Excerpt

## Table of Contents

Abstract

1 Introduction

2 Structure, Principle & basic goals of WAMS

2.1 Structure

2.2 Principle

2.3 Basic functions

3 Voltage Stability

4 WAMS to transient voltage stability

5 Voltage stability index

5.1 Simple index

5.2 Stability Area

5.3 Dynamic safety area

5.4 Research on voltage stability region

5.5 Voltage stability margin

6 Transient detection with WAMS

7 Summary

Reference

## Abstract

Wide-area measurement systems(WAMS) is a system to measure the parameters of the power system and deal with abnormal cases. Transient voltage stability is the ability that the system keeps the voltage normal after fault, which is very important to the power system. In this paper, voltage stability index and the theories on voltage stability as well as WAMS' functions are proposed. The use of WAMS can help system to make quick measures to keep the voltage stability.

## 1 Introduction

Nowadays, the researches on WAMS develops very fast, for the 2 needs of power system[l]: a) the synchronism of time; b) the wide area of space. To satisfy the need of power system, people begin to research Phase Measurement Unit (PMU) and WAMS. PMUs can realize synchronism of time, which is different from Remote Terminal Unit (RTU). So, we can get phases of each node and buses. WAMS is composed of PMU which gather information, then transfers to the center.

## 2 Structure, Principle & basic goals of WAMS

### 2.1 Structure

WAMS is mainly composed of PMUs and center control system. The structure is shown in Fig 1.

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Fig 1 WAMS's structure

Center control system is in the control center, while PMUs are installed in the substation.

### 2.2 Principle

PMU sub stations installed in power plants and substations will be packaged with GPS time scale data and transmits to the data center via a high speed communication network. Data center synchronously processes and store the phase of each sub station, then calculates and gets the unit and the bus' voltage, phase angle, relative units and line power etc. , realizes collection and monitoring of dynamic data of power network, and offers important data sources for monitoring, fault analyzing and controlling the power system. Nowadays, WAMS' functions are turning from only monitoring to online dynamic security assessment, auxiliary pre decision control, emergency control etc.

### 2.3 Basic functions

WAMS has 3 basic functions:

1) WAMS has solved the problem of measurement; It has played a good role in improving the accuracy of state estimation of power system and running status of realtime monitoring system.

2) The key research point is the control of dynamic and transient status.

3) The final goal is to realize wide area closed loop control for power system, and apply powerful safeguard of power system.

WAMS focuses on the following things^{[2]}:

1) Improved power system monitoring and visualization to assist power system operators' real time awareness and help them to prevent grid collapse through better recognition and response to evolving grid events.

2) Authentic system parameter used in power system models and analytical tool to design and operate a more reliable grid.

3) Enhance grid output and application of existing grid assets.

4) Quick and better analysis of a disturbance that impacts the power system especially one that results in a blackout.

## 3 Voltage Stability

The definition of voltage dips is the supply voltage sudden drop to the 0.1 p.u. and 0.9 p.u. of the declared voltage then followed by a voltage recovery ^{[3]}. It usually last for a time between 10 ms and 1 minute just as what has been shown in the Fig. 2. Short circuits and earth faults are the main cause of voltage dips at the wind turbine terminals. If the Wind Farms don't have the ability to withstand voltage dips for a certain time, they will be disconnected from the system and then it will have great impact on the power system and may cause a cascading voltage fall and finally result in the breakdown of part or all the power system.

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Fig. 2 Voltage dips

## 4 WAMS to transient voltage stability

Transient data input can be provided by WAMS. It can calculate time-to-time current flow, and form accurate time-to-time operation conditions. At the mean time, the WAMS system can contain many databases( such as history, time-to-time etc), which can offer quick reliable transient data. Then, the center control system calculates the transient voltage stability index, and reach a conclusion whether it's the fault and whether the fault is serious.

Voltage Stability Monitoring^{[2]}:

Synchrophasor technology facilitates monitoring, prediction and managing of the voltages in the transmission grid. Most transmission lines are operating near their voltage stability limit. By use of Synchrophasor technology the grid operator can in advance visualize the symptoms of voltage collapse and thereby prevent it, when the transmission line voltage stability limits are reached or exceeded.

## 5 Voltage stability index

### 5.1 Simple index

Voltage stability is the ability of the system to maintain a stable voltage when power system is subjected to a disturbance. There isn't a universal criterion on transient voltage stability. Reliability norm proposed by Western Union power system in the United States says, if only one element fails, the time that transient voltage is lower than 80% of normal value shouldn't be longer than 400 ms; if 2 elements fail, the time shouldn't be longer than 800 ms. In China, the index recommended is, after the fault clears, if the voltage below 0.75 p.u. is longer than 1 s, the transientvoltage is unstable. ^{[4]}

Whether the system is easy to lose stability depends mainly on the system's structure and the line's impedance. The criterion below ^{[5]} is based on impedance. The impedance of the system is equal to an impedance(Zj), the load is equal to an impedance. The stability criterion can be defined as

If , the bus voltage Is stable; otherwise It's unstable. When the system Is In transient status, should be the mean value.

There Is another crlterlon^{[6]} based on the Jacobian matrix of the system. Set x as the voltage of a node, tFas the end moment of the fault, as the voltage after the fault ends, h matrix measures the transient voltage stability, It's defined as the left eigenvector of the Jacobian matrix. Q Is the solution of the equation Where lm Is unit mxm matrix.

Where H¡ ¡s the Hessian Matrix of an unstable balance point of the function composed of

Therefore, the criterion Is defined as Where Is the critical point after the fault, decided by , the system Is transient stable, or It's transient unstable.

As for a complex system, we must know where the fault Is can make the system more serious. That Is fault ranking:

1) Scan the fault as N-l,N-2. If the flow doesn't astringe, the fault seems to be serious.

2) If all the flows astringe, then calculate static stability margin Index, the smaller It Is, the more serious the system Is.

3) Aiming at the serious fault (especially ¡n (1)), make plan to keep the system stable.

### 5.2 Stability Area

Theorem l(Hartmann theorem): For a balance point xe, If Jacobian matrix D f(xe) doesn't have an eigenvalue which Is pure Imaginary number( Include 0), the balance point can be linear. If all eigenvalues' real parts are negative, the balance point Is stable.

**[...]**

^{[1]} Ju Ping et al, "Survey of wide area measurement system". Electric Power Automation Equipment, No.7,vol.24,pp 37-41, Jul 2004.

^{[2]} Kingsuk Saha et al."Smart Grid and WAMS in Indian context - A review". 2015 Clemson University PowerSystems Conference (PSC), pp 1-6, Mar 2015.

^{[3]} H. Amarfs, "Power quality solutions for voltage dip compensation at wind farms," in IEEE Power Eng. Soc. General Meeting New York: IEEE, 2007, pp. 512-514.

^{[4]} DL755-2001, Power system security and stability of technical guideline.

^{[5]} M. Kenan, A. BASA, " Investigation of Various Voltage Sag Analyses in Wind Farm through SVC ", Electri. and Electron. Eng. (ELECO), 2011 7th Int. Conf. on, pp. 13-17, 2011.

^{[6]} J. Zheng, J. Li, " Reactive Optimization Control for the Wind Farm with Static Var Compensator (SVC)", Control and Decision Conf. (CCDC), 2012 24th Chinese, pp. 2792 -2795, 2012.

- Quote paper
- Bachen Wang (Author), 2016, Wide-area measurement systems applied to voltage stability, Munich, GRIN Verlag, https://www.grin.com/document/318654

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