TM Phase locked loop system and system level control at ... (1).pdf · distributed renewable energy...

ShankarBabu.P, International Journal of Technology and Engineering Science [IJTES]TM Volume 3 [4], pp: 3301-3309, April 2015

ISSN:2320-8007 3301

Phase locked loop system and system level control

at single-phase bidirectional Converter interfaced

with micro grid system Mr.P.ShankarBabu Dr.M.Gopinath Dr.J.B.V.Subrahmanyam,

Associate Professor, Dept. of EEE, Professor, Dept. of EEE, Professor, Dept. of EEE, M.R.EC (Autonomous), Bharath University, C.J.I.T,JANGAON.

Hyderabad, India. Chennai TELANGANA

Abstract—This paper proposes a complete modeling and

control system for a bidirectional, single-phase,

multifunctional PWM converter for residential power level

distributed renewable energy and grid connected microgrid

system applications. A simple control structure was used to

cover all of the required operating modes, including stand-

alone mode(SAM), grid-connected mode(GCM) inverter, and

rectifier mode. A new single-phase PLL and active islanding

detection algorithm was also proposed for system-level

operation in order to meet IEEE standard 1547. The resulting

control structure is very simple and presents robust, low

transient responses even for extreme load steps between no-

load and heavy-load conditions. The transition between modes

was also seamlessly achieved, as predicted, due to the

common inner current-loop that have all operating modes.

The proposed system is simulated using MATLAB.

Index Terms—Stand-alone (SAM), grid-connected (GCM),

Phased Locked Loop (PLL)

I. INTRODUCTION (HEADING 1)

With pressing demand for and limited production of

chemical energy resources, such as petroleum and natural gas,

energy problems have become more urgent, and have become

a focus of people around the world. According to the latest

reports from British Petroleum, the world reserves of

petroleum now represent 41 years of production at the current

rate [1]. So-called “energy crisis” has been a hot topic

recently.

The direct consumption of chemical energy resources

has significant environmental impacts. In particular, the

emission pollution from vehicles, which leads to global

warming, pushes governments to make efforts towards the

development of clean renewable energy resources, such as

wind, bio-gas, and solar energy to address energy and

environmental problems.

Renewable energy technologies such as solar power,

wind power, hydroelectricity, micro hydro power, biomass and

biofuels could be used to replace conventional chemical

energy resources. In 2006, about 18% of global energy

consumption came from renewable resources. Wind power is

growing at the rate of 30% annually, with a worldwide

installed capacity of over 100 GW [3], and is widely used in

several European countries and the United States.

1.2 RENEWABLE ENERGY SYSTEMS OVERVIEW:

Currently, industrial countries generate most of their

electricity in large centralized facilities, such as coal, nuclear,

hydropower or natural gas-powered plants. These plants have

excellent economies of scale, but usually transmit electricity

over long distances. Most plants are built this way due to a

number of economic, healthy, safety, logistical,

environmental, geographical and geological factors. For

example, coal power plants are built away from cities to

prevent their heavy air pollution from affecting the populace;

in addition, such plants are often built near collieries to

minimize the cost of transporting coal.

Distributed generation is another approach to the

manufacture and transmission of electric power. It reduces the

amount of energy lost in transmitting electricity because the

electricity is generated very near where it is used, perhaps

even in the same building. This reduces the size and number of

power lines that must be constructed [9]. If renewable energy

resources are used as distributed generation resources, the

distributed hybrid power systems can also be referred as

renewable energy systems. Figure 1.1 illustrated a typical

renewable energy system with a conventional utility grid.

Figure 1.1: Large scale renewable energy system

interacted with utility grid


ISSN:2320-8007 3302

Distributed hybrid power systems (DHPS) consist of

ac and dc sub-systems connected to various load types, where

the DG resources can be either dc or ac sub-system-based

[10]. A self-sustainable energy system has been built in the lab

of Centre for Power Electronics Systems (CPES), and was

interconnected with a solar converter, a utility grid, and load in

both the power and communication sense; in addition, a wind

converter was wired to be part of the system.

The critical component of this system is the bi-

directional converter, which connects the dc and ac sub-

systems together, and connects the system with the utility grid.

Figure 1.2 shows a probable single-phase DHPS with energy

storage on the dc side and other renewable energy resources

through the system.

Figure 1.2: Single phase AC/DC interactive renewable energy

system

As we can see in Figure 1.2, the bi-directional single-

phase converter in a distributed hybrid power system should

fulfil the following modes of operation:

1. Stand-alone inverter mode: When the grid is lost, the

converter regulates the ac bus voltage and frequency to sustain

the ac load, while the renewable energy resources or energy

storage on the dc side provide power. The ac-side renewable

energy resources would act as current sources.

2. Grid-tied inverter mode: When the grid is connected, the

converter acts as a current source to source or sink power to

the grid to balance the power flow between the dc and ac

subsystems, while one of the dc resources regulates the dc bus

voltage.

3. Grid-tied rectifier mode: When the grid is connected, the

converter regulates the dc bus voltage to sustain the dc load

while all the dc-side renewable energy resources run as current

sources.

4. Grid-tied charger/discharger mode: When the grid is

connected, the converter charges energy storage elements,

such as a battery pack. When the grid is lost, the battery

discharges supplying power to sustain both the dc and ac load.

II. MODELING AND DESIGN OF MULTI FUNCTION

SINGLE-PHASE CONVERTER

2.1 CONVERTER MODELING:

2.1.1 Modeling of Full-bridge Switches:

The standard single-phase full-bridge converter

topology consists of two phase legs. Each phase leg has two

solid-state devices in series. Based on the functionality of the

semiconductor switch, the solid-state switch can be

represented as a single-pole-single-throw (SPST) ideal switch

[43] when losses, parasitics and the interior structure are

ignored, which is shown in Figure 2.1

Figure 2.1: Ideal switch representation

The switching function of the SPST switch is provided below.

Thereby, one phase leg switches can be represented as a

single-pole-double-throw (SPDT) ideal switch with the same

modelling method as shown in Figure 2.2.

Figure 2.2: Ideal phase leg switches representation

2.1.2 Modeling of Stand-alone Inverter Mode:

Applying the ideal SPDT instead of one phase leg

switch gives the simplified inverter topology illustrated in

Figure 2.3.


ISSN:2320-8007 3303

Figure 2.3:Ideal model of stand-alone inverter mode

2.1.3 Modelling of Grid-tied Inverter Mode:

The grid-tied inverter topology illustrated in Figure 2.4 is

actually the same as the stand-alone inverter mode. The

difference between them is that the ac capacitor is removed

because its dynamics are ignored by the connection with the

strong grid source.

The switching model of the grid-tied inverter mode is as

follows

Figure 2.4: Ideal model of grid-tied inverter mode

2.1.4 Modelling of Grid-tied Rectifier Mode:

The topology of this mode is illustrated in Figure 2.5. It is

clear to see that the capacitor is moved from the ac side to the

dc side to stabilize the dc voltage and improve the transient

response.

Figure 2.5: Ideal model of grid-tied rectifier mode

2.1.5 Modeling of Grid-tied Charger/Discharger Mode:

Figure 2.6 shows the circuit of the charger/discharger

mode. The difference between this mode and the rectifier

mode is that with the charger/discharger mode, a charging

inductor is hooked up on the dc side in series with an energy

storage element, such as a battery.

Figure 2.6: Ideal model of grid-tied charger/discharger mode

2.2 CONTROL STRUCTURE:

The control structures for each mode are shown in Figures 2.8

to 2.11.

Figure 2.8: Stand-alone inverter mode control structure


ISSN:2320-8007 3304

Figure 2.9: Grid-tied inverter mode control structure

Figure 2.10: Grid-tied rectifier mode control structure

Figure 2.11: Grid-tied charger/discharger mode control

structure

We can see that all modes have an inner-line inductor

current loop. This paper proposes that all modes share the

same inner current loop. In order to combine the inner current

loops, the current loop dynamic response should be checked

particularly at the crossover frequency. The proposed control

structure is selected as double loop feedback controller as

shown in Figure 2.12. The inner loop is selected as the ac

current of the line inductor to achieve fast dynamic response

for input disturbances, and the outer loop is designed with

different compensators to regulate the desired control

variables, such as ac voltage, dc voltage and dc charging

current.

Figure 2.12: Single-phase converter control structure

2.4 GENERIC INNER CURRENT LOOP DESIGN:

Figure 2.13 presents a simplified topology with all

probable modes of operation of the passive components. Our

main objective is to see the possibility of building a generic

inner current loop for all modes of operation.

Figure 2.13: Single-phase ac-dc renewable energy systems

2.4.1 Stand-alone Inverter Mode:

From the small-signal model, we know that the

control-to-current small-signal transfer function at stand-alone

inverter mode is:

The plant transfer function is a two-order system which has

double-pole, left half-plan zero.

2.4.2 Grid-tied Inverter Mode:

The control-to-current small-signal transfer

function at grid-tied inverter mode is


ISSN:2320-8007 3305

III. PROPOSED METHOD

3.1 INTRODUCTION:

In Recent years, due to the growing concern with

energy shortage and network stability, the concepts of

distributed generation (DG), microgrid systems [1], [2], dc

nanogrid systems [3], [4], and ac/dc hybrid power systems

[5]–[7] have all become progressively more popular;

especially with the decreasing costs of various clean

renewable energy sources (RES), such as: wind, solar, and

fuel-cells to name a few and more adoption of dc powered

residential loads, such as solidstate lighting. These DG

systems would be connected to the utility grid under normal

operating conditions, but also have the additional capability to

sustain a local system (micro- or nanogrid) by sourcing power

directly from the renewable energy sources and energy storage

devices if necessary to make grid transmission level black-

and brownouts seem transparent to the local system loads.

Figure. 3.1. Single-phase AC/DC interactive renewable energy

system

Distributed hybrid power systems (DHPS) consist of

ac and dc subsystems connected to various load types, where

DG resources can be connected on the ac or dc systems [6].

The critical component for such a system is the ac/dc

bidirectional, pulsewidth-modulation (PWM) converter that

connects the ac and dc subsystems together and to the utility

grid. The diagram in Fig.3.1 illustrates an example of a single-

phase , DHPS with renewable energy sources throughout the

system.

3.2 SYSTEM MODES OF OPERATION:MODELING

AND CONTRO STRUCTURE:

Before the control structure is designed and

implemented, the converter system needs to be modeled.

Specifically, the full bridge, multifunctional PWM converter

with different possible ac or dc configurations is shown in Fig.

3.2, where idg is the current flowing from the dc DG resources,

and its average model of full-bridge is described in (3.1) and

(3.2).

(3.1)

(3.2)

As seen in (3.1) and (3.2),VAB ,idc and dab are the

average terminal voltage of the full bridge, average dc-link

current, and average duty-cycle varying between -1 and 1,

respectively. Notice that if vdc is constant, the terminal voltage

VAB is only a function of the duty-cycle dab . The differential

equations describing the average model of the full-bridge

converter may then be derived as follows:

(3.3)

(3.4)

(3.5)

The average and small-signal models for the different

modes can be derived by combining (1)–(5). Notice that in

GCM, the dynamics of the ac capacitor can be ignored due to

the stiff grid, just as the dynamics of the dc-link capacitor can

be ignored because of the constant dc-link voltage during

SAM. With the control architecture selected to be a double-

loop feedback system, as shown in Fig. 3.3, the inner loop is

used to regulate the ac line inductor current. In order to

achieve fast dynamic responses from a wide array of

disturbances, the inner loop will need to be designed with high

bandwidth, while the outer loops regulate different control

variables depending on the operating mode.

Inclusion of all control features, such as digital delays

and sensor filters should also be included. Each sensor filter is

assumed to be a second-order, low-pass-filter, Hfilter(6). A one

switching-cycle (Tsample) delay, Hdelay(7), is modeled in the

modulator to approximate the digital computation and A/D

conversion delay. The modulator gain is assumed to be unity.

(3.6)


ISSN:2320-8007 3306

Fig.3.2 Control structures under different modes of operation

(3.7)

IV. PSIMULATION RESULTS

4.1 SIMULATION MODEL OF BI-DIRECTIONAL

PWM CONVERTER IN GRID CONNECTED

INVERTER MODE

4.1.1 MEASUREMENT

4.1.2 CONTROL CIRCUIT

4.1.3 SIMULATION OUTPUT OF BI-DIRECTIONAL

PWM CONVERTER IN GCM-INVERTER MODE


ISSN:2320-8007 3307


PWM CONVERTER IN GRID CONNECTED

RECTIFIER MODE

4.2.1 MEASUREMENT


PWM CONVERTER IN GCM RECTIFIER MODE


PWM CONVERTER IN STAND INVERTER MODE

4.3.1 MEASUREMENT


PWM CONVERTER IN STAND ALONE INVERTER

MODE


ISSN:2320-8007 3308

4.4 SIMULATION MODEL OF EXTERNAL VOLTAGE

CURRENT CONTROL

4.4.1 MEASUREMENT

4.4.2 SIMULATION OUTPUT OF EXTERNAL

VOLTAGE CURRENT CONTROL


ISSN:2320-8007 3309

V. CONCLUSION

This paper proposed a complete modeling

and control system for a bidirectional, single-phase,

multifunctional PWM converter for residential power level

distributed renewable energy and grid connected microgrid

system applications. A simple control structure was used to

cover all of the required operating modes, including stand-

alone (SAM), grid-connected (GCM) inverter, and rectifier

mode. A new single-phase PLL and active islanding

detection algorithm was also proposed for system-level

operation in order to meet IEEE standard 1547. The

resulting control structure is very simple and presents

robust, low transient responses even for extreme load steps

between no-load and heavy-load conditions. The transition

between modes was also seamlessly achieved, as predicted,

due to the common inner current-loop that all operating

modes have.

In future the proposed concept can be

implemented with artificial neural network (ANN) for

voltage and current control, and the proposed concept can

be implemented in real time with different loading

conditions.

REFERENCES

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interfacing power quality compensator for three-phase

three-wire microgrid applications,”IEEE Trans. Power

Electron., vol. 21, pp. 1021–1031, Jul. 2006.

[2] F. Katiraei and M. R. Iravani, “Power management

strategies for a microgrid with multiple distributed

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1821–1831, Nov. 2006.

[3] D. Boroyevich, I. Cvetkovic, D. Dong, R. Burgos, F.

Wang, and F. C. Lee, “Future electronic power

distribution systems—a contemplative view,” inProc.

2010 IEEE Int. Conf. Optim. Electr. Electron. Equip., pp.

1369–1380.

[4] D. Salomonsson and A. Sannino, “Low-voltage DC

distribution system for commercial power systems with

sensitive electronic loads,”IEEE Trans. Power Del., vol.

22, pp. 1620–1627, Jun. 2007.

[5] T. Zhou, P. Li, and B. Francois, “Power management

strategies of a DC-coupled hybrid power system in a

microgrid for decentralized generation,” inProc. 2009 Eur.

Conf. Power Electron. Appl., pp. 1–10.

[6] E. Ortjohann, A. Mohd, A. Schmelter, N. Hamsic, and M.

Lingemann, “Simulation and implementation of an

expandable hybrid power system,” inProc. 2007 IEEE Int.

Symp. Ind. Electron., pp. 377–382.

[7] I. Cvetkovic et al., “Future home uninterruptible

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