STATCOM DAMPING CONTROLLER FOR POWER SYSTEM

Abstract

Owing to various reasons like ever-increasing demand of electric power, deregulation, restrictions on construction of new transmission corridors etc., the modern power systems now-adays are forced to carry quite a large amount of power, thereby operating in a much stressed condition. Under this stressed condition, power transfer between two areas over a tie line often exhibits low frequency inter-area oscillations. Low frequency oscillations were first observed in Northern American Power Network in early 1960's during a trial interconnection of the Northwest Power pool and Southwest power pool. Later they have been also reported in many other countries. As the low frequency oscillations constrain the power carrying capacity of the line, it threatens power system security as well as limits the most efficient utilization of the transmission system. Therefore, this problem has caused wide concern and attracted the attention of the researchers since 1960s. In the early days, the excitation control of the synchronous machines was the only remedy available for controlling these oscillations. Later, power system stabilizers (PSS) were developed and added to the excitation system loop to enhance the damping of these low frequency oscillations. However, a PSS is generally more effective in damping local modes, but in certain cases it may not be sufficient to provide the necessary damping for inter-area oscillations. Therefore, for enhancing the power system security, development of most effective method of damping the inter-area mode of low-frequency oscillation is still a major area of research today. Now, in the last one and half decades power electronic device based equipments, commonly known as Flexible ACTransmission (FACTS) controllers, have been implemented in various parts of the world to achieve better utilization of the existing power grid. Because of their capability of fast power control, these FACTS controllers can be used effectively to damp out the inter-area oscillations. Among the various FACTS controllers, Static Synchronous Compensator (STATCOM) is one of the most prominent equipments. It is a shunt-connected equipment whose main objective is to control the bus voltage. However, by using the voltage controller only, the STATCOM may not be able to damp out the inter-area oscillations effectively and in some cases may even aggravate the problem. Therefore, an auxiliary damping controller is often needed to be designed, which would provide a superimposed damping signal to its voltage loop. Because of the above requirement, researchers around the world have started paying attention to the problem of design of damping controller for STATCOM in recent past. In the literature, different techniques have been developed for designing such controllers for STATCOM. Careful review of the different control strategies proposed in the literature reveals that many techniques have been validated on relatively smaller test systems (SMIB ortwo area 4 machine system). The techniques validated on large system, have either used; a) PSS on all the machines or b) multiple FACTS devices or c) multiple remote input signals. Moreover, in many instances, the robustness properties ofthe proposed control schemes have not been demonstrated over a range ofoperating conditions. Therefore, there is still a need for developing a STATCOM control strategy that is simple, easy to implement, robust, utilizes only local input signals and has proven stability properties. Also, the developed technique should not require any other stabilizing device like PSS or other FACTS devices for stability improvement. This work attempts to address this issue. For this purpose, in this thesis, use ofperiodic output feedback technique (POFT) and Fast Output Sampling Feedback Technique (FOSFT) is proposed for designing adamping controller for a STATCOM. Basically POFT and FOSFT are two different types of multirate output feedback techniques. In POFT, the rate of application of the control input signal is higher than the output sampling rate. On the other hand, in FOSFT, the output signal is sampled at a faster rate than that of the control input signal. As already shown in the literature, the controller designed by POFT as well as FOSFT is both robust and of simple structure, i.e., the controller is essentially a constant gain matrix, which can be made effective over arange of operating conditions. Although these techniques have been applied for designing power system stabilizers (PSS) but to the best of the knowledge of the author, the application of these techniques has yet not been reported for designing damping controller for STATCOM in the literature. The first step for the design of damping controller is calculation of the initial operating condition(s) of the system, which can be obtained through load flow analysis. Traditionally, a STATCOM has been represented in apower flow study as aPV bus. In this approach, generally 11 the internal losses in the STATCOM (both switching losses as well as the ohmic losses of the step down transformer) are neglected and as a result, the specified real powers at these PV buses are set to zero. As the internal losses are neglected, this method, although very popular, is not very accurate. To address this issue, in the literature, different methodologies have been suggested to consider the STATCOM losses in the load flow solution. In one approach, only the ohmic loss of the step down transformer has been considered but the switching loss has not been taken into account. In another procedure the Jacobian matrix needs to be modified to incorporate the STATCOM and its losses and therefore, this method is likely to increase the computational burden. To overcome the above limitations, in this thesis, an improved load flow technique has been developed, in which, both the switching losses and the transformer ohmic losses are taken into consideration. Moreover, in the proposed technique, no modifications in the main load flow Jacobian matrix are required, which, in turn, ensures good convergence characteristics of this method. Also, to reduce the computational burden, the main load flow equations and the STATCOM equations are solved sequentially. The effectiveness of this proposed technique has been tested on modified IEEE 30 bus and IEEE 57 bus power systems and the developed method has exhibited good convergence characteristics. After the initial operating conditions were computed, to begin with, POFT has been applied to design the STATCOM damping controller in a single machine infinite bus (SMIB) environment. Towards this goal, the detailed linearised state space model of a SMIB system incorporating a STATCOM has been developed. With this model, initially the damping controller with remote generator speed as stabilizing signal has been designed for the STATCOM. The performance of the developed controller has been validated through detailed non-linear simulation studies. For this purpose, a five cycle, self-clearing fault has been considered at the generator terminal and the performance of the controller was found to be satisfactory for damping the low frequency oscillations. Although the performance of the damping controller with remote signal has been found to be adequate, for practical implementations, local signals available at the STATCOM terminal are preferred. Now, because of theuse of local signal as stabilizing signal, the contribution of the system inputs onthe system outputs (the matrix 'D' in the linearised state space representation of the power system containing STATCOM) cannot be neglected. As a result, in this thesis, the 111 existing POFT has been extended to incorporate linear systems with non-zero 'D' matrix. The appropriate local output feedback signal has been chosen through Right Half Plane (RHP) zeros and Hankel Singular Value (HSV) analysis. Subsequently, the damping controller has been designed with this extended periodic output feedback technique. From detailed non-linear simulation studies carried out in a SMIB system, the efficacy of the developed local signal based controller has been found to be quite satisfactory for improving the systemdamping. Apart from the POFT, another variation of the multi-rate output feedback technique, namely, the FOSFT has also been applied for designing the damping controller for the STATCOM. For this purpose, initially this method has been used inthe SMIB system. Using the developed linear model, the damping controller with remote generator speed as stabilizing signal has been designed and the performance of the developed controller has been found to be satisfactory upon validation through detailed non-linear simulation studies. Further, to design the controller using FOSFT with locally available stabilizing signals, necessary theoretical modifications have also been incorporated in the existing FOSFT. The performance of damping controller designed with the modified FOSFT has also been found to be quite satisfactory from detailed non-linear simulation studies carried out in the SMIB environment. After establishing the suitability ofthe two control techniques for designing the damping controller for STATCOM with local stabilizing signal in a SMIB system, the design of damping controller for multi-machine system has been carried out in Chapter 6. Towards this, initially the linearised state space model of a multi-machine power system with STATCOM has been developed. Subsequently, a procedure for selecting the STATCOM location and the suitable stabilizing signal, based on the evaluation of Right Half Plane (RHP) zeros and Hankel Singular Values (HSV) of the resulting linearised state space system, has been presented. The performances of the developed damping controllers have been thoroughly tested under different operating conditions in 3 machine and 10 machine power systems. It is observed that the controllers are quite capable ofdamping out the oscillations effectively in the power system. To summarize, the major contributions ofthis thesis are as follows: • Asimple yet efficient load flow method for power system with multiple STATCOMs has been developed. The method is also capable ofincluding STATCOM losses. IV Existing periodic output feedback and fast output sampling feedback control techniques have been extended for linear systems with non-zeros 'D' matrix. The suitability of these techniques for enhancing system damping has been first tested on SMIB system. The performance of the controller has been evaluated with remote and local stabilizing signals under different operating conditions through detailed non linear simulation studies, and found to be satisfactory. For a multi-machine system, a systematic procedure for deciding the proper STATCOM location and the suitable stabilizing signal has been developed based on RHP zeros and HSV of the linearised state space model. The damping controllers have been designed using periodic output feedback and fast output sampling feedback control techniques using local stabilizing signals. The performance evaluation of the controllers on two multi-machine systems through detailed non liinear simulation studies indicates that a STATCOM alone is capable of efficiently damping out system oscillations even in the absence of PSS.