Calculation and Compensation of PCC Voltage Variation Using a Grid Connected Inverter of a Wind Turbine in a Weak Grid

Transcription

1 Calculation and Compensation of Voltage Variation Using a Grid Connected nverter of a Wind Turbine in a Weak Grid Ji-Hoon m, Seung-Ho Song Dept. of Electrical Engineering in Kwangwoon University, Seoul, , Korea hipihipiyo@kw.ac.kr, ssh@kw.ac.kr Abstract n a hybrid power system where wind power generators are installed in a weak grid, the prediction, investigation and compensation study about the power quality is necessary. Especially, the voltage variation is one of the most important issues for the application of high penetration of wind farm in a week grid. As part of an experimental program to evaluate wind turbine generator performance on the isolated diesel power system, a simplified model for voltage deviation calculation is developed and tested according to the installation capacity of wind power. A method of voltage compensation is proposed using the reactive power output which can be calculated analytically. Results of calculation and compensation algorithm are compared to mitigate the voltage variation problem. As a result the prediction error of the voltage variation at (Point of Common Coupling) was lower than 1% of the actual value. The voltage variation is ively compensated using the proposed method and the required increase of the inverter capacity for the compensation is calculated.. SMPLFED MODELNG OF THE POWER SYSTEM N SAPS- SLAND A. Wind Turbine Modeling The installed wind turbine is a 1kW PMSG (Permanent Magnet Synchronous Generator) type with PWM inverter for grid connection. As shown in Fig. 1 the wind turbine can be modeled as a controllable current source with variable wind speed if the inverter performs current control successfully with power factor control function. The installed WECS has unity power factor with variable output current (Power) according to the change of wind speed. [6]. NTRODUCTON n Korea, most of the isolated grid is powered by the diesel generations system. Such system has several problems such as a high cost of fuel with extra transportation fee and negative environmental s.[1] As an alternative of this problem, the wind power generation looks very attractive especially in such cases if the partial loads can be powered by the clean wind energy. To reduce the cost of energy and maximize the saving the high penetration ratio is desirable as large rating of the wind energy conversion system as possible. But the parallel operation of wind turbine with conventional diesel generator would affect the conventional grid. The larger wind power generation system capacity is, the more influence on the grid exists. From the field measurement of power quality according to national rule and the international standard, EC614-1, the voltage variation at was the most critical item. [] n order to study power quality from aspect of voltage variation, this paper proposed a simple and helpful prediction model of voltage variation as the rated power of a wind turbine is increased and reactive power control strategy to compensate voltage variation. A simplified simulation model using PSCAD/EMTDC and a T-equivalent circuit model of the local power system is developed and evaluated. [3~5] Fig. 1 Simplified modeling of a grid-connected wind turbine B. Grid System Modeling in Sapsi-island A simplified block diagram of the power system in Sapsiisland is shown in Fig.. A wind turbine is connected to the grid at the which is a low voltage side of TR3 of which the power rating is 15(kVA). The total power rating of four diesel generators is 9(kW) and the average load of the island is 1kW. The minimum capacity of the operating diesel generator is 3kW in the island. Some parameters are listed in Table 1 and utilized to build up the simplified simulation model of the power system in the island. The simulation model consists of a diesel generator, power lines, transformers, loads and a wind turbine.

2 Table Equivalent impedances in generator voltage level. Symbol Descriptions Value deal Generator Regulated Voltage Source 38(V) T1 TR1 series impedance 75[kVA] (Ω) T TR series impedance 1[kVA] (Ω) T3 TR3 series impedance 15[kVA] (Ω) line High voltage line impedance. 7.13(Ω) L1 mpedance of Load (Ω) L mpedance of Load (Ω) Fig. A Simplified diagram of the power system in Sapsi-island. mpedance of Wind Turbine Connection (Ω) Table 1 Some parameters of the power system in Sapsi-island. Symbols Descriptions Ratings Diesel Generator Synchronous generator with voltage regulator and electronic governor (Total number of unit is 4.) 15(kVA) units, 3(kVA) units TR1 38/66[V] Transformer 75(kVA) TR 66/38[V] Transformer 1(kVA) TR3 66/38[V] Transformer 15(kVA) Wind Turbine 3-bladed PMSG with full power inverter Single phase 1(kW) Fig. 4 Simulation about voltage variation versus wind turbine output power. T L1 Fig. 3 mpedance diagram of the power system in Sapsi-sland. The circuit diagram and impedance parameters for the simplified simulation of Sapsi-island are shown in Fig. 3 and Table respectively. A PSCAD/EMTDC simulation model shown in Fig. 5 for the simplified Sapsi-island is developed using these parameters. The diesel generator is simplified as an ideal voltage source in the diagram because the feedback controller of the diesel generator regulates the output voltage ively. The variation of voltage is plotted according to the output power of the wind turbine in Fig. 4. [7~9] Fig. 5 The power system PSCAD model in Sapsi-sland.

3 . ANALYSS OF VOLTAGE VARATON DUE TO WND TURBNE CAPACTY Regardless of the complexity of actual power system network, the equivalent circuit diagram of T-shape can be derived at the for the simple calculation of voltage deviation due to the wind power change. The T-equivalent circuit has three branches one is for the conventional voltage source and the other is for the current source of wind power and the last represents loads on the system as shown in Fig. 6. The parameters in Table 3 are the equivalent impedances in generator voltage level for the T-equivalent circuit. Voltage Regulator Governer Diesel Generator V G P G Q G G V L P L Q L Local Loads = + (1) LL = L load G LL () LL + G V = V + (3) V = V x (4) θ (5) ΔV = V Where V L load G G x + V x cosθ + Vx Equivalent Line mpedance between Equivalent mpedence of Load Output Current of Wind Turbine Equivalent Line mpedance between Wind Turbine and Terminal Voltage of the Grid load and (6) Equivalent Line mpedance between nternal Combustion Power Plant and P Q V Wind Turbine Fig. 6 Proposed T-equivalent circuit model of an isolated power system with wind turbine. Table 3 mpedances for the T-equivalent circuit model. Symbol Descriptions Value deal Generator Regulated voltage source / Base voltage 38(V) G LL Equivalent line impedance at to the diesel generator Equivalent load impedance at including the line impedance for the load connection Equivalent impedance at for the connection of wind turbine (Ω) (Ω) (Ω) The equivalent load impedance in (1) includes the load impedance and the line impedance for the load connection. f we define the parallel impedance of G and LL as the ive impedance, as (), the voltage deviation at the (Point of Common Coupling) is analytically expressed as the ive line impedance at the multiplied by the current from the wind turbine in (3). Also it s shown that the value of the impedance of connecting cable between the and the wind turbine has no on the voltage at the. [1] n case of low voltage-side connection of wind turbine, the voltage deviation at can be significant problem because the other local loads can be connected to the and the ive line impedance becomes high because it includes the impedance of the grid-connection transformer, TR3. Fig. 7 voltage variation phasor diagram by wind turbine output power. To calculate the amount of voltage variation ΔV, the phasor vector diagram analysis is used as shown in Fig. 7. The voltage without wind power, V is considered as the reference voltage as (4). t is assumed that the current vector of the wind turbine is aligned to the phase angle of V in (4), which is achieved in many grid connected inverters with unity power factor control function. So the angle of the voltage deviation vector is determined by the ive impedance, as shown in Fig. 7. As a result, the amount of voltage variation can be calculated V -V by the subtraction of the magnitude of V from the magnitude of V in (6). Note that the magnitude of V can be different as the phase angle of varies even though the size of and is the same. The resultant voltage vector, V is changed from the original V both in magnitude and angle. n Fig. 8 the maximum value of voltage variation is plotted according to (6). The maximum voltage variation is almost proportional to the amplitude of when the output power of the turbine is fixed as 1kW in Fig. 8. f the is fixed as the value from the Table 3, the voltage variation increase with the increase of the installed wind power rating.

4 9 Maximum value of Voltage Variation versus 8 Maximum Voltage Variation(V) Amplitude of ( ) (a) Maximum Volue of Voltage Variation versus Wind Turbine Rated Power 7 (b) 6 Maximum Voltage Variation (V) Wind Turbine Rated Power (kw) (b) Fig. 8 Prediction of voltage variation according to the wind turbine rated power and (a) voltage variation versus with constant wind turbine output power (1(kW)) (b) voltage variation versus wind turbine rated power with constant (indicated in Table 3) V. REACTVE POWER COMPENSATON (c) Fig. 9 voltage variation by supporting reactive power (a) voltage compensation phasor diagram using adding reactive power. (b) voltage compensation phasor diagram due to inverter capacity. (c) voltage compensation phasor diagram due to voltage allowance range V _new V _old V (a) V = _comp θ The proposed method of addition of reactive output current of the wind turbine is shown in Fig. 9 for the compensation of the voltage variation at. As shown in Fig. 9 (a), the reactive current component, which is perpendicular to the active current vector, is added to the voltage vector and the resultant voltage vector of the can be located on the circle of the zero variation of voltage. The magnitude of the required reactive current component voltage vector is depends on the magnitude of the active current multiplied by the ive impedance. However the relationship is not linear and the solution can be calculated from the system of equations of the circle and the line as (9). f the active current is quite large due to the high wind speed, the required reactive current cannot be supplied by inverter due to the current rating of the semiconductor devices. Two current limit circles of the

5 inverter system are shown in Fig. 9 (b). One is the circle with 1% of the rated current limit and the other is with 15%. The larger inverter current rating, the wider range of voltage compensation can be obtained. How much of over-sizing of the inverter is necessary for the perfect compensation of the voltage variation in full variable inverter-based wind power generation system? f we have a range of permissible voltage variation, it also can be considered in the calculation of the required magnitude of reactive compensation. For example, if it is allowed that.3% of voltage variation at, the range, m, is 5(V) and the required magnitude of compensation can be reduced from 13.5V to 7.7V as shown in Fig. 9 (c). The required reactive power of the wind turbine for the compensation of voltage is plotted according to the generation power amount in Fig. 1. f the range of allowed voltage variation, m=5[v], the required reactive power can be reduced as shown in solid line in Fig. 1. _comp = V θ Where Q Q comp x sin θ ( Vx + m) ( Vx cos + _comp Required Output Reactive Current of Required Output Reactive Power of nverter nverter Amplitude of Required Voltage Vector for Compensation ) (9) Additional Current Rating (%) Required Additional Current Rating of nverter m 5 Active Power Output(kW) Fig. 11 Calculated inversing ratio of inverter capacity versus voltage variation margin and wind turbine rated power. For the compensation of the voltage, the additional amount of current rating of the inverter is calculated and compared in Fig. 11. As the total current rating is calculated from the square root of the sum of the active and reactive current components, the amount of additional current rating is not proportional to the required reactive power. With given, the additional current rating is 13% for the perfect compensation of the voltage during the active power output of 1kW. With the proper margin of voltage variation, the additional current rating can be reduced as shown in the graphs in the right side of the Fig. 11. = (1) _comp _comp _comp Q = V (11) Fig. 1 shows required wind turbine reactive power for the proper voltage variation versus wind turbine rated power using (9). f voltage variation margin is low, the more reactive power is required. Required Wind Turbine Reactive Power(kVar) Required Wind Turbine Reactive Power m=(v) m=5(v) V. COMPARSON OF VOLTAGE VARATONS Fig. 1 shows the 1kW wind turbine installed in Sapsiisland, Korea. Fig. 13 shows the measured voltage data against the output power of wind turbine. The sampling frequency is 1 Hz. The center line represents the ive impedance at and the dotted lines shows the variation margin due to the voltage regulation error which can be recognized during the zero output power generation. [11~13] All three results of voltage deviation at with the same operating condition of wind power generation, when wind turbine output power is 5.5(kW), are compared in Table Wind Turbine Rated Power(kW) Fig. 1 Required wind turbine reactive power due to different voltage variation margin. Fig. 1 1kW Wind turbine in Sapsi-island.

6 Voltage[V Voltage Variation Wind Turbine Output[W] Fig. 13 voltage variation amplitude by wind turbine output power. Table 4 Comparison of the voltage deviation with the same wind power generation. Voltage deviation Model at [V] PSCAD /EMTDC simulation model 6.4 T-equivalent circuit model (Proposed circuit for prediction of voltage variation) 6.1 Measurement data (averaged) 6.9 V. CONCLUSON n a small isolated wind-diesel hybrid system, the voltage variation problem is investigated for the power quality evaluation according to the variable wind power generation capacity. The steady state voltage error exists during the generation and the amount of the voltage deviation depends on the equivalent line impedance and the load impedances. To minimize the voltage deviation at, more efforts to minimize the equivalent short circuit impedance such as the selection of better location of installation are necessary. Using the proposed T-equivalent circuit model, faster and easier evaluation of the voltage variation at is possible in early design stage of wind farm installation. f the voltage variation during the wind power generation is unavoidable, the proposed voltage compensation method can be applied using exact calculation of the required reactive power components. The current limit of the inverter is investigated for the better understanding of the compensation limit. t is analytically shown that the additional amount of reactive current can be calculated with consideration of the margin of the voltage variation. REFERENCES [1] H.-N. Jang, S.-D. Kim, A Pre-Feasibility Test of ntroducing Renewable Energy Hybrid Systems Case Studies for 3 Off-Grid slands, Environmental and Resource Economics Review, Volume 15, No. 4, September 6, pp.693~71 (in Korean) [] Power Electronics Laboratory of Kwangwoon University, "A Study about Assessment of Power Quality as Measurement of Power Quality with a Small Size Wind-Diesel Hybrid Power System", Journal of Power Electronics, 8.1, The Final Report of the Project on Consugnment [3] J.-J. Kim, S.-H. Song, Grid Connection Simulation Model of Variable Speed Wind Turbine Using PM Synchronous Generator, CPE 4, October 4, pp.649~654 [4] Kannan Rajendiran, W. W. L. Keerthipala, C. V. Nayar "PSCAD/EMTDC Based Simulation of a Wind-Diesel Conversion Scheme" [5] J. T. Bialasiewicz, E. Muljadi, S. Drouilhet, G. Nix "Modular Simulation of a Hybrid Power System with Diesel and Wind Turbine Generation" Windpower '98 Bakersfield, CA April 7-May 1, 1998 [6] Excel-S 1kW Turbine Specs.3.pdf [7] S.-J. Kim, S.-J. Seong, "A Simple Prediction Model for Voltage Variation Due to Active Power Fluctuation of a Grid Connected Wind Turbine", Journal of Power Electronics, 9.1, p85-p9 [8] S.-J. Kim, J.-H. m, S.-J. S.eong, S.-H. Song, Prediction Model for Voltage Variation due to Active Power Fluctuation of Grid Connected Wind Turbine [9] S.-J. Kim, S.-J. Seong, "A Simple Prediction Model for Voltage Variation Due to Active Power Fluctuation of a Grid Connected Wind Turbine", Journal of Power Electronics, 9.1, p85-p9 [1] J. H. m, S S. Song, Modeling and Analysis of Voltage Variation due to the Wind Power Fluctuation in an solated Grid, WWEC 9, 9. 6 [11] EEE 1547, EEE Standard for nterconnection Distrubuted Resources with Electric Power Systems, 3 [1] EC614-1 Wind Turbines Part 1 Measurement and Assessment of Power Quality Characteristics of Grid Connected Wind Turbines [13] Stavros A. Papathanassiou, Fritz Santjer, "Power Quality Measurements in an Autonomous sland Grid With High Wind Penetration", Transactions on Power Delivery vol.1, EEE, 6.1, p18~p4s ACKNOWLEDGMENT This paper is the outcome of a research center of breakthrough technology program supported by the Ministry of Knowledge and Economy (MKE).