Design of PV power station remote monitoring system data acquisition device

Proceedings of the 2011 International Conference on Advanced Mechatronic Systems, Zhengzhou, China, August 11-13, 2011

Design of PV Power Station Remote Monitoring System Data

Acquisition Device

Junming Xiao, Pengcheng Liu, Lingyun Jiao, Haiming Zhu, Yinghu Du

Abstract-This paper mainly introduced the basic structure

and design process of hardware and software of PV power station remote monitoring system data acquisition device, collected and monitored the photovoltaic power generation

system voltage, current, temperature, gave the bidirectional

current sensing circuit, temperature detection circuit, serial communications interface circuit and remote communication diagram of PC and single chip microcomputer, the conditioning circuit of inverter AC voltage and current, data acquisition flow chart and so on. Aiming at the large scale

parallel centralized development trend of PV power station, using group control technology, in the case of low sunshine data acquisition centralized control inverter to make inverter turns operation, greatly extend the service life of the inverter. The system could complete the data acquisition and processing of PV power station operating parameters and the instantaneous value of electrical equipment operating status, realized dynamic curves and report shows and print function of realize real-time and historical data. Keywords: PV power station; remote monitoring; data acquisition; group control technology; bidirectional current sensing circuit

I. INTRODUCTION

Energy is the essential economic foundation of human life. Reliable, safe supply of energy and clean, efficient energy utilization is the basic guarantee to realize the economic and social development. People pay more and more attention to solar energy, which is an inexhaustible clean energy, PV draws the most attention [1, 21. In order to improve the efficiency of photovoltaic power generation system and understanding the local solar energy resources and the natural environment, photovoltaic system must be monitored and controlled [31. Traditional photovoItaic monitoring system is generally close monitoring maintenance staff on site need to keep continuously monitoring the data shown in equipment and making the corresponding treatment. A large part of the current constructions of photovoltaic power station are in remote areas where the environment is not suitable for long-term duty. To solve these above problems, this paper adapts the way of combination of telephone lines and RS-232 bus to transfer data. With the existing telephone network, remote monitoring the operation of PV power station, making the PV power station built in remote areas do not need staff on duty. Save of manpower resources has great significance for further promotion and application of PV power station.

JM. Xiao, P c. Liu, LY Jiao, H.M. Zhu, YH. Du are with the School of Electrical and Information, Zhongyuan University of Technology, 41 Zhongyuan Road, Zhengzhou, China, 450007. [email protected](JM. Xiao), [email protected](P.c. Liu).

978-0-9555293-7-5/11/$25.00

II. THE STRUCTURE AND FUNCTION OF

REMOTE MONITORING SYSTEM

Voltage and current signal acquisition

module

Luminous intensity signal acquisition

module

Temperature signal acquisition module

Reset and watch-dog circuit

Fig.l System hardware structure

Output control and alarm si nal

As figure 1 shown is the system hardware structure diagram, its core is micro-controller. The whole control system consisted of data acquisition, AID conversion and reset and watch-dog circuit, output control, clock chips, serial communication, LCD display and print drives and other peripheral circuit. Functions as follows:

(1) Monitoring DC current: current of photovoltaic cells and battery;

(2)Monitoring AC voltage and current: inverter output voltage and current;

(3)Temperature testing: environmental temperature, battery surface temperature, room temperature and battery surface temperature;

(4)Collecting PV power system environment parameter, battery array, electrical equipment operation parameters and the grid parameters;

(5)The system could real-time monitoring the operation state of PV power station. When the equipment malfunction it can immediately issued a warning signal, promptly notify the staff to handle the accident [41.

(6)ln the process of battery charge and discharge, the system can effectively monitor the various states of battery array. With the existing telephone network to remote monitoring PV power station, realize the function of remote control, telemetry, remote communication and remote adjustment;

367

(7)Storing the operation information of PV power station, realize the graphic display and printing of real-time data and historical data.

Except the general function, the system using group control technology, through centralized control of data acquisition to make inverter "rotate the leadership", inverter turns into operation in the case of low sunshine. This can improve the efficiency of low sunshine, extend the life of the inverter, achieved the n+x redundancy, increasing capacity prediction and energy management functions.

III. TWO-POINT SAMPLING ALGORITHMS

There are two ways to acquire power station AC according to the different sampling signals, they are DC sampling and AC sampling [51. DC sampling algorithm is simple and easy to filter. However, it has the problem of large investments and maintenance complex. AC sampling has the advantages of good real-time, phase distortion, less investment and easy to maintain. AC sampling have wide range of application, there are many algorithms according to the different application. It can be divided into sinusoidal model algorithm and the non-sinusoidal periodic model algorithm in accordance with the model function [61, in the sinusoidal model algorithm there are a single point of sampling, two sampling algorithm, etc. This article adopts two sampling algorithm.

Suppose u, = Urn sine OJt + rp)

u, = Urn sine OJt + rp + 90°)

U,2 +U22

= U"� sin2 (OJt + rp) + U:, cos' (OJt + rp)

= U� = 2U2

:. U =

�u" ;U22

. . rt:+t: SImIlarly 1 = �� P, Q acquisition:

u, i, + u,z, = Urn sine OJt + rp )1rn sine OJt -¢ + rp) +

(I)

(2)

(3)

(4)

(5)

U,,, sine OJt + rp + 90°)1", sine OJt -¢ + rp + 90°) (6)

= UJm COS¢ = 2P

UJ, - u,i, = Um sine OJt + rp )1m cos( OJt - ¢ + rp)

- U'" cos( OJt + rp)1", sine OJt - ¢ + rp)

= U 1 sin n\ = 2Q III III r

IV. SYSTEM HARDWARE DESIGN

A. Bidirectional current sensing

(7)

The photovoltaic cell and battery both have charge and discharge mode in PV power generation system [71. In order to prevent from being overcharge and over discharge, PV

978-0-9555293-7-5/11/$25.00

monitoring systems need to have bi-directional current sensing function to measure battery charge and discharge current.

�---1"'"0""i1JT O,..,.l.i..!J +---+

GND

Fig.2 Bidirectional current sensing circuit

In this paper, two unidirectional current-sense amplifiers MAX4173 were used to compose bi-directional current-sense circuit. The block diagram was shown in

Figure 2. VREF is set as zero detection voltage, when part

A works, the circuit output voltage is higher than V REF '

�IS+ > �IS-' battery power for the load; when part B works, the circuit output voltage IS lower

than �I"F ' �IS+ < �IS-' the battery is charged. The

bi-directional current sensing circuit using the capacitor stability control loop in feedback branch. The GND of Part B was modulated by MAX4230 to ensure the device supply voltage required for normal operation.

B. Inverter AC output voltage and current conditioning circuit

Inverter output voltage and current are AC signals, before signal into the microcontroller must go through a second-order low-pass filter filters out higher harmonics, the conditioning circuit shown as Figure 3.

Fig.3 Current voltage measurement conditioning circuit

C. Temperature detection circuit Temperature is one of the important parameters of PV

power plant remote monitoring system. In a given light intensity, the change of PV battery temperature will affect the battery output power. PV cell temperature increases 1°C,

368

the efficiency of battery down about 0.45% and the life of it will come down [8].

C2

FigA Temperature detection circuit

AS Figure 4 shown is bridge temperature detection circuit of two-wire connection. TL431 and potentiometer VRI regulate to produce reference voltage. RI, R2, VR2, and PtlOO constitute a measure bridge. When PtlOO and VR2 are not equal, bridge output differential pressure signal. The signal amplified by the LM324 op amp turn into an expected size voltage signal, the voltage signal can connect directly with AD converter chip. In the differential amplifier circuit

R3=R4, R5=R6, magnification=R5/R6, amplifier circuit is

negative feedback method. Adjust potentiometer VR2 can change the temperature of the zero set, adjust R5 / R6 ratio can change voltage signal magnification, adjust the resistance of R 1 and R2 can change the size of the differential bridge output.

D. Luminous intensity detection circuit

Detection of fixed luminous intensity is very important;

luminous intensity is an important parameter of stationary

power generation efficiency in power generation. According

to the rule of short-circuit output current of solar cells and

solar radiation intensity is proportional, we select a piece of

solar cell exposed in sunlight and customized by standards of

luminous intensity as the light intensity detection sensor

input, we can determine the intensity of sunlight at this time

according to the size of short-circuit current. The method of

KTIAIP-type low current sensors are used to detect

short-circuit current of light battery. The detection principle

is shown in Figure 5.

I ,-----�- +r---�-.

KTli\/P

+ M

Fig.S The circuit of the examination oflight strength

978-0-9555293-7-5/11/$25.00

The output signal of the balanced current sensor is the

form of current, and the test current is proportional to the

measured current. When the primary current is lA, the M

output of the measuring current are 100mA. If you remove

the voltage output in the form, the user must series a resistor

between end of M and the zero point of the power supply;

check the voltage according to the size of the selected

resistance, the limit of resistance can be determined as

follows:

E-Vce-hRi R max = -----------

1�'

Where E is Power supply voltage,

Vee is saturation voltage of power tub,

h is output signal current,

Ri is sensor resistance.

R1 200

s vee

Fig.6 MCU reset circuit

E. Reset and watchdog circuit

8051

vee

RST

vss

(8)

In the start or the end, the program needs to begin from

scratch, while all the machine registers, I/O interfaces, and

so on all must be reset. When the microcontroller pins ALE

and PSEN output high, RST pin up to high-level, the

microcontroller reset. If high-Ievel ofRSTNPD is generated

directly trom instantaneous power-on, it's power-on reset; if

generated by pressing the button, it's manual reset. Reset

circuit is shown in Figure 6.

In SCM control system, the power, electromagnetic

interference often cause the program to run exception, and

even into the "death cycle", and finally cause system failure.

The hardware watchdog circuit is added to ensure the

reliable operation. Circuit diagram as shown below:

369

RCT

vee

NE56604

CT 1---'-------, �� ����----�����

eLK f--''-----+--M TO lK

"lID C30

Fig.7 Watchdog circuit

v. SYSTEM SOFTWARE DESIGN

Software design is one of the key factors for remote monitoring; system uses modular programming, mainly completed for data acquisition and processing of PY power station remote monitoring system; the storage and transform of system parameters; the dynamic curve display of station important parameters; report display and print of system operation parameters and instantaneous state, and so on. As Figure 8 shown is the data acquisition flow chart.

Set the acquisition parameters

N

Fig.8 Data acquisition f10w chart

A. AID conversion subroutine

The PV power station remote monitoring system introduced in this paper uses high-precision DCIDC module

as the output to provide the AID converter analog reference voltage VRE, the AID converted output of the input

analog signal is shown in the following formula:

RESULT = 1023 X VsIGNAL -ANGND

VREF-ANGND (9)

Where VllioF is reference voltage of AID converter

analog, ANGND is analog ground of AID conversion

circuit.

978-0-9555293-7-5/11/$25.00

AID conversion is carried out by the AID conversion flag. AID conversion flag bit is set in the timer interrupt routine, time interval between two AID conversions is about 1

second. AID conversion process flow chart is shown in

Figure 9.

370

Conversion channel number­

AL register

Set the value of AL register

with Portl

Read state of Port 1

State of Port I is

onsistent with the AL?

y

Set AID conversion time register

Set AID command register, and

start AID conversion

Read AID conversion

status register

Conversion N

N

>--------" complete

y

Read AID conversion result

Calculate the conversion results

corresponds to the voltage, current,

temperature, light intensity values,

de osit into the corres onding variables

Fig.9 AID conversion process flow chart

B. Key processing subroutine

The system has key circuits and related routines to display multiple parameter values. It can judge whether the trigger button is pressed and cumulate number of times of the trigger button depending on which to display different parameters used to achieve more data inquiry.

In this subroutine, the key state level will be determined at first, if the key state of high I, it doesn't trigger and returns the main program; key triggers at the state of low level 0, meanwhile, number of key trigger states plus I(the system shows nine values, so the key states are designed to 9 times) . Followed by cycle, until the number of key states to 9, then a query loop has all ended, the number of key states back to 0 in preparation for the next query cycle. As Figure 10 shown is the key processing subroutine flow chart.

Judge key state

Judge trigger number

Exit button processing procedure, return to

main program

key=J, untriggered

keyflag=9

Keyflag reset

Fig.10 Key processing subroutine flow chart

C. LCD display subroutine

In LCD display subroutine, number of buttons trigger state will be determined at first. If the key triggered once, then the subroutine section FRESHl will be called (In which can call

other display update subroutines to inquiry other parameters according to the number of buttons trigger state); if it doesn't trigger while the button's state is I, then it read the data collected within the address data channel 2 and send into the

temporary storage unit, two figures of each temporary address were sent to the cache by calling DI SRAM subroutine. At last, display subroutine is called to display the value.

According to the number keys triggered to enter different update subroutine, take a single digital signal by collection of different data, adjust the level of the digital, computing

978-0-9555293-7-5/11/$25.00

and put into the display buffer, and finally run the display routines. Subroutine flow chart was shown in Figure II.

� ____ ---, ____ � Display buffer

]] the character have shown

N

pointer add J

Fig.11 LCD display subroutine flow chart

D. Serial communication protocol

MCS-SI series MCU interface is a full-duplex serial communication interface, you can simultaneously send and receive data and it can also be programmed by software to set its baud rate and frame format.

Serial Port of MCS-Sl series can be configured into 4

kinds of work ways by using software. This data acquisition system use work way 1 whose baud rate was determined by the timer TI, at this time, the timer TI operates in timer /

counter work way 2. To make data communication between the microcontroller and PC to be successful, we must ensure that they both have the same communication parameters. In

this paper we set the baud rate of 9600 baud, no parity, 8

data bits and using I-bit stop bit. The SCM serial interface communication code used in this paper as outlined below:

371

void in it_rs232 ( v o id )

{ S CON=Ox S O; #S erial p o rt m o del,

allo w ed to receive

TMOD= TMOD

T H I=OxFD;

TL l=O xFD;

E S=I;

EA =I;

T RI=I;

}

Ox2 0; #T im er T I , m o d e2

#En a b le serial p o rt interru p t

#En a b le total interru p t

#S tart tim er

VI. EXPERIMENTAL RESULTS

Figure 12 (a) and 12 (b) shows the experimental result, figure 12 (a) is light intensity and system power output comparison chart of a sunny day, figure 12 (b) is light intensity and system power output comparison chart of a cloudy day. By observing the waveform of figure (a) and figure (b), we can see the system output power subject to the impact of changes in light intensity. In the real system, open circuit voltage of photovoltaic cells and light intensity proportional to the logarithm relationship. The battery electric current will also change obviously with the increase of light intensity, the output power of PY array increased rapidly with light intensity increased.

�.oo

�.oo

";_ 800

.."'700 .

--- � --- � -------- r ---

____

1 _______ _

I ----1------------1----------- --,-------- ---- -,-------- -

--- -+ --- ---.j --------

aA sunny day

I' ,

ii<600- - -- - --- - ---

�:- - -- - --- -

"

, I

" , �

---1- --I----�---.:

'40 ----------- -,.

"

--I----f-------- - --I----r-----:

-- 1----1----:: -- 1---- 1-----

b A cloudy day

Fig� 12 Light intensity and system power output comparison chart

978-0-9555293-7-5/11/$25.00

VII. CONCLUSION

The popularization and application of PV power station promote the development of PY power station remote monitoring system. This paper mainly described the basic structure of data acquisition, introduced the design process of hardware and software. System used the method of telephone lines and RS-232 bus combination for data transmission, adapted the existing telephone network to achieve the remote monitoring of PV power station. Experiment results show that the system has high reliability and stable operation, has important significance for the further promotion and application ofPY power station.

REFERENCES

[1] YTsur and A.Zemel, "Long-term Perspective on the Development of Solar Energy," Solar Energy, vol. 68, no. 5, pp. 379-392,2000.

[2] D.Y.Goswami, "New and Emerging Developments in Solar Energy," Solar Energy, vol. 76, no. 1, pp. 33-43,2004.

[3] Qingzhi Wu and Xiao Sun, 'The Data Acquisition and Communication System Based on LabV1EW for PV Power Station," Micro computer information, vol. 18, no� 5, pp� 31-33,2002

[4] Liwei Li, Wang Ying and Shuzhe Bao, "Development of the Intelligent Surveillance and System of PV Power Station," Power sources, vol. 31, no. 1, pp. 76-79,2007.

[5] Yanlei Zhao, Yong Qian and Zibin Yin, "The Analysis of Two-point Sampling Alogrithm and Program Design ofAC Sampling in a Power Station Monitor System," Power System Technology, no. 5, pp. 37-38, 2002.

[6] Hong Zhang and Chengmei Wang, "Comparison of Normal Alternating Sampling Methods in Electric Power System," North china electric power, no. 1, pp. 64-66, 1999.

[7] Zhenghao Zhang, Hong Chen and Zhaodong Jin, "Design of Data Collection and Monitoring Instrumentation Based on PV Power Station," Electrical Measurement &Instrumentation, vol. 46, no. 528, pp. 48-50,2009.

[8] Gang Yang, Ming Chen, Shaoxiong Yao and Guanwen Li, "Development of Data Acquisition Facility for the Photovoltaic Power Station," Micro computer information, vol. 25, no. 4-1, pp� 86-88, 2009�

372