Series Voltage Boost Technology
For Low Power Distribution Voltage Regulation
A. Chandler
R. Krogstad
A. Lee
K. O'Dell
Department of Electrical Engineering
Arizona State University
Tempe, AZ 85287 USA
Abstract This paper presents simple and low
cost AC voltage regulator that utilizes series
boost technology. The basic idea is to utilize
adjustable ac voltage source in series with the
supply voltage to make the sum of the source
voltage and the series boost voltage constant as
seen by the load. The circuit does not require
any kind of synchronization with the source
mains. Voltage sags up to 30 percent have been
accommodated for a line voltage of 120 volts
and load up to 350 watts (a typical household
appliance rating). Laboratory tests of a prototype
are reported. An innovative portion of this
design is the magnitude and phase feedback
control of the AC series voltage. The basic
components of the regulator design are a series
boost and a control circuit for the series boost.
The series boost voltage is developed from a
transformer that is fed by the original supply.
The device complies with ANSI standard C 84.11989 for class A secondary distribution voltage
regulation.
Keywords Voltage regulation; series boost;
power quality; distribution engineering.
1. Introduction
This paper concerns the subset of power
quality engineering that deals with voltage
regulation. The scope is limited to voltage sags:
specifically voltage sags to -30% reduction of
rated voltage at the secondary distribution level.
The objective is to examine series voltage boost
technologies for applications in the 350 W class
for residential applications. The concept of the
phase controlled, series boost electronic voltage
regulator is to use a voltage source in series with
the supply voltage to make the sum relatively
constant as seen by the load,
Vload = Vsource + Vseries
|Vload| = regulated quantity
G. Heydt
K. Nigim
Department of Electrical
Engineering
Birzeit University
Birzeit, West Bank
The series boost voltage is developed from a
transformer that is fed by the original supply.
The series boost transformer is a standard two
winding transformer, in which the control circuit
excites one winding. The other winding is in
series with the supply. The objective is to sense
voltage sag and vary the series boost voltage to
achieve proper phase and magnitude. The
voltage regulator is divided into two principal
parts: voltage sensing and series voltage control
circuitries. The voltage sensing circuit
determines when sag occurs and the magnitude
of that sag. It then sends a signal to the control
circuit. The voltage applied to the primary side is
therefore controlled by the triac switch through
the delay of the firing instant (α) with respect to
the ac cycle. By chopping part of the cycle, the
rms primary voltage is then somehow smoothly
controlled which adjusts the series boost
configuration.
2. Voltage regulation
Voltage
regulation
has
become
increasingly important for computers and
computer-controlled loads. Millions of dollars
can be lost because of drop in voltage lasting a
few cycles. “Approximately 80 percent of
electrical maintenance staff activity can be
equated to treating the effects of transient
phenomena
on
electronic
equipment.
Maintenance costs are a controlled item for all
companies, and real efforts should be made by
employees to reduce costs in their areas of
concern.” [1]
Traditional voltage regulators often
used electromechanical and electromagnetic
technologies. As an example, the trip coil
voltage regulator is designed to hold in relay
contacts by maintaining a control voltage on the
relay coil during voltage sag. The device works
similar to contemporary designs in that it
maintains voltage for about 25 percent loss of
input voltage. The device works within a subcycle response time and it can be designed for
120, 240, and 480-volt AC systems [3].
Alternatively, there is a range of voltage
1
3 Series voltage boost technology
Series boost technology entails the use
of a series transformer that is used to boost the
supply bus voltage. Figure (1) shows the basic
concept. This technology has been used in high
power regulation applications in the form of a
dynamic voltage restorer (DVR) and other types
of regulators. References [8-9] are a sampling of
recent reports on series boost technology.
In the series boost configuration, the
excitation of the series transformer is obtained
from a number of alternative controllable voltage
sources. For purposes of the proposed design, a
triac chopped wave is obtained from the supply
bus, and the chopped wave is applied to the
excitation winding of a series transformer. By
varying the triac control, the series boost
obtained is controllable. Feedback control is
used to render the circuit operable as a voltage
regulator.
VSerie
Control
Sense
Vs
ac –ac
chopper
Load
Figure (1) Basic series boost voltage regulation
technology Phase Coordinated Waveforms.
The premise of this design project is to
maintain a constant voltage to a load during
voltage sag. During a voltage sag condition, this
design utilizes voltage-sensing circuitry that will
control the gate firing instant of the triac.
The control circuit is designed to sense
the voltage sag of the ac supply. The sensed
signal rectified and is compared with a reference
voltage that is set to present 120 V. The output
voltage of the comparator is then amplified and
integrated to minimize sudden change
overshooting. The output is then compared with
unity triangular waveform synchronized to mains
supply half cycle. The result of the two
waveforms crossing is pulse width modulated
voltage signal in synchronism with the supply.
As the sensing voltage drops, so the error
increase, decreasing the delay angle allowing
much of the ac portion to be applied across the
boost primary windings. The boost level depends
on the phase and rms amplitude of the boost
transformer primary voltage. Phase is important
in this stage of the control circuit, and the phase
and amplitude of the triac voltage is controlled in
a feedback arrangement to regulate the load
voltage [5-6]. The transformer primary voltage
as a function of the gate delay angle is given by
equation (1).
V(α) = kVp [1/(2π). (π - α + ½ sin (2α)]½
(1)
Where,
k is the step down transformer ratio.
Figure (2) shows the rms triac output voltage for
various DC control voltage inputs in a typical
phase controlled configuration.
The portion
of this graph that is most important for the
design is the linear region between 3 and 8 Vdc
control voltage for the selected phase controller
circuit. The voltage sensing circuit is designed
to operate in this linear region.
140
120
Output voltage (Vrms)
regulator configurations that employ transformer
tap changing or rotating generator excitation
control. These technologies have the advantage
of high power convenience, but the
disadvantages of slow speed, large size, and high
maintenance requirements.
Several regulator technologies are based on
rectifier - inverter topologies. A voltage dip
proofing inverter uses a large storage capacitor to
maintain voltage during voltage sag. It supplies
the load with a semi-sinusoidal wave voltage
equal to the main power source.
It was
originally designed for inductive loads with low
power factor. The device can be manufactured
for 120 or 240 volt AC systems [3].
Constant voltage transformers (CVTs)
utilize a resonant capacitor circuit in the
secondary to maintain a constant voltage during
voltage sags. The device has a sub-cycle
response time and can be used for a wide range
of voltages and loads. CVTs need to be
oversized to approximately double the load value
for appropriate voltage compensation.
The
output of a CVT can collapse when the inrush
current gets too high [3].
100
80
60
40
20
0
0
1
2
3
4
5
6
7
8
9
10
DC control voltage (Vdc)
Vreference
Figure (2) Triac rms voltage output for various
DC control voltages
2
Figures (3) and (4) show the output
wave shapes of the phase controlled triac circuit.
These wave shapes help illustrate the operation
of the triac as phase controlled bilateral switch in
which the typical voltage waveform are as shown
in figures (3) and (4). These figures are included
to appreciate the wave shape distortion of the
triac chopped voltage: a clear disadvantage of
this voltage regulation method is the high level
of bus voltage distortion at the load. The primary
voltage waveform is assumed to follow the same
pattern at the secondary side of the transformer.
However, as the secondary side voltage is in
series with the supply and at a fraction of the
primary injected voltage, the overall distortion to
the load voltage is acceptable.
The source unregulated voltage is sensed through
voltage sensory circuit that convert the ac value
to dc level allowing detection of voltage sags of
both the positive and negative half cycles.
The output of the sensing part of the control
varied from –9 to –6.5 Vdc for input voltages of
120 to 90 Vac. Amplification of the control
signal by a factor of two is needed to
accommodate the proper feedback gain enough
to cover 80% of the control range. The control
and power circuit designed is shown in fig. (5).
Figures (6) and (7) shows the Pspice
simulation response of sensing part of the circuit
for line voltages between 90 and 120 Vac. This
illustrates linear control.
1 6 .7 m s
58 V
Figure (3) Triac voltage waveform at 3 Vdc
control voltage (α≤π/2)
Figure (5) Schematic of voltage sensing circuit
9
Control voltage (Vdc)
8
165 V
16.7 m s
7
6
5
4
3
2
1
0
90
95
100
105
110
115
120
Line voltage (Vac)
Figure (4) Triac voltage waveform at 6 Vdc
control voltage (α≥π/2)
In the series boost configuration, the
regulated voltage is given by the following
equations:
For α o < conduction period < 180 o, then
(2)
Vo(ωt) = Vs(ωt) + Vo(α)
And for 0o < conduction period < α o, then
(3)
Vo(ωt) = Vs(ωt) = Vp . sin(ωt)
Figure (6) PSpice response of the voltage
sensing circuit
4 Voltage sensing configuration
3
Voltage Sag Response
20
Output Control
Voltage
15
10
5
0
90
100
110
120
Supply Voltage (Vrms)
Figure (7) Voltage sensing circuit test results
5 Design boundaries
Another factor that was considered in
the consideration of the boost transformer was
the series reactance and its effect on the overall
circuit. The transformer used in the prototype
design had a leakage reactance of about 2.3
ohms.
The limitations of the power
transformer used as the series element are size
and weight. The larger the VA rating of the
transformer, the larger and heavier the
transformer will be. A power transformer can be
manufactured for almost any combination of
voltage between the primary and the secondary
windings. The transformer chosen for this
design was a 120/52 Vac transformer. At a
supply voltage magnitude of 90 Vac, this
transformer produced a boost voltage of 38 Vac.
This voltage accounted for any voltage drop due
to the series reactance of the transformer and for
the drop in supply voltage.
5. Experimental results
The triac control circuit, the voltage
sensing circuit and the series boost transformer
AC
control
Transformer
An objective of the design of this
regulator was potential application for residential
loads.
The 350 W design specification
corresponds to 2.91 A at 120 Vac. The load side
of the transformer had to be designed for the
proper boost voltage under worst-case
conditions.
This meant that a 90 Vac input to the
control side would provide the 30 Vac boost to
the supply voltage to maintain 120 Vac to the
load. Therefore, to regulate up to 30% of supply
drop, the transformer primary to secondary turns
must be,
kmax > 4
(4)
Set voltage
reference
+ Vdc
+
Vdc
Regulat
ed
power
supply
- Vdc
A
Set min.
delay
dc delay instant
control
Phase controlled PWM
generator
TEA1007
Set
max.
delay
Gate voltage
pulse
Tria
c
Series Boost
Transformer
1: k
Unregulate
d ac source
AC
AC
Load
Rectifier Output
25
ac synchronization
signal
30
were combined into a prototype for testing and
debugging.
Figure (8) is a schematic
representation of the entire circuit.
The response of the regulator is
illustrated by results of a series of partial load
and full load tests. The IEEE 141-1993 standard
for minimum and maximum voltage is –4.0%
and +9.5% for a 120 Vac supply. The ANSI
C84.1-1989 standard for the 120 Vac nominal
voltage range is a maximum voltage of 126 Vac
and a minimum voltage of 114 Vac. Figures (9)
to (11) shows the results of a 40 W,100W and
320 W load test respectively.
The figures illustrates several key
points. The design maintains voltage within
IEEE and ANSI standards down to a supply of
80 Vac. The second point that can be illustrated
is the versatility of the gain adjustment on the
voltage sensing system. This adjustment is
external to the device and can be adjusted at any
time to obtain the desired voltage. This could be
part of regulating the load voltage feedback loop.
Several test were made to debug the controller to
ensure precise performance of the regulation as
the source is reduced to –30%.
Soft start
35
+
Vdc
40
Figure (8) Series boost voltage regulator
control schematic
4
The experimental work indicated
successful self-regulation through the control of
the triac-firing instant with respect to the source
voltage periodical half cycles. The circuit does
not require any kind of synchronization. The
voltage across the load was maintained at 120V
from 100% down to 70% of the source supply
rms level.
Figure (11) Output (load) voltage of a series
boost voltage regulator for a 320 W load
A series of experiments was conducted
to inspect the waveform of the prototype. The
boost voltage however, is only a partial sine
wave and when added to a full sign wave, creates
an increase in peak voltage and a distorted wave
shape.
130
Gain Adjusted at 120 Vac Supply
125
Output Voltage (Vrms)
120
115
240 V
Gain Adjusted at 90 Vac Supply
110
105
100
16.7 m s
95
90
80
85
90
95
100
105
110
115
120
Supply Voltage (Vrms)
Figure (9) Output (load) voltage of a series
boost voltage regulator for a 40 W load
Figure (12) Output voltage waveshape (load
voltage) of the prototype at 120 Vac supply with
a 100 W load
130
125
115
180 V
110
D e cre as ing S u pp ly V oltag e
Inc rea sin g S up ply V olta ge
105
100
16.7 m s
95
90
80
85
90
95
100
105
110
115
12 0
S up ply V o ltage (V rm s)
Figure (10) Output (load) voltage of a series
boost voltage regulator for a 100 W load
Figure (13) Output voltage waveshape (load
voltage) of the prototype at 90 Vac supply with a
100 W load
130
125
120
162 V
Output Voltage (Vrms)
Output Voltage (Vrms)
120
115
110
G a in A d jus ted a t 1 20 V a c S u pp ly
G a in A d jus ted a t 9 0 V ac S up ply
16.7 m s
105
100
95
90
80
85
90
95
100
105
S u p p ly V o ltag e (Vrm s)
110
115
120
Figure (14) Output voltage waveshape (load
voltage) of the prototype at 80 Vac supply with a
100 W load
5
The distorted wave particularly (α≥π/2)
has the disadvantage of high crest factor which is
defined;
Crest factor = Vo-peak / Vrms,
(4)
The high factor as well as high harmonic
distortion is basic disadvantages of the simple
triac chopper design. However, if voltage
regulation is required for 15% of the source, the
output-regulated voltage is almost sinusoidal.
Figures (12)-(14) show the combination of the
boost voltage and supply voltage sine waves
delivered to a 100 watt load as the control
voltage demand varies to maintain constant
output.
6. Recommendations for future work
The main recommendations for future
work and improvement of the proposed regulator
design are:
• The regulator transient response should be
measured
• The response time of the regulator should be
identified and compared to general
residential needs
• Alternative filtering methods should be
studied
• The present design shows some dependence
of the regulation on the load level. Adding a
second, very low gain feedback loop could
accommodate this.
• The response of inductive loads should be
investigated (e.g., induction motor loads)
7. Conclusions
The objective of this design project was to
inexpensively build a phase controlled, series
boost voltage regulator.
The regulator
specification is in the 350 W class intended for
residential applications. Demonstrated in this
paper is a series configuration that employs a
simple triac chopper as the control element.
Laboratory tests are included to illustrate the
regulator response. The main disadvantage of
the design is the high level of voltage waveform
distortion that is a function of the triac firing
instant and the transformer step down ratio. The
main advantages are low cost, simple design, and
tight voltage control. The laboratory prototype
complied with ANSI C 84.1-1989 class A for
supply voltage sags down to 90 Vac.
[2] P. Z. Fang, “Voltage Sag Support,” Power
Electronics and Electric Research Center, August
2000, http://www.ornl.gov/etd/peemc/pemi5.htm
[3] T. La Rose, “Voltage Sag Mitigating
Devices,” presented at Microchip Technology
Inc., Chandler, Arizona, February 2000.
[4] SGS-Thomson, X. Durbecq, “Control by a
Triac for an Inductive Load, How to Select a
Suitable Circuit,” Application Note D89AN308,
pp. 1-14.
[5] K. A. Nigim, G. T. Heydt, “A Single Phase
AC
Phase
Controlled
Series
Voltage
Conditioner,” submitted for publication to the
Journal of Electric Machines and Power
Systems, 2001.
[6] B. M. Bird, K. G. King, “Introduction to
Power Electronics,” John Wiley and Sons, New
York, 1992, pp. 151-171.
[7] Velleman Kit NV, “K8003, DC Controlled
Dimmer” Gavere, Belgium,
http://www.velleman.be
[8] G. T. Heydt, W. Tan, T. LaRose, M. Negley,
“Simulation and analysis of series voltage boost
technology for power quality enhancement,”
IEEE Transactions on Power Delivery, v. 13,
No. 4, October, 1998, pp. 1335 – 1341.
[9] A. Ghosh, G. Ledwich, "Structures and
control of a dynamic voltage regulator (DVR),"
IEEE Power Engineering Society Winter
Meeting, January, 2001, v. 3, pp. 1027 -1032
Biographies
Adam Chandler is a senior in the Department of
Electrical Engineering at Arizona State
University. He is from Phoenix, AZ.
Ronald Krogstad is a senior in the Department
of Electrical Engineering and an engineer at
Microchip Technologies in Tempe. AZ.
Ashley Lee is a senior in Electrical Engineering
at Arizona State University. Ms. Lee is from
Phoenix, AZ.
Kelly O'Dell completed her BSEE degree at
Arizona State University in May, 2001. Ms.
O'Dell is presently a graduate student at ASU.
She is from Phoenix, AZ.
Gerald T. Heydt is a Professor and Center
Director at Arizona State University.
Khaled A. Nigim is a Visiting Fullbright Fellow
at ASU. He is from Gaza, Palestine.
References
[1] Catalog No. EP-2000, Waveform Correction
Absorber, Environmental Potentials Inc., Carson
City, NV, 2001.
6