POSSIBILITIES OF VOLTAGE CONTROL IN ISLANDED DISTRIBUTION SYSTEMS WITH

PHOTOVOLTAIC POWER SOURCES

Authors: Ing. Jan Veleba, Ing. Vladislav SíťařDepartment of Electric Power Engineering and Environmental Engineering

Faculty of Electrical EngineeringUniversity of West Bohemia in Pilsen, Czech Republic

Overview

• Introduction

• Modelling of OLTC Transformers in N-R Method

• Modelling of Var Limits in N-R Method

• Modelling of PV Sources in Steady State

• Case Study – Modified IEEE 123-Bus Radial System

• Evaluation of the Results

• Conclusion

Voltage Control

• Why is it important in distribution networks?– installations of renewable and other dispersed power sources– highly stressed operation modes of the system– deregulated electricity market policies– realization of the Smart Grid concept

• Possible threats?– bus voltages out of operational limits– insufficient level of reactive power generation

• Solutions:– remotely operated OLTC transformers and capacitor banks,

synchronous condensers, etc.

• Our task:– to find optimal settings of devices above to meet voltage targets

and maintain all remaining bus voltages within the limits

• Model of OLTC and fixed-tap transformers:

Modelling of OLTC Transformers in N-

R Method

( )

( )( ) ( )

( ) ( )iikkikkiikkikikki

ikkiikkiikikiikikik

ikkiikkiikkiik

tVVBVBBQ

tVVBtVBBQ

tVVBPP

θλθ

λθθ

λθθ

−+++−=

−−++−=

−−−=−=

cos/

cos//

sin/

2

0

2

0

• Scenario 1: tap magnitude is within its limitsupdate vector:

• Scenario 2: tap magnitude exceeds its limitupdate vector:

• Entire process in the N-R method1] In iteration 0, all controlled voltages set to the targets

2] Iteration number increased by one

3] Power mismatches and Jacobian calculated, updating process accomplished

4] Check for tap limits, updating process of matrix A, back to 2]

5] Rounding process of the taps and vars, solution re-convergence

Modelling of OLTC Transformers in N-

R Method

[ ]TNkikkN VVtVVV ,...,,,...,,,..., 113232 +−θθθ

[ ]TNkkkN VVVVVV ,...,,,...,,,..., 113232 +−θθθ

• Bus Type Switching Logic for Var Limits

1] Forward logic

2] Backward logic

For capacitor banks, rounding process to closest integer

(regulation step) must be applied when converging

Modelling of Var Limits in N-R Method

min

max

min

max

if

if

GiGi

GiGi

Gi

Gi

GiQQ

QQ

Q

QQ

<

>

=

<==

>==

=sp

iiGiGi

sp

iiGiGi

sp

ii

VVQQ

VV QQ

VV

AND

OR

AND

if

min

max

Modelling of PV Sources in Steady

State

• PV plant modelled as a PQ power source:

– V-A characteristic of the source:

– Short-circuit current correction:

– No-load voltage correction:

– Additional formulas:

– Final formula:

)e(II t

OC

mV

VV

SC

−

−= 1

( )011 CCaSC TTKGCI −+=

( ) 3

0

4

03000

1 K

GG

CCCC

aa

eK

-TTCVV

−

−+=

002 15273 15273 CCaaC t.TGC.tT +=++=

/ekTV Ct =

( ) )e(pVIcP t

OC

mV

VV

SC

−

−−= 1100/1 l

Modelling of PV Sources in Steady

State

• P-Q diagram of the PV source

Source: Prokop, L., Misak, S.: Hodnocení provozu fotovoltaické elektrárny, EPE konference, VŠB - TU Ostrava, Dlouhé Stráně, 2011.

Case Study – Modified IEEE 123-Bus

Radial System

Source: http://ewh.ieee.org/soc/pes/dsacom/testfeeders/

• Other modifications 1/2

– 5 PV power plants incl. 6/0.4 kV fixed-tap transformers

– 3 OLTC isolation transformers

– 2 additional isolation 2 MVA fixed-tap transformers

Case Study – Modified IEEE 123-Bus

Radial System

Bus No. 119 124 125 126 127

Nominal

power

[kWp]

80 100 90 70 80

From-To Bus

No.

Nominal

power [kVA]

Tap settings

(min/max/step)

Targeted

voltage [pu]

018-021 2000 0.9/1.1/0.00625 0.96

013-115 3000 0.9/1.1/0.002 0.98

060-116 2000 0.9/1.1/0.008 0.96

• Other modifications 2/2

– Voltage-controlled devices - overview

Case Study – Modified IEEE 123-Bus

Radial System

Bus

No.

Typ

e

Active

power

[kW]

Min/max var

limits

[kVAr]

Specified

voltage

[pu]

008 G - - 0.95

076 G 2000 -350/650 0.95

044 SC 0 -75/150 0.94

101 SC 0 -50/100 0.96

064 CB 0 0/7x50 0.97

094 CB 0 0/6x100 0.96

110 CB 0 0/10x50 0.96

Input Data for PV Power Plants

Source: http://re.jrc.ec.europa.eu/pvgis/apps4/pvest.php#

Input Data for Loads

Source: http://www.ote-cr.cz/

• Voltage conditions w/o and w/ voltage control

Evaluation of the Results

5 10 15 200.88

0.9

0.92

0.94

0.96

0.98

1

Time [hours]

Vo

lta

ge

Ma

gn

itu

de

[p

u]

5 10 15 200.88

0.9

0.92

0.94

0.96

0.98

1

Time [hours]

Vo

lta

ge

Ma

gn

itu

de

[p

u]

• Optimal settings of OLTC transformers

Evaluation of the Results

5 10 15 20

0.95

0.96

0.97

0.98

0.99

1

1.01

1.02

Time [hours]

Ta

p M

ag

nitu

de

[p

u]

• Optimal numbers of capacitor steps activated

Evaluation of the Results

5 10 15 20

0

1

2

3

4

5

6

7

8

9

Time [hours]

Ste

ps o

f S

witch

ed

Sh

un

t C

ap

acito

rs [

-]

• Var generations of voltage-controlled devices

Evaluation of the Results

5 10 15 20-0.01

-0.005

0

0.005

0.01

0.015

0.02

Time [hours]

Re

active

Po

we

r G

en

era

tio

n [

pu

]

• MVA Loadings of Fixed-Tap/OLTC Transformers

Evaluation of the Results

5 10 15 20

10

20

30

40

50

60

70

Time [hours]

Fix

ed

-Ta

p/O

LT

C T

ran

sfo

rme

r L

oa

din

g [

%]

• Total active power losses

Evaluation of the Results

5 10 15 200.8

0.85

0.9

0.95

1

1.05

1.1

1.15

1.2

1.25

1.3

Time [hours]

To

tal A

ctive

Po

we

r L

osse

s [

%]

• Voltage control of the islanded distribution power system with PV power sources has been simulated using various types of voltage-control devices

• Voltage conditions have been significantly improved in the modelled time interval of the year

• In future work, the optimization of total active power losses and reconfiguration problem will be also included

Conclusion

Thank you for your attention