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DEVELOPMENT OF PHASE SHIFTING TRANSFORMERS

FOR SCOTTISH AND SOUTHERN ENERGY plc

Krishnamurthy Vijayan, Waldemar Ziomek, Juan Carlos Garcia Willi Felber

Pauwels Canada Inc., Winnipeg, Canada Felber Engineering, Austria

INTRODUCTION

Reliability of power systems are strengthened by large

interconnected networks. Effective control of power flow

in these networks is essential for improving stability,

enhancing reliability and optimizing capabilities of such

large interconnected systems. The voltage variation

provided by load tap changers in transformers can help to

control only reactive power flow where as Phase Shifting

Transformers (PSTs) are proven solution for control of real

power flow in interconnected systems.

In 2004, Pauwels Canada Inc. successfully developed 240

MVA and 150 MVA PSTs for Scottish and Southern Energy

plc, Scotland. This paper describes briefly the general

principle and basic types of PSTs. It also describes specific

details on the design, manufacture and testing process of

the PSTs delivered to Scottish and Southern Energy plc.

PRINCIPLE OF PSTs

Purpose

PSTs are primarily used to control flow of real power inparallel lines or interconnected systems by introducing a

phase angle shift between the Primary (Source) and

Secondary (Load) terminals. This is achieved by providing

a boost voltage in quadrature (perpendicular) to line

voltage.

Application

Consider the current distribution between two parallel lines

as shown below

When power flows between these two systems, Individual

line loading depends on the impedance of the lines. More

power flows through the path with lower impedance. This

may cause individual line overloading and is usually

contrary to efficient system operation. If it is desired to

balance the current by increasing i1

by Di, and therefore

decreasing the current in line 2 to i2-Di, it is necessary to

introduce a circulating current Di in the system. This can

be done with the introduction of a PST in series with one

of the lines (see Fig. 2). An expression for the circulatingcurrent is

(1)

Where DV is the quadrature boost voltage provided by the

PST. This voltage needs to be almost perpendicular to the

line voltage so that control of real power flow can be

achieved. By using on load tap changers in PSTs DV can

be varied or even reversed so that a full range of power

flow control is possible between parallel lines.

Another important application of PSTs is the control of

power flow between two large independent grids. The flow

of active power between two interconnected systems is

given by (2).

VS

i1 Z1

i2Z2 iTOTAL

VL

Fig. 1 Current distribution in parallel lines

Fig. 2 Current distribution in parallel lines with PST

i1+i Z1

i2-i Z2 iTOTAL

VS VL

VLVPSTPST

1 2

Vi

Z Z

=

+

V1 V2

P

System 1 System 2

Fig. 3 Two independent grids

( )1 2 sinVVP Z

= (2)Therefore, the flow of real power between two systems

can be effectively controlled by varying the phase angle

difference (j).

Equivalent circuit and phasor diagram

Before understanding the phasor diagram it is essential to

understand the following terminologies

Advance phase angle : The phase angle that results when

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the PST's Load terminal voltage leads the Source voltage

terminal. This condition produces an increase in the line's

source to load power flow. Fig. 4 shows the vector diagram

for this case.

Retard phase angle: The phase angles those results when

the Load terminal voltage lags the Source terminal voltage.

This condition produces a decrease of the line's source toload power flow.

Like any transformer, PSTs have inherent impedance which

varies with the phase angle. Generally, PSTs have their

minimum impedance at zero phase shift which increase

with the phase angle. PSTs can be considered as voltage

sources with internal impedances that vary with phase angle

shift.

As can be seen in the vector diagram, under load condition,

the phase angle shift is affected by the PST's internal

impedance. The internal phase shift b of the PST can be

calculated as [2]

(3)

Where cos(f) is the load power factor.

The effective phase shift under load is given by

These relations not only have impact on the transformer

design but also on the selection of load tap changer. The

PSTs rated phase-shift is generally defined at no-load but

as can be seen from the equations, it is not possible to

achieve this phase angle under load in advance position.

Also if the no-load angle is exceeded in retard position,

over excitation will occur in parts of PST.

TYPES OF PSTs

The different types of PSTs can be better understood if

certain terminologies are defined.

Symmetrical PST: Under no-load the magnitude of the

PST's source and load voltages are equal, independently

of the phase angle between them.

Quadrature: It refers to the boost voltage introduced by

the PST being perpendicular to the line voltage at one

terminal.

The basic types of PSTs can be Symmetrical or non-

symmetrical, they can be single core or dual core and they

can be single tank or dual tank.

There are also many other types or design possibilities

which makes the subject of PST development very

challenging.

Types of Phase shifting transformers

Non-symmetrical quadrature type

iLRPST+jXPST

VS VL VLeff

VS

iLXPST

iL

VL iLRPST

VLeff

Load

No Load

Fig. 4 PST equivalent circuit and phasor diagram

( ) ( )

( ) ( )

cos sinarctan

sin cos

L PST PST

L L PST PST

I X R

V I X R

= +

ADVANCE

LOAD NO LOAD =

RETARDLOAD NO LOAD = +

Phase shifting

transformer

Quadrature

symmetricalQuadrature

non symmetrical

Single core Two core

Single Tank Dual tank

Fig. 5 Types of PSTs

Fig. 6 Asymmetrical PST

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Session I Design For Reliability

In this type of PST the quadrature voltage is achieved by

connecting the regulating winding of phase B to the Delta

connection point of phase A and B and so on. The

advantage of this type is that it has no exciting transformer.

The major disadvantage of this type is the tap changer and

regulating winding being directly exposed to system

disturbances. Special measures are required to ensure their

impulse withstand capability.

Symmetrical quadrature type single-core

This is an extension of the non-symmetrical type whichcan be achieved with an additional tap changer.

Two-core symmetrical quadrature PST

This is the standard and classical solution with series and

exciting transformers each built on its own core but with

windings electrically interconnected.

Single tank/Dual tank

These are possible design options for 2 core designs.

Depending on size, voltage class, etc., the Series and

Exciting transformers could be housed in the same tank or

in separate tanks. Obviously housing in separate tanks has

the disadvantages of being more expensive and that the

inter-connection between the units needs to be redone at

site.

PSTs FOR SCOTTISH AND SOUTHERN ENERGY

plc

Application

In the Scottish power system, two double-circuit 275kV

parallel lines run north to south via the east coast to

transport Hydroelectric and Wind energy. One double-circuit 132kV line runs north to south via the west coast

and a single 132 kV line runs north to south via the east

coast for the same purpose. When there is a fault on one of

the 275kV lines the power tries to flow through the 132kV

system since its impedance is smaller under that condition.

This leads to over loading of the 132kV lines.

In this case three PSTs were needed to control the active

(real) power flows in the non faulty lines so that each of

these lines could be used to its rated capacity without over

loading the 132kV lines.

The PSTs will be bypassed using bypass switch undernormal condition. Thus there is possible impulse condition

with both Source and Load terminals connected together.

Ratings and special requirements

Basic ratings

Site1 (Fiddes substation)

1 x 150 MVA, 132 kV class, 50 Hz, 550 kV BIL with

phase angle shift under load of +/-15, using load tap

changer with +/-8 steps

Site2 (Errochty substation)2 x 240MVA, 132 kV class, 50 Hz, 550 kV BIL with phase

angle shift under load of +10/-20, using load tap changer

with +/-8 steps

Special requirements

l Transformers to comply with IEC 76 standards.

Requirements specific to PSTs to be as per IEEE

C57.135 [2], which are the only available standards

for this kind of applications.

Fig. 7 Symmetrical PST

Fig. 8 Quadrature-booster PST

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l Dual core with separate series and exciting

transformer, Quadrature booster designs preferred.

l Dual tank design anticipated due to transportation

limitations. A maximum height of 4.87 m and weight

of 220 T is allowed on the roads approaching the

final site in UK. Single tank designs meeting these

transportation limits are acceptable.l Use of non-linear voltage limiting devices to control

internal voltages to be avoided.

l 115% chop wave test required

l Special impulse test required with both Source and

Load terminals connected together to simulate the

PST bypass condition.

l The ratio of zero sequence to positive sequence

impedance to be smaller than 9 (Z0/ZP< 9)

l 110% over voltage and frequency variation of 47.5

to 52 Hz

Design approach

l A classical design concept of a two-core symmetrical

Quadrature booster was applied

l An extremely compact design concept was applied

in order to house both, series and exciting units in a

single tank while meeting size and weight

transportation requirements. Such compactness was

achieved by using tall and slim design concept with

low impedance. Special care was taken to assure

withstand capability against short circuits.

l Series transformer and exciting transformer are built

on separate 3 leg cores with electrical interconnection

between them

l A tertiary delta winding was introduced in the

exciting transformer in order to meet the zero

sequence impedance requirement

l Use of a special dual multi-layer design on the series

transformer delta winding to achieve controlled

dielectric stresses during lightning impulse condition.

l Center-fed exciting winding so that the high voltageconnection with the series winding is made at the

center, away from core ground potential. The two

transformer heights and dimensions were closely

matched to facilitate the interconnection inside the

tank

l Fully interleaved exciting winding

l Special 2 layer tap winding with leads connected to

high speed resistance type tap changer.

Design details

Basic design

The winding disposition on the two cores is indicated

below.

Impedance calculation and loss control

FEM based leakage flux analyses were used to determine

size and position of tank shunts and clamp shunts.

Impedance calculations were performed with the help of

field calculation tool based on Bessel functions.

a) Series transformer b) Exciting transformer

Dielectric design

Impulse calculations using LC network based software

were performed for each of the following conditions: a)

Impulse on S (or L) terminal with the other terminal

grounded b) Impulse on S & L terminals connected

(bypassed PST). For condition b) the center connection

point voltage was estimated to rise to 1034kV during the

550 kV full wave impulse test.

Electrostatic stress analyses were performed for all critical

locations and special contouring and insulation components

were used wherever necessary.

DELTA

SERIES

LTC2

LTC1

TERTIARY

EXCITER

Series unit Exciting unit

Fig. 9 PST windings layout

Fig. 10 Electromagnetic FEM simulations.

a) b)

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Design reviews were conducted at every stage of the

development process.

Manufacturing

l All windings and core were built to close dimensional

tolerance

l Pre-fit of the units inside tank was necessary to make

measurements for final interconnection at center.

l After final housing of the units, special contoured

high voltage inter connection was done by skilled

person inside the tank. Connections inspected and

approved by Quality Assurance and Engineering by

physical inspection inside tank.

l Special manufacturing process evolved by buildinga prototype for the dual multi-layer type delta

winding.

Photograph below shows the two units assembled and

housed inside the common tank.

Testing

Special features of some of the factory tests are highlighted

here

Phase angle measurement

Phase angles were measured according to proposed method

in IEEE C57.135 [2]. The phase shift was also recorded inoscilogram, see figure below.

Dielectric tests

Lightning impulse

Low voltage recurrent surge impulse tests were done with

both series and excitation units electrically connected to

verify impulse voltage calculations. Some of these

measurements were done with both assemblies outside the

tank and inside the tank. The measured values were close

to calculated values.

Fig. 11 Typical stress plot: Series transformer main gap

at highest stress region

Fig. 12 Series and exciting transformers assembled

inside tank

Fig. 13 S and L wave shapes when S leads L

S

N

L

S

N

L

Fig. 14 Lightning impulse test on S (or L) terminal and

on S+L terminals (bypassed PST)

Fig. 15 Low voltage recurrent surge impulse-test.

Impulse response at the exciter T connection point

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Besides impulse test on S and L terminal, special lightning

impulse test including 115% chop wave condition were

applied to (S+L) connected together. The calculated and

RSG test results at the T point for 1.2/50s full wave

impulse applied to (S+L) is shown in Fig. 15. The

maximum values by test are lower than calculated values.

The difference is attributed to a) Measurement for this case

was done in air outside tank and b) Our calculations do

not consider damping.

Induced voltage test

For this test the unit was energized through a set of

temporary test bushings brought out of the tap windings.

Both 1-phase and 3-phase induced tests were conducted

as per IEC standard [3].

Loss measurement

In PST the loss distribution in series and exciting units

vary with the phase angle. Typical loss distribution of the

measured values for 240MVA unit is shown in table.

Temperature rise test

With single tank design, temperature rise test for obtaining

oil rise values do not pose any difficulties. But measurement

of gradients of each of the windings needs special methods.

Gradient of each winding including the LTC winding was

measured either by direct or indirect measurements.

1. Resistance of Series winding of ST can be measured

directly between S & L ( R+R= 2R in the diagram

below)

2. For the Exciting winding it will be an indirect

measurement as follows

S & L terminals to be connected & resistance to neutral

was measured. This will give (R/2 + Re) ohms. Since R is

known, Re can be calculated.

3. Resistance of Delta winding & LTC winding are

measured using temporary test bushings connected

to LTC.

Test frequency

The phase shifters were designed for frequency of 50Hz

to suit the Scottish system. However the test frequency at

the Pauwels Canadian plant was 60Hz and hence

conversion factors were applied to the tests results in orderto get guaranteed design parameters at 50Hz. Useful

guidance was taken from IEEE Tutorial for the conversion

factors [3].

Test summary

All the tests on both the ratings were very successful. All

three units passed all tests first time right.

l The measured phase angles were within 1% of the

guaranteed values.

l The maximum P.D level was 103 pC.

l The maximum hot spot temperature was 68.50C.l The special zero sequence impedance ratio

requirement of Zo-open /Z positive was fully met.

(Photo of unit under test)

CONCLUSIONS

As can be seen the design and development of Phase

shifting transformers are unique. It was a challenge to meet

Table 1 Summary of tested losses on 240 MVA PST

Series Exciting Total

unit unit

No load loss kW

Zero phase shift 0 26.8 26.8

Maximum phase shift 21.2 26.8 48

Load loss kW

Zero phase shif 224.3 0 224.3


Total loss kW

Zero phase shif 224.3 26.8 251.1


Series winding on ST

Excitingwinding Re []

R []R []

S L

N

Fig. 16 T network used to calculate the exciting winding

resistance

Fig. 17 240 MVA phase shifter on site.

Courtesy of Scottish and Southern Energy plc

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the transport limits with the single-tank design. Considering

various advantages it was worth the efforts since this

approach gives the best satisfaction to the ultimate

customer. All the three units have reached site and they

are presently undergoing field trials.

REFERENCES

1. W. Seitlinger, "Phase Shifting Transformers,

Discussion of Specific Characteristics", CIGRE

Session 1998

2. IEEE Std C57.135-2001 "IEEE Guide for the

Application, Specification, and Testing of Phase-

Shifting Transformers", IEEE Power Engineering

Society, 2002

3. E.G. teNyenhuis, R.S. Girgis, "50Hz to 60Hz

conversion factors for transformer performance

parameters". IEEE/PES Transformer Committee

Tutorial session April 16,2002

4. IEC standard 60076 IEC: 2000 "Power transformers".

5. Axel Kramer "On-Load Tap-changers for Power

transformers by (MR publication)"

6. Scottish Hydro-Electric Transmission Ltd"Specification for Quadrature booster transformers".

Enquiry No:C33995/01

ACKNOWLEDGEMENTS

The authors express their sincere thanks to Scottish and

Southern Energy plc., for providing the system details and

site photographs.

The authors also acknowledge the support provided by the

management and colleagues at Pauwels Canada Inc.,

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