Impact of Increased Penetration of Solar PV on Small Signal Stability of Power System

Transcription

1 July 5 Impact of Increased Penetration of Solar PV on Small Signal Stability of Power System LINEAR DYNAMIC MODEL: Figure shows the configuration of a single-machine infinite bus power system connected with a large-scale PV power generation plant. Typical voltage-current characteristic of PV generation is non-linear, given by the following expression []. 4, Assistant Professor Moradabad Institute of Technology, Moradabad, Uttar Pradesh. Abstract This paper presents the impact of large scale photovoltaic (PV) generation on power system small signal oscillation stability. A linear Phillips-Heffron model of a single-machine infinite-bus power system integrated with a PV power plant is established and Simulation are carried out in MATLAB Simulink software. The results obtained indicate that power system oscillation stability can be affected negatively by large scale integration of solar PV. There exists an operational limit of the PV power plant as far as system oscillation stability is concerned. Beyond the operational limit, the PV generation supplies negative damping torque, thus damaging system oscillation stability. Hence for the safe penetration of PV generation into power systems, the operational limit of oscillation stability of the PV power plant must be considered. INTRODUCTION: With the increase in world population and reduction in natural resources, power utilities are facing challenges to provide adequate energy with acceptable cost while maintaining carbon emission level. Due to this the demand for renewable energy has increased significantly. Among various types of renewable energy sources solar energy has become the most promising due to the following factors: ) increasing efficiency of solar cells; ) manufacturing technology improvement; and ) economies of scale [].Meanwhile, more and more PV modules have been and will be connected to utility grid in many countries. Solar PV generators can be deployed more flexibly, in terms of the location, than other renewable resources such as wind and solar thermal generators []. Small scale installation of solar PV panels can be roofmounted on commercial or residential buildings or ground-mounted close to load. Large scale solar PV generators are normally built at locations remote from load centres given the need of large area of lands. As observed in generation interconnection processes, small scale solar PV generators range from less than one MW to MW while the capacity of large-scale solar PV generator project can spread from less than MW to MW.Solar PV interconnection can be at different voltage levels. Large-scale solar PV generators typically interconnect at the transmission levels due to their higher capacity. Small-scale solar PV generators mostly choose to interconnect to the closest distribution or sub-transmission system. With many countries having ambitious renewable energy targets in next -5 years, integrating these solar PV into the power system. Small numbers of renewable generation cause few or no problems to the grid, but as the percentage of renewable generation grows, power quality, protection, and safety concerns associated with PV need to be evaluated []. Distribution systems are typically designed to supply unidirectional power flow (from substations to loads), the interconnection of PV generation needs to be studied to ensure that its potential impacts are fully understood. Historically, the large majority of research in this field has centred its attention to studying impacts of conventional distributed generation (reciprocating engines, small hydro, biomass, etc) on the operation and planning of distribution systems. However, it is worth noting that conventional distributed generation is not intermittent in nature, therefore most of its impacts can be investigated through steady state analyses. Due to the intermittency of its primary resource, Solar PVgeneration impacts also need to be analysed by means of dynamic studies. So in this work we want to investigate the impact of Increased Penetration of Solar Photovoltaic Generators on the small signal stability of the power system.

2 July 5 where T is the junction temperature, N s and N p number of cells in series and parallel respectively, n ideality factor, I r the irradiance, k Boltzmann s constant, I sc short-circuit current and I saturation current and q charge of the electron. Figure : Single Machine Infinite Bus Power System With Solar PV Generation Figure shows the test system. In this system a photovoltaic power plant is connected to a single machine infinite bus power system by two-stage topology of inverter systems. The PV power plant actually generates fluctuating DC voltage as output. It is connected to conventional power system in the same manner PV generation is connected to grid i.e. through a two stage topology of inverter system. In order to enhance and smooth the DC output voltage, the DC/DC inverter is used. The maximum power point tracking (MPPT) is also installed with PV cells in order to extract as much power from them. The DC/DC inverter controls the injection of DC current I dc by controlling the duty cycle ratio dc. The enhanced and smooth DC output voltage is converted in to AC output voltage by implementing DC/AC inverter. In order to regulate the exchange of active and reactive power between the PV plant and rest of the conventional power system, the DC/AC inverter is employed with pulse width modulation (PWM) control. The commonly-used dynamic equations of the generator for the study of power system oscillation stability are. ()..() () 4

3 July 5 Where P V i V i ( x i ) i ( E ' x ' i ) i E ' i ( x x' ) i i (4) t td tsd tq tsq q tsq tsd q d tsd tsq q tsq q d tsd tsq E E ' ( x x' ) i q q d d tsd V V V t td tq From figure we have From the above equation we find.(5) In d-q coordinate from equation 5..(6)....(7) The dynamic model of DC/DC inverter can be simplified as: I pv V pv dc V L dc Neglecting DC/DC converter losses But, dc....(8) Hence; We can write all this as..(9) Ac voltage at terminal of the DC/AC inverter can be expressed in d-q coordinates of the generator as Where k is the converter ratio depend upon the inverter structure. V dc is the DC voltage across the capacitor C dc. Power at input terminal of DC /AC Converter is the product of Controlled output voltage of inverter and current DC/AC output current fed to capacitor 4

4 July 5 Power at output terminal of DC /AC converter is the product output voltage of AC voltage at terminal of DC /AC converter and current flowing through transformer If neglecting converter losses than = Thus in d-q coordinates But as So; Therefore, On substituting the values of as well as, we have..() DC/AC converter having modulation ratio m and phase of the PWM algorithm through which voltage control can take place. Thus m ( )( ) K ac s..() s sref m V V K s V V ( )( )..() dc dc dcref where Kac (s) and Kdc (s) is the transfer function of the ac and dc voltage controllers respectively.the MPPT function of the DC/DC converter can be modelled as ( )( ) c c K pv s.() pv pv max d d P P A linear dynamic model is obtained by linearising the non linear equations - around an operating condition. The linearized model is given below:- = + + [ ]= 44

5 International Journal of Engineering Technology Science and Research July 5 Figure :Linearized Phillips Heffron model of solar power plant integrated in the single machine infinite power system PERFORMANCE OF THE SYSTEM: Linearized Phillips Heffron model of solar power plant integrated in the single machine infinite power system so obtained is simulated using MATLAB Simulink software. The simulation model is as shown in figure. In this study we consider that total load has been fixed. But the mixer of conventional power P t (obtained from grid and transmission system) and photovoltaic power P pv (obtained from solar power plant) can vary in amount such that it meet the total load demand. Now we obtain various results by varying the both powers in various amounts as follows: Case when P t =9% and P pv =% Case when P t =8% and P pv =% Case when P t =7% and P pv =% Case 4 when P t =6% and P pv =4% Case 5 when P t =5% and P pv =5% Case 6 when P t =4% and P pv =6% Case 7 when P t =% and P pv =7% Case 8 when P t =% and P pv =8% Case 9 when P t =% and P pv =9% The results are as shown in figure. When Pt=.9 and Ppv= Case when P t =9% and P pv =% 45

6 International Journal of Engineering Technology Science and Research July 5 When Pt=.8 and Ppv= Case when P t =8% and P pv =%\ When Pt=.7 and Ppv= Case when P t =7% and P pv =% When Pt=.6 and Ppv= Case 4 when P t =6% and P pv =4% When Pt=.5 and Ppv= Case 5 when P t =5% and P pv =5% 46

7 International Journal of Engineering Technology Science and Research July 5 When Pt=.4 and Ppv= Case 6 when P t =4% and P pv =6% When Pt=. and Ppv= Case 7 when P t =% and P pv =7% When Pt=. and Ppv= Case 8 when P t =% and P pv =8% When Pt=. and Ppv= Case 9 when P t =% and P pv =9% Figure: Simulation of the example power system with different solar PV power penetrations 47

8 July 5 The results so obtained shows that as the penetration levels of Solar PV are increasing power system oscillations are also increasing. With the fixed load at the infinite busbar and the more power contributed by the PV generation the worse impact of PV generation on system oscillation stability was observed. CONCLUSION:- This paper investigates the impact of increased penetration of solar PV in the Conventional generation. In this paper a detailed mathematical modelling of solar PV power plant integrated in the single machine infinite bus power system is presented. The System is simulated for various combination of powers obtained from the conventional power plant and the Solar PV power plant at a fixed load at the infinite busbar.the results so obtained shows that as the penetration levels of Solar PV are increasing power system oscillations are going worse and posing a limit on the integration level of Solar PV power in the conventional power system. Appendix I The values of the photovoltaic power generation integrated with single machine infinite bus system is taken as: ) Transmission line values =. p.u., =. p.u., =. p.u. ) Generator basic values: =. p.u, =.47 p.u, =. p.u,h =.9 p.u,d =.5 p.u.,t do = 5 p.u ) PV Power plant basic values: q =,K=.8,n=,I scr =. A,V oc =.6 V,s= W/,T= 98 K,T r = 98 K,N s = 8,N p =, L dc =.5 p.u. 4) Automatic voltage regulator values : T A =. p.u., K A = p.u. 5) Intial load conditions: =. p.u., =. p.u., =. p.u.,c dc =. p.u Appendix II K = (X q X d ) {I tsd (X q +X q ) (V b cosδ) + I tsq (X d + X d ) (V b sinδ)} + E q {(X q + X q ) V b cosδ} K = (X q -X d ) {I tsq X d } + E q I tsq K = + (X d -X d ) X d K 4 = (X d -X d ) (X d + X d ) (V b sinδ) K 5 = {( ) X q (X q + X q ) (V b cosδ) ( ) X d (X d + X d ) (V b sinδ)} K 6 = {( ) (-X d X d )},,,,, K pdc = (X q -X d ) {I tsq.x d (m k sinψ) I tsd X q (m k cosψ)} - E q {X q m k cosψ} K pψ = (X q -X d ) {I tsd X q (m k V dc sinψ) + I tsq X d (m k V dc cosψ)} - E q {X q m k V dc sinψ} K pm = (X q -X d ) {I tsq X d (k V dc sinψ) I tsd X q (k V dc cosψ)} E q {X q k V dc cosψ} K qdc = (X d X d ) X d (m k sinψ) K qψ = (X d -X d ) X d (m k V dc cosψ) 48

9 July 5 K qm = (X d -X d ) X d (k V dc sinψ) K vdc = - {( ) (m k cosψ) X q X q + ( ) (m k sinψ) X d X d } K vψ = {( ) X q X q (m k V dc sinψ) ( ) X d X d (m k V dc cosψ)} K vm = - {( ) X q X q (k V dc cosψ) + ( ) X d X d (k V dc sinψ)} K dm = ( ) [m k V dc sinψ cos (X d -X q ) - k (I sq sinψ I sd cosψ)] K dψ = ( ) [{(m k cosψ) V dc X d + (m k sinψ) V dc X q ) + (I sq m k cosψ I sd m k sinψ}] C= where a = - ( ) C= C = ( ) [{(X d + X d ) (m k cosψ) (V b sinδ) + (X q + X q ) (m k sinψ) V b cosδ] C 4 = ( ) [(m k cosψ) X d ] C 5 = ( ) [m k cosψ sinψ (X d -X q )] C 6 = = b = b = b + k dc b 5 b = {-(X s ) (X q + X q ) (V b cosδ) + j (X s ) (X d + X d )(V b sinδ)} b = {X s X d } b = {( + X s X q ) (m k cosψ) + j ( + X s X d ) (m k sinψ)} b 4 = {(+ X s X q ) (k V dc cosψ) + j (+ X s X d ) (k V dc sinψ)} b 5 = {-( + X s X q ) (m k V dc sinψ) + j ( + X s X d ) (m k V dc cosψ)} References: [] Rangy Sunny and Robins Anto, Harmonics Control and Performance Analysis of a Grid Connected Photovoltaic System International Conference on Advanced Computing and Communication Systems (ICACCS - ),, Coimbatore, INDIA,Dec. 9,. [] Yi Zhang, Songzhe Zhu, Robert Sparks, Irina Green, Impacts of Solar PV Generators on Power System Stability and Voltage Performance IEEE Power and Energy Society General Meeting,pp - 7,. [] Schoene J., EnerNex, Knoxville, Zheglov V., Houseman D., Smith, J.C., Photovoltaics in distribution systems Integration issues and simulation challenges IEEE Power and Energy Society General Meeting, pp. 5,. [4] Julio Romero Agüero, Steve J. Steffel, Integration Challenges of Photovoltaic Distributed Generation on Power Distribution Systems, IEEE Power and Energy Society General Meeting, pp. 6,. [5] S. Dahal, N. Mithulananthan and T. Saha, An approach to control a photovoltaic generator to damp low frequency oscillations in an emerging distribution system, in Power and Energy Society General Meeting,.IEEE, pp. -8,. [ 6] Rodrigo Hartstein Salim and Rodrigo Andrade Ramos, A Model-Based Approach for Small-Signal Stability Assessment of Unbalanced Power Systems, IEEE Transactions on Power systems, vol. 7, No. 4, pp. 6 4,. 49

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