W225-10022
June, 2010
Regulator Efficiency:
Considerations for Design and
Loss Evaluations
Craig A. Colopy
Global Technology Manager, Step-Voltage Regulator Products
Cooper Power Systems
2300 Badger Drive
Waukesha, WI 53188
www.cooperpower.com
P: 877.CPS.INFO
Cooper Power Systems is a valuable trademark of Cooper Industries in the U.S. and other countries.
You are not permitted to use the Cooper Trademarks without the prior written consent of Cooper Industries.
©2010 Cooper Industries. All Rights Reserved.
Table of Contents
Introduction ...................................................................................................................2
Step-Voltage Regulator Types......................................................................................3
No-Load and Load Loss Profiles..................................................................................6
How Regulator Construction Affects Losses and Short Circuit Strength ............... 12
How to Evaluate Losses of Step-Voltage Regulators ............................................... 15
Obtaining and Applying A & B factors....................................................................... 17
June 25 2010
1
Introduction
No-load and load losses are a real cost –
Evaluation of losses in step-voltage regulators is increasingly a major consideration
when evaluating competitive alternatives. A significant amount of focus is directed to the
efficiencies of transformer related products. These efficiencies have a direct impact on
the energy required to supply quality power to end users.
The initial cost of a voltage regulator should not be the only evaluation point when
comparing one voltage regulator manufacturer with another, as the initial price of a
voltage regulator is not a true reflection of the total cost that will be realized over the life
of the unit. Maintenance, installation and operation, technical support, dielectric strength,
short circuit strength, and losses are just some of the areas that should be evaluated in
addition to the first cost for a true measurement when comparing voltage regulator
manufacturers’ offerings. This paper addresses one of those areas: loss evaluation.
Loss evaluation formulas have existed for years for distribution transformers and for a
limited amount of voltage regulators. These formulas have cost factors (A and B) for the
no-load and load losses with the factor applied to the no-load value. Below is a simple
example of a total ownership cost (TOC) evaluation. The ownership cost of a Cooper
Power Systems (CPS) offering was significantly lower even though the first cost was
higher.
Exhibit A
CPS
Alternate
Manufacturer
First Cost
$10,500
$10,000 (-5%)
ILLUSTRATIVE EXAMPLE
Cost of Losses
Total Ownership Cost
$9,000
$19,500
$11,250 (+25%)
$21,250 (+9%)
Losses are related to the quality of the design –
Simply stated, poor designs typically lead to higher losses. Design efficiencies are
directly related to values of no-load and load losses. Indirectly, loss levels can be used
for comparing thermal capabilities and short circuit withstand strengths between
competitive alternatives. Losses, thermal characteristics, and short circuit withstand
strengths are all dependent on the active material content, internal impedance, and the
types of bridging reactor and main core and coil constructions.
Therefore, it is very important that commonly used design techniques and their
implications on losses be fully understood if realistic and comparable evaluations are to
be made. Of even greater importance is an appreciation of the design techniques that
are used and to incorporate these designs techniques as part of the evaluation between
competitive alternatives.
June 25 2010
2
Step-Voltage Regulator Types
Single-phase step-voltage regulators use reactive switching to regulate load voltage
providing nominal ±10% regulation. This is accomplished with the use of 32 approximate
5/8% voltage steps. Basically, the step-voltage regulator is a tapped autotransformer.
The secondary (series) winding is in series with the line, while the primary (shunt)
winding is connected phase-to-neutral for 4-wire grounded systems and phase-to-phase
for 3-wire ungrounded systems. The series winding connection to the shunt winding is by
way of the reversing switch of the tap changer. A raise or lower mode is dependent on
the polarity of that connection.
The eight approximate 1¼% taps of the series winding and center tapped bridging
reactor provide for the individual 5/8% incremental voltage adjustment. In all, there are
33 positions that include Neutral, sixteen lower positions (1L, 2L, 3L, 4L, etc.), and
sixteen raise positions (1R, 2R, 3R, 4R, etc.).
Voltage regulators are designed and manufactured using three basic types of
construction dependent on the voltage and current ratings as well as the manufacturers’
and users’ preference. These are defined by the voltage regulator IEEE Standard
C57.15 – 2009 as Type A, Type B, and Series Transformer.
Type A step voltage regulators are used for a majority of substation applications. The
tapped series winding is located on the load side of the shunt winding, and by adjusting
those taps the output load voltage changes. The load current flows in the section(s) of
series winding that is in the circuit. The current that flows in the shunt winding is
dependent on the tap position that relates to this amount of series winding. The
maximum amount of current that can flow in the shunt winding is approximately 10% of
the series winding current that occurs at the 16 raise and 16 lower tap positions.
With Type A construction (Figure 1), the core excitation varies between 90 to 110% of
rated voltage due to the unregulated source supply voltage connected directly across the
shunt (excitation) winding of the voltage regulator. A separate voltage transformer is
used on the load side of the regulator to provide a voltage supply for the control and tap
changer motor. The voltage supply for the control is used for establishing a reference
voltage in which the actual load voltage is compared to for possible system voltage
correction based on programmed control settings. The use of the tapped series winding
in conjunction with the tap-changer reversing switch provides the Type A voltage
regulator a ±10% voltage regulation range about the nominal rated voltage by operating
in either a “raise” or “lower” mode.
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Figure 1―Type A Step-Voltage Regulator
Type B step-voltage regulators are predominately used for distribution lines outside of
the substation and lateral circuits. The tapped series winding is located on the source
side of the shunt (excitation) winding, and by adjusting those taps the output load
voltage changes. The source side current that flows in the series winding is a sum of the
rated load current plus the excitation current that flows in the shunt winding. The value
of series winding current is dependent on the tap position with the maximum amount of
current occurring at the 16 raise tap position. This value is approximately 110% of the
rated load current. At the 16 lower tap position the value is approximately 90% of the
rated current. The current that flows in the shunt winding is also dependent on tap
position. The maximum amount of current that flows in the shunt winding is also at the
16 raise tap position in which the value is approximately 10.5% of rated load current. A
lower amount of current, approximately 8.5% of rated load current, flows in the shunt
winding at the 16 lower tap position.
With Type B construction (Figure 2), the core excitation is established by the voltage and
bandwidth settings due to the shunt winding connected directly across the regulated load
voltage circuit. A utility winding in the main coil coupled with the shunt winding is used to
provide a voltage supply for the control and tap changer motor. Type B construction due
to the nature of the design cannot provide the same amount of maximum voltage
regulation for the extreme raise and lower positions. Minimally, 10% voltage regulation is
provided at the maximum raise position while the maximum lower position is
approximately -8.5% voltage regulation.
June 25 2010
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Figure 2―Type B Step-Voltage Regulator
A typical Series Transformer construction (Figure 3) is used for large substation
applications in which the nominal load current value exceeds the high end tap changer’s
maximum continuous (thermal) current rating. A series transformer is placed in series
with the line in lieu of a tapped series winding. A tapped secondary winding of a shunt
(excitation) transformer is connected to the tap changer. A division in load current occurs
at the series transformer with a portion of it seen by the tap changer contacts. The
voltage tapped off of the secondary is reflected back into series transformer for line
voltage correction. The resulting voltage and current seen by the tap changer are within
its design limitations. The unique designs of the series and shunt transformers and their
location with respect to each other have a significant effect on the no-load and load loss
values. The CPS Series Transformer design is similar in nature to a Type A design due
to the fact the unregulated source voltage is applied across the primary of the shunt
(excitation) transformer. A separate voltage transformer is used on the load side of the
regulator to provide the voltage supply to the control and tap changer motor. Voltage
regulation provided by Series Transformer-Type A construction is ±10%.
Figure 3―Series Transformer Step-Voltage Regulator
June 25 2010
5
No-Load and Load Loss Profiles
Typical load loss profiles for the Type A, Type B, and Series Transformer designs are
shown in Figures 4, 5, and 6 respectively. In each case the highest loss position is
established as the 100% level with other position’s having losses at a percentage of this
maximum loss value. Losses for all tap positions are based on rated current. Regulators
are rated and cooled for the highest loss position at rated load.
100%
% Load Loss of Max Loss
90%
80%
70%
60%
50%
40%
30%
20%
10%
0%
16R 14R 12R 10R 8R
6R
4R
2R
N
2L
4L
6L
8L
10L 12L 14L 16L
8L
10L 12L 14L 16L
Tap Position
Figure 4―Type A Load Loss Profile
100%
% Load Loss of Max Loss
90%
80%
70%
60%
50%
40%
30%
20%
10%
0%
16R 14R 12R 10R 8R
6R
4R
2R
N
2L
4L
6L
Tap Position
Figure 5―Type B Load Loss Profile
June 25 2010
6
100%
% Load Loss of Max Loss
90%
80%
70%
60%
50%
40%
30%
20%
10%
0%
16R 14R 12R 10R 8R
6R
4R
2R
N
2L
4L
6L
8L
10L 12L 14L 16L
Tap Position
Figure 6―Series Transformer Load Loss Profile
The process of moving from one tap to an adjacent tap consists of closing on a circuit on
the adjacent tap before opening the circuit on the previous tap. The tap changer
movable contacts move through stationary taps alternating in eight bridging and eight
non-bridging (symmetrical) positions. Figure 7 shows the two movable tap changer
contacts on a symmetrical (even) position; the center tap of the reactor is at the same
potential.
Figure 7―Reactor Circuitry – Symmetrical (Even) Tap Position
An asymmetrical position, as shown in Figure 8, is realized when one tap connection is
open before transferring the load to the adjacent tap. Thus all load current flows through
June 25 2010
7
one-half of the reactor, magnetizing the reactor, and introducing a reactance voltage
drop in the circuit. This transfer takes around 25-30 milliseconds for completion. At a
minimum, three current zero opportunities are required for arc extinction.
Figure 8―Reactor Circuitry – Asymmetrical Position
Figure 9 shows the movable contacts in a bridging (odd) position. Voltage change is
one-half the 1¼% tap voltage of the series winding because of its center tap and
movable contacts located on adjacent stationary contacts.
Figure 9―Reactor Circuitry – Bridging (Odd) Tap Position
A circulating current caused by the two contacts at different positions (reactor energized
with 1¼% tap voltage) is limited by the reactive impedance of this circuit. Two opposing
requirements must be considered in the design of the amount of reactance to which the
value of the circulating current is set. First, the circulating current must not be excessive.
June 25 2010
8
Second, the variation of reactance during the switching cycle should not be so large as
to introduce undesirable fluctuations in the line voltage. The reactor has an iron core with
gaps in the magnetic circuit to set this magnetizing circulating current value based on the
applied tapped voltage. The value of this circulating current also has a very decided
effect on tap changer’s arc interruption ability and contact life.
During a tap change operation, voltage and magnetic flux within the reactor (preventive
autotransformer) may shift rapidly as the moving contact leaves one stationary contact
and closes on another stationary contact at a different voltage. In some cases the core
will temporarily saturate when the contacts close, resulting in an inrush current through
the reactor circuit. The magnitude and duration of this inrush is dependent upon the
geometric structure of the reactor, the number of reactor turns used, the operating flux
density of the reactor core, the X/R ratio of the system at the point of regulator
application, the system source impedance at the point of regulator application and the
timing of contact closure. Abnormal contact wear may result during contact closure
depending on the frequency and amount of asymmetrical inrush current.
The no-load loss is a result of the excitation of the main core and the reactor and is
dependant on the tap position and the applied voltage. IEEE Standard C57.15 states the
no-load loss values for the specified tap positions are to be based on rated voltage
applied across the shunt winding. Factors that cause differences in the no-load losses of
voltage regulators of the same design include variability in characteristics of the core
steel, mechanical stresses induced by the manufacturing process, variation in gap
structure and core joints, and variability of reactor core gaps.
A graph of no-load losses per tap position is shown in Figure 10. In this case, the odd
steps have the highest losses due to the reactor excited by the section of the series
winding that it is bridged across as shown in Figure 9. The even steps only reflect the
excitation of the main core; the reactor is not energized. The highest loss positions are
established as the 100% level with other positions having losses as a percentage of this
maximum loss value.
% No Load Loss of Max Loss
100
90
80
16
L
14
L
12
L
10
L
8L
6L
4L
2L
N
2R
4R
6R
8R
10
R
12
R
14
R
16
R
70
Tap Position
Figure 10―No-Load Loss Profile without Equalizer Winding
June 25 2010
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An alternative for the voltage regulator design is the use of an equalizer winding. The
equalizer winding is physically located in the outside section of the main coil and is
inserted by way of the tap changer into a closed circuit with the reactor on symmetrical
(even) steps, and with the reactor and a section of the series winding on bridging (odd)
steps. The voltage of the equalizer winding is approximately half of the voltage of a
section of the series winding or 5/8% of the nominal voltage. The connection of the
equalizer winding into the reactor circuitry for bridging (odd) steps is set up so that the
reactor sees only the difference in voltage between the series winding section and the
equalizer winding. On symmetrical (even) steps, the voltage of the equalizer winding
energizes the reactor. Figures 11 and 12 shows circuitry involving the equalizer winding
and the reactor for symmetrical (even) and bridging (odd) steps.
Figure 11―Equalizer Winding and Reactor Circuitry – Symmetrical (Even) Tap Position
Figure 12―Equalizer Winding and Reactor Circuitry – Bridging (Odd) Tap Position
June 25 2010
10
This equalizer winding reduces the maximum amount of voltage exciting the reactor to
almost half, permitting the use of a smaller reactor and decreasing the amount of
difference in no-load loss values between the bridging (odd) and symmetrical (even) tap
positions. Equalizer windings are used in the majority of Cooper Power System’s
available ratings. It has the additional benefit of reducing the interrupting load during a
tap change resulting in minimized and balanced erosion for all contacts. The amount of
reactor energization for all of the tap positions is dependent on the number of turns used
in the equalizer winding and the series winding sections.
% No Load Loss of Max Loss
Any tap position, even or odd, can have the maximum value of no-load losses at the
applied rated voltage. A graph of no-load losses per tap position of a typical design using
equalizer windings is shown in Figure 13. Notice that for this example the even tap
positions have the maximum value for the no-load losses and that a majority of the odd
tap positions have the same value as the even tap positions. The difference between the
minimum and maximum values is minimized resulting in a more even distribution of noload losses for the 33 tap positions. Each rating with an equalizer winding has a unique
no-load loss pattern.
100
90
16
L
14
L
12
L
8L
10
L
6L
4L
2L
N
2R
4R
6R
8R
10
R
12
R
14
R
16
R
80
Tap Position
Figure 13―No-Load Loss Profile with Equalizer Winding
June 25 2010
11
How Regulator Construction Affects Losses and Short
Circuit Strength
Cooper Power Systems uses its 55 years of experience in the design and application of
voltage regulators to develop unique techniques in the construction and manufacturing
process to address all of the life cycle related costs such as losses, dielectric withstand,
short circuit withstand and tap changer contact life.
A bridging reactor is required to maintain continuity during a tap change and to provide
impedance for limiting the amount of current to be interrupted by the tap changer. The
no-load losses and contact life are significantly affected by the amount of impedance
used as well as by the type of reactor construction used. The reactor construction also
affects the dielectric and short circuit strength of the voltage regulator. Figures 14 and 15
show the two types of reactor constructions available: shell type and core type.
Figure 14―Shell Type Reactor
Figure 15―Core Type Reactor
June 25 2010
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Cooper Power Systems uses a core type reactor construction to minimize the no-load
loss value for all tap positions. A benefit of the core type reactor design is that the two
coils are interlaced resulting in minimal impedance-to-load current flow and therefore
minimal voltage drop. The use of these interlaced coils with aluminum strip winding also
ensures superior short circuit withstand strength. Figure 16 shows a dismantled view of
the CPS core type reactor. The 4-gap structure of the core keeps the no-load losses to a
minimum due to the reduced thickness of the individual gaps which keeps fringing
magnetic flux to a minimum. Shell type constructions (two cores, one coil) typically have
one or two larger gaps maintaining the same amount reactive circulating current
resulting in a greater fringing magnetic flux causing higher no-load losses and higher hot
spots in the reactor windings.
Figure 16―CPS Core Type Reactor
In addition, CPS isolates their reactors from ground, as shown in Figure 17, resulting in
superior dielectric strength and a smaller compact design for even lower no-load losses.
Figure17―Ground Isolated Core Type Reactor
Minimization of losses related to the main core and coil assembly is accomplished by
pressing the sides of the main coil, as shown in Figure 18, during the bake out process.
June 25 2010
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Superior short circuit withstand strength is also a byproduct of this type of manufacturing
process. All wasted space is removed which translates to a lean design that uses active
material efficiently and keeps losses to a minimum.
Figure 18―Hot Pressed Main Coil
For Type A substation regulators, CPS uses a unique series-shunt-series
coil construction, shown in Figure 18, placing the halves of the tapped aluminum strip
series winding on the inside and outside of the centered shunt (excitation) winding. The
results are lower stray losses minimizing the load loss values and greater short circuit
strength due to improved coupling between windings and the use of an axial force
deterrent winding (aluminum strip series winding) on the outside of the coil.
The result of the unique CPS design techniques described above is a voltage regulator
that has the lowest losses within the smallest package having superior short circuit
withstand and dielectric strength.
June 25 2010
14
How to Evaluate Losses of Step-Voltage Regulators
When losses are evaluated, the tap positions shall be specified per IEEE Standard
C57.15-2009, paragraph 5.8.3.
Total losses of voltage regulators designed as Type A or Type B are the sum of no-load
and load losses defined as follows:
NLL = (NLLN + NLL1R)/2
where:
NLL = Average No-Load Loss (W)
NLLN = No-Load Loss in the “Neutral” position (W)
NLL1R = No-Load Loss in the “1 raise” position (W)
and:
LL = (LL16R + LL15R + LL15L + LL16L)/4
LL = Average Load Loss (W)
LL16R = Load Loss in the “16 raise” position (W)
LL15R = Load Loss in the “15 raise” position (W)
LL15L = Load Loss in the “15 lower” position (W)
LL16L = Load Loss in the “16 lower” position (W)
Total losses of voltage regulators designed with series transformer are the sum of noload and load losses defined as follows:
NLL = (NLLN + NLL15R + NLL16R)/3
where:
NLL = Average No-Load Loss (W)
NLLN= No-Load Loss in the “Neutral” position (W)
NLL15R= No-Load Loss in the “15 raise” position (W)
NLL16R= No-Load Loss in the “16 raise” position (W)
and:
LL = (LL16R + LL15R + LL15L + LL16L)/4
LL = Average Load Loss (W)
LL16R = Load Loss in the “16 raise” position (W)
LL15R = Load Loss in the “15 raise” position (W)
LL15L = Load Loss in the “15 lower” position (W)
LL16L = Load Loss in the “16 lower” position (W)
The load losses of a voltage regulator are losses resulting from rated load current
carried by the voltage regulator at the specified tap positions. Load losses include I²R
loss in the windings due to this rated current and stray loss due to eddy currents induced
June 25 2010
15
by leakage flux in the windings, core clamps, tank walls, and other conducting parts.
Load loss is also called winding or copper loss.
No-load losses are those losses resulting from the excitation of the voltage regulator by
applying rated voltage directly across the shunt winding at the specified tap positions.
The no-load loss of a voltage regulator consists primarily of the iron loss in the voltage
regulator main and reactor cores, and the I²R and stray loss due to reactive circulating
current within the reactor and equalizer winding circuit. No-Load loss is a function of the
magnitude, frequency, and waveform of the impressed voltage. No-load loss is
sometimes referred to as core or excitation loss. CPS applies rated voltage up to 35 kV
at rated frequency, 50 or 60 Hz, across the shunt (excitation) winding to ensure the noload values measured are accurate and that the least amount of calculation is involved
for correction purposes.
Total ownership cost (TOC) for a step-voltage regulator then is calculated in the same
manner as for a transformer, using the no load and load loss, as follows:
TOC = (KFC • FC) + (KNLL • NLL) + (KLL • LL)
where:
FC = first cost (price)
KFC = factor applied against first cost
KNLL = factor applied against the average no load loss, expressed in dollars per watt
KLL = factor applied against the average load loss, expressed in dollars per watt
IEEE C57.15 standard stipulates that the test of a voltage regulator shall be subject to
the following tolerances: “the no-load losses of a voltage regulator shall not exceed the
specified no-load losses by more than 10%, and the total losses of a voltage regulator
shall not exceed the specified total losses by more than 6%. Failure to meet the loss
tolerances shall not warrant immediate rejection but lead to consultation between
purchaser and manufacturer about further investigation of possible causes and the
consequences of the higher losses.”
June 25 2010
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Obtaining and Applying A & B factors
Below is an example of a spreadsheet using typical economic values for obtaining and
applying A and B factors to typical values of price and losses quoted in a contract.
The initial cost of a voltage regulator is one of many parts of a true evaluation when
comparing one voltage regulator manufacturer to another. One of those parts, loss
evaluation, is a piece of the total cost realized over the life of a voltage regulator. It is a
true reflection of design efficiency and of the techniques involved in the manufacturing
process.
June 25 2010
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