# Auxiliary stabilizer of STATCOM based on the state feedback theory.

INTRODUCTIONRecently development in the field of high-power electronic has made Flexible AC Transmission System (FACTS) devices viable and attractive for power system applications. FACTS devices have been widely used for different purposes in power systems such as stability improvement, voltage control, power control, etc (Nilsson, 1995; El-Sadek et al., 1997; Mihalic and Papic, 1998; Oliveira et al., 1999; Fang and Ngan, 2000; Hingorani et al., 2000; Kazemi and Sohrforouzani, 2006; Taher et al., 2010; Nguimfack-Ndongmo et al., 2014; Zamora-Cardenas et al., 2014). Paper (Nilsson, 1995) discusses that the AC transmission system is emerging as a key recourse for the electric utilities. It is increasingly being stressed by wheeling demands for which it may not have been built. However, the power flows in a network are not easily controlled because line parameters, which determine the distribution of power in the system, are not easily changed. Power electronics based systems which can provide the operators the needed flexibility in controlling the system power flows have been in use for some time and more such systems are being developed. These systems are broadly referred to as FACTS which stands for flexible AC transmission system. This paper discusses the implications of FACTS technologies for the security and reliability of the AC transmission system and discusses how the new systems fits into the existing systems for control of the power system as a whole. Paper (Zamora-Cardenas et al., 2014) proposes a practical approach to incorporate synchronized phasor measurements into a weighted least squares state estimation algorithm suitable for electric networks containing flexible AC transmission systems (FACTS) controllers. Equations relating the branch current phasors to FACTS controllers' state variables are derived from first principles in order to directly append synchronized phasor measurements of currents and voltages to the set of measurement data collected from the supervisory control and data acquisition system to estimate the equilibrium point of the electric power system. The phase angle measured by a perfect phasor measurement unit at one of the buses is selected as the global reference for the estimation, such that the voltage phasor measurements provided as data to the estimator correspond to the angle differences between all remaining voltage phasor measurements and the reference angle. Furthermore, the synchronized current phasor measurements are transformed to rectangular coordinates to enhance the convergence properties of the proposed state estimation approach. In both cases, the variance of the new set of synchronized measurements is calculated based on the uncertainty propagation theory. The proposed approach simultaneously upgrades the estimated values of the state variables of FACTS devices and the electric network for a unified estimation of the system state. Lastly, numerical simulations with two real-life electric power networks are reported to demonstrate the prowess of the proposed approach. Paper (El-Sadek et al., 1997) discusses that a flexible AC transmission system (FACTS) is a proposed new technique for balancing unbalanced three phase arc furnace loads. Although reactances of conductors can be balanced by traditional methods, the method proposed here is a unique method for balancing inevitably unbalanced arc resistances. Compensator controller elements and relations between their variables and the actual firing angles are derived. The influence of furnace load balancing on its characteristics is discussed.

Current paper aims at investigating an auxiliary stabilizer of STATCOM based on the state feedback theory. Single machine infinite bus power system which is installed with STATCOM is considered as case study. Simulation results demonstrate the effectiveness of the proposed method to damp out the oscillations.

Test System:

Figure 1 shows a single machine infinite bus power system installed with STATCOM. A non-linear dynamic model of the system is derived by disregarding the resistances of all components and the transients of the transmission lines and transformer of the STATCOM. The nonlinear dynamic model is given as (1).

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (1)

A linear dynamic model is obtained by linearising the non-linear model around the nominal operating condition. The linear model is given as (2).

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (2)

Where,

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]

[DELTA][m.sub.E]: Deviation of pulse width modulation index [m.sub.E] at shunt inverter. By controlling [m.sub.E], the output voltage of the shunt converter is controlled. [DELTA][[delta].sub.E]: Deviation of the phase angle at shunt inverter. By controlling [[delta].sub.E], the exchanged active power between STATCOM and power system is controlled. The dynamic model of the system in the state-space form is obtained as (3).

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (3)

The stabilizer is mainly provided to increase damping in power system. This controller is generally made as a lead-lag compensator. Figure 2 shows the block diagram of conventional stabilizer. In this paper, state feedback is used to design stabilizer. This method is introduced in the next section.

State feedback:

The state of a dynamical system is a collection of variables that permits prediction of the future development of a system. We now explore the idea of designing the dynamics a system through feedback of the state. We will assume that the system to be controlled is described by a linear state model and has a single input. The feedback control will be developed step by step using one single idea: the positioning of closed loop eigenvalues in desired locations. Figure 3 shows a diagram of a typical control system using state feedback. The full system consists of the process dynamics, which we take to be linear, the controller elements, K and kr, the reference input, r, and processes disturbances, d. The goal of the feedback controller is to regulate the output of the system, y, such that it tracks the reference input in the presence of disturbances and also uncertainty in the process dynamics. An important element of the control design is the performance specification. The simplest performance specification is that of stability: in the absence of any disturbances, we would like the equilibrium point of the system to be asymptotically stable. More sophisticated performance specifications typically involve giving desired properties of the step or frequency response of the system, such as specifying the desired rise time, overshoot and settling time of the step response. Finally, we are often concerned with the disturbance rejection properties of the system: to what extent can we tolerate disturbance inputs d and still hold the output y near the desired value (Astrom and Murray, 2010).

State feedback theory or pole placement, is a method employed in feedback control system theory to place the closed-loop poles of a plant in pre-determined locations in the s-plane. Placing poles is desirable because the location of the poles corresponds directly to the Eigen-values of the system, which control the characteristics of the response of the system. The system must be considered controllable in order to implement this method. The state space model of system can be represented as (4) and then, the poles of the system are the roots of the characteristic equation given by (5).

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (4)

[absolute value of (sI - A)] = 0 (5)

Full state feedback is utilized by commanding the input vector u as (6).

[u.bar] = -K[x.bar] (6)

Eventually, the state space equations can be derived as follows;

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (7)

The roots of the state feedback system are given by the characteristic equation as follows:

det [sI - (A - BK)] (8)

Comparing the terms of this equation with those of the desired characteristic equation yields the values of the feedback matrix K which force the closed-loop Eigen-values to the pole locations specified by the desired characteristic equation.

Simulation Results:

As stated before, in this paper, state feedback is used to design supplementary stabilizer of STATCOM. The output signal [DELTA][omega] is assumed as input of feedback and AmE is considered as output of state feedback. Simulation results are carried out following 5-cycles three-phase short circuit at bus 1. Figure 4 shows the generator speed following disturbance. It is clear that the oscillations are damped out and system stability is passed after 15 seconds. Figure 5 indicates the rotor angle following 5-cycles three-phase short circuit at bus 1. This figure shows that rotor angle is driven back to nominal value after disturbance. Figures 6 and 7 show the voltage angle and magnitude at bus 1 respectively. The voltage stability is clear in these figures. Figure 8 shows the stabilizer output signal following disturbance. It is clear that stabilizer injects extra signal to the STATCOM and provides more damping to the system.

Conclusions:

In this paper, state feedback theory was applied to design stabilizer based on the STATCOM. A single machine infinite bus power system installed with STATCOM was considered as case study. Simulation results demonstrated that the designed stabilizer could guarantee the system stability following large signal disturbances such as three-phase short circuit.

ARTICLE INFO

Article history:

Received 25 January 2014

Received in revised form 12 March 2014

Accepted 14 April 2014

Available online 5 May 2014

REFERENCES

Astrom, K.J. and R.M. Murray, 2010. Feedback systems: an introduction for scientists and engineers: Princeton university press.

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Fang, W. and H. W. Ngan, 2000. "A robust load flow technique for use in power systems with unified power flow controllers," Electric Power Systems Research, 53(3): 181-186, 3/1/.

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Nguimfack-Ndongmo, J.D.D., G. Kenne, R. Kuate-Fochie, 2014. "A simplified nonlinear controller for transient stability enhancement of multimachine power systems using SSSC device," International Journal of Electrical Power & Energy Systems, 54(0): 650-657, 1/.

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Zamora-Cardenas, E.A., B.A. Alcaide-Moreno, and C.R. Fuerte-Esquivel, 2014. "State estimation of flexible AC transmission systems considering synchronized phasor measurements," Electric Power Systems Research, vol. 106, no. 0, pp. 120-133, 1//.

Reza Saki, Somayeh Yarahmadi, Faranak Nikabadi, Vahid Chegeni

Department of Electrical Engineering, Doroud Branch, Islamic Azad University, Doroud, Iran

Corresponding Author: Reza Saki. Islamic Azad University, Department of Electrical Engineering, Doroud, Iran.

E-mail: sakii@iau-doroud.ac.ir

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Author: | Saki, Reza; Yarahmadi, Somayeh; Nikabadi, Faranak; Chegeni, Vahid |
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Publication: | Advances in Natural and Applied Sciences |

Date: | Jul 1, 2014 |

Words: | 1833 |

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