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Journal of Energy and Power Engineering 7 (2013) 2156-2163

A Real-Time Power Controller for Grid-Connected

Inverters in LV Smart Microgrids

Kourosh Sedghisigarchi1, Yadollah Eslami2 and Asadollah Davari3

1. Electrical and Computer Engineering Department, California State University, Northridge 91330, USA

2. Department of Engineering Technology, West Virginia University Institute of Technology, Montgomery 25136, USA

3. Electrical and Computer Engineering Department, West Virginia University Institute of Technology, Montgomery 25136, USA

Received: January 24, 2013 / Accepted: June 07, 2013 / Published: November 30, 2013.

Abstract: This paper presents a real-time power flow controller for VSIs (voltage source inverters) interfaced to low voltage microgrids. The proposed controller is modular, flexible, intelligent, inexpensive, portable, adaptive and designed to positively contribute in low voltage microgrids in which the lines R/X ratio is greater than the transmission lines. Therefore, the proposed control strategy is developed for operation in distribution lines. The controller strategy is different from the conventional grid-connected inverters which are designed based on transmission line characteristics. This controller, using a Texas Instrument general purpose DSP (digital signal processor), is programmed and tuned using MATLAB/SIMULINK in order to enhance self-healing, reliability and stability of the grid. This general purpose controller makes proper decisions using its local measurements as the primary source of data. The controller has the capability of communicating with the adjacent controllers and sharing the information if/when needed. The power flow output of the inverter is tested for both islanded and grid-connected modes of operation. The inverter positively contributes to active and reactive power supply while operating in grid-connected mode. The proposed control method has been implemented on a Texas Instrument DSC (digital signal controller) chip and tested on a hardware test bench at the Alternative Energy Laboratory at WVUIT (West Virginia University Institute of Technology). The system’s experimental results verify the validity and efficiency of the proposed controller. Key words: Distribution generator, inverter, LV (low voltage) microgrid, power flow controller, digital signal controller.

1. Introduction

Proper integration of DGs (distributed generators) to

the grid at LV (low voltage) levels promises a reliable,

efficient, modular and secure microgrid [1].

Distributed generators interfaced to LV grids can be

divided into two major categories: inverter-based and

non-inverter based ones. Advances in power electronic

devices make inverter based DGs, such as fuel cells,

photovoltaic arrays and storage, devices more flexible

than constant output power generators.

The balance between the supply and demand must be

satisfied in both islanded and grid-connected modes to

Corresponding author: Yadollah Eslami, Ph.D., assistant

professor, research fields: digital circuits and systems and smart microgrids. E-mail: [email protected].

ensure stable operation of a microgrid. To achieve this

goal, the inverter of each grid-connected DG must be

controlled coordinately to participate efficiently in low

voltage power networks such as microgrids [1].

Stable operation of microgrid becomes more

substantial when it is islanded and dependent only on

its own DGs. Fast, intelligent and coordinated control

of advanced DC/AC inverters is required to guarantee

the voltage and frequency stability of isolated

microgrids. Advances in power electronic devices

enhance microgrids efficiency, power quality, voltage

stability, reactive power support and loss reduction. A

modular and applicable platform for decentralized

control applications requires proper control strategies.

The active and reactive power control schemes

proposed in literature for inverter based DGs are

D DAVID PUBLISHING

A Real-Time Power Controller for Grid-Connected Inverters in LV Smart Microgrids

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mostly based on the assumption that the power lines are

inductive (R/X << 1) [2-9]. The proposed control

methods for high voltage levels are not feasible for the

LV networks [10-12], in which the lines are highly

resistive. In fact, the line resistance to the line reactance

ratio is a large number (R/X >> 1). The power flow

control methods presented in Refs. [11, 12] use a

virtual inductance or virtual frequency-voltage frame

to achieve this goal. Integration of single phase electric

vehicles to unbalanced LV microgrids is discussed in

Ref. [13].

Today, inverter based DGs operate at unity power

factor in grid-connected mode. In fact, they contribute

only to active power control, which may end up

lowering the power factor at the PCC (point of

common-coupling) [8, 9].

This paper presents a real-time power flow portable

controller for VSIs (voltage source inverters) that are

interfaced to low voltage microgrids. This modular

controller can be programmed by decentralized control

algorithms in order to enhance self-healing, reliability

and stability of the grid. The main objective is to be

able to implement and test decentralized control

algorithms on this controller.

The power flow control strategies for distributed

generators operating in both islanded and connected to

LV networks are presented. The DSC (digital signal

controller) based controller uses a PLL (phase-locked

loop) to synchronize the distributed generator with the

grid while operating in the islanded mode. After

synchronization, the active and reactive output powers

to the grid are controlled. The controller measures the

inverter output voltage and current as well as the

voltage at the point of common-coupling and sends the

proper switching signals to the grid-connected inverter.

The control schemes are simulated for the non-unity

power factor in MATLAB/Simulink.

Control power flow strategy for inverter based

distributed energy resources operating at LV networks

are introduced [14]. The developed model is simulated

in MATLAB/Simulink [15] using the

SimPowerSystems toolbox [16]. The proposed control

method has been implemented on a Texas Instrument

DSC chip and tested on a hardware test bench at the

Alternative Energy Laboratory at WVUIT (West

Virginia University Institute of Technology). The

system’s experimental results provided in Section 4

verify the validity and efficiency of the proposed

controller.

2. Active/Reactive Power Control in LV Networks

Fig. 1 shows the schematic of a DC source

distributed generator connected to the low voltage

microgrid using a VSI (voltage source inverter) [17]. A

transformer is needed to isolate and amplify the output

AC voltage of the inverter as shown in Fig. 1.

The DG in Fig. 1 is able to operate in both isolated

and grid-connected modes. While the DG is not

connected to the grid, it supplies power only to its local

load. After synchronization and closing of the breaker,

both the DG and grid provide power to the available

loads (in this case local load). The interconnected DG

needs to be properly controlled in order to supply the

required active and reactive power. The output current

of the DG and active/reactive power outputs to the grid

can be expressed as Ref. [18]:

I = (V1-E)/(R + jX) (1)

P = Re (V1I*) (2)

Q = Im (V1I*) (3)

Inverter output voltage (V1) and grid voltage (E)

vectors specified in Fig. 2 can be written:

V1 = V1 E = E0 (4)

Pdg = P + PL (5)

Qdg = Q + QL (6)

where:

Pdg, Qdg: inverter active and reactive power;

PL, QL: load active and reactive power;

P, Q: active and reactive power flow to grid.

P = (V1/(R2 + X2)) [R(V1 - Ecos) + XEsin] (7)

Q = (V1/(R2 + X2)) [X(V1 - Ecos) - REsin] (8)

In HV (high voltage) transmission lines where (X >>


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Fig. 1 An inverter based DG system interfaced to LV electric network.

Fig. 2 Phase diagram of inverter voltage and grid voltage.

R), and assuming a small value for angle (sin = ;

cos = 1), Eqs. (7) and (8) can be simplified to Ref.

[17]:

P = (V1E/X) (9)

Q = (V1 - E)V1/X (10)

Therefore, the DG active and reactive output power

flows to the grid (P, Q) can be controlled by phase

angle () and voltage magnitude (V1 - E) differences.

However, in low voltage distribution lines (where

R >> X) the active/reactive power control equations in

HV transmission line networks described in Eqs. (5)

and (6) are not applicable anymore. Using the line data

for the LV networks given in Ref. [11], Eqs. (2) and (3)

can be simplified to:

P = V1(V1 - E)/R (11)

Q = (-V1E/R) (12)

As shown in Eqs. (11) and (12), the output active

power depends on the voltage magnitude difference (V1

- E). On the other hand, the reactive power is a function

of the phase angle difference between V1 and E (). The

above equations may not be accurate enough for the

control purposes of distribution lines in which R/X ratio

is close to 1 or not large enough. The methods

suggested by Refs. [12, 13] are possible alternatives for

these types of low voltage electric networks.

3. Inverter Control Strategy

Fig. 1 displays the overall closed loop system for an

inverter based DG which was developed in

MATLAB/Simulink using the SimPowerSystems

toolbox. In this development, DC output voltage is

converted to an AC voltage using a voltage source

inverter. The output AC voltage is filtered using a low

pass filter (L-C-L) to obtain a pure 60 Hz sinusoidal

signal.

The proposed controller is designed to operate in

both islanded and grid-connected modes. In standalone

or islanded mode, the DG provides electric power to its

own local load. In this case, the DG output voltage is

equivalent to the local load voltage which needs to be

kept within the standard margins [9]. The inverter

output voltage remains synchronized with the grid at

the point of common connection. In grid-connected

I

E

RI

V1

jXI


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mode, the output active and reactive power of the

inverter is controlled based on Eqs. (11) and (12).

Also, the DG active and reactive power outputs to

the LV grid are controlled using the grid connected

controller sub blocks shown within the dashed line in

Fig. 1. The VSI (voltage source inverter) control modes

are proposed by the first author in Ref. [17].

4. Hardware Implementation

The Delfino F28335 control card by Texas

Instruments [19] is used to implement the controller.

The card uses a TMS320F28335 digital signal

controller which is a powerful DSC in the TI C2000

family. The control card provides 16 analog inputs with

12-bit resolution and up to 12.5MSPS (mega samples

per second) capability per input [20]. It also provides

up to 88 programmable digital input/output pins, with

plenty of internal modules that simplify its use in many

control applications, especially in the smart grid

control application. There are three reasons for

choosing this board for the implementation:

First, it is a small and cheap DSC board in

comparison with the other DSCs available from TI.

Second, it includes sufficient input/output pins and

internal modules to implement any controller of

reasonable size for smart grid applications. Third, it is

supported by the Embedded Coder package [21]

available from Mathworks for MATLAB and Simulink.

This package converts a MATLAB or Simulink model

into a C program that can be loaded into the DSC

memory by using Code Compose Studio [22], the TI

provided IDE package for interfacing to its DSCs. This

last feature makes the design, modification and

debugging process of any model in Simulink fast and

easy. The Simulink model used for this experiment is

shown in Fig. 3. The DSC resources used in this

application are three analog inputs (A0 to A2) and one

ePWM (enhanced pulse width modulation) module

(two ePWM output signals). The analog inputs are used

to read the inverter current and voltage and the grid

voltage into the controller, as shown in Block A of Fig.

3. The DSC analog inputs amplitude range is limited to

0 V to 3 V, and it is one of the limitations of the analog

inputs of this DSC. To address this limitation, the

transducers from the Lab-Volt test bench are used to

convert the grid and inverter voltages and currents to a

low voltage within ±2.5 V. Then, a custom designed

circuit is used to further attenuate the transducer

outputs by a factor of 2 and then add a bias voltage

(Vbias = 1.34 V) to adjust them to the appropriate range

of 0 V to 3 V. The input signals are then sampled at a

rate of 10 kHz and with 12-bit accuracy.

The DSP board comes with six ePWM modules,

which are powerful blocks for the inverter control

applications. For this experiment, only one ePWM

module is used. The module produces the sawtooth

carrier signal internally and just needs a sinusoidal

input to produce the PWM IGBT drive signals as

indicated in Block B of Fig. 3.

The amplitude and the frequency of the internal

sawtooth are programmable; hence, the block can be

used for various output frequencies and modulation

indices.

A simplified diagram of the hardware setup at our

Alternative Energy Laboratory is illustrated in Fig. 4.

This test bed is built of Lab-Volt modules including an

IGBT inverter, a data acquisition unit, smoothing

inductors, capacitors, a transformer, current/voltage

sensors and resistive and inductive loads as shown in

Fig. 5. A synchronization relay is located between the

grid voltage and the inverter.

Once the output voltages are synchronized, the

breaker closes using the normally open and normally

closed relays on the Lab-Volt protection relay bench

and the synchronization relay. The test bed system

parameters are given in Table 1.

5. Case Study

The system has been tested for various load types,

sizes and different set point values. The test results for

two case studies are given as follows:

Case 1: Pref = 16 W and Qref = 8 V,


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Fig. 3 The developed SIMULINK model used for hardware test.

Fig. 4 Single line diagram of the hardware setup.

Fig. 5 Hardware setup at alternative energy lab of WVUIT.


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Table 1 Hardware setup parameters.

Q controller parameters Ki = 0.02 Kp = 0.001

P controller parameters Ki = 0.02 Kp = 0.2

Line parameters R = 600 Ω L = 0.26 H

LC filter Lf = 0.4 H Cf = 4.4 uF

DC voltage input Vdc = 136 V

Fig. 6 Inverter and grid output voltages in synchronization mode.

local load: RL = 600 Ω (resistive load);

Case 2: Pref = 5 W and Qref = 10 V,

local load: RL = 36 Ω, L = 1.6 H (inductive load).

Fig. 6 shows the inverter output voltage and the grid

voltage in synchronization mode. Fig. 7 shows the grid

(E1) and inverter output voltage (E2) graphs in grid

connected for case 1.

As Expected, the inverter output active power (P2)

has reached its set point value (Pref = 16). Summation

of P1 and P2 minus the line losses (RI2) is equal to load

active power (P3). The inverter output reactive power

has been transferred 100% (Q2 = -Q1 + QXL) to the grid

since the local load is purely resistive and Q3 = 0.

In case of a resistive-inductive (RL) local load, both

inverter active and reactive power have reached their

set point values as demonstrated in Fig. 8. The load

current has an 82° phase shift with respect to the load

voltage since the load is highly inductive; therefore P3

is a small number while Q3 is a large value. The load

reactive power has been supported by both the grid and

inverter.

The output voltage, current and power rms values for

the inverter, grid and load are indicated using the

Lab-Volt digital meter. As expected, the inverter

output voltage varies at the point of connection while

controlling the power output. The voltage variation

should be limited within standard margins to avoid

undesired interruptions.

6. Conclusions

This paper proposes a general purpose power flow

controller for inverter based distributed energy

generators connected to low voltage networks. Active

and reactive control techniques of DGs in both

grid-connected and standalone modes of operation for

LV networks are developed. The distributed generator

inverter uses a PLL to achieve synchronization and to

control the active and reactive power outputs of the DG

connected to the LV grid.

The models are developed in MATLAB/Simulink

using the SimPowerSystems toolbox and tested for

unity and non-unity power factor scenarios. The

Fig. 7 Graph and digital display screens for purely resistive local load (R = 600 Ω).


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Fig. 8 Graph and digital display screens for RL local load (R = 36 Ω, X = 1.6 H).

proposed control method has been implemented on a

C2000 Texas Instrument digital signal controller and

tested on the small scale test bed at the Alternative

Energy Lab of WVUIT for both resistive and inductive

local loads. The experimental results confirm the

validity and accuracy of the proposed method.

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