possibilities of voltage control in islanded distribution systems with photovoltaic...
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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
Q
<
>
=
<==
>==
=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
• 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