1
Grid Connected Solar PV and Reactive Power in
a Low Voltage Distribution Network
Dean Condon
Ergon Energy, Technology Development Group
e-mail: dean.condon@ergon.com.au
Abstract— Proof of concept reactive power testing to
influence the voltage levels on the Low Voltage distribution
network is performed at the Ergon Energy Solar Skate Park
using Grid Connected Inverter Energy Systems.
Index Terms—Grid connected, Photovoltaic, Power, Reactive
Power, Solar.
I. INTRODUCTION
T
HIS investigation looks at how reactive power from grid
connected Solar Photovoltaic (PV) inverter energy
systems can be used to influence the levels of voltage on the
Low Voltage (LV) distribution network. Ergon Energy (EE)
owns and operates a 100kW Solar PV system on Magnetic
Island, which was installed through the Townsville
Queensland Solar City Project. The system is known as the
Solar Skate Park (SSP), because the solar panel roof structure
covers an existing skate park. During the design phase of the
project, a PV voltage rise model was produced with the SSP
connected to the adjacent LV network. This model predicted
a 18V rise of the LV at the SSP and resulted in a change to
the connection arrangement. The new arrangement has the
SSP connected to the Medium Voltage (MV) network via a
dedicated distribution transformer, - which results in a low
impedance connection to the network. The inverters used at
the SSP have the ability to produce both real and reactive
power. The ability of the SSP to produce and consume
reactive power and its corresponding impact on the LV
network voltage was tested whilst still connected to the
dedicated transformer – low impedance connection and with
the SSP connected to adjacent LV network – high impedance
connection.
II. SYSTEM CHARACTERISTICS
The SSP PV system is connected to a dedicated 315kVA
11000/415 V distribution transformer. The adjacent LV
network is supplied from a 200kVA transformer and services
75 mostly domestic customers. The Solar Skate Park can be
connected to the adjacent LV network through 2 sets of
manually operated LV links. At the time of performing this
analysis, the adjacent LV network transformer number was
TVS 55 and hence for this report – will be known as TVS 55.
A. Solar Skate Park
The SSP PV system consists of 7 x SMA Sunny Tripower
grid connected 3 phase inverters, connected to a common ac
bus. The reactive power control of these units is managed via
a SMA power reducer box, which sends command
information to the SMA Webbox via an Ethernet connection,
which sends the command to individual inverters via a RS485
network. Through changing values in the Power Reducer
web page the inverters can be requested to supply reactive
power (act as a capacitor) or consume reactive power (act as
an inductor). The SSP connection to the grid is through 20m
of 3 phase 70mm² Cu cable that is connected to the 315kVA
pole mounted transformer. This equates to a connection
impedance of 0.00128 + j0.00039ω. The inverters have the
maximum voltage limit factory set at 270V above which the
inverter instantaneously disconnects from the grid.
B. Distribution Network
The adjacent TVS 55 network consists of both
underground (UG) and overhead (OH) sections. Shown
below in Fig. 1 is a single line diagram of the low voltage
distribution network of TVS 55 and the SSP. The 100 kW
SSP is on the right of the open point links. The backbone
overhead conductor is imperial 7/104 Cu and the
underground cable is 95mm² Al. The distance between the
SSP and TVS 55 is approximately 400m. This equates to a
connection impedance of 0.1058 + j0.1324ω, - considerably
larger than the connection impedance for the dedicated
transformer.
There are also 3 locations where voltage is recorded, 1)
SSP, 2) 150m from the SSP towards TVS 55 and 3) TVS 55.
The voltage values are averaged over a 10 minute period.
2
Baseline Voltage Levels - Unity pf
254.00
Voltage (V)
252.00
250.00
248.00
246.00
244.00
242.00
23:00
22:00
21:00
20:00
19:00
18:00
17:00
16:00
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13:00
12:00
11:00
9:00
10:00
8:00
7:00
6:00
5:00
4:00
3:00
2:00
1:00
0:00
240.00
Time
Avg Vln A
Avg Vln B
Avg Vln C
Figure 3: Voltage levels with pf = 1.0
A. pf = unity
The power factor of all inverters is kept at unity. Figure 2
shows the power output and it can be seen that the real and
apparent power are equal and the reactive power is zero.
Reactive Power Testing - Unity pf
100.00
80.00
60.00
40.00
20.00
23:00
22:00
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11:00
10:00
9:00
8:00
7:00
6:00
5:00
4:00
3:00
0.00
-20.00
-40.00
-60.00
-80.00
-100.00
Time
Avg kW
Avg kvar
Avg kVA
Figure 4: Power values with pf = 0.8, under excited
The voltage profile is shown below in Figure 5 and has a
flatter profile with a maximum midday value of
approximately 245V.
90.00
80.00
Baseline Voltage Levels - Consume vars, pf=0.8
70.00
60.00
50.00
254.00
40.00
252.00
30.00
Voltage (V)
20.00
10.00
23:00
22:00
21:00
20:00
19:00
18:00
17:00
16:00
15:00
14:00
Time
244.00
242.00
23:00
22:00
21:00
20:00
19:00
18:00
17:00
16:00
15:00
14:00
13:00
12:00
11:00
10:00
Time
Figure 2: Power values with pf = 1.0
Avg Vln A
The voltage levels are shown below in Figure 3 and the
profile shows low morning levels, rising to a midday max and
dropping in the afternoon – consistent with PV power output.
The maximum midday value is approximately 248V.
9:00
8:00
7:00
6:00
5:00
4:00
240.00
Avg kVA
3:00
Avg kvar
246.00
2:00
Avg kW
248.00
1:00
13:00
12:00
11:00
9:00
10:00
8:00
7:00
6:00
5:00
4:00
3:00
2:00
1:00
0:00
0.00
250.00
0:00
Power (kW, kvar and kVA)
100.00
Reactive Power Testing - Consume vars, pf=0.8
2:00
The ability of reactive power produced by the inverters to
influence levels of the LV network was first tested whist the
SSP was connected to the dedicated transformer. Three cases
will be examined below, A) power factor (pf) is unity, ie 1.0,
B) power factor equals 0.9 and C) power factor equals 0.8.
Values shown in graphs are averaged over a continuous 7 day
period and are from different weeks over a 6 week period,
thus some real power values may be different from a change
in solar radiation.
1:00
REGULATION
0:00
III. REACTIVE POWER TESTING, PROOF OF VOLTAGE
B. pf = 0.8, under excited
The power factor of all inverters was changed to 0.8 under
excited – that is consuming reactive power. Figure 4 below
shows the power output and shows maximum midday values
of real power ~ 72 kW and apparent power ~ 90 kVA. The
reactive power has a maximum midday value of ~ 54 kvar.
Voltage (V)
Fig. 1: Single Line Diagram of TVS 55 Low Voltage distribution network
Avg Vln B
Avg Vln C
Figure 5: Voltage levels with pf = 0.8, under excited
C. pf = 0.8, over excited
The power factor of all inverters is changed to 0.8 over
excited – that is producing reactive power. Show below in
Figure 6 is the power output and shows maximum midday
values of real power ~ 75 kW and apparent power ~ 95 kVA.
The reactive power has a maximum midday value of ~ 57
kvar.
3
the top and the time period is shown at the bottom.
23:00
22:00
21:00
20:00
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2:00
1:00
100.00
90.00
80.00
70.00
60.00
50.00
40.00
30.00
20.00
10.00
0.00
0:00
Power (kW, kvar and kVA)
Reactive Power Testing - Export vars, pf=0.8
Figure 8: Inverter connection sequence
Time
Avg kW
Avg kvar
Avg kVA
Figure 6: Power values with pf = 0.8, over excited
The voltage profile is below in Figure 7 and now has a
curve similar to the PV output. The profile has a maximum
midday value of approximately 250V.
Baseline Voltage Levels - Export vars, pf=0.8
254.00
Voltage (V)
252.00
250.00
248.00
246.00
244.00
242.00
23:00
22:00
21:00
20:00
19:00
18:00
17:00
16:00
15:00
14:00
13:00
12:00
11:00
10:00
9:00
8:00
7:00
6:00
5:00
4:00
3:00
2:00
1:00
0:00
240.00
The first 2 inverters were turned on at 11:30am, at
11:40am U3 was turned on, then at 11:50am U2 disconnected
from the grid. The power factor of all inverters was adjusted
to 0.9 under excited and at 12:00pm U3 and U4 were turned
on. At 12:20 U5 was turned on and then U1 and U2
disconnected from the grid. The power factor of all inverters
was adjusted to 0.8 under excited the connection sequence
continued. At 12:50 the last inverter was turned on and all
inverters remained on for approximately 30 mins, before all
inverters were turned off and power factor restored to unity.
The SSP was disconnected from TVS 55, reconnected to the
dedicated transformer and inverters turned on.
Time
Avg Vln A
Avg Vln B
Avg Vln C
Figure 7: Voltage levels with pf = 0.8, over excited
The above testing proves the concept that reactive power
can be used to influence the levels of voltage on the LV
network. The change in maximum midday value from 245V
to 248V for the under excited case was 3V (1.25%) and 248V
to 250V for the over excited case was 2V (0.83%). These are
small changes, however the SSP connection to the network is
very low impedance and hence would only expect to see small
changes.
The following sections describe the results from the
voltage recorders on the day of testing. Data has been limited
to the time period of 10am to 3pm and power factor is 0.9
from 12:00-12:20 and 0.8 from 12:30-13:30.
A. Results from SSP – V1
Show below in Figure 9 is the real power, reactive power
and power factor during the testing period. Once all inverters
were on, with a power factor of 0.8, the real power maximum
value is 77 kW and the reactive power maximum value is 59
kvar. The real power is incrementally increasing as the
inverters remain on and don’t disconnect as a result of high
voltage.
Solar Skate Park - Reactive Power Testing
Shown below in Figure 8 is the inverter connection
sequence. There is 1 row for each inverter, labeled U1 to U7.
Green indicates the inverter is on, red indicates the inverter is
off and orange indicates the inverter was on, but tripped off
caused by high voltage. The power factor of all inverters is at
1
80
0.9
60
0.8
0.7
40
0.6
20
0.5
0
0.4
-60
15:00
14:50
14:40
14:30
14:20
14:10
14:00
13:50
13:40
13:20
13:10
13:00
12:50
12:40
12:30
12:20
12:10
12:00
11:50
11:40
11:30
11:10
11:00
10:50
10:40
10:30
10:20
-40
10:10
-20
10:00
Power (kW and kvar)
The SSP has the unique ability to manually disconnect
from the dedicated transformer and connect to the adjacent
TVS 55 network. By making this connection change, the
impacts of having a large PV system connected to the end of a
high impedance LV network can be tested. On 13th
September 2011, the SSP was connected to TVS 55 to
conduct testing and at the end was reinstated to the dedicated
transformer. The following sections describe the events
observed and voltage levels measured at the 3 voltage
recorders.
100
Power Factor
IV. REACTIVE POWER TESTING, HIGH IMPEDANCE NETWORK
0.3
0.2
0.1
-80
0
Time
Real Power (kW)
Reactive Power (var)
Power Factor
Figure 9: Power and power factor at SSP, while connected to TVS 55
Figure 10 below shows the voltage profile at the SSP
during the testing period. Maximum values for C phase
voltage were recorded near 265V. But these are averaged
over a 10min period and instantaneous values would have
been at 270V to cause inverters to disconnect from the grid.
During the period when all inverters were on with power
4
factor at 0.8, maximum power was being produced and the
voltage is relatively low at 255V. This shows that the voltage
has reduced from over 265V – likely 270V, down to 255V by
having the power factor at 0.8, representing a 10-15V (4% 6%) change. This would indicate a higher level of voltage
regulation from the reactive power in a LV network with high
impedance.
system.
C. Results from V3
Voltage recorder 3 was installed at transformer TVS 55,
approx 400m from the SSP. Shown below in Figure 13 is the
voltage profile for the day of the test.
Voltage Levels - at Transformer
250
249
15:02
14:52
14:42
14:32
14:22
14:12
14:02
13:52
13:42
13:32
13:22
13:12
13:02
12:52
12:42
12:32
Time
15:00
14:50
14:40
14:30
14:20
14:10
14:00
13:50
13:40
13:20
13:10
13:00
12:50
12:40
12:30
12:20
12:10
12:00
11:50
11:40
11:30
11:10
11:00
10:50
10:40
10:30
10:20
10:10
10:00
235
12:22
12:12
12:02
11:52
11:42
11:32
11:22
241
240
240
11:12
245
11:02
243
242
10:52
250
10:42
246
245
244
10:32
255
10:02
Voltage (V)
260
248
247
10:22
Voltage (V)
265
10:12
Voltage Levels - Solar Skate Park
Vln A
Vln B
Vln C
Time
Vln b
Vln c
Figure 10: Voltage at SSP, while connected to TVS 55
B. Results from V2
The location of Voltage recorder 2 was 150m from the SSP
towards TVS 55. The recorder was installed out the front of
the rural fire brigade, hence some of the graphs are titled with
rural fire brigade. Shown below in Figure 11 is the voltage
profile for the day of the test.
Voltage Levels - Fire Brigade
Baseline Voltage Levels - at Transformer
250.00
249.00
248.00
247.00
246.00
245.00
244.00
243.00
242.00
15:02
14:52
14:42
14:32
14:22
14:12
14:02
13:52
13:42
13:32
13:22
13:12
13:02
12:52
12:42
12:32
12:22
12:12
12:02
11:52
11:42
11:32
11:22
11:12
11:02
10:52
10:42
10:32
10:22
10:02
260
10:12
241.00
240.00
265
Voltage (V)
Shown below in Figure 14 are baseline voltage levels
during the same time period. These values are averaged over
a 7 day period, the week after the test date.
Voltage (V)
Vln a
Figure 13: Voltage at V3, with SSP connected to TVS 55
Time
255
Avg Vln A
250
Avg Vln B
Avg Vln C
Figure 14: Baseline voltage at V3, with SSP connected to TVS 55
245
240
15:00
14:50
14:40
14:30
14:20
14:10
14:00
13:50
13:40
13:30
13:20
13:10
13:00
12:50
12:40
12:30
12:20
12:10
12:00
11:50
11:40
11:30
11:20
11:10
11:00
10:50
10:40
10:30
10:20
10:10
10:00
235
Time
Vln A
Vln B
Vln C
Figure 11: Voltage at V2, with SSP connected to TVS 55
Shown below in Figure 12 are baseline voltage levels
during the same time period. These values are averaged over
a 7 day period, the week before the test date.
Baseline Voltage Levels - Fire Brigade
Voltage levels at the transformer seem lower during the
time of testing, compared to baseline values. Most likely
caused by different levels of load on the transformer.
D. Power at Transformer
The recorder at TVS 55 - V3 - also has the ability to
record real, reactive and apparent power. Shown below in
Figure 15 is the apparent power measured on the day of test.
At approximately 12:30pm the apparent power peaks at 39
kVA, which is higher than the traditional evening peak.
265.00
Voltage (V)
260.00
255.00
250.00
245.00
240.00
15:00
14:50
14:40
14:30
14:20
14:10
14:00
13:50
13:40
13:30
13:20
13:10
13:00
12:50
12:40
12:30
12:20
12:10
12:00
11:50
11:40
11:30
11:20
11:10
11:00
10:50
10:40
10:30
10:20
10:10
10:00
235.00
Time
Avg Vln A
Avg Vln B
Avg Vln C
Figure 12: Baseline voltage at V2, with SSP connected to TVS 55
Figure 15: Apparent Power (kVA) at TVS 55
The midday values of voltage are approximately 3V higher
compared to the baseline data. Indicating the high SSP
voltage has impacted the network some 150m from the PV
Shown below in Figure 16 is the real power measured at
the transformer. It can be seen that the real power goes
5
negative during the middle part of the day – thus creating
reverse power flow which would be sent back into the 11kV
network. The reverse power flow peak of 30 kW occurs at
approximately 12:30pm.
required to be supplied by the network to enable the SSP to
remain connected to the grid. This was an increase in
reactive power of 500%. If the additional reactive power was
required at a time of high load, then it is possible that the
network would be required to supply a large amount of
current for the real power requirements and also a large
amount of current for the reactive power requirements – thus
having a double current impact on the network and possibly
causing overload issues for the network. This scenario is
unlikely to occur because during the period of high loads the
network voltage would tend to be lower and hence less
reactive power would need to be supplied by the network to
lower the voltage.
Figure 16: Real Power (kW) at TVS 55
Shown below in Figure 17 is the reactive power measured
at the transformer.
The peak of 25 kvar occurs at
approximately 12:45pm, and there is 23 kvar measured at
12:30pm.
Ergon Energy is not proposing to run inverters at fixed
power factors because it would exacerbate the low voltage
problem during high network load periods. Rather, Ergon
foresees the final solution being a voltage controlled system,
where the LV voltage determines the amount of reactive
power injected or absorbed by the Inverter. Ergon Energy
intends to conduct further reactive power testing at this site
with the aim of investigating control strategies, including the
following two 1) Reactive Power as a function of Voltage
Q(V), and 2) Power reduction as a function of Voltage P(V).
VI. ACKNOWLEDGMENT
Figure 17: Reactive Power (kvar) at TVS 55
Shown below in Figure 18 is the reactive power profile
over 30 days. It can be seen that the usual reactive power
value is approximately 5 kvar. This indicates that on the day
of testing, there was a 500% increase in reactive power to
support the 77 kW of real power at the SSP.
Figure 18: Reactive Power (kvar) at TVS 55 - over 30 days
V. CONCLUSION
The ability of reactive power to influence the voltage levels
on the Low Voltage distribution network was shown for both
a low impedance and high impedance network. The reactive
power produced a voltage change of approximate 1% in the
low impedance network and 5% change in high impedance
network. In the high impedance network case, 25 kVAr was
This work was supported in part by the Australian
Government Solar Cities program.
The author gratefully acknowledges the contribution of M.
Wishart for his assistance with this report.