1 Submitted at the IEEE Power & Energy Society General Meeting, Boston, July 2016.

Paper# 16PESGM0961

Unique Capabilities of Sen Transformer:

a Power Flow Regulating Transformer
Kalyan K. Sen Mey Ling Sen
Sen Engineering Solutions, Inc. Sen Engineering Solutions, Inc.
Monroeville, PA, USA Monroeville, PA, USA
senkk@ieee.org senml@ieee.org

pendent P and Q power flow control [1]. Impedance regula-

Abstract — Two decades ago, a renewed interest in the devel- tion, implemented by the UPFC, provides the most optimized
opment of power flow control technology resulted in the creation control of power flow in a transmission line.
of unified power flow controller (UPFC) with the use of voltage-
sourced converters due to the availability of high power semicon- The benefits of the independent P and Q control are enormous.
ductor switches, such as 4500 V, 4000 A-rated Gate Turn-Off
For just the right amount of Q flow, required to maintain the
thyristors. Lessons learned from this experience continue to rein-
force the true needs of a power flow controller, i.e., high reliabil-
voltage stability, the rest of the rating of the line, up to its
ity, high efficiency, low-cost, component non-obsolescence, high thermal limit, can be used to carry the revenue-generating P
power density, interoperability and portability while providing and, therefore, reduce losses in the transmission lines due to
the optimal power flow control capability. The Sen transformer the reduction of Q flow. Reduction of Q flow in the generator
(ST) provides these qualities while enhancing the controllability and step-up transformer reduces their losses and increases
in an electric power grid by using functional requirements and efficiencies, and free up their capacity to carry more P. Addi-
cost-effective solutions. Utilities that are looking for ways to en- tionally, when the independent P and Q control is used, it re-
hance the controllability in their power grid by voltage regula- lieves grid congestion, limits power flow in a line, does not
tion, phase angle regulation, line impedance regulation, fault-
trip a fully-loaded line when it is needed the most to avoid a
current limitation, and much more can benefit from using the ST.
possible blackout and directs power flow through the desired
Index Terms—FACTS, Smart Grid, SMART Power Flow Con- transmission paths to ensure the highest overall efficiency, and
troller. lower wholesale electric market costs to loads.

I. INTRODUCTION Early power flow controllers employed basic technologies for

the corrective voltage injection. This includes transformers,
B asic power flow control technology can be of two types:
non-voltage-sourced converter (non-VSC) and VSC. Ex-
amples of a non-VSC type include transformer and load
capacitors, and reactors. Later designs used power electronics
to achieve much greater optimization using the independent P
and Q power flow control algorithm.
tap changers (LTCs) that are used in voltage regulating trans-
former (VRT) and phase angle regulator (PAR), reac-
A paradigm evolved that independent P and Q power flow
tors/capacitors switched by breakers, static var compensator
control algorithm required power electronics VSC-based tech-
(SVC) and thyristor-controlled series compensator (TCSC).
nology, resulting in the typical Flexible Alternating Current
The VSC-type consists of static synchronous compensator
Transmission Systems (FACTS) controllers of the last two
(STATCOM), static synchronous series compensator (SSSC)
decades. It is well recognized that power electronics-based
and unified power flow controller (UPFC).
solutions offer superior characteristics in resolving power
quality issues [2]. But is power electronics really required in a
Common to most power flow controllers are two basic sec-
utility application for power flow regulation?
tions:
1. A control computer or programmable logic controller
The objective of this paper is to present unique capabilities of
(PLC) that implements power flow optimization algo-
the Sen transformer (ST), a power flow regulating transformer,
rithms.
which implements modern techniques, i.e., independent P and
2. A high-power circuit that is capable of creating the correc-
Q power flow control, using time-tested components, such as
tive voltage vector, determined by the power flow optimi-
transformer and LTCs.
zation algorithm.
II. BACKGROUND
When one power flow control parameter – voltage magnitude,
phase angle or reactance – is regulated, the result is a simulta- The heart of the independent P and Q power flow control
neous control of active (P) and reactive (Q) power flows in a technique is not the voltage regulation or phase angle regula-
transmission line; but impedance regulation results in an inde- tion or reactance regulation; it is the impedance regulation and
the most cost-effective topology is the shunt-series where a
series unit is connected back-to-back to a shunt unit through

978-1-5090-4168-8/16/$31.00 ©2016 IEEE

2 Submitted at the IEEE Power & Energy Society General Meeting, Boston, July 2016. Paper# 16PESGM0961

which the exchanged power at one unit’s AC terminals flows shared link to and from the same transmission line under com-
to the other unit’s AC terminals freely [3]. If a power electron- pensation. These exchanged active and reactive powers (Pexch
ics-based solution is used for this purpose, two back-to-back and Qexch) emulate in series with the line a capacitor (C) or an
DC-to-AC converters with a joint DC link make it more cost- inductor (L) and a positive resistor (+R) or a negative resistor
effective than two back-to-back AC-to-AC converters with a (−R). A positive resistor (+R) absorbs active power from the
joint AC link. line; a negative resistor (−R) delivers active power to the line.
A capacitor (C) delivers reactive power to the line and, in the
Fig. 1 shows a simple power transmission system with a send- process, increases the power flow of the line. An inductor (L)
ing-end voltage Vs (i.e., Vs ∠δs), a receiving-end voltage Vr absorbs reactive power from the line and decreases the power
(i.e., Vr ∠δr), the voltage VX (i.e., Vs – Vr) across the line reac- flow of the line. Therefore, the compensating voltage is actual-
tance (X), a series-connected compensating voltage Vs’s (i.e., ly an impedance emulator that modifies the effective imped-
Vs’s ∠β), the modified sending-end voltage Vs’ (i.e., Vs’ ∠δs’), ance (both resistance and reactance) of the transmission line
and the line current (I). The active and reactive power flows at between its two ends, which modifies the sending-end voltage
the sending end are Ps and Qs, at the modified sending end are to be of a specific magnitude and a phase angle that results in
Ps’ and Qs’, and at the receiving end are Pr and Qr, respective- an independent control of P and Q power flow in the line. In-
ly. dependent P and Q power flow control can be optimized so
that the useful P flow is maximized while the less desirable Q
Ps , Q s Ps', Q s' Pr , Q r flow is minimized in the controlled path.
Vs's
VX In 1998, impedance regulation method was demonstrated for
I
the first time using a power electronics-based solution at
American Electric Power’s Inez substation. The power elec-
X tronics-based FACTS controllers are capable of providing
Vs Vs' Vr responses in the range of milliseconds [2]; however, the expe-
riences have shown that the needed response time is in seconds
P exch in most utility applications [4]. Therefore, it is desirable to
Q exch redesign the independent power flow controller to meet the
functional requirements of providing responses in seconds,
Fig. 1. A simple power transmission system with a series-connected which will make it less expensive than the power electronics-
compensating voltage, Vs’s. based solution. This was the motivation to develop the ST.

In the early 1990s, Westinghouse Engineers experimented with Lessons learned from the use of a UPFC continue to reinforce
implementing the compensating voltage, generated by a VSC the true needs of a power flow controller – high reliability,
as shown in Fig. 2. This circuit diagram, known as unified high efficiency, low cost, component non-obsolescence, high
power flow controller (UPFC), consists of two units – shunt power density, interoperability and portability while providing
and series: the shunt unit is a VSC that is connected to the line the optimal power flow control capability. The ST provides
through a coupling transformer; the series unit is also a VSC these qualities while enhancing the controllability in an electric
that is connected to the line through a coupling transformer. power grid by using functional requirements and cost-effective
The VSCs are connected together at the joint DC link capaci- solutions.
tor.
III. THE SEN TRANSFORMER (ST) CONCEPT
Vs's The ST uses a shared magnetic link between primary and sec-
ondary windings as shown in Fig. 3a. A three-phase voltage is
Shunt P exch Series applied in shunt to three primary windings that are Y-
Vs Unit Unit Vs' connected and placed on each limb of a three-limb, single-core
transformer. On the secondary side, three induced voltages
from three windings that are placed on three different limbs are
combined, through series connection of the associated wind-
ings, to produce the compensating voltage (Vs’s) for each
Unified Power Flow Controller phase. The number of active turns in the three windings is var-
ied with LTCs. As a result, the composite voltage becomes
Fig. 2. Unified power flow controller (UPFC). variable in magnitude (Vs’s) and variable in phase angle (β) in
the range of 0º and 360º as shown in Fig. 3b. The modified
The series-connected compensating voltage (Vs’s = Vs’ – Vs) is sending-end voltage (Vs’) with variable magnitude (Vs’) and
of variable magnitude and phase angle and it is also at any variable phase angle (δs’) stays confined within the circle.
phase angle with the prevailing line current. Therefore, it ex-
changes active and reactive powers with the line. The ex- Components needed to build a Sen Transformer are transform-
changed active power (Pexch) flows bidirectionally through the er core, windings, LTCs, transformer oil, tank, instrument

978-1-5090-4168-8/16/$31.00 ©2016 IEEE

3 Submitted at the IEEE Power & Energy Society General Meeting, Boston, July 2016. Paper# 16PESGM0961

transformers, computer control or PLC that implements power The transformer and LTCs-based power circuit results in much
flow optimization algorithms. lower initial cost, long term stable technology, wide availabil-
ity of expertise for operation and maintenance, proven reliabil-
ity of transformer and LTCs, and equal independent power
Shunt Series flow control functionality in comparison to a VSC-based solu-
Unit Vs's Unit tion.

Vs The ST is more than 99% efficient, since power flow through

Vs' the ST encounters only one stage of loss (Fig. 3a); in contrast,
there are four stages of losses – two in the VSCs and two in the
coupling transformers – in the UPFC (Fig. 2).

The concept of independent power flow control has been

P exch demonstrated in 1998. SEN Engineering makes it compact and
Q exch affordable. The ST footprint is practical for relocation with
low cost. The dimensions of a typical ST would be
10’X30’X20’, which is a fraction of a footprint of a UPFC
Sen Transformer installation. Comparison of sizes and footprints between a
(a) UPFC and a ST are shown in Figs. 4 and 5, respectively.

The figure shows the world’s first UPFC at AEP’s Inez substa-
I tion. The figure shows the substation; the inverter hall, control
Vs's room and the station battery are located inside the building; the
shunt, series, spare and intermediate transformers, breaker and
β switches, and heat exchangers are located outdoors. The relia-
bility of this system is not known, since it is not used. Note
that the power circuit consists of over 10,000 discrete compo-
VX Vr nents. Most components are obsolete and spare parts are simp-
ly not available.

Vs
Vs'
δ'

(b) δ
δr δs δ s'

Fig. 3. (a) Sen Transformer and (b) the related phasor diagram.
100’

IV. PROPOSED SOLUTION AND ITS ADVANTAGES Fig. 4. Westinghouse-built UPFC at the AEP Inez substation [5].
An alternate way to implement the shunt-series topology is to
use a ST. Both UPFC and ST use the same computer control
methodology and algorithms for independent P and Q power
flow optimization. However, the UPFC consists of a power
electronics VSC-based power circuit. But the ST consists of a
transformer and LTCs-based power circuit; it uses no power
electronics for most applications.

The power electronics VSC-based power circuit results in

higher initial (installation) cost and recurring cost (operation, 10’
loss and maintenance), rapid obsolescence, specialized exper-
tise for operation and maintenance, reliability issues and non- Fig. 5. Size and footprint of a comparably-rated ST.
portability.

978-1-5090-4168-8/16/$31.00 ©2016 IEEE

4 Submitted at the IEEE Power & Energy Society General Meeting, Boston, July 2016. Paper# 16PESGM0961

Following are the two most popular topologies of ST. The ST = 9 instead of 3+9 = 12. Therefore, the practical magnetic rat-
with an autotransformer is the most cost-effective topology ing of the ST is only 9/6 = 1.5 pu for 120ο range of operation,
that interfaces two transmission systems of different voltage instead of two pu for 360ο range of operation. Note that by
levels and implements independent power flow control as using six secondary windings, one of the six operating regions
shown in Fig. 6. (0ο to 120ο, 120ο to 240ο, 240ο to 360ο, 300ο to 60ο, 60ο to
180ο, and 180ο to 300ο) can be selected.
Vs'sA IA
Vsr c A 345 kV VsA

a1
0
Vs'A
A VSC-based compensating voltage is capable of injecting a
IB
Vsr c B 345 kV 4 voltage in series with a line between 0º to 360º. For a design of

4
138 kV b1

0
Vs'B
a 20%-rated compensating voltage, the least-utilized operating

4
IC
Vsr c C c1

0
50 Vs'C
points are at 0º and 180º, since there may be a limit on how
Vs'sB
much the modified sending-end voltage can be changed from
VsB

a2
0 its nominal value. The ST eliminates this waste by using a
A 4 lesser number of taps in this operating area and lowers the cost

4
20 138 kV b2
0 of the ST further. The related operating points are shown in
4
c2

0
0
Fig. 9.
0
0

B VsC Vs'sC
20

0 IA
Vs'sA
20

a3 VsA
4 0
4
50

C b3 a1 Vs'A
0
50

IB
4

VsB 4

4
c3
0

0
Vs'B
EXCIT ER UNIT COMPENS AT ING VOLT AGE UNIT IC
VsC c1
Vs'C
Fig. 6. ST’s compensating voltage unit is connected to the stepped-
Vs'sB
down voltage of a transmission line.
0
a2
A 4
Applications with more than 230-kV voltage level requires a b2
two-core design where the taps are not exposed to high

0
voltages as shown in Fig. 7.
Vs'sC
IA
C B
VsA Vs'A
IB

4
b3

0
VsB Vs'B
IC

4
c3

0
VsC Vs'sA Vs'C
EXCIT ER UNIT COMPENSAT ING VOLT AGE UNIT

0
a1
4
Fig. 8. ST for voltage compensation in the range of 0ο through 120ο.
4

A b1
0

c1
β =0ο
0

Vs'sB

0
C B a2
4
4

b2
0

c2
0

Vs'sC

EXCITER UNIT

0
a3
4
SERIES UNIT β =120 ο
4

b3
0

c3 VsA
COM PENSATING VOLTAGE UNIT
β =120 ο
Fig. 7. ST configuration using taps with lower voltage and current Vs
ratings. V sC B

Often time, the need is to increase or decrease the power flow

in a certain transmission line, but not both. In that case, the ST
configuration can be further simplified to provide a 120º con-
β =120 ο β =0ο
trol range rather than a 360º control range. For a 120º control β =0 ο
range of operation, only three primary windings and six sec-
ondary windings are needed for the power circuit as shown in Fig. 9. Modified sending-end voltage points for operation within the
control range of 0ο and 120ο.
Fig. 8. Therefore, the total number of windings required is 3+6

978-1-5090-4168-8/16/$31.00 ©2016 IEEE

5 Submitted at the IEEE Power & Energy Society General Meeting, Boston, July 2016. Paper# 16PESGM0961

If voltage regulation is also needed in addition to the power VI. CONCLUSION

flow regulation, particularly when the short-circuit impedance Increased transmission capacity can be obtained either by
at the point of compensation is high, a separate shunt reactive building new transmission lines, which is a long and costly
power compensator may be used along with the ST as shown process or by harnessing the dormant capacity of the existing
in Fig. 10. This topology provides an independent P and Q transmission lines. This might be a quicker and cheaper option,
power flow control and voltage regulation, without using any using the ST, which is a functional requirements-based, cost-
VSCs. effective solution that is reliable, efficient, free from compo-
nent obsolescence, portable, and based on proven technology
When the performance of ST and UPFC are compared, the ST of transformers and LTCs to meet today’s utilities’ power flow
offers the same independent power flow control as the UPFC. control needs. Implementation of the ST technology does not
• ST with mechanical LTCs provides an independent power require each independent power flow control application to be
flow control with a response time in seconds. nearly a research project, as is often the case with power elec-
• Power electronics-based UPFC provides an independent tronics solutions.
power flow control also in seconds, since its response is
slowed down in most utility applications [6]. The ST technology meets the immediate need of the utility in
terms of maximizing the revenue-generating P power flow
while providing the highest efficiency. Successful installation
of a Sen Transformer will give utilities the capability of a full-
featured power flow controller that independently regulates the
P and Q power flows in a transmission line. This technology is
based on functional requirements to meet the power industry’s
pressing need for the most economical ways to transfer bulk
power along a desired path. Flexible power routing with the
use of the ST creates immediate capacity to absorb alternative
Sen Transformer energy sources into the grid.

The relatively low cost of the ST technology may allow wide-

SVC spread implementation of independent power flow control
across the grid, which has been impossible to be achieved with
Fig. 10. Voltage regulation with SVC and independent power flow the high cost of a UPFC solution. Also, the biggest obstacle of
regulation with ST.
the UPFC is its thousands of discrete components that are
simply not available as spare parts due to component obsoles-
V. SUMMARY
cence, according to the experience gained in the last two dec-
The ST uses a transformer and LTCs to create a compensating ades. The ST technology is suitable to create a power flow
voltage that is at any angle with the prevailing line current and, controller market with the right technology at the right price.
therefore, acts as virtual impedance that enhances the control-
lability in an electric power transmission system by voltage VII. REFERENCES
regulation, phase angle regulation, line impedance regulation,
[1] K. K. Sen, “Practical Power Flow Controller Brings Benefits of
fault-current limitation, and much more, all in one unit. Power Electronics to the Grid,” How2Power Today, March
2015 issue, [Online]. Accessed on January 30, 2016.
The best features and the lessons learned from the available http://www.how2power.com/newsletters/1503/articles/H2PTod
power flow control techniques are used to define the ST as ay1503_design_Sen.pdf?NOREDIR=1
follows: [2] K. K. Sen, “Overview Of Voltage Regulation Schemes For
• High reliability with the lowest number of components Utility And Industrial Applications,” How2Power Today, Sep-
• Impedance control feature using a shunt-series topology tember 2015 issue, [Online]. Accessed on January 30, 2016.
http://www.how2power.com/newsletters/1509/articles/H2PTod
• Adequate response time for utility applications without the ay1509_design_Sen.pdf?NOREDIR=1
unnecessary use of high-cost power electronics [3] M. L. Sen and K. K. Sen, “Introducing the SMART Power Flow
• Lowest installation cost Controller - An Integral Part of Smart Grid,” paper no. 103,
• Lowest operating cost with minimum maintenance and IEEE Electrical Power and Energy Conference, Oct. 2012.
losses [4] K. K. Sen and M. L. Sen, “SMART Power Flow Controller for
• Minimum equipment rating and minimum footprint so that Smarter Grid Applications,” paper no. 15PESGM1910, IEEE
it can be relocated easily when the system needs change Power & Energy Society General Meeting, July 2015, Denver,
• Free from component obsolescence (note that power elec- USA.
[5] N. G. Hingorani and L. Gyugyi, “Understanding FACTS: Con-
tronic systems become outdated every 10-15 years)
cepts and Technology of Flexible AC Transmission Systems,”
• Interoperability so that components from various suppliers IEEE Press, 2000.
can be used, resulting in a global manufacturing standard, [6] K. K. Sen and M. L. Sen, “Introduction to FACTS Controllers:
ease of maintenance, and ultimately lower cost to con- Theory, Modeling, and Applications,” IEEE Press and John
sumers. Wiley & Sons, Chapter 9, 2009.

978-1-5090-4168-8/16/$31.00 ©2016 IEEE