Static Voltage Stability Analysis under Different Accident Conditions

With the development of power electronic devices, the research of Flexible AC Transmission System (FACTS) in regulating line power flow and improving static voltage stability is more in-depth. This paper discusses the static voltage stability of power grid with new energy taking into account the distributed static series compensator (DSSC), and evaluates the power flow control efficiency of DSSC from the aspect of system static voltage stability. First, the equivalent circuit and working principle of DSSC are analyzed, and the power flow equations are obtained based on the equivalent power injection model of DSSC; Secondly, based on the power flow calculation equation, the efficiency evaluation index that can reflect the static voltage stability of the system is proposed, and the impact of new energy with different capacities on the static voltage stability is calculated and analyzed; Finally, the proposed method is verified by the simulation analysis of IEEE-24 bus system and actual power grid: the proposed method is fast and simple, and has certain engineering application value.

Optimal allocation of Var source involves optimal location and determination of the size of the Var source. The purpose is to find the right tradeoff between the investment cost of the new Var source and the benefits in system operation derived from the presence of the additional reactive compensators. Traditionally, the locations for placing new Var sources were either simply estimated or directly assumed. This paper categorizes the literature relevant to optimal allocation of shunt dynamic Var source SVC and STATCOM, based on the voltage stability analysis tools used. Those tools include static voltage stability analysis ones such as P-V and V-Q curve analysis, continuation power flow (CPF), optimization methods (OPF), modal analysis, saddle-node bifurcation analysis, and dynamic voltage stability analysis ones such as Hopf bifurcation analysis and time-domain simulation. Static voltage stability analysis techniques are also used in dynamic Var planning. At the end of the paper, the advantages of static and dynamic voltage stability analysis tools are summarized. (7 pages)

With the rapid development of the Chinese power grid, its dynamic behavior becomes increasingly complex. How to prevent voltage instability and voltage collapse at the receiving-end system has become an urgent problem at present. To prevent voltage collapse, the key is to determine the stability margin at the operating point of the power system, and the load margin is the most basic and widely accepted voltage stability margin indicator. Considering the role of medium and long-term slow-acting elements in the actual system that affect the system voltage stability, this paper combines the static voltage stability analysis with the transient, medium, and long-term dynamic voltage stability analysis based on whole process simulation, and uses time-domain simulation methods to evaluate the voltage stability of the actual system. By analyzing the various dynamic characteristics of generator overexcitation limits and transformer on-load regulation on the development process of voltage-collapsing accidents, this paper proposes an evaluation index for the whole process dynamic stability margin of the receiving-end system. On this basis, the whole process dynamic defense system of the power grid is also constructed. The proposed method is validated by using a provincial power grid in China as an example.

The large-scale integration of photovoltaic (PV) power can bring a greatly negative influence on the grid-connected system’s voltage stability. To study the static voltage stability (SVS) of PV grid-connected systems, the traditional SVS index, L-index, was re-examined. It was firstly derived and proved that the PV active output Ppv is proportional to the voltage phase angle of the PV station’s POI (Point of Interconnection), based on a simplified two-node system integrated with a PV station operating in PV (active power—voltage) mode or PQ (active power—reactive power) mode with unit power factor. Then a novel voltage stability sensitivity index LPAS-index was proposed that takes the derivative of the L-index with respect to the POI’s voltage phase angle, so as to reflect the influence degree of Ppv on the SVS of each load node. A SVS zoning analysis method for the PV grid-connected system was designed according to the classification results of load nodes based on the proposed LPAS-index, the power grid can be zoned into three kinds of areas that reflect different correlations between the SVS and Ppv: strong correlation, moderate correlation and weak correlation. Since the LPAS-index is less impacted by Ppv, the SVS zoning results are relatively unchanged. On the basis of a classic 14-node system with PV, the practicability of the zoning analysis method was verified. The simulation results show that the PV access point in general falls within the strongly or moderately associated area with Ppv. When most of the load nodes fall within the weakly associated area with Ppv, it is not necessary to consider the impact of Ppv and load power is still the main influencing factor on the SVS. In the multi-PV case, owing to the expansion of areas more affected by Ppv, an excessive Ppv can cause adverse influence on the SVS of the whole power grid; and an effective PV power-shedding measure is proposed to solve this problem. The proposed SVS zoning analysis method can be used for reference by power grid dispatchers.

A reactive power planning method based on improved particle swarm optimization (IPSO) is presented. This method can improve the quality of initial particles and eliminate the boundary oscillation in traditional particle swarm optimization. The voltage stability margin index (VSI) is used to rank the load buses. Then reactive power compensators are installed on the buses with weak voltage stability. The maximization of voltage stability margin is included in the objective function of reactive power planning to improve system voltage stability. Its application in IEEE 30‐bus system illustrates that the real power loss is depressed with voltage stability margins increased simultaneously. Copyright © 2009 John Wiley & Sons, Ltd.

Comprehensive evaluation of voltage stability based on EW-AHP and Fuzzy-TOPSIS

This paper presents a methodology for selection of location of static VAR compensator based on static voltage stability analysis of a power system. The analysis presented here uses the L-index of load buses. This includes voltage stability information of a normal power flow in the range of 0 to 1(no load to voltage collapse). An approach has been developed to select a suitable size and location of static VAR compensator as required in an IEEE-14 bus system. The STATCOM and SVC are used in the continuation power flow process for static voltage stability analysis to enhance stability margin.

With the large-capacity DC transmission and various types of power electronic equipment widely used in power systems, the voltage stability problem has once again been closely watched. Voltage stability can be divided into static, transient and dynamic stability. The static voltage stability accident, Generally speaking, the grid load is slowly increased, causing the load terminal bus voltage to slowly drop. Near the critical value, any small disturbance will cause an irreversible sudden drop in the load bus voltage, this is a voltage collapse. The article first analyzes the static voltage stability margin index and related technical standards. Comparative analysis of several static voltage stability margin calculation methods. Second, take the planning grid of a very large city as an example. Using PSS/E software to simulate the static voltage stability margin of the system under various conditions. Analysis of the static voltage stability margin characteristics of the urban power grid. And the reactive power compensation optimization scheme is proposed for the weak voltage link.

In the static voltage stability analysis, the dynamics of the components in the power system are not considered and the creditability of the results obtained is more or less affected. Based on the continuation algorithm, this paper investigates the dynamic stability using the small signal analysis method. The stability type is distinguished by the modal participation factors of the state variables. Firstly, the equilibrium manifold is traced by a locally parameterized continuation method. Then, the dynamic stability of each equilibrium point is analyzed using the small disturbance analysis method considering the components dynamics. The system state matrix is calculated numerically by applying small perturbation to each state in turn. Simulation results show remarkable differences in the stability critical points obtained by the small signal perturbation dynamic analysis and the static analysis. Also a region of dynamic voltage stability is found in the lower portion of the PV curve.

An electric power system sometimes fails because of disturbances that occur unexpectedly, such as the uncontrolled loss of load that developed from cascading blackout. Which make stability through a little of under voltage load shedding should work. The development of phasor measurement unit(PMU) makes network supervision possible. The information obtained from PMU is synchronized by global positioning system(GPS). There are many real-time algorithms which are monitoring the voltage stability. This paper presents the study on the VILS(Voltage Instability Load Shedding) using PMU data. This algorithm computes Voltage Stability Margin Index(VSMI) continuously to track the voltage stability margin at local bus level. The VSMI is expressed as active and reactive power. The VSMI is used as an criterion for load shedding. In order to examine the algorithm is effective, applied to KEPCO system.

Power systems are being operated close to their voltage security limit which makes their reliable operation more challenging. Voltage stability has been a major problem faced by utilities now a days. Many blackout have been reported around the world related to voltage instability problem. Many voltage stability analysis methods have been presented up till now. Here artificial neural network based approach for voltage stability analysis of power system is presented. Voltage stability analysis of power system has been done in terms of voltage stability margin. Voltage stability margin is calculated by estimating distance from current operating point to maximum voltage stability point according to loading parameter. Principal Component Analysis is an efficient feature selection method used here to find few input variables required to estimate voltage stability margin with sufficient accuracy and execution speed. Major drawback of previously published work is they need to train new neural network whenever change in system topology occur which is very time consuming process. Therefor in this paper single ANN has been prepared for estimating VSM for several system configuration. Feed forward neural network and radial basis function neural network have been used for voltage stability analysis of power system. The final result indicate that proposed method provide an efficient and compact ANN model for different system configuration as well as base case operating condition with lowest possible input required.

The power system is experiencing a higher penetration of renewable energy generations (REGs). The short circuit ratio (SCR) and the grid impedance ratio (GIR) are two indices to quantify the system strength of the power system with REGs. In this paper, the critical short circuit ratio (CSCR) is defined as the corresponding SCR when the system voltage is in the critical stable state. Through static voltage stability analysis, the mathematical expression of the CSCR considering the impact of GIR is derived. The maximum value of CSCR is adopted as the critical value to distinguish the weak power system. Based on the static equivalent circuit analysis, it is proved that the CSCR is still effective to evaluate critical system strength considering the interactive impact among REGs. Finally, we find that the GIR can be neglected and the SCR can be used individually to evaluate the system strength when SCR>2 or GIR>5. The correctness and rationality of the CSCR and its critical value are validated on ADPSS.

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In new distribution systems, the proportion of distributed renewable energy generation is gradually increasing. Existing methods for static voltage stability analysis struggle to accurately reflect the impact of distributed generation output. First, this paper introduces robustness indices to quantify the impact of active and reactive power output fluctuations from distributed generation on load buses, based on the L-index. Second, a static voltage stability indicator system is constructed. Based on the characteristics of the indicators, definitions for voltage stability weak bus, active power vulnerable bus and reactive power vulnerable bus are established. Third, an optimization model to enhance static voltage stability is developed specifically for these buses. The Pareto front of the solutions obtained by it based on the NSGA-III algorithm includes the optimal solutions under different index preferences, and then it is achieved by adjusting the power output of the power source. Last, the effectiveness of the indicator system and the enhancement methods are validated through the IEEE 118-bus system.

To address the issue of static voltage stability in distribution networks with high proportion distributed pv generation(DG) during fault, this paper establish a fault equivalent model which consider the characteristics of PV equipment during low-voltage ride-through(LVRT). The model derives the constraints on the PV current phase angle required to maintain static voltage stability. Subsequently, the impact of DG capacity at the point of common coupling and the electrical distance between the distribution network and the AC power source on the PV output current phase angle constraint is quantitatively analyzed. On this basis, a current control method is proposed: when there is a voltage stability risk in the system, the active and reactive current injection from the PV is adjusted, while under no voltage stability risk, the low-voltage ride-through control strategy remains unaffected. Finally, simulation examples based on the Matlab/Simulink testing platform verify the validity of the analysis and the proposed control strategy.