1

Power Transformer Application for

Wind Plant Substations
IEEE PES Wind Plant Collector System Design Working Group

Contributing Members: M. Bradt, M. R. Behnke, W. G. Bloethe, C. Brooks, E.H. Camm, W. Dilling, B. Goltz,
J. Li, J. Niemira, K. Nuckles, J. Patiño, M. Reza, B. Richardson, N. Samaan, J. Schoene, T. Smith, I. Snyder,
M. Starke, R. Walling, G. Zahalka

Abstract—Wind power plants use power transformers to step • Loading patterns of wind plant transformers are
plant output from the medium voltage of the collector system to significantly more variable, which is due to the
the HV or EHV transmission system voltage. This paper intermittency in the primary energy sources (wind
discusses the application of these transformers with regard to the
speed), than typical power transformer applications,
selection of winding configuration, MVA rating, impedance, loss
evaluation, on-load tapchanger requirements, and redundancy. with a relatively low load factor (20% to 40% load
factor, from field experience).
Index Terms—Wind generation, power transformers, • The value of wind-generated energy that is
substations. transformed by and lost within the transformer is
greater than typical transformer applications due to the
I. INTRODUCTION incentives and mandates related to renewable power

W
generation.
IND power plants that are too large for direct
connection to a local distribution system, are • The value of transformer reliability (frequency of
interconnected to HV or EHV transmission systems. failure) becomes less significant than availability
Today, the vast majority of wind generation is interconnected (capacity loss times duration of loss) due to the
to the utility grid at the transmission level. This relatively low contribution of the wind plant to system
interconnection is accomplished using one or more power generation capacity requirements. However, it should
transformers to step-up plant output, from the medium voltage be recognized that low transformer reliability can
level used for the plant’s collector system to the transmission directly result in low availability when the mean time
system voltage level. These transformers, along with to replace or repair is long. Transformer replacement
switchgear, protective relays, metering, reactive compensation and major repair lead times tend to be long, and
equipment (in some plant designs), and other equipment outage times can be particularly long for wind plants
needed to perform the interconnection and control and protect in remote or offshore locations
the collection system, are located in a substation. • The transformer must usually provide a ground source
The functional requirements for wind plant substation to both the transmission and collector systems.
transformers are to reliably transfer power to the transmission This paper describes the unique considerations of wind
system, provide ground sources for the transmission grid and power plant substation transformer application. Some use the
collector system, and maintain acceptable collector system term “wind plant step-up transformer” to indicate the
voltage (sometimes aided in this function by the controllable transformer between the MV collector and HV/EHV
reactive power output of the wind generators or supplemental transmission voltage levels. However, wind plants also have
reactive compensation equipment) at the least life-cycle cost, small step-up transformers at each wind turbine generator
including losses as well as initial capital costs.
(WTG) to transform unit output from the voltage level of the
The application considerations for a wind power plant
generator to the collector system voltage. Thus, the term
substation transformer have elements in common with
“step-up” is ambiguous in usage. For this reason, this paper
conventional power plant step-up transformers, as well as
elements in common with primary distribution substation will use the term wind plant substation transformer
transformers. However, there are a number of unique aspects exclusively.
of the wind plant application that strongly influence substation
transformer application, including: II. WINDING CONNECTIONS
The choice of winding connections for wind plant
substation transformers is constrained by the necessity to
provide ground sources, and sometimes also by requirements
to provide zero-sequence isolation.

978-1-4244-6547-7/10/$26.00 © 2010 IEEE

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A. Ground Source Requirements generally requires use of 200 kV BIL equipment. While
All transmission systems in North America, as well as most substation-class equipment of this insulation level is widely
transmission systems around the world, are designed to be available, it is generally cost prohibitive to apply 200 kV BIL
effectively grounded. This means that the ratio of driving substation class equipment throughout the wind plant
point zero-sequence reactance to positive-sequence reactance collection system.
(X0/X1) must be less than three, and the ratio of zero-sequence B. Winding Configuration Alternatives
resistance to positive sequence reactance (R0/X1) must be less
As previously discussed, the grounded-wye/delta winding
than one at any point in the transmission system [1]. If a
configuration for wind plant substation transformers is
power plant were to be interconnected such that it did not
generally not favored due to the necessity of providing wind
contribute a zero-sequence shunt admittance (ground source),
plant collector system grounding via alternate means. The
its positive sequence short-circuit contribution would reduce
exception is where this winding configuration is mandated by
the system’s positive sequence reactance, X1, without
the transmission system operator. Some Canadian utilities
providing a commensurate reduction of its zero sequence
have required grounded-wye/delta wind plant substation
reactance, X0. As a result, the X0/X1 and R0/X1 driving point
transformers in order to completely block the zero sequence
impedance criteria might not be met at points in the
currents from any wind plant collector faults from being seen
transmission system near the wind plant interconnection.
by the utility ground relays.
Excessive temporary overvoltages could result during ground
A delta (HV/EHV side)/grounded-wye (MV side)
faults due to insufficient grounding.
transformer is generally not acceptable to transmission system
An additional consideration is the possible contingency
operators due to the absence of a ground source contribution
involving the power plant, and a portion of the transmission
to the transmission system. There have been some situations
system, becoming isolated from the remainder of the grid. If
where the existing grid X0/X1 ratio is sufficiently low that
the power plant provides no ground source, then the isolated
interconnection of a wind plant with this transformer
subsystem may be completely ungrounded. Extreme
connection has been deemed tolerable, when combined with
overvoltages could result.
protection schemes that ensure that sufficient grounding is
Therefore, most transmission system operators require that
maintained under all conditions.
power plants present a ground source to the transmission
A grounded-wye/grounded-wye connection does not
system. For conventional power plants, this is typically
provide a substantial zero sequence admittance; thus it does
accomplished using a grounded-wye/delta step-up
not create a ground source. This connection, however, will
transformer. This transformer configuration, however, does
transmit a ground source existing on one side of the
not present a ground source to the collector system. A
transformer to the other side. In general, this winding
previous Wind Plant Collector Design Working Group paper
connection is not used because it does not create a ground
[2] provides substantial discussion regarding collector system
source.
grounding requirements.
A possible exception, however, is when a grounded-wye/
Some grounding must be provided for the collector system
grounded-wye three-phase transformer, wound on a three-leg
in order to avoid extreme overvoltages due to the repetitive
core, is used. Because the return path for zero sequence flux
interruption and restriking of low-current arcing ground faults
in this transformer design is outside of the core, the zero-
that can occur in ungrounded medium voltage systems.
sequence shunt impedance is moderately low. Typically, the
Actually, such a system without intentional grounding is not
zero-sequence shunt impedance is on the order of one per-unit
truly ungrounded, but rather it is grounded via the capacitance
on the transformer base, and this may be a sufficient ground
of the collector cables and lines; a highly dangerous condition.
source in some applications. In the normal operating
Grounding can be provided by a supplemental device, such
condition, with the wind plant interconnected to the
as a grounding transformer. However, if the collector system
transmission grid, the zero-sequence driving point impedance
is to be effectively grounded, the grounding transformer must
of the transmission system reflects through the transformer
be very large as it must present a shunt zero-sequence
and usually transfers sufficient grounding to allow the MV
reactance that is less than three-times the sum of the
collector system to be considered effectively grounded. If the
substation power transformer leakage reactance plus the
wind plant should become separated from the grid, the
minimum driving point positive-sequence reactance of the
grounding source provided by the three-leg three-phase
transmission grid at the point of interconnection. An
grounded-wye/grounded-wye substation transformer may be
impedance-grounded collector system is a possible option that
sufficient to maintain effective grounding considering the
allows use of a smaller grounding transformer. However, an
relatively low short-circuit current capacity of the wind
impedance-grounded system will generally require an increase
turbines in the plant. On the transmission voltage side during
in the insulation levels of all the collector system equipment.
the islanded condition, grounding also may be sufficient.
The maximum basic insulation level (BIL) available for many
The isolation of the wind plant zero sequence from the
of the distribution-class components used in a collection
transmission system zero sequence provided by this type of
system, such as separable cable connectors, is 150 kV. This is
transformer is only partial, and may not be acceptable to the
problematic for a 34.5 kV collector system as coordination
transmission system operator from the standpoint of
with this maximum available BIL usually requires an
transmission system protective relaying coordination.
effectively-grounded system. Insulation coordination with an
impedance-grounded 34.5 kV (nominal voltage) system
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A grounded-wye/grounded-wye transformer with a delta particular loading and temperature conditions of the specific
tertiary provides a low impedance grounding source to both wind plant. IEEE Standard C57.91-1995 provides transformer
the transmission system and to the wind plant collector thermal and insulation aging models which can be used to
system. Zero sequence isolation of the wind plant from the model the specific application, allowing the transformer
grid is not complete, but is usually sufficient. It is for this specifier to determine a transformer rating that can provide
reason that this connection seems to be predominately selected adequate insulation life [3].
for wind plants in the US. Often, the delta tertiary is Because the substation power transformer is critical to the
unloaded, and is solely present to provide a low-impedance wind plant’s revenue stream, transformer rating specification
ground source. In this case, the delta winding may not even should be performed conservatively. Although the principles
be brought out to external bushings (embedded delta tertiary). of transformer thermal loading and intentional overloading,
In other applications, the delta tertiary is used for supplying according to C57.91, are well accepted in the utility industry,
station service load. the institutions financing a wind plant may require a full
When the ratio of transmission voltage to wind plant capacity transformer MVA rating.
collector voltage is less than approximately three, it may be
feasible to consider an autotransformer. An autotransformer IV. TRANSFORMER IMPEDANCE
constrains the main windings to the grounded-wye There tends to be a “natural impedance” of a transformer
configuration, but a delta tertiary may be specified. An that is dependent on the MVA rating, nominal voltages,
autotransformer is physically smaller, and less expensive, than insulation levels, and manufacturer. The manufacturer can
a two-winding transformer because only a portion of the produce a transformer with natural impedance at less cost than
transmitted power is magnetically transformed. The ratio of if a greater or lower impedance is specified. Unless the design
the physical MVA of an autotransformer to the MVA of a of the wind plant dictates otherwise, it is preferable to specify
two-winding transformer having the same throughput capacity an impedance near this value.
is equal to the transformer’s co-ratio. The co-ratio is the A greater impedance than this natural value may need to be
difference between the high and low side voltages, divided by specified to reduce the maximum short-circuit current on the
the high-side voltage. wind plant MV collector bus. High short-circuit current can
be an issue for large wind plants where a single transformer is
III. TRANSFORMER MVA RATING used. Alternatives to an artificially-high power transformer
Power transformers have a self-cooled rating, and usually impedance include designing with a two-transformer design
one or more forced-cooled ratings. The most conservative having a split MV bus, specification of higher short-circuit
approach to wind plant substation transformer MVA rating current rated MV equipment, or inclusion of current-limiting
selection is to specify a transformer with the maximum self- reactors in the design.
cooled MVA rating equal to the plant’s rated real power Wind turbines need to have a specified system strength in
output, divided by the minimum required power factor. order to meet performance specifications, such as low-voltage
Transformer MVA ratings do not present a hard limit to the ride-through, and to avoid instabilities. The system strength
loading MVA that can be safely applied. Load current causes seen by the wind turbines is defined by the grid driving point
heating of the transformer winding, which in turn causes impedance, substation power transformer impedance, as well
aging of the transformer insulation. Loss of transformer as the collector system impedances, including individual unit
insulation life is a highly non-linear function of transformer transformers. In situations where the transmission grid is
winding temperature, as well as the duration of exposure. A particularly weak (high impedance), a design alternative may
period of excess winding temperature causes accelerated be to specify a reduced substation power transformer
aging, but aging progresses at less than the nominal rate when impedance.
the winding temperature is less than the nominal full-load
value (110 °C at the hottest spot on the winding). Thus, V. TRANSFORMER LOSS EVALUATION
retarded aging during low-load periods tends to offset All transformers have load loss, which is proportional to the
accelerated aging caused by overload periods. square of the current loading, and no-load loss that is present
Transformer rating is based on continuous loading at the whenever the transformer is energized, whether it is loaded or
MVA rating, at an ambient temperature of 30 °C. Wind plant not. (No-load losses vary in a non-linear relationship to
output, however, tends to be highly variable, and plants are applied voltage. However, because voltage is usually
infrequently required to operate continuously at the minimum maintained within a defined range, no-load losses are typically
power factor. Thermal time constants of a power transformer considered fixed, without consideration of voltage variation.)
are typically hours long, thus tending to smooth the winding In addition, a power transformer will also have auxiliary
temperature rise caused by a variable load. Wind plants in losses (fans and sometimes pumps) that increase step-wise
many locations rarely reach full output during weather with loading.
conditions that produce high ambient temperatures, and In a wind plant, these losses decrease the metered power
maximum loading tends to occur primarily during the cooler delivered to the utility grid, and thus decrease the plant
seasons. All of these factors make it technically feasible to revenue. In addition, no-load losses are present even when
select a transformer MVA rating that is less than the the wind is below cut-in speed, and the plant is not generating
maximum wind plant MVA. To do so, however, requires power. In this situation, the power flows from the utility grid
careful analysis of the transformer aging that occurs for the
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to supply the no-load losses. Depending on tariffs and particularly those where the wind turbines are capable of
agreements, this power may need to be purchased at a cost per providing voltage regulation functionality, do not need
kWh rate that is greater than the value of energy sold by the OLTCs. Where possible, OLTCs should be avoided. They
plant to the market or power purchase agreement recipient. add substantially to transformer costs and maintenance
It is in the long-term interests of the wind plant owner to requirements, and decrease transformer availability.
select a transformer that balances initial capital costs and the Application considerations which drive the need for OLTC
present worth of losses accrued over the life of the specification include:
transformer. Instead of specifying a certain loss levels, the • Unusual range in transmission system voltage,
preferable means to achieve this optimization is to allow the typically greater than the usual 0.95 to 1.05 p.u.
competing power transformer vendors to optimize their range
designs given specified loss evaluation factors. • Very long collector feeders, where the collector bus
The no-load loss factor, typically called the “A factor” in voltage must be decreased with increasing power
the utility industry, is the amount of initial transformer capital output, to compensate for the impedance voltage
cost increase that justifies a unit of no-load power loss rise along the feeders and maintain adequate voltage
reduction. The load loss factor, typically called the “B range at all of the wind turbine generators.
factor,” is the amount of initial transformer capital cost • Wind turbines that are not capable of regulating
increase that justifies a unit of power loss reduction at rated collector system voltage using variable reactive
load. The unit of power typically used for a power power generation and absorption.
transformer is one kW, and the typical unit for a distribution-
type padmount transformer (such as used for individual wind On-load tapchangers are typically applied to the
turbine unit step-up transformers) is one Watt. The total transformer winding having the greatest voltage variation.
evaluated cost of the transformer is: Where an OLTC is used to compensate for transmission
Initial Cost + A × No-Load Loss + B × Load Loss (1) voltage variation, the OLTC is on the HV/EHV winding.
Off-load tapchangers are routinely applied to power
Each transformer manufacturer has different manufacturing transformers. They allow taps to be changed only with the
costs and design tradeoffs, yielding a different relationship transformer de-energized.
between transformer price and losses. Using the A-B factor
methodology allows the manufacturers to compete with each VII. MULTI-TRANSFORMER APPLICATIONS
other on the common basis of total life-cycle cost to the
For large wind plants, multiple substation power
owner. This results in a far better optimization of design than
transformers may be considered. The unit cost (cost per
specification of a given kW loss limit.
MVA) of power transformers tends to decrease with MVA
The A and B factors are specific to a given wind plant
rating. All other considerations aside, a single substation
project. Proper calculation of these factors considers the wind
power transformer provides the lowest cost solution.
plant load-duration curve (diurnal curve), project financing,
However, there are a number of considerations that can drive
taxes, value of sold and purchased energy, as well as other
application of multiple wind plant substation transformer;
technical and financial factors. A previous Wind Plant
either as multiple transformers in a common substation, or
Collector Design Working Group paper [4] details the
construction of multiple substations dispersed within the wind
derivation and calculation of A and B loss evaluation factors.
plant. These considerations are discussed below.
Compared to typical power transformer loss evaluation
factors, the factors for a wind plant may differ substantially A. Practical Constraints
due to a number of reasons. The loading factor of a wind Very large transformers are difficult to ship, due to their
plant substation transformer is less than a typical power physical dimensions and weight. Many wind plants are
transformer which would tend to reduce the B factor. located in remote areas, distant from heavy-duty roads and rail
However, the higher value of wind-generated energy due to lines. The logistical costs of transporting a very large
incentives (e.g., Production Tax Credit) and mandates transformer may offset the benefits of scale of using a very
(Renewable Energy Standards), plus the reduced effective cost large transformer.
of capital investment due to the special tax depreciation Another constraint relates to the practical limits on the
schedules for wind plant equipment, tends to result in a larger current rating of the transformer on the medium-voltage side.
than normal B factor. The high value of produced energy, and Load currents above 3000 A pose difficulty with regard to the
possibly the higher cost of purchased energy for non- ratings of commonly-available switchgear and the capacity of
generating hours, combined with the tax depreciation impacts practical bus conductor sizes. While equipment is available to
result in A factors that tend to be far greater than typical handle higher currents, the associated cost premiums may
transformer applications. offset the transformer cost advantage of a single large
transformer.
VI. LOAD TAPCHANGER APPLICATIONS
B. MV Collector System “Reach”
On-load tapchangers (OLTC) are specified for some wind
plant substation power transformers to compensate for Wind generation inherently requires a minimum geographic
transmission system voltage variations, or to provide means to area per MW of generation capacity, in order to sufficiently
adjust the collector bus voltage. Many wind plants,
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separate turbines to minimize aerodynamic wake effects. compare single and multi-transformer designs, and to
Except in flat areas, terrain features dictate the placement of determine transformer MVA ratings for multi-transformer
wind turbines, and in practice, may require mean distance designs, should consider the probability of transformer failure,
between turbines to be far greater than the theoretical mean time to repair or replace, value of energy output,
minimum. A large wind plant MW rating inherently results in transformer losses (a single, full-sized transformer generally
the wind plant covering a large geographic extent. Use of a has lower no-load and load losses than two, half-sized
single substation in a large wind plant requires transmission of transformers), transformer costs, costs of ancillary substation
power from wind turbines at the outer extent of the plant to equipment, and financial parameters including tax
the substation via MV lines. This affects the cost of the MV implications. Ancillary substation costs in a multi-transformer
design include additional buswork, switchgear, and protection.
collection system, increases losses, and may create voltage
regulation issues. The costs of an extended MV collector
system can offset the benefits of scale derived from a single
substation design, and at some size, the extent of the MV
collector system becomes technically impractical.

C. Availability and Reliability

In typical utility and industrial applications, the need for
reliability often drives the use of multiple transformers. Wind
plants differ, however, in that reliability (frequency of outage)
is not a critical metric, but availability (available energy not
delivered) is critical to plant financial viability.
A design with two transformers, each with 50% of required
total capacity, has a much lower chance of complete outage
(two unit failure) than the failure rate of a single transformer.
However, the chance of a single unit failure is twice that of
one large transformer, but each failure results in loss of 50% Figure 1. Illustration of constrained capacity.
capacity. Viewed simplistically, the transformer capacity
availability is the same with either the 2×50% or 1×100%
options. D. Constraints on Contingency Operation
However, the nature of wind generation makes this In collection system designs where the wind power plant is
comparison somewhat more complex. Wind plants operate served by more than one co-located substation transformer, it
most of the time at much less than rated capacity. During is common practice to configure the substation such that each
outage of one transformer, the remaining half capacity in a transformer serves a number of feeders but the two groups are
2×50% design is sufficient to transform the entire wind plant not normally operating in parallel (split bus design). This
output for much of the time, and requires only partial limits the short-circuit level on the MV bus, which could be
curtailment of wind plant output for other hours. As a result, otherwise very large if the transformers were paralleled. In
a 2×50% transformer capacity design can yield substantially the event of a transformer outage, the bus tie can be closed
more than 50% of the available energy output when one of the and the wind plant operated via the remaining transformer.
transformers is unavailable, without any transformer There are a number of issues that need to be addressed when
overloading. This is illustrated by Figure 1, which shows a considering this contingency mode of operation.
typical wind plant cumulative generation duration curve. If the wind turbines are capable of sufficient short-circuit
Potential output exceeding 50% capacity is lost during outage current contribution, operation of all turbines with the bus tie
of one transformer, shown by the shaded region, but this area closed may exceed the short-circuit current limitations of the
is less than half of the total area under the generation duration equipment on that bus. In this case, some turbines may need
curve. (This is an average result. A transformer outage to be taken out of service during closed bus tie operation.
during a period of consistent high wind could cause loss of up Removal of wind turbines from service also can be a means
to 50% of the available energy. Likewise, a failure during a used to curtail loading of the in-service substation transformer
low wind period could have minimal impact on plant output.) to an acceptable level.
During outage of one transformer in a two-transformer The driving point impedance of the wind plant, relative to
design, intentional overloading of the remaining transformer is the total rating of the wind turbines connected to the tied bus,
an option that can also be considered. Using the thermal and is inherently reduced during operation with one substation
insulation life models provided in [3], an overload limit can be transformer out of service with the bus tie closed, and all wind
determined that results in an acceptable loss of life during the turbines in operation. Attention must be given to ensure that
period during unavailability of the other transformer. the resulting short-circuit ratio is within the specifications for
Another option in a two-transformer design is to rate each the wind turbines. Some wind turbines may need to be taken
transformer greater than 50% of the total required transformer out of service to meet this constraint.
capacity, such that greater wind plant output can be obtained Operation with a tied MV bus fed from one transformer can
during a single transformer outage. An economic analysis to also greatly change the resonant characteristics of the wind
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plant. That is due to the fact that the series and parallel
resonances frequencies will be shifted from the designed
points to other values. Frequency scans should be performed
for all modes of operation of the substation transformers and
the modes of operation where resonance problems can occur
at low order odd harmonics should be avoided.

VIII. REFERENCES
[1] IEEE Guide for the Application of Neutral Grounding in Electrical
Utility Systems—Part I: Introduction, IEEE Standard C62.92.1-2000.
[2] IEEE PES Wind Plant Collector System Design Working Group, "Wind
Power Plant Grounding, Overvoltage Protection, and Insulation
Coordination" Proceedings of the 2009 IEEE Power and Energy Society
General Meeting.
[3] IEEE Guide for Loading Mineral-Oil-Immersed Transformers, IEEE
Standard C57.91-1995.
[4] IEEE PES Wind Plant Collector System Design Working Group, “Wind
Power Plant Substation and Collector System Redundancy, Reliability,
and Economics" Proceedings of the 2009 IEEE Power and Energy
Society General Meeting.