Grid Connected PV System

APAP2011 www.apap2011.org

*Guo XiaoYun (email: [email protected])

Real-time and grid-connected control of PV power system

GUO XiaoYun, CHEN JinMei, LIU QiHui

Abstract: This paper analyzes the equivalent model of photovoltaic cells and then discusses the output characteristics of

pho-tovoltaic cells. Using the RTDS (Real Time Digital Simulator) to build a real-time simulation system of grid-connected photo-voltaic power generation that includes three-phase grid-connected PV systems, maximum power point tracking (MPPT) me-thods for PV arrays, grid-connected control of PV system, inverter control and filter design meme-thods. This model achieves the maximum power tracking by the method of the photovoltaic cells self-optimization perturbation and observation based on the buck circuit. According to the control target of three-phase PWM inverter designing a amplitude and phase control system which makes the network systems realize separately control the active power output and reactive power output, in the mean-while the DC bus voltage can be limited at the reference value. The simulation results show that the system cannot only track the maximum power point of photovoltaic cells quickly, accurately and effectively, it can realize the stability control of the DC bus voltage. Further, fit out the maximum power curves at different environmental changes (include the Ambient temperature and Illumination intensity) to verify the output characteristics of photovoltaic cells; enhance the grid-connected PV system ap-plications generally.

Keywords: PV grid-connected, MPPT, BUCK control, amplitude and phase control, perturbation and observation

1 Introduction

In modern society, all countries of the world competed de-veloping green renewable energy, the solar with its unique advantages such as inexhaustible, clean, no pollution, no regional limits and so on, becomes the focus of attentions. It has widely used in power generation, water supplying, heating and satellite energy etc, transforms the entire nation into a low-carbon society. Due to the important status of solar power in the global power energy system, as voltaic cell components’ price spiraling lower and photo-voltaic technology development, solar resource has the po-tential to be the main alternative energy source to replace traditional ones in the future.

Control of grid-connected photovoltaic power generation system is a comprehensive and coordinated process, which involves the cooperation between equivalent model of pho-tovoltaic cells, system optimization, tracking control, DC/DC transform and grid-connected inverter technology orderly. Targeted control of grid-connected photovoltaic power generation system: Ref. [1] proposed a control strat-egy which based on maximum power point tracking and combine the three-phase PV grid voltage-controlled to maintain the system voltage stability; Ref. [2] according to

the three-phase bridge circuit theory, design a three-phase grid-connected PV system control. Ref. [3] combined the PV grid-connected control and reactive power compensa-tion control into one, constitute a PV grid power condicompensa-tion- condition-ing system to improve power quality and reduce power loss. Targeted real-time problem simulation of traditional photovoltaic power generation system, A real-time grid-connected photovoltaic power generation simulation system is built based on RTDS, qualitative analysis of the output characteristics of photovoltaic cells, and add the maximum power tracking control algorithm based on BUCK circuit in the simulation system which can realize the photovoltaic power generation system maximum power tracking. Using amplitude and phase of three-phase inverter control achieve the PQ decoupled, it not only stabilizing the DC bus voltage, also verified the output characteristics of photovoltaic power generation system by simulation and fitting.

2 System Description

Photovoltaic power generation system is a new electricity generating system that directly converted solar photovoltaic effect of radiation into electric. A basic set of photovoltaic power generation systems typically include photovoltaic cells, MPPT controllers, inverters and grid, which is shown



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in Figure 1.

Figure 1 Structure of photovoltaic power generation system

Taking into account the requirements of engineering pre-cision, Figure 2 shows a practical equivalent circuit of a single model of photovoltaic cells, it is described as a con-stant current source which current is Iph paralleled with a

positive diode. In this figure the Rs is series resistance which ideal situation is 0; Rsh is the shunt resistor which ideal

situ-ation is infinite. In the actual, the Rs is hoped as small as possible and Rsh as large as possible.

ph I ID I_r sh R S R I

Figure 2 Single model equivalent circuit of photovoltaic cell

According to the equivalent diagram, photovoltaic cells V-I characteristics of a single equation can be calculated. I IphID Ir (1) ( ) 1000 ph sc T ref S I I u C u T T (2) In the formula: ( ) 1 s q V IR AKT D o I I e  ª º  « » « » ¬ ¼ (3) 3 g qE AKT o D I C T e (4) s r sh V IR I R  (5) Calculated˖ ( ) 3 ( ) 1000 1 g s sc T ref qE q V IR s AkT AKT D sh S I I C T T V IR C T e e R   u  u  ª º   «  » « » ¬ ¼ (6) In the formula: I——Load current;

V——Battery terminal voltage; Iph——Photocurrent;

S——Light intensity(W/m2);

T——Battery temperature(K); ID——Diode reverse current;

RS——Series resistance (RS=0.0419)˗

Rsh——Shunt resistor (Rsh=2000)˗

CT——Temperature compensation coefficient (1.6mA/K)˗

Eg——Bandgap voltage(1.13eV);

K——Boltzmann constant (1.38e-23)J/K);

q——Electronic energy(1.6*10-19C);

CD——Temperature coefficient(CD=10.0);

A——P-N junction coefficient of semiconductor device in

photovoltaic cells (A=1.11);

Isc——Short-circuit current when S=1000W/m2, T=Tref (Isc=3A);

As can be obtained from the above equation, the output current of solar photovoltaic has a close relationship with light intensity S and cells temperature T that is a kind of nonlinear function; it is obviously that the battery output power P with S and T also exist such nonlinear relationships. Figure 3(a), (b) are the I-V and P-V characteristic curve which at the same cell temperature but different light inten-sity conditions. Figure 4 (a), (b) are the I-V and P-V cha-racteristic curve which at the same light intensity but dif-ferent cell temperature conditions (Where the battery tem-perature and ambient temtem-perature is approximately the rela-tionship: T=Tair+0.035), and the current-axis corresponds

means the battery short-circuit current Isc, the intersection

with the voltage axis means the battery open circuit voltage.

2 1kW m/ 2 0.75kW m/ 2 0.5kW m/ 2 1kW m/ 2 0.75kW m/ 2 0.5kW m/

Figure 3 I-V and P-V characteristic curves at the same cell temperature

and different light conditions

According to Figure 3 (a), (b), in a certain battery tem-perature and different light intensity circumstances, the output characteristics of photovoltaic cells has the following characteristics:

1) Output short-circuits current linearly increased followed light intensity increases.

2) In Figure 3 (a), before A point the output current is ap-proximately think as the short-circuit current of photovol-taic cells, showing a constant current state; and when the voltage exceeds the A point, the output current decreased rapidly, showing a constant voltage state.

3) The maximum power alters with light intensity(S), the greater the light intensity the maximum power point higher to move up, rising trend is shown as red in Figure 3 (b).

0 Cq 25 Cq 50 Cq 0 Cq 25 Cq 50 Cq

Figure 4 I-V and P-V characteristic curves at the same light and different

cell temperature conditions

According to Figure 4 (a), (b), in a certain light intensity but different battery temperature, the output characteristics of photovoltaic cells has the following characteristics: 1) The temperature has little effect on the output short-circuit current of photovoltaic cells, when the temper-ature rise the output short circuit current increased slightly. 2) The maximum power output point of the battery rises while the temperature falls which shown in Figure 4 (b) in red.

3 MPPT control

Figure 5 shows the BUCK power converter topology. The circuit can simulate any light intensity, temperature and load conditions on the electrical performance of photovoltaic cells. BUCK converter in the power tube control by the V-I characteristic curve of photovoltaic cells, the control strate-gy as shown in Figure 6 the LC filter design parameters are as follows:

Figure 5 BUCK circuit

lim 2 (1 ) (1 ) ( ) 8 s CAP s o o T L V I T V C L V G _G G ­ t  ° ° ® _ ° t ° _' ¯ (7)

Where  is the duty cycle of BUCK circuit, VCAP is the

input voltage, TS is the switching cycle, Ilim is the minimum

current of BUCK circuit between the continuous and dis-continuous current mode, Vo is the output voltage, Vo is the maximum output voltage variation allowed. Sampling of

VCAP and ICAP then calculate the optimal input voltage by

MPPT algorithm, compared with the actual value through the PI regulator to get the control signal of power ment of BUCK circuit, control strategy of power manage-ment is shown below.

Figure 6 BUCK circuit control strategy

There are many MPPT algorithms such as the incremen-tal conductance method, neural networks, curves fitting, and the perturbation and observation method. The essence of strategy is a dynamic self-optimizing process which detect-ing the real-time of the output power of photovoltaic cells through some control strategy, using a certain control algo-rithm predicts the largest photovoltaic power output may be at a certain situation, by changing the duty cycle of the power tube to meet the maximum power output require-ments. In this paper, solving the maximum power point current and voltage by using perturbation and observation method (P & O) which flow shown in Figure 7. The process is as follows: preliminary design a photovoltaic cell operating voltage firstly. Then periodic disturbance output voltage of the PV cells by adjusting the duty cycle of the power tube to, for example to increase it, then compare the former and the later output power of the PV, if output power increase, then dP/dV> 0 which demonstrate that the photo-voltaic cells work at the left of maximum power point and should be maintained in the next cycle increasing the output voltage of photovoltaic cells; on the other hand, if the out-put power decreases that is dP/dV <0, shows that the battery is working on the right side of the maximum power point, what means the current perturbations will make the operat-ing point away from the direction of maximum power point. Though, we should change the direction of disturbance so that the output of photovoltaic cells voltage decreases. Re-peated adjusting, approaching the operating point of PV maximum power point finally.

Figure 7 Flow program of perturbation and observation method

4 Convert control

The main circuit of three-phase PWM inverter is shown in Figure 8, constituted by three phase inverter, series reactors and DC link capacitance. Its output directly connected to the grid and the input connected MPPT controller of photovol-taic systems. There are direct current control and indirect current control methods, the former need to detect the AC side current. Through the current regulator to keep the ac-tual value of AC current followed the reference value in order to control the power factor; the latter control the reac-tive power and acreac-tive power system by controlling the tage amplitude and the phase deviation with the supply vol-tage at the AC side, what called the amplitude and phase control.

Figure 8 Three-phase inverter

There are two control objectives of three-phase PWM inverter: one is transfer energy at the maximum power fac-tor; another is to control the DC bus voltage stably. The control strategy goal is to generate the required modulated wave signal for PWM signal, thus can directly take mod-ulate wave signal amplitude and phase as the control objec-tives, utilize it generated the control signal what needed.

AC

u

u

L

I

I

u

u

u

M

[

Figure 9 Single-phase equivalent circuit and vector graphics of the

in-verter

Figure 9 shows the single-phase equivalent circuit and phase diagram of three-phase PWM inverter. The resistance is small that can be neglected. The amplitude and phase control get the carrier voltage signal by controlling the am-plitude of PWM and the angle  that lag the power grid vol-tage u AC, indirect control the inverter current. Further reckoned get that the inverter reactive power changes influ-ence the AC inverter output voltage amplitude, and the power inverter active power determines the change of the angle . Based on this, it can be referred Phase and Ampli-tude Control as a PQ decoupling control. To meet the con-trol objectives, according the proportional relationship be-tween the active power and the DC bus voltage of system, we take the DC bus voltage as the reference value of PI regulator which control the modulating wave voltage angle, take the value of reactive power as the reference value of PI regulator which control the modulating wave voltage am-plitude. The control strategy is in Figure 10.

Figure 10 Inverter Control Strategy

5 Simulation results

Table 1 and Table 2 are photovoltaic cells parameters and filter parameters in the real-time simulation system based on RTDS

Table.1 Solar cell parameters

Parameters Value Open circuit voltage Voc 21.7V

Short-circuit current Isc 3.35A Maximum power point voltage 17.4V Maximum power point current 3.05A

Serial Number 36

Parallel Number 1

Reference temperature 25 C

Reference light intensity 1000 W/m2

Table.2 P & Q parameters and the LC filter parameters Parameters Value Sampling cycle 5ms Voltage step 10V L 0.001H C 0.08F

As Figure 11 shows, the fitting result is similar to the curve of maximum power in Figure 3(b), verify the PV curve that the maximum power increased as the light inten-sity S enhanced.

Table. 3 The maximum battery power at different light intensity

Circumstance Battery voltage (kV) Maximum power(W)

T=25/S=400 0.770 19.82 T=25/S=600 0.800 30.93 T=25/S=800 0.830 41.70 T=25/S=1000 0.835 52.20 T=25/S=1200 0.840 63.50 T=25/S=1400 0.843 74.58 0.74 0.76 0.78 0.8 0.82 0.84 0.86 0.02 0.03 0.04 0.05 0.06 0.07 0.08 P-V POWER

Figure 11 T = 25 C, the fitting curve of PV when light intensity

changes

Table 3 shows the maximum power output of PV cells under different light intensities. Fitting according to the Table 3, Figure 12 to Figure 17 shows the amounts change when the battery temperature T= 25 C, light intensity S make a step change vary from 1000W/m2 to 1200W/m2. Figure 13 shows the maximum active power output of PV cells. The results show that the system can quickly adjust the maximum power output as the light intensity step changed. Figure 14 shows the entire output power curve of PV system, in which active and reactive power appears small fluctuations as light intensity step changed. Figure 15 shows the reference voltage Vref and actual voltage VCAP get by

MPPT algorithm in real-time calculation, the results showed that perturbation and observation method (P & O) can quickly get the right reference voltage Vref which

corresponding to the maximum power of the PV cells under different environmental conditions, then through the PI regulator get the actual voltage. Figure 16 shows the

enlarged curves of VCAP and Vref to verify the effectiveness

of perturbation and observation method (P & O), the simulation voltage step is 10V. Figure 17 shows the DC bus voltage curve, verified that amplitude and phase inverter control could stabilize the DC bus voltage. Above simulate results demonstrate that the system's maximum power increases with the light intensity once again.

Simulation: Light intensity S step changed from 1000W/m2 to 1200W/m2.

Figure 12 Light intensity and battery temperature

Figure 13 PV active power

Figure 14 PV output power

Figure 15 The reference voltage Vref get by MPPT algorithm and the actual voltage VCAP

Figure 16 Magnified of Vref and VCAP

Figure 17 DC bus voltage fluctuation

6 Conclusion

This paper analyzes the equivalent model of photovoltaic cells and studies the output characteristics of photovoltaic cells. Based on the RTDS (Real Time Digital Simulation) built a model that realizes the real-time maximum power tracking control MPPT by using the system P & O perturba-tion and observaperturba-tion method. According to the structure of photovoltaic grid-connected systems, adopting the ampli-tude and phase control to achieve DC bus voltage stable control, improved the system power factor by the PQ de-coupled method, as a whole reached the MPPT-voltage

con-trol. Simulation results show that the system can track the photovoltaic cells quickly, accurately and effectively, also realize the stability of the DC bus voltage control. The maximum power curve fitted at different environmental changes verified the output characteristics of photovoltaic cells. Enhance the overall applications of grid-connected PV system.

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