Control of Low Voltage Microgrid

in Autonomous Operation Mode

Michał Małaczek, Irena Wasiak

8th EPNet, Szklarska Poręba, 19-21 September 2016

Presentation Outline

Introduction

Objective of the paper

Microgrid operation modes

Proposed control strategy

Microgrid simulation model

Simulation results

Conclusions

2

Typical structure of the Microgrid

3

Grid

Energy Storage

Wind

Turbines

Generators

Photovoltaic

Panels

Loads

Microgrid

DC

AC

AC

DC

Variable speed

source

Distributed generation units installed in

Microgrid

4

High speed source

Microgrid

DC

AC

AC

DC

Photovoltaic Panel

Microgrid

DC

AC

Objective of the paper

It could be expected from the microgrid to ensure sufficient

generation capacity, controls and operational strategy to supply

loads after being disconnected form the supplying network even if

the microgrid delivers electricity with less power and lower quality.

With an appropriate control strategy of the sources there is a

possibility to provide power supply to local customers even if any

disruption occurs in the power system.

The major purpose of the work is to demonstrate the

control strategy for autonomous mode of microgrid operation.

5

Grid-connected mode

When a microgrid operates in grid-connected mode:

No direct voltage and frequency control is required

Utility grid constitutes a reference voltage and frequency source

Energy sources are controlled as current sources

Interface converters are Voltage Source Converters (VSC)

operating in current controlled mode (CC-VSC) in

synchronization with the suppling voltage, according to P-Q

control strategy

6

Grid-connected mode

Control algorithms can be delivered in various coordiante systems,

such as:

αβ0 – stationary reference frame

abc – natural reference frame

dq0 – synchronous reference frame

7

Q

Controller

abc

dq

PWM

InverterSource

iabc

uabc PLLΘ

Pref

P

Qref

idref

iqrefController

id

iq

abc

dq

abc

dq Θ

uqud

iq

id

Θ

Controller

Controller

udref

uqref

uqud

iq

idP

Q

Power

Calculation

Θ

Block diagram of P-Q control in dq reference frame

Autonomous mode

The objectives of control strategy in autonomous microgrid

operation is to:

Ensure power balance

Maintain the required values of voltage amplitude and its frequency

at the point of connection with the supplying network

In order to meet these requirements it is necessary to have “grid-

forming” unit within the microgrid which has adequate power reserve to

cover power demand changes.

Such unit is assigned to regulate the voltage at the PCC and set

the system frequency. It employs voltage controlled mode for voltage

source converter (VC-VSC) and follow U-f strategy.

8

Autonomous mode

Control algorithm for VC-VSC according to U-f control strategy

9

PWM

InverterSource

Urms

Urms refController

miReference

Voltages

Determination

uaRef ubRef ucRef

Virtual

PLL

Θset

Θset

Block diagram of U-f control

Autonomous mode

Under autonomous operation mode microgrid can be controlled

according to two possible control strategies:

Single Master Operation (SMO)

Muliti Master Operation (MMO)

In the case of SMO strategy when two or more energy units are

connected to the microgrid operating autonomously, only one of

these can be the source of reference voltage. This unit follows the

U-f control, while the other sources implement the P-Q control.

10

Autonomous mode

MMO strategy is applied if two or more sources actively

participate in microgrid voltage and frequency control.

In such a case a frequency-droop and voltage-droop control is

derived that share active and reactive powers between sources.

11

Proposed control strategy

The proposed control strategy assumes that:

All energy sources within the microgrid are CC-VSC controlled

independently on the operation mode.

In autonomous microgrid operation SMO strategy is applied and

the source of reference voltage is the energy storage.

The control of energy storage changes from P-Q in grid-

connected regime to U-f in autonomous operation.

Operation of the microgrid is coordinated by central controller

12

Proposed control strategy

The central controller can performs the following functions:

Keeps constant active power exchange with the network during

grid connected mode of microgrid operation (optional)

Identifies the conditions for island creation

Changes the storage control from P-Q to U-f

Determines set points for MT during autonomous operation

Identifies the condition for reconnection of the microgrid with the

network

13

Microgrid Simulation Model

14

Microturbine

Energy Source

Energy

Storage

Load

PV

Panels

VSC

VSC

VSC

Utility

Grid

YKY 5x 50 mm2

L = 300 m

Transformer63 kVA

15 / 0,4 kV

500 MVA

15 kV

YKY 5x 25 mm2

L = 50 m

YKY 5x 25 mm2

L = 100 m

YKY 5x 50 mm2

L = 150 m

Energy SourceCentral

Controller

PCC

Microgrid Simulation Model

15

Model of PV source in PSCAD program

a b c n

A

B

C

iARef

iBRef

iCRef

3 P

hase

RM

S

URMS

I0Ref

Reference currents

forming

uA

URMS

iARef iBRef ICRef

Pget

uA

Active Power

Determination

Pgen

uB

uC

uB

uC

Microgrid Simulation Model

16

Microturbine model in PSCAD program

abc

IcF

IbF

IaF

1

2

3

2

5

2

4

2

6

2

2

2

G1 G3 G5

G4 G6 G2

IFdc

1 [uF]

1 [uF]

1 [uF]

1.0

[o

hm

]V

icin

v

Iac

ibin

v

iain

v

UaF

UbF

UcF

uabFLcouple

+

Lcouple

+

Lcouple

+

Control System DQ

uAccsuBccsuCccsiAccs iBccsiCccs

G1 G2 G3 G4 G5 G6

Firing pulsePset

Qset

udref uqref uqud

udref

uqref

uq

ud

uAccsuBccsuCccs

Simulation results

17

Active power flows within microgrid

Simulation results

18

Reactive power flows within microgrid

Simulation results

RMS voltage at the PCC (pu)

19

Simulation results

Voltage frequency within the microgrid

20

Conclusions

The idea of proposed control allows to continue load supply after

microgrid disconnection from the utility network

The applied strategy assumes that energy storage maintains the

reference voltage in the microgrid. Thus, the control of energy

sources can remain the same as in grid-connected mode and

uninterrupted transition from grid-connected to islanding

operation is possible.

The central controller is designed which determine set points for

the controllable source to cover load power changes in the

microgrid. As a result the storage power is maintained close to

zero which makes the autonomous operation of the microgrid

possible in longer-term

21

Thank you for your

attention

22