Optimizing Reactive Compensation
for Wind Farms: Meeting Today's Utility
and Regulatory Requirements
A White Paper by American Superconductor Corporation
How Dynamic VAR Technology Enables Wind Farms
to Meet Grid Interconnection Requirements
Executive Summary
W
ind energy is one of the
and furthermore once they have
fastest-growing sources
been switched off they must wait
of electricity in the
five minutes until they can be
United States and around the world.
re-energized in order to allow their
The growing importance of wind
trapped charge to dissipate.
power has subjected it to greater
Accordingly, it is often difficult
scrutiny and more rigorous
to maintain optimum amounts
operational standards than ever
of reactive compensation for
before. As wind-generated power
any length of time using switched
transitions from boutique status to a
capacitor banks alone. Second, in
full-fledged power source, it has
some instances, switching banks
become apparent that the industry
of capacitors to regulate voltage
needs smarter and more appropriate
levels has been reported to cause
solutions to address common
excess stress on the wind turbine
voltage regulation and dynamic
gearboxes.
voltage stability-related interconnecCapacitor banks typically offer
tion requirements.
the lowest first-cost option for the
Advances in dynamic volt
control of voltage on a scheduled
ampere reactive (VAR) technology,
basis, and will undoubtedly remain
coupled with innovative applications
a central element of almost every
and services, eliminate the drawwind farm reactive compensation
backs of traditional voltage and
system. However, wind farm
power factor control methods and Wind farm generation costs have fallen by 50
interconnection requirements often
percent over the last 15 years, moving closer to the
enable wind developers to meet
can not be satisfied with capacitor
cost of conventional energy sources, according to
today’s more stringent and specific the Global Wind Energy Council.
banks alone. Optimizing reactive
interconnection requirements. This
compensation for wind farms merits
white paper explores the background issues, available a wider perspective that addresses the physical interconnecsolution alternatives, including the installation of a dynam- tion with utility grids, specific interconnection regulatory
ic reactive compensation system manufactured by American requirements, the business relationship with the utility itself,
Superconductor (AMSC) and operational considerations for and the cost of operation and ownership of wind farm
addressing voltage and power factor control in the context equipment. In extreme cases, especially in weaker areas of
of a wind farm and its interconnection to the grid.
the grid, deficient interconnection schemes can even affect a
Historically, wind farm operators have employed wind farm’s megawatt output and jeopardize revenues by
mechanically-switched capacitor banks to regulate voltage forcing wind farms off-line. In this broader context, it
at the point of utility interconnection. However, due to its is very important to design the reactive compensation
highly and continuously variable nature, wind-dependent system after careful analysis of the grid dynamics at the
technology is poorly served by this traditional approach. point of interconnection.
There are two reasons for this. First, wind turbines,
especially many of the induction type, can draw large
amounts of reactive power (VARs) from the grid.
This dependency triggers frequent remedial action to
maintain voltage levels within the tolerances established by
regulatory requirements. Although wind energy is variable,
capacitors are able to switch only fixed amounts of VARs,
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Executive Overview
of Voltage and Power
Factor Control
This overview section discusses basic
background information and issues,
and is intended to provide the framework for understanding the problems
that Dynamic VAR technology can
resolve and the extended benefits that
it provides.
What Are VARs?
Power consists of two components:
real power and reactive power. Real
power, which is the functional element
that can do work (driving machines,
lighting lights, etc.) exists when the voltage and the current are in phase with
each other. Reactive power, on the other
hand, exists when the voltage and the
current are out of phase by 90 degrees.
Although reactive power is unable to
provide actual working benefit, it is
often used to adjust voltage; so it’s
a useful tool for maintaining desired
voltage levels. Every AC transmission
system always has a reactive component, which can be expressed as “power
factor.” If the power factor is low and
inductive (due to the wind turbine or
other electrical equipment), then VARs
are being drawn off of the grid, which
reduces the system voltage. If the power
factor is capacitive, then VARs are being
added to the grid, which raises the
system voltage. Some method is needed
to manage power factor by injecting or
absorbing VARs as necessary in order to
maintain optimum voltage levels and
optimize real power flow.
How is Voltage Commonly
Controlled?
Traditionally, the easiest and leastcost way to manage VARs is to install
shunt capacitor or reactor banks
on
the
transmission
system.
Calculations determine how many
VARs are needed at any given point,
and
appropriately
sized
banks
“Wind energy has now
reached the milestone of 50GW
of worldwide installed capacity
and the industry is ready for a
broader roll out. Wind energy has
the maturity, clout and
global muscle to deliver deep cuts
in CO2, while providing a hedge
against fluctuating fossil fuel
prices and reducing energy import
dependence.”
Arthouros Zervos
Chairman, Global Wind Energy Council
September 2005
of capacitors are strategically placed
(usually
rated
in
MegaVARs;
5 MegaVARs or 10 MegaVARs, for
example). As voltage levels fluctuate,
capacitor banks are switched on or off
to either inject more VARs into the
system or remove them, as required. The
effect is that the system voltage is maintained within tolerances established by
the transmission owner as well as
regulatory requirements.
Capacitor Bank Switching
Stresses Wind Farm
Equipment
Although a cheap way of compensating for VAR losses, capacitor bank
switching also results in an immediate,
abrupt step-change in the voltage on the
grid or the bus to which they are connected. The step-change instantaneously
increases the torque, or twisting force,
on a wind turbine gearbox. The variable
nature of wind generation itself often
triggers an extremely high number of
the switching events that in some cases
can begin to affect the reliability of the
gearboxes. Like all induction motors,
many induction-type wind turbines
draw VARs off of the grid in amounts
that fluctuate with changes in wind
speed at the turbines. This, in turn, can
cause an unacceptably large voltage
drop at or near the wind farm interconnection point with the grid. So, given
that these VARs need to be
compensated in order to maintain
voltage, it is not uncommon for a large
site to experience fifty to a hundred
switching events a day. In some cases
the resulting gearbox stresses eventually
take a toll, accelerating maintenance
cycles of the gearbox.
Remote Locations
Complicate the Issue
The size of capacitor banks that can
be used is governed by the strength of
the grid or bus. Conventionally, the size
of step-change in voltage from a switching event must be kept below a certain
percentage of total voltage (typically
two percent or below). A step change in
voltage of any larger magnitude can
potentially cause problems with other
equipment in the substation at the wind
farm. While first cost considerations
may drive a preference for solutions
employing fewer and larger capacitor
banks, local conditions often require the
use of a series of smaller banks relative
to the strength of the grid at that
location. Given that most wind farms
are located in remote areas, the typical
grid to which they connect is quite often
relatively weak at that point because
these grids are isolated from the generation sources.
The Business Side
of Voltage and
Power Factor Control
The party responsible for regulating
the voltage at the wind farm is typically
the wind farm owner. With the increasing prevalence of wind generation in
recent years, the issue of grid connection
requirements for wind farms has come
under heightened scrutiny by regulators
as well as regional transmission organizations and reliability councils. As a
result of new rules such as those issued
by the Federal Energy Regulatory
Commission, wind farm owners in the
United States are responsible for complying with more stringent and specific
requirements related to voltage control
and high or low voltage ride-through
(the ability of the wind turbines to stay
3
®
4
AR
rV
pe
Su
or
In recent years, AMSC’s D-VAR
technology has become a preferred and
innovative solution to address grid
interconnection requirements associated
with wind farms. In addition to addressing grid connection requirements, the
D-VAR system provides additional
operational benefits such as mitigating
step-voltage changes. It does this by
using advanced power electronics, often
in combination with traditional capacitor banks, to dynamically inject or
absorb precise amounts of VARs into
the system. Where capacitor bank
switching alone is a binary on-off
action, dynamic voltage control is more
akin to a radio volume control with
fluid, continuously adjustable levels.
D-VAR technology offers an
economic strategy for complying with
interconnection requirements that also
can act as a two-way shock absorber,
not only resolving VAR demand and/or
voltage control issues created by the
wind farm, but also enhancing
the ability of sensitive wind turbine
generators to avoid tripping off-line due
= Wind Farm
®
D-VAR® Systems: An Ideal
Strategy for Wind Farm
Applications
D-VAR® solution
designed for wind
farm power factor
control, voltage
regulation and low
voltage ride through.
SuperVAR® synchronous
condensers designed
for optimal steady-state
voltage regulation and control.
R
VA
D-
connected to the grid during voltage
disturbances) for their wind farm than
was the case in the early years of the
emerging wind power industry. That
responsibility is, in turn, driving wind
farm owners and turbine manufacturers
to incorporate dynamic VAR technology
into their projects to enable compliance
with these standards.
Regardless of who is responsible,
all the parties have a vested interest in
effectively and economically meeting the
grid connection requirements. The
ultimate objective, of course, is to ensure
that wind farms provide a consistent,
dependable source of real power generation while operating at peak efficiency and
uptime with manageable cost of ownership. But also important to the utility is the
power quality and reliability delivered to
their other customers on the grid.
= Transformer
to common voltage disturbances that
occur on the transmission grid. Keeping
wind turbine generators on-line has
proven to be, in some locations, a significant problem with today’s wind farms,
and dynamic VAR technology is often
worth the investment for this reason
alone.
For wind farm owners, the D-VAR
solution delivers several significant side
benefits beyond ensuring compliance
with standards. The elimination
of switching-related stress on the
gearboxes in some cases reduces
maintenance requirements and extends
the life of the equipment. Furthermore,
because sudden voltage disturbances on
the collector bus are mitigated, by using
this solution, the wind farm
has enhanced ability to ride through
transient high or low voltage conditions.
This maximizes the megawatt output
and increases revenues.
For utilities, the D-VAR system
eliminates large VAR demands and the
resulting voltage swings caused by
uncompensated wind farm operation.
With this system in place, the wind farm
looks to the utility much more like a
= Capacitors
conventional synchronous generator,
in terms of the ability to dynamically
control voltage. This mitigates or eliminates the need to install capacitor banks
on the transmission system to control
voltage. In cases where capacitor banks
are called for, solutions may involve a
smaller number of units with larger
ratings — leading to lower costs for the
utility with the added ability of the
D-VAR system to offset these larger step
voltage changes and smoothly switch
capacitor banks.
With its integral control system, the
patented D-VAR system can be customfitted to specific wind farm facilities.
For example, a small (8 MVA) D-VAR
device can be combined with a number
of low-cost, medium-voltage capacitor
banks to create an integrated, effective
voltage and power factor control system
for a wind farm.
AMSC has also developed DVC™
(Dynamic VAR Compensator) solutions
and
SuperVAR®
machines
that
can address similar issues as the
D-VAR system. DVC systems have
been developed to address the need
for large-scale solutions requiring
hundreds of megaVARs (MVARs) of
reactive
compensation
connected
directly to the transmission grid.
AMSC’s DVC solutions are based on the
widely successful D-VAR platform.
They are a hybrid STATCOM/SVC
solution that utilizes inverter-based
FACTs (Flexible AC Transmission
Systems) technology similar to D-VAR
systems along with proprietary fastswitched capacitors and reactors.
SuperVAR machines use standard
synchronous condenser frames and
stator coils paired with advanced powerdense rotor coils made from AMSC’s
superconductor wire. The result is a
synchronous condenser that is more
efficient than conventional rotating
machines — without the high rotor
maintenance costs typical of older,
conventional synchronous condensers.
SuperVAR machines are specifically
designed for continuous, steady-state
dynamic VAR support, with lower
standby losses, higher output, and
greater reliability than conventional
synchronous condensers.
SuperVAR machines are costeffective solutions that can provide tight
voltage regulation and power factor
correction to alleviate fluctuating voltage and VAR demands at wind farms.
How D-VAR Systems Work
D-VAR systems are dynamic reactive
power sources of the flexible AC transmission system (FACTS) classification.
As depicted in Figure 1, D-VAR devices
are installed on the wind farm collector
bus, continuously monitor the voltage at
the point of grid interconnection, and
take precise, instantaneous action as
necessary. The variable output of the
D-VAR device is typically the first
source used to regulate voltage. As additional compensation is required, the
patented control system of the D-VAR
system will switch a capacitor bank (or
reactor) in or out. At the exact moment
of switching, the DVAR device instantaneously injects (or absorbs) the same
amount of VARs as the capacitor bank,
thereby eliminating the step voltage
change that would otherwise occur. The
D-VAR system then resumes its normal
voltage regulation mode, dynamically
injecting or absorbing VARs as required.
Notably, because of the capacitive and
inductive capabilities of the D-VAR
system it can handle a significant
percentage of the events that would
otherwise traditionally trigger a capacitor-bank switch; the annual number of
capacitor switching events can be
reduced. This results in less maintenance
time and lower cost of ownership of the
capacitor bank switches or breakers.
In addition to the operation inside
the wind farm that Figure 2 shows, the
D-VAR system can also help protect the
wind farm from voltage disturbances
that normally occur on the transmission
grid (such as voltage sags or swells). The
sensing and control scheme of the
D-VAR system continuously monitors
the voltage at the wind farm collector
bus or point of connection to the transmission grid. When the voltage rises or
Figure 1: Typical Dynamic VAR System and Capacitors Connected to a Wind Farm. The dynamic
VAR system shown in the figure continuously monitors the collector bus and/or transmission
grid voltage of a typical wind farm to ensure that the voltage remains within the utility specified
range. Continuous voltage regulation is accomplished by a combination of VAR injection or
absorption from the dynamic VAR system, and by controlled, seamless switching of capacitor
banks. In addition, the system mitigates voltage transients that typically originate on the
transmission grid and can cause the wind turbine generators to trip off-line.
Figure 2: Example of a tight voltage profile maintained through varying wind conditions at an
existing 130MW wind farm using a D-VAR system solution.
5
falls to a level outside a preset target or
bandwidth, the D-VAR system responds
instantaneously. It injects or absorbs
sufficient reactive power to fully offset
the event, maintaining the voltage inside
the desired bandwidth levels.
Considerations in
Developing a Site-Specific
D-VAR Solution
Although there are some factors that
are common to all wind farm sites, each
has unique aspects that must be considered when specifying the voltage and
power factor control solution. Some
wind farms will be fortunate enough to
interconnect with a very strong grid that
can tolerate the VAR demand of the wind
farm without the assistance of anything
more than simple capacitor banks. Other
wind farms may be small enough that
their operation does not negatively affect
the transmission grid or they fall below
the minimum MW size to which less
stringent standards apply.
“Wind power's rapid growth
provides what is potentially the
quickest and best supply-side
option to ease the natural gas
shortage.”
Arthouros Zervos
Chairman, Global Wind Energy Council
September 2005
These situations are somewhat
unusual, however, due to two aspects of
the basic economics of wind energy.
First, scale economies are driving the
construction of larger wind farms that
have higher output relative to the local
capacity of the grid. Second, wind farms
by their nature tend to be located in
remote areas where power delivery grids
are not as strong. As a result of these
factors, many wind farms require a voltage control strategy both to ensure compliance with regulatory interconnection
standards as well as to ensure optimal
operation. Factors to consider include
the strength of the local grid connection,
the size of the wind farm, the type of
wind turbine generator and the inter-
6
connection specification of the grid
owner. The best way to develop
a strategy for voltage control is to do
a site-specific analysis. This includes a
detailed technical analysis of the grid
strength and requirements, and results
in an in-depth report with recommendations.
American Superconductor’s staff of
very experienced utility transmission
planners will provide these studies free
of charge or obligation. The company
makes these services available to its
customers because it believes it is the
first step in understanding system
requirements that will lead to recommendations for the best and most
economic solution to meet these needs.
Some Cost of
Ownership Factors
Wind generation facilities are
long-term investments, so in order to
accurately evaluate the reactive compensation options, ongoing and cumulative
cost of ownership must be factored
in
with
the
initial
cost
of
infrastructure elements. Most often this
includes a calculation of the cost of
running a system over time, taking
into account losses and efficiencies.
These losses include the amount of
power necessary to operate the balance
of plant equipment, including reactive
compensation solutions — power that
must be paid for.
For example, as with almost all
power equipment, cooling is a consideration. With power electronic devices,
the two basic choices are liquid cooling
and air cooling. Air cooling, of course, is
much less complicated because it does
not require plumbing, pumps, etc., and
the ongoing operational and maintenance costs are also substantially lower
because the more simple air-cooled
system draws significantly less auxiliary
power than a conventional liquid
cooled system.
Another factor to consider is the
flexibility of the control system. This is
not only important in the initial phases
of the installation but also after the
wind farm is up and running because it
governs how closely the equipment can
be further tuned to the specific wind
farm conditions and requirements. One
important item to consider in designing
a wind farm is the willingness of the
supplier to customize their product.
Conditions are also likely to change
over time, and the inherent capability of
the control system to be adapted as the
wind farm changes or expands is also
critical. In addition, like all similar
equipment, dynamic VAR devices have
maintenance requirements. Is the manufacturer able to offer sufficient levels of
support? For example, some systems
such as the D-VAR system or DVC
devices from AMSC can be monitored
24/7/365 by the manufacturer, either at
very modest cost or as part of an annual maintenance package. This kind of
service reassures wind farm owners that
the devices are working properly and
providing the protection and reliability
they expect. Other services should
include full hardware and software
support specific to the installation. The
maintenance factor is part of the
ongoing and lifetime cost of ownership.
Summary
It is very commonplace for today’s
generation of wind farms – larger-scale
operations located in remote areas of the
grid – to require more sophisticated
voltage control strategies than their
early predecessors. Wind farms also
can have unique circumstances and
operational considerations that warrant
evaluation of the merits of traditional
“voltage control by capacitor banks”
approach. Employing dynamic VAR
devices, sometimes in combination
with control of capacitor banks,
eliminates many of the negative
consequences of traditional solutions.
Both the wind farm developer/owner
and ultimately the utility benefit from
this approach.
The D-VAR system
advantages for the
wind farm include:
• Grid interconnection standards
are met.
• Voltage sags or swells originating
from the transmission grid are
mitigated. This enhances the ability
of the wind farm to stay online and
helps to prevent nuisance tripping
of the wind turbine generators.
This also helps maximize the power
output of the wind farm which
leads to increased revenues.
• Step-voltage changes due to local
or remote capacitor-bank switching
are mitigated, or eliminated, thus
preventing excess gearbox stress
or failure.
• Capacitor-bank switching events
are minimized, which reduces
switch maintenance costs.
•Overall grid interconnection costs
are minimized.
The D-VAR system
advantages for the utility:
• Large VAR demands are eliminated,
as are the resulting voltage swings
caused by uncompensated wind farm
operation. The wind farm maintains
a consistent, smooth voltage profile.
• In some cases, the need to install
additional capacitor banks on the
transmission system is eliminated.
• If transmission capacitor banks are
installed for any reason, their impact
to the local wind farm is minimized.
About American
Superconductor
Corporation
American Superconductor (AMSC)
is the manufacturer of the D-VAR and
DVC dynamic reactive compensation
systems which provide voltage support
to utility transmission and distribution
systems. In addition to wind farms,
AMSC D-VAR systems are also being
used worldwide to address a variety of
grid-related problems such as voltage
instability, power transfer constraints
and steady-state voltage regulation.
AMSC is the world’s principal
vendor of high temperature superconductor (HTS) wire and large rotating
superconductor machinery, and it is a
world leading supplier of dynamic
reactive power grid stabilization
products. AMSC's HTS wire and power
electronic converters are at the core of a
broad range of new electricity transmission and distribution, transportation,
medical and industrial processing
applications, including dynamic reactive
power grid stabilization solutions, large
ship propulsion motors and generators,
smart, controllable, superconductor
power cables and advanced defense
systems. The company’s products are
supported by hundreds of patents
and licenses covering technologies
fundamental to Revolutionizing the
Way the World Uses Electricity™.
More
information
is
available
at http://www.amsuper.com.
American Superconductor and
design, AMSC, POWERED BY AMSC,
Revolutionizing the Way the World Uses
Electricity and DVC are trademarks
and D-VAR and SuperVAR are
registered trademarks of American
Superconductor Corporation. All other
trademarks are the property of their
respective owners.
© 2006 American Superconductor Corporation
7
For more information, please contact:
American Superconductor
Two Technology Drive
Westborough, MA 01581
toll-free USA/Canada: +1 800 315 3319
tel +1 978 842 3362
fax +1 978 842 3364
info@amsuper.com
www.amsuper.com
© 2006 American Superconductor Corporation. All rights reserved. Printed in USA.
American Superconductor and design, AMSC and Revolutionizing the Way the World Uses Electricity are trademarks of American Superconductor.
All other trademarks are either property of American Superconductor or property of their respective owners.
WF_WP_0306