Control of the Power Flow in an Energy System Based on Grid

Connected with Photo Voltage Generator

ROZANOV Y.K., KRIUKOV K.V.

Department of electric and electronic apparatus

Moscow Power Engineering Institute (Technical University)

111250, Moscow, Krasnokazarmennaya Street, 14

RUSSIA

Abstract: - Power system on base solar source of energy connected with grid can have many functions using

modern power electronics achievement as full controlled power devices and digital control [1]. Using different

transformations from three coordinate systems to two and p-q theory one can realize different modes of

operating AC/DC converter to control of power flow in the system.

Key words: - photovoltaic energy system, DC/DC converter, AC/DC converter, rectifier, inverter, reactive

power compensator, active filter, power conditioner, digital control system.

1 Introduction

Recently it is increasing an interest to use

renewable energy. There are many renewable

energy sources but solar energy has most popular.

It can be effect of developing power electronics

during last year. There are many new devices based

on which converters, regulators and commutation

switches with many functions can be constructed. It

can be improved photovoltaic systems which can

have many functions such as improving power

quality on load bus, uninterrupted power on it,

active filter of high harmonics current or voltage,

reactive power compensation, changing direction of

power flow (to grid or inversely to accumulator

battery), etc. Using full controlled and fast recovery

devices with microprocessor control we have

possibility to realize all this functions in one energy

system. Digital control of power flows can change

operating mode less than one half-period of power

net’s frequency. Further it can carry out fast

calculation of any parameters of regime. This paper

presents middle power photovoltaic energy system.

It’s shows possibility of power electronics using in

the system.

2 Arrangement and modes of the

system operating

Main components of the system and their

connection are shown on Fig.1.

Photovoltaic cells connected in series and joint in

groups, which connected in parallel. Output of

these modules connected to input of series

connected buck-boost DC-DC converters. The

circuits of converter may be Cuk or it modifications

having good efficiency factor and wide voltage

regulation range. It’s used as maximum-power-

point tracker (MPPT) for the PV modules. Also it’s

regulated battery charger current as photovoltaic

system needed in energy storage unit during

periods of low solar irradiation. From battery side

DC bus connected with DC/AC converter

(converter I on Fig.1) based on full controlled

devices as MOS or IGBT. Also, capacitor

connected in parallel with battery on DC side of

DC/AC converter for shunting high frequency

pulsations. The AC side of the converter connected

with grid. For middle power it is three-phase

converter fulfilled on base of three-phase bridge

topology. It operating with PWM and can provide

current in four quadrants of complex coordinates. It

means that power flow can have any direction.

Other words using PWM we can control phase

current on AC side of the module and change its

direction and character of the power flow.

Fig. 1

Arrangement of the energy system

12th WSEAS International Conference on CIRCUITS, Heraklion, Greece, July 22-24, 2008

ISBN: 978-960-6766-82-4

41

ISSN: 1790-5117

We can regulate voltage on load adding in

system low power (~1/10 S

nom

) AC/DC converter

(converter II on fig.1), which adds small amount of

voltage with corresponding phase angle trough

transformer.

The converter I also can operate as reactive

power generator i.e. source of power with lagging

or lagging current to implement static compensator

and active filter regulating reactive power and

generating high frequency harmonics. It can

compensate current distortion in the grid. It is

important in supplies nonlinear load.

In common case the system can operate in the

following regimes:

1. PV modules generate electric energy, but grid

failure. In this case power provided to vital load.

But control system taking into account the state of

charge of battery for charging it if it’s necessary. In

this case control system operating using algorithm

of operation, which used information about

battery’s state of charge. Control system of DC-DC

also taking into account information about MMP.

Priority in operation determinates by controlling

algorithm. Converter II used as power conditioner

for loads adding in series required value of the

instant voltage level.

2. The grid providing power to all loads including

emergency. Converter I used as reactive power

compensator and active filter of loads. It’s

corrected power factor of loads for grid. Also it can

charge battery operating as rectifier, changing

reference current of the converter’s control system.

The converter II is used as power conditioner for

loads.

3. In case of deficiency of power in PV and grid the

energy system operate as uninterruptible system

from battery.

Power circuit of the control converter I fulfilled

on IGBT transistors. Fairly simple drive circuitry

and constant technology development make

transistors more advantageous than various types of

controlled thyristors up to high power range. It

utilization enables use of medium and high

frequency PWM techniques as well as variable

hysteresis modulation. For the converter II one can

use transistors MOS type as it has small power.

3 AC/DC converters control system

Recently, more prospective is microprocessor

based control system. Control systems of

converters I and II are similar. Therefore here

converter’s I control is described only. Fig. 2 shows

converter I with control and sensing systems.

Converter’s control provides different modes of

operating including reactive power and distortion

of currents compensation. In this case is rational to

Fig. 2 Converter AC/DC with control and sensing

systems

use the theory of instantaneous power known as the

“Akagi-Nabae theory” [4]. This theory is known as

pq-theory based on Clark transformation, i.e.

abc/αβ. According to it forward abc/αβ

transformation is as follows:

(1)

Matrix (1) coefficient can be chosen (2/3 here.

From (1) the resulting vector is:

(2)

Consequently, reverse Clark transformation is as

follows:

(3)

According pq-theory. One can show real and

imaginary power in αβ frame as follows:

(4)

In [4] corresponds to instantaneous real power as

traditional approach, where q(t) is not the same as

traditional reactive power.

According p and q-theory one can be represented

(5)

12th WSEAS International Conference on CIRCUITS, Heraklion, Greece, July 22-24, 2008

ISBN: 978-960-6766-82-4

42

ISSN: 1790-5117

where

and

– average components that

correspond to fundamental active and reactive

power, consequently;

and

alternative

components, caused by harmonics. Thus, reference

signals for currents

and

from this can be

received.

Reverse Clark transformation results reference

currents three-phase system:

(6)

One of the main advantages of pq-theory is that

there is no need for phase synchronization. Also,

average components p and q can be easily detected

(filtered) by means of low-order low-pass filters.

The latter makes positive impact on system

dynamic and stability.

Though, there is no need in synchronization,

non-synchronized rotating two-axis frame is

necessary Rotating at fundamental frequency it

enables to obtain average components p and q as

constant values. Αβ frame can also be used as

rotating frame in the first place. There is also an

option to use stationary αβ frame and another

separate rotating frame afterwards. The latter way

was chosen for the control system design in the

paper. In the Fig.3 the first part of control system

block diagram is shown. After voltages and

currents abc/αβ transformation in stationary frame

(3/2 symbol in the Fig.3) instantaneous reactive

power is calculated. Then its average detected by

means of LPF. In the other control system channel

active power reference signal component is

obtained from DC bus error by means of PI

regulator. Then reference current components

calculation in αβ frame takes place in both

channels. After that p- and q- reference signal

components are summed up for α and β axis and

taken to transformation in rotating frame.

Fig.3 Reference signal generation (part I)

Part II of the control system block diagram

shown in Figure 4. In it there are two PI regulators

are used to work with error between output current

signal and reference current signal in rotating the

dq-frame. Each PI-regulator operates for one axis:

d or q. Preliminarily, abc/αβ and αβ/dq

transformations applied to convert output signal. So

at the output of PI there is a command signal in dq-

frame. Then reverse αβ/dq and abc/αβ

transformations applied to obtain command signal

that is ready for modulation (see Fig.4).

Fig. 4 Reference signal generation (part II)

As far as active is concerned it is necessary to

have synchronized transformation to preserve

accurate harmonics detection. Synchronization is at

fundamental frequency. Control system channel

responsible for load harmonic detection that utilizes

synchronous abc/dq transformation is presented in

Fig.5.

Fig.5 Load harmonic currents detection channel

As seen from it detected fundamental component

is cancelled out of load current leaving harmonics

only for reference. For synchronization phase-

locked loop (PLL) is commonly used. Another

channel is responsible for DC bus voltage. Since

reference signal contains as control and identical to

the one in converter control system (see Fig.3).

Since reference signal contains as low so high

frequency hysteresis modulation is preferable.

Hysteresis modulation provides good dynamics. In

order to maintain constant switching frequency

variable hysteresis parameter techniques can be

used. Figure 6 shows load and grid currents when

AC/DC converter starts operating.

12th WSEAS International Conference on CIRCUITS, Heraklion, Greece, July 22-24, 2008

ISBN: 978-960-6766-82-4

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ISSN: 1790-5117

Fig.6 Converter AC/DC as reactive power

compensator (starts operating at 0ms to compensate

inductive load).

4 Conclusion

Recently are actual using alternative sources of

energy. Between them is the solar energy, which

has larger popular. Photovoltaic sources connected

with grid have structure with power electronic

arrangement can be multifunctional without deep

change it. One can control power flow in such

system with different goals make it universal local

station of power energy.

The main part of it structure is power electronics

converter AC/DC fulfilled on base full controlled

devices commutate in four quadrant of complex

plane and digital control system. This converter can

operate modes of rectifier, inverter compensator

reactive power, active filter and so on.

One can make controlling processes simple and

fast calculating in three-phase systems using p-q

theory, abc/αβ and abc/dq transformations, digital

filters, etc. Results of modeling these processes

show and justify it.

References:

[1] J .Dixon, L. Moran, M, Ortuzar, R, Carmi, P.

Barriuso and H. Flores, “Static VAR Compensator

and Active Power Filter with Power Injection

Capability, Using 27-level inverters and

Photovoltaic Cells”, IEEE ISIE 2006, July 9-12,

Montreal, Quebec, Canada, pp.1106-1111.

[2] D. Chang, J. Kin, and S. Sul, “Unified Voltage

Modulation Technique for Real Time Three-Phase

Power Conversion”, IEEE Transactions on

Industry Applications, Vol. 34, №2 1998, 374-378.

[3] H. Akagi “New trends in active filters for power

conditioning”, IEEE Transactions on industry

applications, Vol.32, №6 1996, pp.1312 i322.

[4] K. Huosung, H. Akagi, “The instantaneous power

theory on the rotating p-q-r reference frames,”

Power Electronics and Drive Systems Conference,

1999, pp.422-427.

12th WSEAS International Conference on CIRCUITS, Heraklion, Greece, July 22-24, 2008

ISBN: 978-960-6766-82-4

44

ISSN: 1790-5117

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