1
PQ and PV Control of Photovoltaic Generators in Distribution Systems Sarina Adhikari, Student Member IEEE , Fangxing Li, Senior Member IEEE , Yan Xu, Senior Member IEEE , Huijuan Li, Member IEEE
Abstract— Abstract— The increasing concerns in environmental issues and depleting fossil fuel reserves lead to a growing interest towards renewable energy based distributed energy resources (DERs) like photovoltaic (PhV) generators. With maximum power point tracking (MPPT) controls, the efficiency of the PhV systems can be optimized. A method of MPPT control based on a power balance between the DC and AC sides in a two-stage PhV configuration is proposed in this paper. With proper controls of PhV inverters, these generators are capable of producing necessary amount of reactive power so as to provide required voltage support. Similarly, the amount of active (P) and reactive (Q) power can be controlled to match the local load profile. The MPPT control is integrated with voltage control to fulfill the PV control objective. The control algorithms have been tested in an IEEE 13-bus distribution feeder. The simulation results clearly demonstrate that the proposed control approaches for PhV generators is capable of maintaining the PhV node as a PQ or PV bus with active power being controlled at MPP. Index Terms—Photovoltaic generators; active and reactive power control; Maximum power point tracking(MPPT); voltage control
I. INTRODUCTION DERs are the generating units or energy storage elements located close to the load centers with a capacity range of 10 kW to 50 MW [1]. The growing concerns on depletion of fossil fuels supplemented by related environmental issues and rising threats to energy security and power system stability together act as the major drivers for the deployment of DER in modern power systems. DERs like solar photovoltaic (PhVs) and wind, are clean technologies and are capable of providing ancillary services in the form of reactive power through power electronics interface, in addition to the active power. The increased penetration of these technologies in the modern distribution system calls for the improved controls to provide required ancillary services to support the grid. The DER interconnection standard IEEE1547 is also being modified to address a Volt-Var support through DERs and renewable energy resources and other interconnection issues in a newer version IEEE 1547.8[2].
S.Adhikari and F. Li are with Department of Electrical Engineering and Computer Science, The University of Tennessee, Knoxville, TN 37996, USA. Y. Xu and H. Li are with Power and Energy Group, Oak Ridge Ridge National Laboratory, Oak Ridge, TN 37831,USA. Contact: F. Li, 1-865-974-8401,
[email protected].
Several methods of controlling the active and reactive (a.k.a. nonactive) power injection from the DER using power electronics interface have been proposed [3]. These include modified Newton Raphson algorithm, the use of back to back converters and the application of the widely accepted method of Park’s transformation in the dq0 domain. The application of Proportional, Integral and Derivative (PID) controllers in developing the dynamic voltage control models of DER with power electronics interfaces is discussed in [8]-[9]. On the other hand, due to the intermittency and variability of the renewable energy resources, the power output of these resources should always be maintained at the maximum possible point through Maximum Power Point Tracking (MPPT) controls to ensure the efficient operation. One of the several methods to find MPP is to plot the power versus voltage (P-V) curves and noting down the maximum point [12]. The present work uses this method to find out the MPP for its simplicity to focus on our proposed controls, although there are other methods found in literatures like Perturb and Observe (P&O) and incremental conductance (IC) methods [14]-[17]. In this paper, first, a PQ control algorithm is proposed for controlling active and reactive power injections from PhV generators connected to a three-phase distribution system. Second, a PV control method is proposed for tracking the maximum power of a solar PhV array connected to a distribution system. It includes the control of both PhV array power output (MPPT control) and the inverter output for maximum active power and voltage (PV) control. This is a simple yet effective MPPT and inverter control algorithm which is unique in controlling the DC side voltage by satisfying the power balance objectives at the DC and AC sides without an additional control loop. Also, the specialty of both the control methods is that they are entirely developed in the abc reference frame with the measurement of the control variables based on instantaneous power. This greatly simplifies the controls in avoiding the hassles of conversions between reference frames if abc and dq0. Also, it is not sensitive to measurement noise since the proposed approach is based on measurements of the past cycle rather than at a particular time instant. The rest of the paper is organized as follows. Section II presents the background of the current research. Section III describes the proposed PQ control algorithm used in this study and the proposed method of PhV MPPT and voltage controls. Section IV shows the simulation results of both the control methods described above. Section V summarizes the major contributions of this work.
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II. RESEARCH BACKGROUND 10
10 2
A. Modeling of Solar Photovoltaic Array
1000 W/m 2
The commonly accepted solar cell model is a one diode model [20]. This work uses the single diode model of the solar cell to model the Kyocera KC200GT solar array, which is shown in Fig. 1. This solar module is chosen in particular in order to easily validate the simulated I-V curve with the experimentally available curve from the datasheet. The I-V characteristics of a solar array, as shown i n [20], are represented by (1).
− 1− = − exp = "#$% ! ! " 1&'()*+)' , 1)-./ 0$ -567 1& * )213*4* , 1) 8 8 ,
(1)
where and are the photo current and the diode saturation currents, respectively. is the thermal voltage of the array, is the number of cells connected in series for greater output voltage, is the Boltzmann constant ( ), T (Kelvin) is the temperature of the p-n junction of the diode, and q ( ) is the electron charge. Also, and in (1) are the equivalent series and shunt resistances of the array, respectively; and a is the ideality factor usually chosen in the range 1 a 1.5. Here a is taken as 1. The photocurrent of the PhV array depends linearly on the solar irradiance and the cell temperature, as shown by (2) [20].
!
!
99
= :;< >#? @@A ;< 2+B >#
(2)
Here, is the photocurrent at the standard test condition (STC, and 1000 W/m2), is the short circuit current/temperature coefficient, is the dtifference between the actual and nominal temperature in Kelvin, G is the irradiation on the device surface, and Gn is the nominal radiation, both in W/m2.
;<
800 W/m
8
8
2
600 W/m
2
400 W/m 6
2
200 W/m
) A ( I
) A6 ( I
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4 25 oC o
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50 C 75 oC 100 o C
0
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30 35 V (Volts)
0
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30 35 V(volts)
a) b) Fig. 2. The I-V characteristics of Kyocera KC200GT from simulation with a) varying irradiance at a cell temperature of 25°C and; b) varying cell temperature at 1000W/m 2.
B. PhV System Configurations
A system configuration of PhV with an inverter interface considered for PQ and PV control is shown in Fig. 3. For MPPT control, a DC-DC booster circuit is added before the inverter stage as shown in Fig. 3b. An instantaneous active power and nonactive power theory [18] [19] is employed to develop the control algorithms. The PhV system is connected in parallel with the grid through a coupling inductor Lc. The coupling inductor filter out the ripples in the PhV output current. The connection point is referred to as PCC as referred above, and the PCC voltage is denoted as vt . The rest of the system denotes the IEEE 13-bus distribution feeder which is simplified as a substation with the feeder equivalent impedance, R + jω Ls. The PhV energy source is connected to the DC link of the inverter with a capacitor C dc. The PhV energy source is the active power source, and the capacitor is the reactive power source of the PhV system.
can be calculated based on (3).
;< = !C
(3)
+
Fig. 1. One diode equivalent circuit of Solar PV.
Using these fundamental equations and parameters from the data sheet, the PhV model is developed and verified with the panel datasheet. The I-V characteristics of KC200GT for different irradiance levels at the cell temperature of 25°C and various cell temperatures for a constant irradiance level of 1000W/m2 as obtained from the simulation are shown in Fig. 2 (a and b) respectively. The similarities of the I-V curves for different conditions with the corresponding curves in the KC200GT panel datasheet prove the validity of the solar panel model in MATLAB/SimPowerSystems.
a)
b)
Fig. 3. Integrated PhV system configuration a) PQ Control; PV Control
b)
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C. Brief Review of Instantaneous Power Definitions
According to the instantaneous power definitions, for a balanced three-phase system, if vt (t ) and vc(t ) denote the instantaneous PCC voltage and the inverter output voltage (harmonics are neglected), respectively, then, the average power of the PhV denoted as P(t ), the apparent power S(t) and the average nonactive/reactive power Q(t ) of the PhV are as given below [19]:
A control scheme is developed accordingly with two feedback control loops as shown in Fig. 4. The inner loop 1 controls the reactive power Q(t ) by controlling the amplitude of vc(t ) while the outer loop 2, the active power P(t ) by controlling the phase angle α of vc(t ). The active power and reactive power are considered as the variables to be controlled. The instantaneous inverter output voltage vc(t ) is controlled to be in phase with the PCC voltage vt (t ). A PI controller PI1 is used to control the magnitude of the inverter output voltage, vc(t ) using (9)
D:E? = F. G- LM H:I?JC:I?KI = OPNN QRST U:E? = :E?C :E? = OPN V. C. − 2C WXQY HC5\ = ]15^Z_ − ZC`5 ab^Z_ − ZC`KEcH:E? Z:E? = :E?C<:E? = V U.:E?− D.:E? = OPN :C WXQY − ?
(4)
(5)
(6)
Here, α is the phase angle of vc(t ) relative to the PCC voltage. P(t ) and Q(t ) in (4) and (6) can be approximated by the first terms of the Taylor series if the angle α is small, as shown in (7) and (8):
D:E? [ OPNN Y Z:E? [ OPN : − C?
(7)
(9)
In (13), 1 is added to the right-hand side of the expression so that when there is no injection from the PhV generator, the PhV output voltage is exactly the same as the terminal voltage. The inverter active power control is realized by controlling the phase angle of the inverter output voltage. The active power control loop is described by (10). The phase angle of vc(t ) is controlled by the PI controller PI2, where Pref is the reference, and Pact is the actual value.
Y\ = .^D_ − DC` . Gb ^D_ − DC `KE
(10)
(8)
III. CONTROL METHODS FOR SOLAR PHV GENERATORS In this section, an active and reactive power (PQ) control algorithm is proposed in the case of solar PhV generators to control the desired amount of active and reactive power as demanded by the local load. Then, a more complicate control approach is proposed for MPPT/PV control for solar PhV generators such that the solar PhV system may act as a PV bus with MPPT. A. Active and Reactive Power (PQ) Control Algorithm
This control method is based on a popularly used Proportional and Integral (PI) controller and hence, is a simple, however very effective measurement based method of Fig. 4. Active power and reactive power control diagram. controlling the power injections from the PhV generator. The three phase instantaneous active and reactive power from the B. PhV MPPT (P) and Voltage (V) Control alg orithm PhV generator can be calculated directly from the This subsection describes the MPPT and voltage (PV) measurement of the terminal voltage, vt and the inverter current, ic. The PQ control problem can be considered to be control algorithm which is shown in Fig. 5. A DC-DC booster the case in which PhV generator is part of an islanded circuit is added in front of the inverter making it is as a two microgrid and it has to generate the power with the set point stage configuration. This MPP operation of PhV can be the provided by the central controller so as to supply some local case when these PhV generators are connected to the utility grid and try to support the grid by supplying the clean power loads. In (7) and (8), with the assumption that the variation of V t and at the same time avoiding the installation of the expensive can be neglected, that is, V t is constant, then the average storage system. Since it is connected to a utility system, the nonactive/reactive power Q(t ) is proportional to the magnitude PhV generator, only a small portion of the total energy of the inverter output voltage vc(t ). However, the average suppliers, is assumed to be dispatched at its maximum power active power P(t ) is dependent on both the amplitude V c and point for economic consideration. With this configuration, the loads equal to the MPP of the installed PhV generator would the phase angle α of vc(t ).
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be supplied by these generators and only the remaining portion of the loads would be supplied by utility.
the look up table for the corresponding irradiance level and cell temperature at first. Then, three different control loops are used for the MPPT and voltage control as shown in Fig. 5. The loop 1 is a MPPT control at the PV array side which uses the reference MPP, P MPPref from the look up table, compares the actual PhV power output (PPV ) with this reference, and feeds this error to a PI controller, PI 1 which outputs the duty cycle δ* for the DC-DC booster such that the array always operates at the referenced point by changing this duty cycle. The equation for this control loop is given by (11). Here, K p1and Ki1 are the controller’s proportional and integral gains, respectively, for this particular control loop.
d\ = 5 \ ^Df_ − D ` g5 \ ab:Df_ − D? KE (11)
Fig. 5. MPPT and voltage (PV) control diagram of solar array. 220 1000 W/m
200
800 W/m
180
600 W/m
) 160 W ( 140 r e w 120 o P 100
400 W/m 200 W/m
2
o
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25 C o
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50 C
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) 150 W ( r e w o P100
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0 0
a)
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b)
Fig. 6. PV Curves of KC200GT panel: a) with varying irradiance; b) with varying cell temperature 200
) W ( t n150 i o p r e w o100 p m u m 50 i x a M
0 200
400
600 2 800 Irradiance (W/m )
a)
1000
220 ) W ( t 200 n i o P r180 e w o P160 m u m i x140 a M 120 20
40 60 80 Cell Temperature (oC)
100
b)
Fig. 7. Relationship of KC200GT panel Maximum Power Point (MPP) with a) irradiance; b) Cell Temperature.
The P-V curves of KC200GT solar panel for varying irradiance and varying temperature are shown in Fig. 6a and 6b, respectively. Also, Figs. 7a and 7b show the relationship between maximum power point (MPP) of the solar panel with irradiance and cell temperature respectively. MPP seems to be directly proportional to the irradiance level and inversely proportional to the panel cell temperature, however, both the relationships are linear. Hence, a look up table with a linear approximation to find the missing data is a reasonable assumption to make. In this work, the MPP is obtained from
At the inverter side, there are three PI control loops in order to transfer this maximum active power to the output of the inverter. The control loops comprise of the inner loop 2 for voltage control at the AC side [10] and the outer loop 3 comprising of other two PI controllers for the control of active power output of the inverter and indirect control of DC side voltage. These control methods are developed for the inverter connected to the IEEE 13 bus distribution feeder. For voltage control at the AC side, feedback PI controller, PI2 is used. As shown in the control diagram in Fig. 5 (loop 2), the PCC voltage is measured and the rms value of vt (t ) is calculated. Then, the rms value V t( t ) is compared to a voltage reference V t* (t ) (which could be a voltage specified by the utility) and the error is fed to a PI controller. The inverter * output voltage V c(t ) is the reference to generate pulse width modulation (PWM) signals to drive the inverter. The output voltage of the inverter is controlled so that it is in phase with the PCC voltage, and the magnitude of the inverter output voltage is controlled so that the PCC voltage is regulated at a * given level V t (t ). The control scheme can be specifically expressed as (12).
HC\:E? = H:E?1 .^\:E? −:E?` . Gb ^\:E? − :E?`KE
(12)
where K P, K I are the gains of the PI controller 1. Note, in (12), 1 is added to the right-hand side for the same reason as described for the value 1 in Eq. (9). The loop 3 is the outermost loop to control the active power output at the inverter side. The basic control methodology is the same as previously described in subsection III-A. However, it comprises of two different control loops to control the active power at the AC and DC sides, respectively. The control of the active power at the DC side is denoted by (13). Here, the reference signal is obtained from the output power of the PhV array, PPV multiplied by efficiency of the DC-DC booster which is taken here as 98% in order to account for the losses in the converter circuit. This reference is then compared with the measured DC power output of the booster, PDC shown
5
in Fig. 5 and the error is fed to the PI controller, PI 3. The output of this loop is a phase shift contribution from the DC side of the inverter. Similarly, another PI controller loop, PI4 is used to obtain the phase shift contribution, required to control the active power at the output of the inverter to be equal to 96% of the PPV. This value of efficiency is considered so as to account for the combined losses of the DC-DC booster and inverter circuits. The equation for this control loop is given by (14). The phase shift contributions from DC and AC sides, and are then averaged as given by (15) to obtain the final phase shift, of the voltage waveform , vc1* which will then generate the voltage reference signal vc* for the inverter PWM.
Y5\ Y.\
Y5\ Y.\ Y\
Y5\ = /:)&h(\ D − Dij ? / a b:)&h( \ D − Dij ?KE
(13) Y.\ = k:)&h* \ D − Dlj ? k a b :)&h* \ D − Dlj ?KE (14) \ \ \ (15) Y = :Y5 Y. ?$2
controlled as given by (19), V DC will be indirectly controlled to a value required to maintain the voltage at the AC side at the referenced utility voltage which would be verified from the results presented in the following section. Note, the controller gain values for the DC active power control loop should follow the pattern of the AC side active power loop so that the phase shifts, and can be reasonably averaged to obtain .
Y\
Y5\ Y.\
IV. SYSTEM CONFIGURATION AND SIMULATION RESULTS The system diagram of the IEEE 13 bus distribution test system is shown in Fig. 8 consists of a substation, 13 buses or nodes, 11 lines, and 8 loads. The loads comprise of a combination of constant impedance, constant current and constant power (ZIP) loads as shown in the figure. The substation is connected to the 115 kV transmission system and the voltage is stepped down to 4.16 kV (RMS, line-to-line) by a distribution transformer (T1). There is one more transformer (T2) which steps down 4.16 kV to 480V to supply a particular load.
Here, the reason behind considering phase shift contributions from both DC and AC side active power is to control the DC side voltage to the desired value. By making and in a close range through the controller gains, it can be assured that the active power at the DC and AC sides is balanced. This coupled with the voltage control loop assures that the DC side voltage is maintained at the value desired by the AC side voltage. This is a very unique characteristic of the proposed control algorithm in that the DC side voltage is controlled indirectly through other control loops. A simple explanation follows next.
Y5\ Y.\
Since the inverter is considered to be 98% efficient, we have:
Dlj = )&h(\Dij Dij = 1&)2)4 \Dlj
which means ,
1* :13? (
)
Substituting the value of P AC ,
Dij = 1&)2)4 \ OPNN Y
:1(?
Fig. 8. Diagram of IEEE-13 bus distribution feeder.
A. PQ Control
That is,
mmN 5& b .bk\ ij = qrnoN
:1h?
From (18), P DC is also dependent on all the parameters at the AC side of the inverter. Hence, it is clear that DC side active power is also directly related to the phase shift of the AC side voltage waveform for the given V t ( t ) and V c(t). Since the voltage at the AC side, V t (t ) is being controlled, the active power control loop at the AC side determines the current being drawn from the DC side and the active power control loop at the DC side assures that the required amount of current, I DC is being drawn from the PhV array. But, since the DC side voltage is dependent on all other quantities being
In order to demonstrate the PQ control algorithm in the IEEE 13-bus distribution feeder, Bus 675 is chosen for study. The P and Q references of the inverter-based PhV generators are taken from the active power load, Pload , and the reactive power load, Qload , of Bus 675. The base case load of Bus 675 is 29kW and 25kVar. Two cases are considered for this study. = • Case1: Pload is increased to a peak value of 43.5kW at t 5 s and Qload is increased to a peak value of 34.5kVar at t = 9 s. • Case 2: both Pload and Qload are increased to the above mentioned values at t = 5 s. Fig. 9 (a and b) shows the active and reactive power profiles of the PhV generator as compared to the local load profile of
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Bus 675 for Case 1. As shown in the figures, both active and reactive load profiles at both loading levels are closely tracked by the PhV generators installed at that bus. The ripples seen in Fig. 9(a) at t = 9s is an effect of reactive load change at that instant. Similarly, Fig. 10 (a and b) shows similar profiles with both the loads increasing at the same instant in time. It is observed that the active and reactive power injection from inverter-based PhV systems closely tracked the load profiles before and after the load increase. Thus, with the proper inverter control proposed in Subsection III-A, the PhV systems can supply the local load through their dynamic behavior. 60 ) 50 W k ( r 40 e w o P30 e v i t 20 c A 10
PPVInverter Pref
0
2
4
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60 ) r a V k ( r 40 e w o p e v i t 20 c a n o N 0
QPVInverter Qref 2
12 t(s) 14
4
a) Active power
6
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12 t(s)
b) Reactive power
Fig. 9. Case 1: Active power and reactive power at Bus 675. 60
60
)50 W k (40 r e w o30 p e v20 i t c A
) r a50 V k ( r 40 e w o p30 e v i t 20 c a n o N10
PPVInverter Pref
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a) Active power
QPVInverter Qref 2
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Fig. 10. Case 2: Active power and reactive power at Bus 675.
B. MPPT and voltage (PV) Control
The proposed MPPT and voltage (PV) control is also tested for a PhV generator connected to the same bus 675 of the IEEE 13-bus feeder. With the topology as shown in Fig. 5, the PhV generator with the booster and inverter is connected to the system. Since, the IEEE 13-bus distribution feeder at Bus 675 is at 4.16kV (RMS, line-to-line), this voltage is stepped down to 480V (RMS, line-to-line) while connecting the PhV array/converters assembly in order to avoid a large DC voltage requirement. Two scenarios are considered for simulation to prove the effectiveness of the proposed control method. • •
Case 1: Change in solar irradiance level Case 2: Change in PhV cell temperature
Both of the above cases directly impact the power output of the PhV array and hence, affect the MPP . The PhV array under study for the proposed PV control has 125 strings with each string having 4 panels in series. The
MPP for a single panel of KC200GT at 1000W/m 2 and 25oC (STC) is 200 W. Hence, the MPP reference power for the entire array at STC is 125*4*200 = 100 kW. The value of MPP reference point for all the values of considered irradiance are obtained from the lookup table approach as described in Section III. Fig. 11 (a through e) shows the simulation results for the case when there is an increase in solar irradiance from 800 W/m2 to 1000 W/m2 at time t = 15s. Fig. 11a shows the PhV array active power output. The MPP reference for the irradiance of 800 W/m 2 is 79.25 kW. With the controller loop 1 in Fig. 5, the duty cycle, for the operation of DC-DC booster is obtained because of which the PhV array is capable of operating at the referenced MPP point as evident from the plot. It is clear that the actual PhV array power output follows the reference very closely. Fig. 11b shows the inverter active power output which clearly reveals the effectiveness of the active power control strategy in tracking the referenced waveform which is about 96% of the output of the PhV array as explained in Section III. Hence, this value is around 4% less than the one shown in Fig. 11a. From Fig. 11c, it is clear that the AC side RMS voltage is maintained at 273V (0.98pu) before and after the change in irradiance. Fig. 11d shows the plot of DC side voltage or inverter input voltage. It can be seen that it is stably maintained in the range of 700V to 800V before and after the irradiance level is increased. This is in the range of a value which is theoretically desired in order to achieve 480VRMSLL at the AC side of the inverter. From theory, which would be: in the present system. Hence, the indirect control of DC side voltage considering the power balance between the AC and DC side of the inverter is clearly demonstrated in this case. Fig. 11e shows the average active power at the DC and AC sides of the inverter. It can be seen that the active power is properly balanced while considering the inverter losses. Hence, the DC side active power is slightly greater than the AC side power.
d\
ij s 1&*'4 \ s 1&*'4 \4() s 3(+
PPft
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) W k (80 r e w o60 P e v i t c40 A
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0.985
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) u p ( e g0.98 a t l o V
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c) Voltage at PCC c) Voltage at PCC (RMS)
d) DC-DC booster output voltage 100 ) 90 W k ( r e80 w o p70 e v i t60 c A 50
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PDC Pinv 10
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PDC Pinv 10
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Fig. 12. MPPT and Voltage (PV) control results with varying cell temperature.
Fig. 11. MPPT and Voltage (PV) control results with varying irradiance.
V. CONCLUSIONS
Fig. 12 (a through e) shows the results for Case 2 when the cell temperature of the PhV array increases from 25 oC to 75oC at time t = 13s at the irradiance level of 1000 W/m 2. There is an inverse relationship between the cell temperature and MPP. Hence, with the temperature increase, the MPP reference decreases from 100 kW to 75.7 kW. Fig. 12a shows the plot of active power output of the PhV array. It is clearly visible that the controls act effectively in tracking the referenced MPP very closely even for the decrease in MPP as in this case. The inverter active power output also closely tracks the reference which is 96% of the PhV power as shown in Fig. 12b. It can be observed from Fig. 12c that the AC side RMS voltage is maintained at 273V (0.98 pu) throughout the simulation period. Most importantly, Fig. 12d shows the effectiveness of indirect DC side voltage control with the proposed method in this scenario as well. The voltage is maintained at around 700V which is a reasonable value according to the discussions in Case 1 above. Fig. 12e shows the active power at the DC and AC sides of the inverter. It clearly proves the effectiveness of the proposed control strategy in maintaining the power balance considering the inverter losses. 120
100
)100 W k ( r 80 e w o60 P e v 40 i t c A20
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a) PhV array active power output 0.984
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b) Inverter active power output 800 700
0.983 ) u p0.982 ( e g a t l 0.981 o V
) V600 ( e g a500 t l o V400
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In this paper, PQ and PV controls for solar arrays are discussed. The contribution of this paper can be summarized as follows: •
•
•
The paper first investigated the capability of active and reactive power control in the IEEE 13-bus distribution test feeder. The dynamic behavior of solar PhV generators in following the local load pattern is presented. It is believed to help in power flow control and in maintaining the voltage stability in future power systems. Next, in this research work, a unique algorithm for MPPT and voltage control for PhV generators is proposed in a two stage configuration. The reference value of MPP is procured by plotting series of PV curves for different irradiance levels and cell temperatures. Then, feeding this data into a look up table to linearly approximate the value of corresponding MPP with respect to the change in these parameters. The DC side voltage is controlled indirectly through the power balance of the DC and AC stages which is a unique characteristic of the proposed method. These controls ensure that the demands are properly served and the voltage at the AC and DC sides are properly maintained at the desired values as required by the utility. Moreover, the MPPT control ensures that the installed solar PhV generators are optimally utilized for the given irradiance level. The simplicity of the proposed control algorithms and popularity of PI controllers support a wide acceptance of the control methods presented.
Note, the proposed control methods, which are based on instantaneous non-active power theory, can be implemented in the grid integrated solar PhV generators for efficient PQ and PV controls. The significance of both the proposed control methods is that these are entirely developed in abc reference frame considering the measurement based approach. Hence, it leads to a simple control approach that avoids the conversions between the reference frames if abc and dq0. Furthermore, the approach is not sensitive to measurement noise as it is
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dependent on the measurements of the past cycle rather than at a particular time instant. The challenge and future work related to the proposed method, however, is the proper choice of controller gains for a good response speed without any over or undershoots which could be one of the future directions of this research. Solutions such as adaptive control proposed in [11-12] may be combined to the control approaches proposed in thi s paper. VI. ACKNOWLEDGEMENT This work was financially sponsored by the Office of Electricity Delivery & Energy Reliability, U.S. Department of Energy and conducted at ORNL and UT Knoxville. This work also made use of the Shared Facilities and the Industry Partnership Program supported by CURENT, an Engineering Research Center (ERC) Program of the NSF and DOE.
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[10] F. Li, J. Kueck, T. Rizy, T. King, 2006, “A preliminary analysis of the economics of using distributed energy as a source of reactive power supply,” Oak Ridge National Laboratory First Quarterly Report for Fiscal Year 2006 , Oak Ridge, TN. [11] H. Li, F. Li, Y. Xu, D.T. Rizy, and J.D. Kueck, “Adaptive Voltage Control with Distributed Energy Resources: Algorithm, Theoretical Analysis, Simulation and Field Test Verification.” IEEE Trans. Power Systems, vol. 25, pp. 1638–1647, Aug. 2010. [12] H. Li, F. Li, Y. Xu, D.T. Rizy and S. Adhikari, "Autonomous and Adaptive Voltage Control using Multiple Distributed Energy Resources," IEEE Transactions on Power Systems, In-Press, 2013.
[13] R. A. Messenger. (2010). Photovoltaic systems engineering (3rd ed.). [14] H. Chihchiang and S. Chihming, "Study of maximum power tracking techniques and control of DC/DC converters for photovoltaic power system," in Proc. Power Electronics Specialists Conference,PESC 98 Record. 29th Annual IEEE , vol.1, pp. 86-93 ,1998. [15] J. L. Santos, F. Antunes, A. Chehab, and C. Cruz, "A maximum power point tracker for PV systems using a high performance boost converter," Solar Energy, vol. 80, pp. 772-778, 2006. [16] X. Liu and L. A. C. Lopes, "An improved perturbation and observation maximum power point tracking algorithm for PV arrays," in Proc. IEEE 35th Annual Power Electronics Specialists Conference, PESC 04, vol.3, pp. 2005-2010, 2004. [17] T. Tafticht, K. Agbossou, M. L. Doumbia, and A. Chériti, "An improved maximum power point tracking method for photovoltaic systems," Renewable Energy, vol. 33, pp. 1508-1516, 2008. [18] Y. Xu, H. Li, D.T. Rizy, F. Li, and J.D. Kueck “Instantaneous Active and Nonactive Power Control of Distributed Energy Resources with a Current Limiter,” in Proc. IEEE Energy Conversion Congress & Expo , 2010. [19] S. Adhikari, Y. Xu, F. Li, et. al., “Utility-Side Voltage and PQ Control with Inverter-based Photovoltaic Systems,” in Proc. 18th World Congress of International Federation of Automatic control (IFAC), Milan, Italy, August 28-Sept. 2, 2011. [20] M.G. Villalva, J.R. Gazoli, and E.R. Filho, “Comprehensive Approach to Modeling and Simulation of Photovoltaic Arrays,” IEEE Trans. on Power Electronics, vol. 24, No. 5, pp. 1198–1208, 2009.
I. BIOGRAPHIES Sarina Adhikari (S’ 08) is a Ph.D. student in Electrical Engineering at The University of Tennessee. She received her B.E (Electrical Engineering) from Institute of Engineering, Pulchowk Campus, Pulchowk, Lalitpur, Nepal in 2002 and her M.E. in Electrical Power Systems Management from Asian Institute of Technology (AIT), Pathumthani, Thailand, in 2005. She worked as a Research Associate in Energy Field of Study at AIT after completing her Master’s degree. Her research interests are voltage stability, distributed energy resources, and control of renewable en ergy in distribution systems. Fangxing (Fran) Li (M’01, SM’05) received his Ph. D. degree from Virginia Tech in 2001. He is presently an Associate Professor at The University of Tennessee at Knoxville and the Director of the Education Program of the CURENT research center. He was a principal engineer at ABB Consulting, Raleigh, NC, prior to joining UT. He is a registered Professional Engineer in North Carolina, an Editor of IEEE Transactions on Sustainable Energy, and a Fellow of IET. Yan Xu (S’02, M’06) received her Ph. D. degree in electrical engineering at The University of Tennessee at Knoxville in 2006. She is presently a research staff member in the Power & Energy Systems Group at ORNL. Huijuan Li (S’07, M’11) received her Ph. D. degree from The University of Tennessee (UT) at Knoxville in 2010. She is presently a postdoc research associate with Oak Ridge National Laboratory (ORNL). She won the First Place Prize award at the Student Poster Contest during the IEEE PES General Meeting, Calgary, Canada, in July 2009.
