APPLICATION OF DECOUPLING TECHNIQUE TO SOME POWER SYSTEM PROBLEMS

Abstract

The difficulty of designing a controller for multivariable control system is closely related to the degree of coupling present between the various inputs. The decoupling of multivariablo control system not only simplifies the design, of controllers but also permits to apply the well known singleinput, single-output control system techniques for synthesizing each single loop controller. The decoupling technique has been applied for flight control and chemical process control and is yet to find active application to the power system problems. The power system can be regarded as a multi-input multi-output control system. In general there exists considerable coupling between various control loops under dynamic and even static conditions. Operationally it would be very advantageous in several situations to decouple various interacting loops. Though adjustment of reactive power flow from the machine does not affect the active power flow to a great extent, but the reverse is not true and it woiild be of great operational advantage if these two loops are completely decoupled. Similarly in case of doubly excited synchronous machine, it may be operationally useful if prime mover, angle regulator and voltage regulator could independently control active power, rotor angle and and reactive power. In case of multimachine system it may be worth while to confine(i.e. decouple) the disturbance near one machine affecting the other machines,where as for interconnected system load frequency control(LFC) opera tion it would be desirable to either restrict the area distur bance confined to same area or to control the frequency and tie line powers independently. Further, while decoupling the system purely from such operational point of view is justifiable, at the same time it can give us easily an opportunity to improve the dynamic and even transient performance. The design can take into account even the sensitivity aspect of operating point variation. In the present work noninteractive controllers for some of the power system problems have been developed based on the linear system model..The method of Falb and Wolovich and others' for decoupling multi-input multi-output system is. followed which also helps in locating the closed loop poles. It has been fur ther shown that it often results in superior performance even in transient conditions involving larger disturbances. The work also considers in detail the effect of changes in operating point and suggests remedies if required. The following cases are considered for study. (1) Single machine without voltage regulator and governor (2) Single machine with voltage regulator and governor (3) Multimachine system with voltage regulator and governor (4) Double winding rotor machine with voltage regulator, angle regulator and governor (5) Load frequency control of interconnected power system. It has been found that the decoupling of active and reactive power control loops of a single machine connected to infinite bus (case 1) has not only reduced the strong coupling present between the two control loops of the original system to zero but also improved the dynamic performance of the system because of pole placement. The analysis also revealed that the cross coupling introduced due to changes in operating point are very small compared to the diogonal terms, thus retaining the decoupling property over a wide range of operating condi tions. To see whether the decoupled system can further be desensitized against changes in operating point, two methods (1) Tzafestas et al and (2) Curz and Perkins, are considered for sensitivity analysis. The first method covers for first order changes in any element of the system matrices. The improvement obtained by this method is marginal. The second method increase the order of the system and there is no improvement in the perfornance. The similar improvement in performance as in case 1 al,ove has heen observed for the synchronous machine with voltage regulator and governcr (case 2) with decoupled actave and reactive power control loops. Generally the machine eola tions contain flux linkages as state variables which are not directly measurable. These flux linkages can be expressed^ interms of measurable quantities like field current, terminal voltage etc. by a linear transformation. It has heen notaced at L performance of the system in case *with *U£~£> states has not deviated much from the performance of the system with flux linkages as state variables. The controller which is designed for dynamic conditions is also tested for large system disturbance case. The "»* analysis of the decoupled system for case 2is carried out for a 3Phase fault at the ..chine terminals. The signals obtained Le to feedback matrix, used for decoupling, are utiliseM» . \™i IIP turbine steam inlet valves and the voltage regula- iTLZsZ restrictions are imposed on the field voltage tor. Kealis ^^ ^ 00nslQerat)ie waiving over a wide range oj- -^ Ifalso noticed that the terminal voltage recovery for the ^upled system is better than the other cases. The flow of electrical power between machines can _ cauSe dynamic ™>^J^Z£Z?£Z will turbanoe to other apparatus, .he "» mohlne and t0 h0lp to isolate the dynamics of Individ aisturtanoe. roduce the effect of inter af"^^^ ox each machine The inputs to prime mover a* roltege gui^ ^ ^^ ^ of a multimachine power ****~* ^ maohine independently. STSl iTSTiJSip^ of rotor ang. and ^

voltage control loops of a 3 machine power system(case 3) has reduced the interaction between the machines to zero at nominal operating point and to a negligible value at other operatingpoints. The transient analysis revealed that a 3 phase fault at one of the machine terminals has little effect on the rotor angles of other machines as compared to the original system. For double winding rotor synchronous machine, there are three control inputs namely input to angle regulator, voltage regulator and prime mover. These inputs can be used to control rotor angle, reactive power and active power of the machine independently. It has been found that the decoup ling of the above 3 control loops helps to improve the dynamic performance and to reduce the coupling present In the system to zero at the nominal operating point and to a very small value at other operating points. The transient analysis has revealed that the transient stability of the decoupled system is better than the original system. Two basic criteria are considered for the design of load frequencir control of interconnected power systems: (1) Non-interaction between frequency and tie line power("both stiff tielinc and elastic tie line case are considered) (2) Area autonomy in which every area take care of its load vari ations. With the help of the two unequal area LFC system It has been found that the noninteractive controllers has improved the dynamic performance of the system and also reduced the effect of load disturbance on frequency and tie line power. Thus, it has been shown in the present work that the application of decoupling technique to power system problems can considerably improve the operation and performance of the system over a wide range of operating conditions.