978-1-5386-6159-8/18/$31.00 ©2018 IEEE
Increasing Hosting Capacity of PV Solar Systems
using Smart Inverter Volt-Var Control
Sibin Mohan
Dept. of Electrical and Computer
Engineering
The University of Western
Ontario
London, ON, Canada
Syeed Hasan
Dept. of Electrical and Computer
Engineering
The University of Western
Ontario
London, ON, Canada
Yafet Gebremariam
Dept. of Electrical and Computer
Engineering
The University of Western
Ontario
London, ON, Canada
Rajiv K Varma
Dept. of Electrical and Computer
Engineering
The University of Western
Ontario
London, ON, Canada
Abstract— The steady state voltage rise caused by reverse
power flows and intermittency in renewable power is the main
limiting factor for integration of distributed generators in
medium and low voltage distribution lines. With the
advancement in smart inverter technologies, the Volt-Var control
using the remaining capacity of the inverter can be used to
provide effective voltage control in distribution systems. In this
paper, the efficacy of Volt-Var control on two PV solar farms
connected to a realistic feeder in Ontario, Canada, is
demonstrated for increasing the hosting capacity up to the
thermal limit of the feeder. Implementation of such smart
inverter controls will greatly help in the integration of PV solar
systems in power grids.
Keywords— PV Solar Systems, Smart Inverters, Distributed
Generation, Reactive Power Control, Hosting Capacity.
I. INTRODUCTION
With increasing concern over global warming, the use of renewable energy is growing at a rapid rate around the world [1]. In 2013, over 140 countries agreed to UN’s framework convention on climate change to reduce carbon emission to combat global warming. This boosted the growth of renewable energy systems. Policies like priority dispatch from renewable energy sources, special feed in tariffs, quota obligations and energy tax exemptions has accelerated the growth of harvesting renewable energy sources. In different renewable sources, wind and PV showed highest growth. The installed capacity of solar PV systems has already reached 229GW [2], and that of wind power systems has increased to 486 GW [3].
A commitment was made by the Canadian Government under the Climate change plan for Canada to buy 20% of electricity from renewable energy sources[4]. This policy along with offset programs, procurement through requests for proposals, standard offer and feed-in tariff programs, and legislated renewable portfolio standards accelerated the growth of wind and PV installations in Canada. Over the last ten years, wind power has grown thirty-fold to 12239 MW in 2017, which is estimated to equal about 10 percent of total Canadian potential wind generation capacity. Similarly, solar photovoltaic (PV) has grown substantially, reaching 2206 MW in 2016 from only 10 MW in 2002. Majority of these installations are in Ontario[4].
The large-scale integration of DGs has changed the
structure of the traditional power system and led to many
operational challenges like steady state voltage rise and dips,
voltage flicker, harmonics and resonances, false tripping of
protection equipment, etc. [5], [6]. The intermittency of the
renewable sources causes steady state voltage limit violation.
This increases the number of operation of traditional voltage
control devices like On Load Tap Changers (OLTC) and
switched capacitor banks [7], [8], leading to a reduction in their
lifecycles. Thus, the main limiting factor for the integration of
the DG is the voltage limit violation caused by the reverse
power flow and intermittency in renewable power [9]. A
strategy to estimate the hosting capacity of solar PV in a radial
distribution network considering the voltage limit is proposed
in [10]. Due to the above-stated voltage limit violations, the
hosting capacity of DGs must be limited substantially below
the thermal limit of the distribution lines.
EPRI introduced the concept of smart inverters whereby
Distributed Energy Resources (DER) inverters such as PV
solar inverters can provide multiple functions of real and
reactive power control either autonomously or in response to
utility issued commands [11]. These smart inverter functions
include: volt/var control, volt/watt control, frequency/watt
control, primary frequency control, ramp rate control, etc. [11],
[12]. The grid interconnection standard IEEE 1547 is revised to
include the smart inverter features [13].
The application of volt/var control for providing voltage
support during varying DG power level is demonstrated in
[14]. A volt/var control strategy to minimize the voltage
deviation and overall power loss is demonstrated in [15]. A
review of different volt/var control techniques is presented in
[16]. From various literature [16,17], it is evident that volt/var
control can provide voltage support and can reduce the energy
losses.
With the large-scale integration of PV solar farms, there is
a possibility of having multiple PV solar farms connected to
the same feeder. Due to the steady state voltage limit, the
capacity of distribution systems to connect these solar farms,
known as hosting capacity, can be limited much below the
thermal limit of the feeder. In this paper, the effectiveness of
Proceedings of the National Power Systems Conference (NPSC) - 2018, December 14-16, NIT Tiruchirappalli, India
the application of volt/var control of two PV solar farms
connected to a realistic distribution feeder in Ontario, Canada
for increasing their hosting capacity is investigated.
The rest of the paper is organized as follows. The study
system is described in section II, and the controller is
presented in section III. The simulation results are
demonstrated in section IV and the conclusion of this work is
provided in section V.
II. STUDY SYSTEM
The single line diagram of the study system is shown in Fig. 1. A realistic 27.6 kV distribution feeder in Ontario is considered as the study system. The transformer is rated 115 kV/ 27.6 kV, 100 MVA and has 0.075 pu impedance. The PV Solar Farm 1 is connected at 30 km distance from the substation. The PV Solar Farm 2 is connected 5 km away from the PV solar farm 1. The power output of both the solar farms are varied in this study. The distribution lines are represented by their equivalent pi models. The resistances , , and inductances , , denote the resistance and inductance of distribution lines between Bus 2 and Bus 3, Bus 3 and Bus 4, and Bus 4 and Bus 5 respectively. The total load of 6 MVA at 0.9 power factor is modelled as a constant RL load and is connected at Bus 5, 40 km from the substation.
𝑻𝟏 𝑹𝑻𝑳𝟏 𝑳𝑻𝑳𝟏 𝑳𝑻𝑳𝟐 𝑳𝑻𝑳𝟑 𝑹𝑻𝑳𝟐 𝑹𝑻𝑳𝟑
Bus 1 Bus 2 Bus 3 Bus 4 Bus 5
Load
PV Solar Farm 1 PV Solar Farm 2
Distribution
Substation
Fig. 1. Single Line Diagram of the Study System
III. PV SOLAR FARM CONTROLLER
The single line diagram of a grid connected PV Solar farm
is shown in Fig. 2(a). The PV power is fed to the grid through
a Voltage Source Converter (VSC). The inductance L,
capacitance C and inductance of the ∆-Y transformer acts as
the filter for the VSC output. L and C are designed such that
the total harmonic distortion (THD) is less than 5%. The
resistance and represents the parasitic resistance of
the inductance L and capacitance C, respectively. The voltage
at the point of common coupling (PCC) is measured and
denoted as in Fig. 2. The inverter output current is
represented as .
The architecture of the PV solar farm controller is shown in
Fig. 2(b). The inverter current , and PCC voltage are
measured and fed to the controller. The controller is designed
on the d-q frame. The PCC voltage angle ϴ for the
transformation is calculated by a phase locked loop (PLL).
The , and , are transformed to the d-q frame using abc
to dq transformation [18].
The voltage vector is aligned to the d axis in this model
thus making = 0, to have decoupled control of the active
and reactive power [18]. The active power is proportional to
direct axis current and reactive power is proportional to
quadrature axis current . The DC link controller maintains
the DC link voltage at the MPPT voltage to extract the
maximum power from solar array. The DC link controller
determines the direct axis current reference ( ) to maintain
the DC link voltage at the reference. The quadrature axis
current reference ( ) is generated by the Volt -Var
controller.
A. Volt-Var Controller
The volt-var characteristic of the system shows the
relationship between the voltage and the amount of reactive
power required to maintain the voltage with in the utility
limits. A typical volt-var curve is shown in Fig. 3. The upper
part of the curve represents capacitive reactive power, and the
lower part represents inductive reactive power. The volt var
controller operates the PV solar farm at unity power factor
when the voltage is within the utility limit. The is then
generated from the as per (1). As per [13], the inverters
are oversized to provide 44% reactive power while operating
at rated capacity. The is limited such that the inverter
current limit according to [13] is not violated.
(1)
Sinusoidal Pulse Width Modulation at 10 kHz switching
frequency is used for generating the gating pulses of the VSC.
The modulation index for the switching is calculated by the
inner current control loop.
IV. SIMULATION STUDIES
Load flow studies are done initially to find out the impact
of PV integration on feeder voltage for various X/R ratios of
the distribution line. Volt-var curves are modelled according
to the results of the load flow studies with following
objectives: (i) to increase the hosting capacity while
maintaining the voltage within the steady state limit, and (ii)
inverter current limit not to be violated. The time domain
simulation studies with the developed volt-var controller is
done in MATLAB Simulink to test its effectiveness in
increasing the hosting capacity.
A. Impact of PV Generation on Feeder Voltage for various
X/R ratios of distribution feeder
The impact of PV generation on feeder voltage for various
X/R ratios are studied by keeping the X/R ratio of the
distribution lines at 2.47 and 1.
Proceedings of the National Power Systems Conference (NPSC) - 2018, December 14-16, NIT Tiruchirappalli, India
1) Ratio X/R = 2.47
The resistance , , and inductance , ,
in Fig. 1 are varied to make X/R =2.47. The studies are
performed for the following cases: (i) only PV Solar Farm 1
connected, (ii) only Solar Farm 2 is connected, and (iii) both
PV Solar Farms are connected.
Fig. 4 shows the variation of bus 3 voltage for various PV
power generation levels. The steady state voltage limits of
1.06 and 0.94 are shown by yellow and green line
respectively. Blue line shows the voltage with only PV Solar
Farm 1 connected. Orange line shows the voltage with Solar
Farm 2 connected and black one shows the voltage with both
Solar Farms connected. The PV power is increased till 45
MW.
1.06
0.94
Capacitive
Inductive
% Voltage
% A
vail
ab
le V
AR
0.5
1.5
Fig. 3. Volt - Var Curve [11]
With only PV solar farm 1 connected, the voltage at bus 3
started decreasing for power increase beyond 25 MW. The load
flow solution fails to converge for power greater than 40 MW.
With PV Solar Farm 2 connected, the voltage starts reducing
for 25 MW and load flow solution fails to converge for power
above 40 MW. With both the solar farms producing power, the
voltage at bus 3 starts reducing for both PV producing 20 MW
each and fails to converge beyond 40 MW. The voltage of the
feeder remains within the limit for a broad range of power and
the voltage falls below the lower limit of 0.94 pu for power
above 35 MW.
Fig. 4. Bus 3 Voltage for different PV power level for X/R = 2.47
2) X/R = 1
To study the impact of PV power injection on bus voltage
for a lower X/R ratio, the feeder X/R ratio is kept at 1 and
following studies are done. The studies performed are for the
𝐢𝐪𝐫𝐞𝐟
𝒊𝒒
𝑽𝒅𝒄𝟐
𝐢𝐝𝐫𝐞𝐟 𝑽𝒅𝒄_𝑴𝑷𝑷𝑻𝟐
𝐢𝐝
𝐯𝐝
𝛚𝟎𝐋
𝐕𝐝𝐜𝟐
𝑲𝑷𝟑 +𝑲𝑰𝟑𝑺
6PWM
𝛚𝟎𝐋
𝐢𝐝
𝐢𝐪 𝐯𝐝 𝐯𝐪
𝒊𝒊𝒏𝒗
𝝆
𝒗𝒑𝒄𝒄
PLL
𝐯𝐩𝐜𝐜
𝐯𝐪
min
𝟏.𝟏𝟐 − 𝒊𝒅𝟐
(b)
𝑮𝑨𝑻𝑰𝑵𝑮 𝑺𝑰𝑮𝑵𝑨𝑳𝑺
𝒊𝒊𝒏𝒗 𝒗𝒑𝒄𝒄 ∆ 𝐘
𝐋 𝐑𝐟𝟏
𝐑𝐟𝟐
𝐂
𝐏𝐂𝐂
𝐕𝐒𝐂
𝑽𝒅𝒄
𝑮𝑨𝑻𝑰𝑵𝑮 𝑺𝑰𝑮𝑵𝑨𝑳𝑺
6 𝐒𝐨𝐥𝐚𝐫 𝐏𝐚𝐧𝐞𝐥
(a)
𝑲𝑷𝟏 +𝑲𝑰𝟏𝑺
𝐊𝐏𝟒 +𝐊𝐈𝟒𝐒
abc
dq
Volt-Var
Controller
Fig. 2. Single Line Diagram of a grid connected PV solar farm with the volt var controller
Proceedings of the National Power Systems Conference (NPSC) - 2018, December 14-16, NIT Tiruchirappalli, India
following cases: (i) only PV Solar Farm 1 is connected, (ii)
only Solar Farm 2 is connected, and (iii) both are connected.
Fig. 5 shows the variation of bus 3 voltage for various PV
power generation levels. The steady state voltage limits of
1.06 and 0.94 are shown by yellow and green line
respectively. Blue line shows the voltage with only PV Solar
Farm 1 connected. Orange line shows the voltage with Solar
Farm 2 connected and black one shows the voltage with both
Solar Farms connected. The PV power is increased till 45
MW.
For all the three cases, the voltage exceeded steady state
limit of 1.06 for a very low PV power of 8 MW, which is
much lower than the thermal limit of the feeder.
Fig. 5. Bus 3 Voltage for different PV power level for X/R =1
Fig. 6. Line Current for different PV power for X/R =1
Fig. 6 shows the current flow in distribution line between
bus 2 and 3 for different power generation of PV solar farms.
Blue line shows the line current for only PV Solar Farm 1
connected. Orange line shows the line current for Solar Farm
2 connected and black one shows the line current for both
Solar Farms connected. The thermal limit of the line is 680A,
and is shown by the yellow line. The voltage limits are
violated for PV power generation of 10 MW for all the three
cases as shown in Fig. 5. The line current for the 10 MW
power is approximately 100 A as depicted in Fig. 6, which is
approximately 15% of the thermal limit of the line. This
shows that the DG integration is limited much below the
thermal limit due to the voltage limit violation.
B. Impact of PV Power on Feeder Voltage for various X/R
ratios with Volt-Var Control
The effectiveness of the volt-var control by utilizing the
remaining inverter capacity to increase the real power
generation of the PV solar farm to the thermal limit of the
feeder is presented in this section. From the studies conducted
for system with different X/R ratios, it is observed that the
voltage limits are violated for a lower PV power injection on
system with X/R =1. So the system with X/R = 1 is
considered for this study.
1) Determination of Volt-Var Curve
The volt-var curve characterisitics are determined by load
flow studies in Power world software. The load flow studies
are conducted for the PV power from zero to 45 MW. From
this study, the reactive power required to maintain the voltage
within the steady state limit is found out and the volt-var is
curve is plotted using this data. The volt-var curve for all the
three cases are shown in Fig. 7. From the volt-var curve, the
equations of are derived. For example, the equations
obtained for the case with only PV solar farm 2 is connected is
shown in (2).
(2)
Fig. 7. Volt -Var curve for X/R =1
From Fig. 5, it can be observed that the highest feeder
voltage is 1.32 pu, which occurs for 45 MW power generation
Proceedings of the National Power Systems Conference (NPSC) - 2018, December 14-16, NIT Tiruchirappalli, India
by PV Solar Farm 1. The reactive power required for this
condition is 10 MVA, which is within the limit of 44%
reactive power injection required by the grid code [13]. Thus
it shows that for this study, the volt - var control for the whole
range of PV power genration can be provided without
violating the inverter current limit.
Time domain simulation in MATLAB Simulink is done with
the obtained volt- var characteristics and the results are shown
in Fig. 8. The dotted curves show the voltage at Bus 3 without
voltage control and the thick lines show the voltage at Bus 3
with Volt – Var control. The steady state voltage limits of 1.06
and 0.94 are shown by yellow and green line respectively.
Blue line shows the voltage with only PV Solar Farm 1
connected. Orange line shows the voltage with Solar Farm 2
connected and black one shows the voltage with both Solar
Farms connected. The PV power is increased till 40 MW.
It is observed that, using the volt-var control, the PV
power generation can be increased till 40MW without
violating the voltage limit. Whereas without the volt-var
control, the DG power generation must be limited below 10
MW.
Fig. 8. Bus 3 Voltage for different PV power level with Volt Var Control for
X/R =1
The current through the line between bus 2 and bus 3 for
different PV power generation with volt-var control is shown
in Fig. 9. Blue line shows the line current for only PV Solar
Farm 1 connected. Orange line shows the line current for
Solar Farm 2 connected and black one shows the line current
for both Solar Farms connected. The thermal limit of the line
is 680A and it is shown by the yellow line.
From Fig. 9, it can be observed that the thermal limit of the
line is exceeded for PV power generation of 35 MW. The
volt-var control is able to maintain the voltage within the
steady state limit for this range as noted from Fig. 8.
This study shows that the volt-var control using the
remaining inverter capacity can maintain the voltage within
the steady state limit and thus help in increasing the DG
interconnection till the thermal limit of the feeder.
In this study, PV integration could be increased up to
100% of thermal limit using smart inverter volt-var control.
However, the percent increase of renewable integration on
different distribution systems will depend upon their specific
short circuit level, X/R ratio and thermal limits.
Fig. 9. Line Current for different PV power for X/R =1 with Volt-Var control
V. CONCLUSION
The integration of distributed generators in medium and
low voltage lines is primarily limited by steady state voltage
limit violations. Simulation studies are reported in this paper
for a realistic distribution feeder in Ontario with the smart
inverter volt-var control implemented on two PV solar farms.
In this study, the power generation of PV solar farms is
limited to 15% of the thermal capacity of the feeder due to
violation of steady state voltage limit. With the advancement
of smart inverters and the revised grid codes, the inverters are
oversized to provide 44% reactive power support even at the
rated capacity. By using the remaining inverter capacity, volt-
var control can be provided to increase the grid integration of
distributed generators. It is shown in this study, that with the
volt-var control using the remaining inverter capacity, the
renewable power generation can be increased from 17% to
100% of the feeder thermal limit without violating the steady
state voltage limits. This smart inverter control helps in
increasing the DG integration to the existing feeders without
the needing to install additional expensive voltage regulating
devices.
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Proceedings of the National Power Systems Conference (NPSC) - 2018, December 14-16, NIT Tiruchirappalli, India