USING DISTRIBUTED PV FOR VOLT/VAR CONTROL AND MAXIMUM
DEMAND REDUCTION
Prepared by: Mr David Smyth BEng Elect (Hons), GCertMgt, RPEQ, MIEAust
Director – Evolve Energy Pty Ltd www.evolveenergy.com.au
Director – Energy Innovations Pty Ltd
August 2011
Abstract
energy, that returns to the source in each cycle, is known
as reactive power.
Distributed Photovoltaics (PV) is typically described as a
large number of small scale PV systems that are
connected at various points on the low voltage electricity
distribution network. Whilst the systems provide very
reliable clean energy in a balanced way across the grid,
commonly used systems simply supply energy when the
sun is shining, with no remote monitoring and control or
network performance capabilities.
Rising energy prices, lower component costs and various
incentives has resulted in a significant up take in the use
of this technology for domestic and commercial
applications.
With increased penetration of these
systems issues for distribution utilities are emerging in
the areas of voltage management, systems quality
control, network operations and demand management.
The issues that are arising from the increased
penetration of distributed PV can be managed by the
using smart inverters with remote wireless
communications supplying reactive power (VAR) in
addition to the real power (W).
The relationship between real power, reactive power and
apparent power can be expressed by representing the
quantities as vectors. Real power (W) is represented as a
horizontal vector and reactive power (VAR) is
represented as a vertical vector. The apparent power
vector (VA) is the hypotenuse of a right-angled triangle
formed by connecting the real and reactive power
vectors. This representation is often called the power
triangle. Using the Pythagorean Theorem, the
relationship among real, reactive and apparent power is:
(Apparent Power)2 = (Real Power)2 + (Reactive Power)2
Using this approach distributed PV systems can be used
to proactively manage network voltage and maximum
demand. Smart inverters have the ability to provide
reactive power (VAR) 24 hours a day and hence manage
voltage and demand very close to the point of use at any
time of the day.
Real and reactive powers can also be calculated directly
from the apparent power, when the current and voltage
are both sinusoids with a known phase angle θ between
them:
This paper explores the existing methods used for
volt/var control on electricity distribution networks and
describes how distributed PV can be used to manage
these issues and optimise distribution network
performance.
Reactive Power = (Apparent Power)sin(θ)
Power Triangle
On an alternating-current (AC) power system, voltage
can be controlled by managing production and absorption
of reactive power. There are three reasons why it is
necessary to manage reactive power and control voltage.
First, electricity generation and distribution equipment
are designed to operate within a range of voltages,
usually within ±6% of the nominal voltage. At low
voltages, many types of equipment perform poorly; light
bulbs provide less illumination, induction motors can
overheat and be damaged, and some electronic
In alternating current circuits, energy storage elements
such as inductance and capacitance can produce periodic
reversals of the direction of energy flow. The portion of
power flow that (averaged over a complete cycle of the
AC waveform) results in net transfer of energy in one
direction is known as real power (also referred to as
active power). That portion of power flow due to stored
Real Power = (Apparent Power)cos(θ)
The ratio of real power to apparent power is called
„power factor‟ and is a number always between 0 and 1.
Benefits of controlling VAR
equipment will not operate at all. High voltages can
damage equipment and shorten asset life.
Second, reactive power consumes transmission and
generation capacity. To maximize the amount of real
power that can be transferred across a congested
transmission interface, reactive-power flows must be
minimised. Similarly, reactive-power production can
limit a generator‟s real-power capability.
Third, moving reactive power on the transmission system
incurs real-power losses (line losses). Both capacity and
energy must be supplied to replace these losses.
prime-mover limit. Power station designers recognise
that the generator will be producing reactive power and
supporting system voltage most of the time. Thus
installing a prime mover with the capacity to supply the
full VA rating of the generator would typically result in
underutilisation of the prime mover.
To produce or absorb additional VARs beyond these
limits would require a reduction in the real-power output
of the unit. Control over the reactive output and the
terminal voltage of the generator is provided by adjusting
the DC current in the generator‟s rotating field. Control
can be automatic, continuous, and fast.
Voltage control is complicated by two factors. Firstly,
the transmission system itself is a nonlinear consumer of
reactive power, depending on system loading. At very
light loading the system generates reactive power that
must be absorbed, while at heavy loading the system
consumes a large amount of reactive power that must be
replaced. The second complicating factor is that a
system‟s reactive power requirements are highly
dependant on the nature of the connected loads as well as
the design philosophy and physical layout of the
generation and distribution systems. No two locations
within a distribution system have identical electrical
characteristics so voltage control technologies and
strategies have to be flexible and able to operate
effectively across a range of operating scenarios.
The inherent characteristics of the generator assists in
maintaining system voltage. At any given field setting,
the generator has a specific terminal voltage it is
attempting to hold. If the system voltage declines, the
generator will inject reactive power into the power
system, tending to raise system voltage. If the system
voltage rises, the reactive output of the generator will
drop, and ultimately reactive power will flow into the
generator, tending to lower system voltage.
Common methods of Volt/VAR control
Every synchronous machine (motor or generator) with a
controllable field has the reactive-power capabilities
discussed above. Synchronous motors are occasionally
used to provide dynamic voltage support to the power
system as they provide mechanical power to their load.
Some combustion turbines and hydro units are designed
to allow the generator to operate without its mechanical
power source simply to provide the reactive-power
capability to the power system when the real-power
generation is unavailable or not needed. Synchronous
machines that are designed exclusively to provide
reactive support are called synchronous condensers.
Generation
The most common forms of base load electric power
generators consist of a prime mover (steam, gas turbines
etc) directly connected to an alternator. A generator‟s
primary function is to convert fuel (or another energy
resource) into electric power. Most generators have
considerable control over their terminal voltage and
reactive-power output. The ability of a generator to
provide reactive support depends on its real-power
production. Like most electric equipment, generators are
limited by their current-carrying capacity. Near rated
voltage, this capacity becomes a VA limit for the
armature of the generator rather than a W limitation of
the prime mover. Production of reactive power involves
increasing the magnetic field to raise the generator‟s
terminal voltage. Increasing the magnetic field requires
increasing the current in the rotating field winding.
Absorption of reactive power is limited by the magneticflux pattern in the stator, which results in excessive
heating of the stator-end iron, the core-end heating limit.
The synchronising torque is also reduced when absorbing
large amounts of reactive power, which can also limit
generator capability to reduce the chance of losing
synchronism with the system. The generator‟s prime
mover (e.g., the steam turbine) is usually designed with
less capacity than the electric generator, resulting in the
The voltage regulator will accentuate this behaviour by
driving the field current in the appropriate direction to
obtain the desired system voltage.
Synchronous Condensers
Synchronous condensers have all of the response speed
and controllability advantages of generators without the
need to construct the rest of the power plant (e.g., fuelhandling equipment and boilers). Because they are
rotating machines with moving parts and auxiliary
systems, they require significantly more maintenance
than static alternatives. They also consume real power
equal of approximately 3% of the machine‟s reactivepower rating.
Capacitors and Inductors
Capacitors and inductors (which are sometimes called
reactors) are passive devices that generate or absorb
reactive power. They accomplish this without significant
real-power losses or operating expense. The output of
capacitors and inductors is proportional to the square of
the voltage. Therefore, a capacitor bank (or inductor)
rated at 100MVAR will produce (or absorb) only 90
MVAR when the voltage dips to 0.95pu but it will
produce (or absorb) 110 MVAR when the voltage rises to
1.05pu. This relationship is helpful when inductors are
employed to hold voltages down. The inductor absorbs
more when voltages are highest and the device is needed
most. The relationship is, however, less beneficial in the
more common case where capacitors are employed to
support voltages. In the extreme case, voltages fall, and
capacitors contribute less, resulting in a further
degradation in voltage and even less support from the
capacitors; ultimately, voltage collapses and outages
occur.
state nature of the STATCOM means that, similar to the
SVC, the controls can be designed to provide very fast
and effective voltage control. While not having the shortterm overload capability of generators and synchronous
condensers, STATCOM capacity does not suffer as
seriously as SVCs and capacitors do from degraded
voltage.
STATCOMs are current limited devices, so their VAR
capability responds linearly to voltage as opposed to the
voltage squared relationship of SVCs and capacitors.
This attribute greatly increases the usefulness of
STATCOMs in preventing voltage collapse.
Variation between equipment types
Inductors are discrete devices designed to absorb a
specific amount of reactive power at a specific voltage.
They can be switched on or off but offer no variable
control. Capacitor banks are composed of individual
capacitor cans, typically 200kVAR or less each. The
capacitors are connected in series and parallel to obtain
the desired capacitor-bank voltage and capacity rating.
Like inductors, capacitor banks are discrete devices but
they are often configured with several steps to provide a
limited amount of variable control which makes it a
disadvantage compared to synchronous motor or flexible
AC transmission system (FACTS) controllers.
Static VAR Compensators (SVCs)
An SVC combines conventional capacitors and inductors
with fast switching capability. Switching takes place in
the sub cycle timeframe (i.e., in less than 1/60 of a
second), providing a continuous range of control. The
range can be designed to span from absorbing to
generating reactive power. Consequently, the controls
can be designed to provide very fast and effective
reactive support and voltage control. Because SVCs use
capacitors, they suffer from the same degradation in
reactive capability as voltage drops. They also do not
have the short-term overload capability of generators and
synchronous condensers. SVC applications also usually
require harmonic filters to reduce the amount of
harmonics injected into the power system.
Generators, synchronous condensers, SVCs, and
STATCOMs all provide fast, continuously controllable
reactive support and voltage control. Capacitors and
inductors are not variable and offer control only in large
pre-determined steps.
An unfortunate characteristic of capacitors and capacitorbased SVCs is that output drops dramatically when
voltage is low and support is needed most. The output of
a capacitor, and the capacity of an SVC, is proportional
to the square of the terminal voltage. STATCOMs
provide more support under low-voltage conditions than
do capacitors or SVCs because they are current-limited
devices and their output drops linearly with voltage. The
output of rotating machinery (i.e., generators and
synchronous condensers) rises with dropping voltage
unless the field current is actively reduced. Generators
and synchronous condensers generally have additional
short-term emergency capacity that can be used for a
limited time. Voltage-control characteristics favour the
use of generators and synchronous condensers. Costs, on
the other hand, favour capacitors. Generators have
extremely high capital costs because they are designed to
produce real power, not reactive power. Even the
incremental cost of obtaining reactive support from
generators is high, although it is difficult to
unambiguously separate reactive-power costs from realpower costs. Operating costs for generators are high as
well because they involve real-power losses.
Static Synchronous Compensators (STACOMs)
The STATCOM is a solid-state shunt (parallel) connected device that generates or absorbs reactive
power and is one member of a family of devices known
as flexible AC transmission system (FACTS) device. The
STATCOM is similar to the SVC in response speed,
control capabilities, and the use of power electronics.
Rather than using conventional capacitors and inductors
combined with fast switches, however, the STATCOM
uses phase shifting techniques to electronically
synthesize the reactive power output. Consequently,
output capability is generally symmetric, providing as
much capability for production as absorption. The solid-
Finally, because generators exist primarily to provide
real power, they experience lost opportunity costs when
called upon to simultaneously provide high levels of both
reactive and real power. Synchronous condensers have
the same costs as generators; but, because they are built
solely to provide reactive support, their capital costs do
not include the prime mover or the balance of plant and
they incur no opportunity costs. SVCs and STATCOMs
are high-cost devices, as well, although their operating
costs are lower than those for synchronous condensers
and generators.
Distributed PV
Photovoltaics generate direct current and require
inverters to couple them to the power system. This use of
a solid-state interface between the solar module and the
power system has the potential added benefit of
providing full reactive-power control, similar to that of a
STATCOM. Recent advances in power electronics and
embedded microcontroller technology is enabling
advanced STATCOM functionality to be incorporated
into consumer level grid connected PV inverter products.
These products are becoming known as „smart inverters‟.
periods when there is no real power generation the
inverters full rated capacity can be utilised to inject or
absorb VAR.
During sunny periods when the inverter is producing real
power, additional VAR generation is limited only by the
VA rating of the device. The inverter uses only a small
amount of real power to generate VAR so there is a
minimal reduction in active power generation to
accommodate VAR production. Historically, inverter
manufacturers have focussed on the supply of real power
because tariffs are based on real power usage.
Smart inverters are able to provide a full range of
reactive power output without affecting the simultaneous
active power output (within the overall VA rating of the
device). The energy source, i.e. solar module can be out
of service or even disconnected without impacting the
ability of the inverter to supply reactive power.
Positive network impacts
The advent of cost effective phase-shifting solid-state
power electronics has turned a potential problem into a
benefit, allowing distributed resources to contribute to
voltage control and current reduction very close to the
customer‟s point of supply. The benefits can therefore
be applied across and within the entire distribution
network.
The maximum demand on electricity distribution
networks is the Apparent Power (VA) which is made up
of the vector sum of the Real Power (W) and Reactive
Power (VAR). The current flow on the lines is directly
proportional in magnitude to the apparent power.
Distribution utilities are experiencing large fluctuations
in voltages on their LV networks as a result of the
increased use of distributed PV systems with no reactive
power generation capability. Typically, during the day
when householders are at work and network load is light,
distributed PV systems supply a significant amount of
real power into the grid. This can result in high voltages
being experienced at the extremities of the LV branches
of the distribution substation area. These high voltages
can exceed the high voltage set points of the inverters of
the distributed PV systems resulting in these systems
disconnecting from the grid (no generation). Owners of
the PV systems complain to the suppliers of the PV
systems that they are not generating the levels of energy
expected of the system. This leaves the PV suppliers
with motivation to rectify this by increasing the voltage
cut-out set points. This is a problem for distribution
utilities because this results in network voltages
exceeding statutory limits. Conversely, in the evening,
around evening mealtime the loads are high and the
active power generated from the PV systems is nil. This
results in low voltages being experienced at the
extremities of the LV branches of the distribution
substation area.
Generation of reactive power (VAR) in addition to real
power on demand is clearly very useful to electricity
utilities in also reducing current on the network and
hence demand on the network during peak load periods.
Distribution utilities construct power line assets to safely
meet the demand (VA) for the design life of the assets.
The optimum outcome for the investment is that the
demand does not exceed the rated capacity of the power
line assets until they are replaced due to age (corrosion,
wood rot etc). This can be in the order of 40 years.
With the increased use of air conditioning and other
appliances the maximum demand of networks are
increasing at a higher rate than was predicted when the
power lines were first built. As a result, power line
assets are being replaced much sooner because the load
exceeds the safe capacity of the line which in some cases
can be many years early than planned.
This means that the initial return on investment is not
realised and a large unplanned capital expenditure is
required to construct new lines.
It is important to note that the maximum demand
typically occurs less than 5% of the time so for the
remaining periods the power line assets are actually
underutilised. By reducing the maximum demand when
this occurs significant savings in power line
augmentation can be achieved.
A logical solution is to use the solid state power
electronics of the inverters to absorb VAR when the
voltage is high and inject VAR when the voltage is low.
Traditional distributed PV is useful for reducing demand
when this occurs during sunlight hours but unfortunately
the maximum demand can also occur during inclement
weather or after dark where traditional distributed PV
cannot be used for this purpose.
The amount of VAR that can be supplied is only limited
by the current carrying capacity of the inverter. During
Given smart inverters can be used to supply reactive
power at any time of the day to the rated capacity of the
device this can be very useful in reducing the apparent
power during maximum demand periods. The devices
can be used to reduce maximum demand even if this
occurs at night or during inclement weather.
Some smart inverters also incorporate wireless two-way
communications, which allows real time grid monitoring
and manual or automatic remote control of reactive
power generation. This allows utilities to remotely
monitor the quality of supply very close to the point of
use. Problems can be identified early before customers
experience problems and system outages can be
identified very quickly improving restoration response
times.
Smart inverters therefore have the potential to provide a
new and efficient tool for maximum demand reduction,
voltage management and distribution system monitoring
in addition to the benefits of distributed renewable
energy generation.
Summary/Conclusion
Significant savings can be achieved in the costs of
electricity generation and distribution by effectively
managing voltage and reducing maximum demand.
Distributed PV provides a significant opportunity to
achieve this by using smart inverters to provide remote
monitoring of voltage and the supply of reactive power
(leading or lagging). Reactive power can be supplied in
addition to the real power supply, on demand, at any time
of the day to the rated current carrying capacity of the
device.
By providing the ability to supply reactive power, as
required, at the extremities of the distribution network
voltage control can be achieved at the most important
part of the network, at the point of supply to the
customer. The supply of reactive power at this point can
also be used to reduce current flow for the entire
distribution system resulting in reduced maximum
demand.
Compared to using conventional STATCOM devices
distributed PV has the added advantage of producing
significant amounts of real power resulting in the device
(electronic components) achieving a higher utilisation
factor and improved return on investment.
Using smart grid enabled distributed PV also allows
increased penetration levels of PV to be achieved in
future by providing the tools for improving voltage
control and electricity grid utilisation.
REFERENCES
[1] David I Eromon; John Kueck (2005) “Distributed
Energy Resource (DER) using FACTS, STATCOM,
SVC and Synchronous condensers for Dynamic Systems
Control of VAR
[2] Tom Jauch; “Volt/VAR Management An Essential
“SMART” Function