system reliability. This paper proposes how to deal with wind generators connected to sub-feeders of distribution system in reliability evaluation by means of the network-equivalent approach. The random nature of wind power, which is quite different from the conventional substations generation can not be ignored in reliability assessment. In this paper, wind power uncertainty caused by wind speed is investigated by dividing wind speed into intervals described by the mean output and interval probability. Then load importance is studied by classifying loads into various grades, each with different weight coefficients. It is assumed that wind power supplies to customers on sub-feeders with wind generators within the limits of its random capacity judging by the load grades. The impacts of wind power capacity and different integration locations on distribution networks reliability are also researched. Reliability indices are calculated considering wind generation uncertainty and load importance, revealing the remarkable influence of wind power on distribution system reliability.

Short Term Reliability Analysis for Protection System with Average Unavailability

In the short-term reliability evaluation, the state probability and unavailability are time-dependent. Remarkable error will be introduced by using the steady-state unavailability in probabilistic simulation. In this paper the analytical solution to the instantaneous state probability is derived, and the average unavailability during the evaluation period is proposed. An estimation to the average unavailability is quantified considering the time elapse from the latest transition. The proposed algorithm is validated by a short-term reliability evaluation to the protection system, and influences of the initial state, the evaluation period, the transition rate, the latest transition are studied.

Var dispatch and voltage control with feasible setting to controllable shunt compensators

Traditionally, voltage control capability of the SVCs is described by PV bus model with full capacity available. In this paper, based on the sensitivity model for var dispatch and voltage control for SVCs, contributing factors, e.g. the available control margin, slope, reference voltage, static voltage characteristic of the power system, are studied. The numerical results show that: 1) voltage control capability of SVC is dependent on the var reserve margin, weakened by static voltage characteristics of the load and system, and improved by var dispatch with synchronous machines; 2) controllable voltage drop with limited var reserve is improved by higher slope or lower reference voltage, thus to yield less var output and coordinate SVCs at different locations with different operation conditions.

Equivalent reactance model for multi-module TCSC in power system probabilistic analysis

The thyristor controlled series compensator (TCSC) is possible to improve transmission capability and power system reliability. In the existing transmission capacity model, the state availability and total reactance are decided by the line flow. The line flow is however dependent on the network configuration, and unknown before the probabilistic sampling in reliability analysis. In this paper, the equivalent reactance of the state space of the series-compensated line is defined, which is decided by the control object in short-term analysis, and discrete after the forced outage instead of continuous over the controllable range. The proposed model is applied to power system probabilistic load flow analysis. By comparing the mean, variance and distribution of the line flow, it is found that with desirable compensation degree and TCSCs availability, the series compensated line may have similar power transmission capability as parallel lines.

Research on Under-Voltage Load-Shedding to Prevent Voltage Collapse of Electric Power System

A voltage collapse is defined as the process by which voltage instability leads to a very low voltage profile in a significant part of the system. Under-voltage load-shedding is an efficient scheme to prevent voltage collapse. The paper analyses the mechanism of under-voltage load-shedding preventing voltage collapse, compares distributed load-shedding scheme with centralized scheme, introduces under-voltage load-shedding scheme employment at home and abroad, points out that how to work out a proper plan to efficiently prevent voltage collapse, select the time steps, site and amount of load-shedding lies on the conditions of the special power system itself.

Low-frequency oscillation characteristics of wind power systems with interface including the stator dynamics

To quantify the oscillation characteristics of wind power systems, the stator dynamics of the induction generators is included. An interface including the stator voltage angle is given, changing the network parameter to the dq frame, which proves that the stator dynamics can not be ignored for practical systems with limited synchronous capacity. But the stator parameters can not be chosen as the state variables. The steady-state and dynamic stability models are derived with the proposed interface. It is found that the instable modes in wind power systems do not satisfy the electromechanical loop participation ratio, and can not be classified as the interarea modes among the synchronous generators. The participation factors shows the dependency of the oscillations on the induction generators, which is validated by the time-domain analysis. The dynamic response of the synchronous generators and the loads further validates that the oscillations are close to voltage oscillation instead of angular instability.