Advances in Power Electronics

Volume 2015, Article ID 626731, 12 pages

http://dx.doi.org/10.1155/2015/626731

## Transient Stability Enhancement of Multimachine Power System Using Robust and Novel Controller Based CSC-STATCOM

^{1}Department of Electrical Engineering, RIET, Jaipur 302026, India^{2}Department of Electrical Engineering, MNNIT, Allahabad 211004, India

Received 3 August 2014; Revised 22 December 2014; Accepted 23 December 2014

Academic Editor: Francesco Profumo

Copyright © 2015 Sandeep Gupta and Ramesh Kumar Tripathi. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### Abstract

A current source converter (CSC) based static synchronous compensator (STATCOM) is a shunt flexible AC transmission system (FACTS) device, which has a vital role as a stability support for small and large transient instability in an interconnected power network. This paper investigates the impact of a novel and robust pole-shifting controller for CSC-STATCOM to improve the transient stability of the multimachine power system. The proposed algorithm utilizes CSC based STATCOM to supply reactive power to the test system to maintain the transient stability in the event of severe contingency. Firstly, modeling and pole-shifting controller design for CSC based STATCOM are stated. After that, we show the impact of the proposed method in the multimachine power system with different disturbances. Here, applicability of the proposed scheme is demonstrated through simulation in MATLAB and the simulation results show an improvement in the transient stability of multimachine power system with CSC-STATCOM. Also clearly shown, the robustness and effectiveness of CSC-STATCOM are better rather than other shunt FACTS devices (SVC and VSC-STATCOM) by comparing the results in this paper.

#### 1. Introduction

The continuous developments of electrical loads due to the modification of the society structure result in today’s transmission structure to be faced close to their stability restrictions. So the renovation of urban and rural power network is more and more necessary. Due to governmental, financial, and green climate reasons, it is not always possible to construct new transmission lines to relieve the power system stability problem at the existing overloaded transmission lines. As a result, the utility industry is facing the challenge of efficient utilization of the existing AC transmission lines in power system networks. So transient stability, voltage regulation, damping oscillations, and so forth are the most important operating issues that electrical engineers are facing during power transfer at high levels.

In above power quality problems, transient stability is one of the most important key factors during power transfer at high levels. According to the literature, transient stability of a power system is its ability to maintain synchronous operation of the machines when subjected to a large disturbance [1]. While the generator excitation system with PSS (power system stabilizer) can maintain excitation control and stability it is not adequate to sustain the stability of power system for large faults or overloading occurs near to generator terminals [2].

So many researchers worked on this problem in finding the solution for a long time. These solutions are such as using wide-area measurement signals [3], phasor measurement unit [4], and flexible AC transmission system. In these solutions, one of the powerful methods for enhancing the transient stability is to use flexible AC transmission system (FACTS) devices [5–8]. Even though the prime objective of shunt FACTS devices (SVC, STATCOM) is to maintain bus voltage by absorbing (or injecting) reactive power, they are also competent of improving the system stability by diminishing (or increasing) the capability of power transfer when the machine angle decreases (increases), which is accomplished by operating the shunt FACTS devices in inductive (capacitive) mode.

In many research papers [2, 9–11], the different types of these devices with different control techniques are used for improving transient stability. In between these FACTS devices, the STATCOM is valuable for enhancement power system dynamic stability and frequency stabilization due to the more rapid output response, lower harmonics, superior control stability and small size, and so forth [7, 12]. By their inverter configuration, basic type of STATCOM topology can be realized by either a current-source converter (CSC) or a voltage-source converter (VSC) [13–17]. But recent research confirms several merits of CSC based STATCOM over VSC based STATCOM [18, 19]. These advantages are high converter reliability, quick starting, and inherent short-circuit protection, and the output current of the converter is directly controlled and in low switching frequency this reduces the filtering requirements compared with the case of a VSC. Therefore CSC based STATCOM is very useful in power systems rather than VSC based STATCOM in many cases.

Presently the most used techniques for controller design of FACTS devices are the Proportional Integration (PI), PID controller [20], pole-shifting controller, and linear quadratic regulator (LQR) [21]. But LQR and pole-shifting algorithms give quicker response in comparison to PI and PID algorithm [22]. LQR controller gain () can be calculated by solving the Riccati equation and is also dependent on the two cost functions (). So Riccati equation solvers have some limitations, which relate to the input arguments. But pole shifting method does not face this type of any problem. So pole shifting method gives a better and robust performance in comparison to other methods.

The main contribution of this paper is the application of proposed pole-shifting controller based CSC-STATCOM for improvement of power system stability in terms of transient stability by injecting (or absorbing) reactive power. In this paper, the proposed scheme is used in the multimachine power transmission system with dynamic loads under a grievous disturbance condition (three-phase fault or heavy loading) to enhancement of power system transient stability studies and to observe the impact of the CSC based STATCOM on electromechanical oscillations and transmission capacity. Furthermore, the obtained outcomes from the proposed algorithm based CSC-STATCOM are compared to the obtained outcomes from the other shunt FACTS devices (SVC and VSC-STATCOM) which are used in previous works [23, 24].

The rest of the paper is prepared as follows. Section 2 discusses the circuit modeling and proposed pole-shifting controller designing for CSC based STATCOM. A two-area power system is described with a CSC-STATCOM device in Section 3. Simulation results of the test system with and without CSC based STATCOM for severe contingency are shown in Section 4, to improve the transient stability of the multimachine power system. Comparison among different shunt FACTS devices (SVC, VSC-STATCOM, and CSC-STATCOM) is also described in Section 4. Finally, Section 5 concludes this paper.

#### 2. Mathematical Modeling of Pole-Shifting Controller Based CSC-STATCOM

##### 2.1. CSC Based STATCOM Model

In this section, to verify the response of the STATCOM on dynamic performance, the mathematical modeling and control strategy of a CSC based STATCOM are needed to be presented. So in the designing of controller for CSC based STATCOM, the state space equations from the CSC-STATCOM circuit must be introduced. To minimize the complexity of mathematical calculation, the theory of transformation of currents has been applied in this circuit, which makes the and components independent parameters. Figure 1 shows the circuit diagram of a typical CSC based STATCOM.