Experimental Testing and Model Validation of a Small-scale Generator Set for Stability Analysis.

Transcription

1 Experimental Testing and Model Validation of a Small-scale Generator Set for Stability Analysis. G. Quiñonez-Varela and A. Cruden Abstract The integration of numerous small-scale generators into existing power systems is anticipated to impact the operation, control and protection of such systems. In particular, maintaining voltage and frequency stability within defined limits is more onerous and requires investigation. The effect of protective limiters and characteristics such as the genuine inertia of the generation set must be taken into consideration in planning studies in order to accurately represent the overall dynamic characteristics of distributed generators. This paper focuses on the investigation of these issues by studying a small-scale reciprocating engine/generator set. The experimental procedures used to determine the genuine inertia of the test rig are described, and the influence and importance of considering the action of protective limiters such as voltage-per-hertz (V/Hz) in stability studies is demonstrated. This work is directly relevant to the review of current UK stability limits, and to the generation planning framework supported by the Scottish Executive. Index Terms AC generator excitation, dispersed storage and generation, frequency control, power system dynamic stability, power system simulation, synchronous generator excitation, synchronous generator stability, voltage control. T I. INTRODUCTION HE integration of numerous small-scale generators such as reciprocating engine sets into existing power systems is anticipated to impact the operation, control and protection of such systems. In particular, maintaining voltage and frequency stability within defined limits is more onerous and requires investigation. Consideration is now being given to the revision of system stability limits and the examination of novel coordinating procedures system [1]. Consequently, an accurate representation of the overall dynamic characteristics of generators is crucial in order to properly investigate the interaction among distributed generators, local loads and the utility. G. Quiñonez-Varela and A. Cruden are with the Centre for Economic Renewable Power Delivery, University of Strathclyde, Glasgow G1 1XW, UK. ( gustavo.quinonez@eee.strath.ac.uk; a.cruden@eee.strath.ac.uk). In addition to the typical automatic voltage regulator (AVR), modern excitation systems incorporate a number of control, limiting and protective functions to ensure that both the exciter and the synchronous generator are properly protected against overfluxing, overloading, overheating, etc. Relevant functions are the overexcitation (OEL), underexcitation (UEL) and voltage to frequency ratio (V/Hz) limiters [2], [3]. Although their action is active under normal operating conditions during some relevant system disturbances, such as substantial loss of load, uncontrolled load shedding and short-circuits, their effect can be particularly significant to the ultimate behaviour of the system under these conditions [4], [5]. However, the effect of these devices is usually not taken into consideration in planning studies and, according to available literature [3]-[6], there is little examination of the dynamic performance of these limiters and small-scale distributed generators. Failure to consider the action of such protective limiters may lead to inaccurate assumptions of the generator responses during planning studies [6], [7]. Along with voltage stability, frequency stability is now a crucial consideration in power system analysis, particularly as a consequence of the relatively recent proliferation of distributed generation. One of the parameters that significantly influence the firmness of generators and overall system stability is the inertia values of the generator sets. Distributed generators are in general small-scale (<10MW) with correspondingly small mechanical inertias in comparison to conventional larger turbo gen-sets. The smaller inertias can cause stronger coupling between the system and the localised generation, and due to a smaller damping effect, can be less effective in compensating the oscillations caused locally by load disturbances. This paper focuses on the investigation of these issues by studying a small-scale reciprocating engine generator set. The experimental procedures used to determine the genuine inertia of the test rig are described. Also, the influence and importance of considering the action of protective limiters such as voltage-per-hertz (V/Hz) in stability studies is demonstrated. This work is directly relevant to the review of current UK stability limits carried out by the Distributed Generation Coordinating Group (DGCG) [1], and to the

2 development of the distributed renewable generation planning tool promoted and sponsored by the Scottish Executive. II. MODELLING OF RECIPROCATING ENGINE GENERATORS A reciprocating engine generator has been modelled using the industry standard power system simulator package PSCAD/EMTDC. The engine governor/fuel actuator is based on a model formerly developed by the authors [8]. The synchronous generator is modelled in the state variable form using the equivalent d-q transformation of the generalized machine theory. The excitation system is represented by an IEEE AC5A model, widely implemented by industry and recommended to represent simplified systems with rotating rectifiers [9]. Some parameters of the system were available from the manufacturer, whilst the remainder were obtained analytically and subsequently verified experimentally. As standard models and the recommended practices for excitation system modelling do not include a representation of protective limiters, a V/Hz limiter has been implemented by the authors and studied. Its block diagram is illustrated in Fig. 1 III. EXPERIMENTAL TEST RIG The experimental assembly comprises a four-stroke, threecylinder, spark-ignition gas engine directly coupled to a threephase, 11 kva, four-pole, self-excited synchronous generator. It comprises a V/Hz limiter and underspeed/overexcitation protection. Fig. 2 illustrates the reciprocating engine/generator set, part of a dedicated CHP rig within the University. The engine/generator set supplies a purely resistive load bank rated up to 12 kw with variable loadings achieved by reconfiguring and switching the different resistive components. Fig. 3 illustrates the experimental load system. Time delay V g V ref V s V/Hz VHz lim K VHz 1.0 t d 1 1 sτ VHZ 0 V ref Fig. 2. The experimental engine/generator test rig. V g F g Range comparator Fig. 1. Block diagram of the V/Hz limiter model. The operation of the V/Hz module is rather straightforward. If the per-unit volts-per-hertz ratio V/Hz goes beyond the predetermined limiting value VHz lim, a significant negative output signal forces the excitation down. The strength of the signal is adjusted by the proportional K VHz gain. Before it is passed on to the summing function of the AVR model, a lag function is used to adjust the V/Hz response time, characterised by the time constant VHZ. A range comparator is used to monitor underspeed or overspeed conditions, and to enable the limiter to adjust the excitation level when the permissible volts-to-hertz ratio is exceeded under such conditions. Typically, the operating point of V/Hz limiter under continuous operation lies between 0.95 to 1.05 per-unit. A time delay block was also included. It is used to disable the action of the V/Hz for the duration of the transients during the simulation initialisation stage. Otherwise, the voltage reference in the AVR may be drop down excessively during this period, causing the eventual failure of the simulation run. Fig. 5. The load array, switching panel, and resistive load elements utilised in the experimental rig. IV. EXPERIMENTAL TESTING AND MODEL VALIDATION In any rotating machine, the angular speed change (known as angular acceleration) depends upon the torque imbalance for a given period of time and the moment of the machine inertia. In generators, imbalances are primarily caused by disturbances such as load switching. However, such

3 determination is complicated since the machine rotor is generally not accessible and comprehensive values are usually not available. In this paper two different test procedures are presented to determine the genuine value of inertia of the experimental test rig. 1) ROCOF The first test procedure conducted is based on resolving the rate-of-change-of-frequency (ROCOF) [10] of the system. Frequency variations are caused by power imbalances originated by system disturbances such as intentional load shedding, sudden loss-of load and short-circuits. The ROCOF is estimated using the following relation [10]: df P ROCOF = = (1) dt 2H where df is the deviation of frequency at a time interval dt in p.u., is the imbalance in power caused by a load disturbance in p.u., H is the constant of inertia in seconds. To perform this analysis, the responses of the engine/generator set under different load conditions were recorded. Table 1 presents the values of inertia obtained by averaging several tests trails under 4 different load variations. Fig. 3 illustrates a typical result showing the ROCOF values determined from the experimental responses. dω J = Jα = T m T e dt where T m is the driven applied torque, T e is the electromechanical torque both in N-m. J is the moment of inertia in kg-m 2 and is the deviation of angular speed at dt (angular acceleration,, in rad/s 2 ). In the case where the mechanical load of the engine-generator is driven externally, the electrical torque component is inert. In power system stability studies it is typical to express the value of inertia of a given machine as the ratio of stored kinetic energy at a rated speed to the rated apparent power such [2]: ( ) b 2 (2) 1 J ω m H = (3) 2 S where H is the inertia constant in sec., m is the mechanical angular speed in rad/s and S b is the rated apparent power of the electric machine in kva. Fig. 4 illustrates a typical result values obtained from varying the angular acceleration of the engine/generator set. Fig. 4. Variation of T m and ω during a sudden deceleration of the assembly. Fig. 3. Determination of the ROCOF from full- to half-load ( = p.u.). 2) Angular acceleration To verify the inertia values obtained from the previous test procedure, a second test was conducted. It consisted in driving the experimental engine/generator set externally, and monitoring the deviation of angular speed and the applied mechanical torque during the transition between steady states at different speeds. The inertia value was determined by analysing the mechanical dynamics of the test rig. Utilising the equation of motion of the rotating assembly to equate the applied driving torque to the inertial torque: Table 1 shows the mean values obtained from several test trials during acceleration and deceleration of the mechanical system. The values of J and H were computed by (2) and (3) using the experimental values for T m and α. V. MODEL VALIDATION Using the reciprocating engine/generator set model and the results obtained from the previous tests, various cases were validated by simulating the experimental test rig in PSCAD/EMTDC. The influence of considering the effect of the synchronous generator excitation V/Hz limiter has been especially examined using the model. It was observed that neglecting its effect gives rise to inconsistencies between simulations and the actual response of the system to voltage recovery. Figs. 5 and 6 show two cases using the developed engine/generator model, with and without the action of the

4 V/Hz limiter and the comparison to the experimental responses. TABLE 1. EXPERIMENTAL VALUES OF INERTIA OBTAINED BY THE TWO TEST PROCEDURES Condition J [kg-m 2 ] H [sec.] From full- to half-load * From half- to full-load * From full-load to no-load * From no-load to full-load * Typical acceleration ** Typical deceleration ** Average values: * Determined by ROCOF using (1) ** Determined by angular acceleration using (2) and (3) As demonstrated in the figures, agreeable matching was achieved in these cases. The obvious simulation differences, with and without a V/Hz limiter, displays marked voltage responses differences as illustrated as observed in the previous figures. If not modelled correctly, i.e. including the effect of protective limiters such as a V/Hz, a network containing a number of such small generating sets may suffer many nuisance trips and unnecessary instabilities. The resulting disparity can become relevant during the selection of settings and ultimate coordination of protections of small-scale distributed generation. Using the inertia values obtained experimentally, the accuracy to represent system frequency variations was verified. Fig. 7 shows a case using the developed engine/generator model, and the comparison to the experimental responses. As in the case the terminal voltage evolution, agreeable matching was achieved for the system frequency. Fig. 5. Time evolution of terminal voltage after a load disturbance (from noload to full-load) with and without the action of V/Hz protective limiter. Fig. 6. Time evolution of terminal voltage after a load disturbance (from halfto full-load) with and without the action of V/Hz protective limiter. Fig. 7. Time evolution of the system frequency after a load disturbances (from full- to half-load). VI. CONCLUSIONS The potential stability problems that can arise from the escalating deployment of small-scale distributed generators, such as reciprocating engine/generator sets, is demanding comprehensive investigation of a number of stability issues and to review the existing permissible limits. This requires an accurate determination of the dynamic performance of the system and its appropriate representation in stability studies. This paper has presented two experimental test procedures to determine the genuine inertia of engine/generator sets. The results obtained shown that both are practicable and easy to perform with small units. The experimental results were used to validate the dynamic response of a reciprocating engine/ generator model developed by the authors. Finally, the significance of considering the effect of protective limiters, such as a V/Hz limiter, in stability studies, especially when maintaining voltage and frequency within

5 permissible limits is crucial, has been demonstrated. The results emphasise that neglecting the influence of such devices may lead to an inaccurate assumption of the generator response and consequently lead to inaccurate system stability assumptions VII. ACKNOWLEDGMENTS The authors acknowledge the financial assistance given to Mr. G. Quiñonez-Varela by the Mexican Council of Science and Technology (CONACyT). Also, the support granted by the Scottish Executive to carry on this research work is gratefully acknowledge. IX. BIOGRAPHIES G. Quiñonez-Varela was born in Zacatecas, Zac. Mexico on December 2, He received the BSc (with distinction) in Electrical Engineering in 1996 from the Universidad Autonoma de Zacatecas, Mexico. Currently he is undertaken postgraduate studies leading to a PhD degree at the University of Strathclyde, UK. His research interests are wind and renewable power generation, control and protection of embedded generation and power quality. A. Cruden was awarded his PhD in 1998 from the University of Strathclyde in optical techniques to measure electric current. He was subsequently appointed as a Lecturer and Research Manager for the Centre for Economic Renewable Power Delivery at Strathclyde in 1998, and his current research interests are control and protection of embedded generation, fuel cells and electric vehicles. VIII. REFERENCES [1] Distributed Generation Coordinating Group (DGCG), URL: [2] Kundur, P. Power Systems Stability and Control, McGraw Hill, New York, [3] IEEE Task Force on Excitation Limiters. Recommended Models for Overexcitation Limiting Devices, IEEE Trans. Energy Conversion, vol. 10, no. 4, pp , [4] Ribeiro, J.R. Minimum Excitation Limiter Effect on Generator Response to System Disturbances, IEEE Trans. Energy Conversion, vol. 6, no. 1, pp , [5] Jia, X.M. and Choi, S.S. Design of Volts per Hertz Limiter with Consideration of the Under-excitation Limiter Control Actions, IEEE Trans. Energy Conversion, vol. 16, no. 2, pp , [6] Murdoch, A. et al. Excitation Systems Protective Limiters and Their Effect on Volt/Var Control Design, Computer Modelling and Field Testing, IEEE Trans. Energy Conversion, vol. 15, no. 4, pp , [7] Rifaat, R.M. Independent Power Producers (IPP) Perspectives and Experiences with WSCC Requirements for Generator Model Validation Tests, IEEE Trans. Ind. Applicat., vol. 37, no. 4, pp , [8] Quiñonez-Varela, G., Cruden, A., Grant, A. and Castaneda, A. "Electrical Integration of Wind Turbines into Industrial Power Systems: The Case of a Mining Unit", Proc. of the IEEE Porto PowerTech Porto, Portugal, September, [9] IEEE Recommended Practice for Excitation System Models for Power System Stability Studies. IEEE Std [10] Anderson, P.A. Power Systems Protection, IEEE Press, New York, 2000.