CIGRÉ Regional South-East European Conference
October 10th - 12th 2012, Hotel Hilton, Sibiu, Romania, (RSEEC 2012)
RSEEC2012-A106
Phase Shifting Transformers
Guidance for Planning and Operation
Herbert Schinnerl
Siemens AG Österreich – Transformers Weiz
Austria
SUMMARY
A phase shifting transformer (PST) is a tool to control the active power flow through specific lines in a
complex power transmission network.
Power flow in power systems needs to be controlled, due to technical reasons (e.g. line overloading) and
economical reasons (e.g. committed power transfer at network node). The need is increasing because of
liberalization effects. This can be achieved with a PST.
This equipment is still not very well known, even with the people, who are in the standard transformers
business. In this paper the main differences will be described as well as what has to be considered in writing
specifications, for comparing offers and operating conditions.
The aim of this paper is to gain a better understanding of the topic, provide simple examples and help in
discussions between planning, operating and manufacturing.
It will also be discussed that the rated phase angle is defined under no-load conditions. The phase angle under
load decreases if the power flow should be increased and the phase angle under load increases if the power
flow should be reduced. As the effectiveness is higher in this retard operation this gives a certain planning
optimization for inserting a PST on an optimized point.
Simple formulas and graphs are given, to examine which phase angles can be achieved with throughput power
and number of steps in one unit of the classical two core solution, which is used for high system voltages and
phase angles above 15 to 20 degrees. Special considerations will be given to overload conditions in retard
operation mode. In many cases the available load tap changers determine the limits.
Remarkable overexcitation of the magnetic circuit can arise, when the power flow is reduced. Such demands
should be clearly defined in order to create economical solutions and evaluate comparable bids.
Characteristic differences like impedance, protection, regulating winding and reliability of single core and
dual core solutions will be discussed, as well as agreed suggestions with users for minimizing costs in the
special lightning impulse test with connected load and source terminals.
KEYWORDS
Phase Shifting - Transformer - Guidance
Herbert.schinnerl@siemens.com
1 Purpose and function of a phase shifting transformer
In principle, a PST creates a phase shift between the primary (source) and the secondary (load) side. Usually,
this phase shift can be varied under load, but sometimes it can be made advance (to increase the power flow)
or retard (to reduce the power flow).
A PST changes the effective phase displacement between the input voltage and the output voltage of a
transmission line (adding a voltage perpendicular to the line voltage), thus controlling the amount of real
power that can flow in the line. The reason is that the line impedance is mainly inductive. Standard substation
transformers are inserting a voltage in phase or opposite to the line voltage (changing magnitude of the
voltage), therefore they only have an impact on the reactive power flow.
2 Operational considerations
Variation of load voltage and phase angle due to load current (ohmic components neglected)
jXPST*iL
~
VS
XPST
VL0
iL
VL
Figure 1 – Voltage across PST
In addition to a similar variation of the load side voltage of a standard transformer caused by the impedance a
PST enfaces also a variation of the phase angle under load. According IEC Standard 62032 clause 4.8.1 [2]
the rated phase angle is defined under no-load conditions. If nothing is mentioned otherwise the standard
applies.
The experience shows that for new projects this must always be agreed at the beginning of discussions
because it often causes a lack of a common understanding due to different views of the involved parties.
Where some only specify the phase angle under load, so that the manufacturers can determine the optimal noload phase angle and impedance, others define a required no-load phase angle and impedance because this is
often used in their system studies software for improving their power flows.
Figure 1 shows that the phase angle under load decreases in case of advance operation and increases in case of
retard operation.
The design rating of a PST is approximately proportional to the maximum no-load phase angle, as there is a
higher effectiveness when the PST is used to reduce the power flow (retard operation). This gives a certain
planning optimization to insert a PST at a location for more of these conditions rather than a location for more
advance operation conditions.
2
Figure 2 – the formula behind is an extract from [1]
For a standard transformer the highest in-phase voltage variation (voltage drop) is determined by the lowest
load power factor. For a PST the biggest impact on the change of the phase angle under load is given by the
highest load power factor. This means that the worst case for an advance operation to achieve a certain phase
angle under load is given by a load power factor of 1.0. Otherwise for retard operation lower phase angles
under load will be reached if the load power factor is lower as 1.0. There is space for optimization, if the
detailed loading conditions like advance or retard operation or power factor are known.
A typical curve of achievable phase angles under load is given in the Figure 3 with a no-load phase angle of
40 degrees.
Effect of load on effective phase angle
pf 0.8, 100% In
pf 1.0, 100% In
pf -0.8, 100% In
no - load
45
phase angle (degree)
30
15
0
-15
-30
-45
-60
-16
-12
-8
-4
0
4
8
12
16
tap position
Figure 3
The phasor diagrams clearly show that for retard operation the voltage across the PST increases, which has an
important effect on different design parameters.
3 Tap Changer Application
3.1 OLTC selection
Depending on core design (single-core, dual-core), impedance and winding arrangement the step voltage
increases at retard operation. This is described more detailed in [4].
3
Figure 4 – extract from [4]
If the design is on the limit to be doubled in case of necessary active parts and OLTC’s or not, there are
always discussions due to overload requirements derived from standard transformer applications without step
voltage increase.
For the dimensioning of the OLTC transition resistors, the maximum step voltage is of interest not only at
rated current, but also the step voltage on the maximum overload current must be considered.
The result shows, that if retard operation is required, the load point 2 on Figure 4 under required overload to
be switched must be inside the limit of the rated step voltage at twice the rated through-current of the OLTC.
This means that due to increase of the step voltage in retard operation no overload with twice of the rated
current is possible.
Additional required safety margins, like rated current of the OLTC should be 1.2 times the maximum rated
current of the transformer, can lead to a double of units in the high end application range.
Unlike standard transformers, the overloading of a PST influences the rated values of the transformer, which
is described in IEC Standard 62032 clause 4.8.4 [2] and IEC Standard 60214-2 clause 6.2.11 [3], but often not
reflected in RFQ specifications.
It must be mentioned, that in a specification it should be clearly stated, if a full rated throughput capability for
power flow in either direction and in any tap position is required.
In IEC Standard 62032 clause 4.8.1 [2], it is mentioned that in the retard position the no-load phase angle
should not be exceeded. If nothing is mentioned, the standard applies and the unit must not be designed for
operation at all tap positions in retard mode.
In Figure 3 with achievable phase angles under load, the operation must be limited down to tap position -11 of
total -16 tap positions. The downsizing is higher, if the no-load phase angle is lower than 40 degrees given in
the example.
3.2 Achievable throughput power as a function of no-load phase angle
In general, the regulating winding and therefore the LTC must be designed for the maximum design rating of
the PST. The maximum regulating capacity (switching capacity per step times the number of steps) is limited
by the capacity of available tap changers.
The Power which is needed to reach a certain displacement in phase angle is:
P alpha = 2 * P thr * sin alpha/2
(Formula 1)
The Power is proportional to the throughput power and almost proportional to the phase angle.
P alpha design rating of the series winding respective phase shifting power (MVA)
P thr throughput power (MVA)
alpha no-load phase angle (degree)
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3.2.1 Calculation of the maximum possible throughput power at a given phase angle:
Assumption:
Approximate 5500kVA step capacity of the LTC (includes retard operation – if only advance operation
6000kVA maximum is possible)
P alpha = 5.5 * k * 3 (phases) * 32 (steps)
k….correction factor (function of max. phase angle)
P alpha must be switched by the on-load tap changer
Example: 48 degrees
P alpha = 5.5 * 0.914 * 3 * 32 = 483 MVA
P thr = P alpha / (2*sin alpha/2) = 483/(2*sin alpha/2) = 593MVA
Figure 5 shows the maximum available throughput power as function of the no-load phase angle.
Difference to a transformer:
For a throughput power of 600MVA and an in-phase voltage regulation of +/-15% only 0.15 x 600 = 90
MVA have to be switched by the on-load tap changer.
Throughput power versus no-load phase angle
step capacity 5000 - 6000 kVA, +/32 steps
90
80
no-load phase angle (degree)
70
60
50
5500kVA
5000kVA
6000kVA
40
30
20
10
0
200
400
600
800
1000
1200
1400
1600
1800
throughput power (MVA)
Figure 5
4 Magnetic circuit
For retard operation at the maximum phase angle tap position the voltage across the series winding increases.
This is different to a standard transformer in operation with ohmic resp. ohmic-inductive load. A higher
voltage across the series windings results in overfluxing of parts of the magnetic circuit.
Such a retard operation can be compared to a pure capacitive load of a standard transformer, where there is
also an increase of voltage in this capacitive loaded winding.
This can be seen at transformers for SVC applications (Static Var Compensator).
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Figure 6
Figure 7
Figure 8
Figure 9
Example 1:
500 MVA, 400kV, +/- 20 degrees, impedance 12% at neutral tap and 18% at maximum tap position
P alpha = 2 * 500 * sin 20/2 = 173.6 MVA
Example 2:
Same as Example 1 but +/- 40 degrees
P alpha = 2 * 500 * sin 40/2 = 342 MVA
Winding Arrangement and Connections:
S1
L1 S2
L2 S3
Phasor Diagram:
S1
a
L1
L3
ϕ
a
b
c
S3
L2
c
L3
Primary
S2
b
Secondary (regulating)
Figure 10
If a winding next to the core gets a higher voltage, this winding is linked with the flux in the core, which leads
to an overfluxing of the core leg.
It can be imagined, if the series winding is next to the core, that there is an overfluxing of the core leg in the
region of the increasing of the phase angle under load for retard operation.
From Figure 3 with pf 1.0 and an impedance of 18% there is an increase of 10.2 degrees.
For example 1 there is an overfluxing of approximate 51% (30.2/20.0 degrees)
For example 2 it is only 26% (50.2/40 degrees) but at all a very high value.
Out of this series windings should not arranged as the innermost winding next to the core.
With the most common arrangement for a dual core solution in Figure 10 and the excited winding of the
series transformer next to the core, there is a minimum overfluxing at rated current in the region of the short
circuit impedance of the exciting unit based on its equivalent two winding rating (P alpha).
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Short circuit impedance of exciting transformer is approximately 18-12=6% @500MVA (30MVAr)
Example 1: 17.3% @173.6 MVA (100*30/173.6)
Example 2: 8.8% @342 MVA (100*30/342)
This occurs at rated current, if there is an overexcitation and increases for overload conditions.
Specifications with following requirements can be seen.
Flux Density: At no-load, the maximum flux density in the cores shall not exceed 1.68 Tesla at 100% of
nominal rated voltage.
From the above mentioned examples, under rated conditions flux densities of 1.97/1.83 Tesla occurs in parts
of the magnetic circuit, if bidders only offer according to these minimum requirements.
At exceptional operating conditions (overload and/or overexcitation) saturation of the magnetic circuit will be
reached, which should be avoided in any case.
What can be seen too is that there is an impact on the noise level due to overexcitation levels in different parts
of the core in case of retard operation. No recommendations exist in the relevant standards.
One possible way is, if retard operation is required, to use a higher value of excitation, at least for the series
unit, for the guaranteed noise level measurement.
The increasing depends on a lot of parameters which influences the overexcitation (value of no-load phase
angle, impedance, winding arrangement). Therefore it is difficult to generate a common rule.
5 Parallel operation – out of step operation
For a parallel operation of two phase-angle regulating transformers, the one out of step operation cannot be
avoided because for a very short time the two on-load tap changers do not switch exact at the same time, even
if the switching operation will be initiated at the same time for both OLTC’s. A good rough circulating
current can be easy calculated with the following formula.
Ic = voltage per step / (2 * impedance)
Ic
Voltage per step
Impedance
(Formular 2)
circulating current (ampere)
Voltage per step (volts)
impedance of one transformer (ohms per phase)
Example:
P
Transformer capacity 500 MVA
U
voltage line to line 400 kV
Z
impedance at zero tap position 0.12 per unit (equal to 12 % at 500 MVA, minimum impedance of
the transformer at neutral tap position)
+/- 22 steps
impedance (ohms per phase) = Z * U² / P
impedance (ohms per phase) = 0.12 * 400² / 500 = 38.4
For a no-load phase-angle of 40 degrees the voltage per step at tap position 1 is 7640 V
Ic = 7640 / (2*38.4) = 99.5 A
The rated current at 500 MVA and 400 kV is 721.7 A
Circulating current Ic is 13.8 % of the rated current.
If the switching capacity of the OLTC under rated condition is almost near the maximum step capacity, this
has to be checked by the OLTC manufacturer.
In addition, it can be seen that a parallel operation of two single core phase shifting transformers cannot be
allowed without additional impedances due to too high circulating currents near the neutral tap position,
where the impedance of the transformers is practically zero.
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6 Single core design
The basic question, whether it should be a single core or a dual core design, is still difficult to answer.
However, there is no doubt that the single core design is cheaper, and it has significantly lower losses. In the
following, some characteristic differences of these two designs are described.
Impedance: The dual core solution has a given impedance at zero tap position (e.g. 12%), while the single
core impedance at neutral tap is zero. If additional impedance is necessary depends on the fault capacity of the
system. The symmetrical short circuit of the system should be < 27 kA at that point for a design without
additional impedance. Please note, that without additional impedance, no parallel operation of two such single
core phase shifting transformers is allowed. The circulating current in an out of step operation near the neutral
tap position is too high.
Protection: In the single core design, for a full protection several internal CT's (at leads in the tank) are
required which cannot be located on the bushings, see Figure 11. This CT’s are at the level of the line voltage.
With the increase of voltage, the necessary space increases and for each CT an additional high voltage
insulation system to earth exists. Therefore this solution is not recommended above 145kV.
Difficulties with buried CT’s are located near the transformer core, which saturates under certain service
conditions like faults or switching, are published. In the meantime, several measures can be taken to avoid
any false tripping.
Some utilities do not accept any buried CT inside a tank. In such case no single core design can be offered,
except a protection scheme without internal CT’s is accepted.
OLTC, regulating winding: Further, as the tap winding is directly connected to the line terminals, it can be
necessary for lightning impulse protection to use internal arresters across the tap winding, especially if
switched capacitors are near the phase shifter. A service condition has to be enfaced, where only one or a few
steps of the regulating winding are inserted between the load and the source terminal. This can lead in case of
switching conditions to excessive stresses of this inserted taps, if they are not protected with internal arresters.
A lot of customers do not accept internal arresters. For these customers it is recommended to specify dual core
solutions.
If all these measures have been taken, it is still cheaper than the dual core design even though it adds to the
cost of the single core design. Another important point of view is the expected reliability. It is difficult to put
numbers to reliability issues, but there are some facts which cannot be ignored. It is obvious that in the single
core design there are much more components on a high voltage level (tap changer, tap leads, CT's, etc.) than
in the dual core design. Experience tells that the most likely components to fail are tap leads at high voltage
level. Of course, the design can be made appropriate, considering all possible voltage stresses and according
to well proven design rules, but nevertheless there remains a higher probability of failure because of the great
number of components stressed to the limit.
So it can be seen, if it is a critical project, where the cost of failure is huge, the suggestion is to stick to a well
proven design concept, which is the dual core design, and also to choose a well proven reliable manufacturer.
Differential protection scheme for single core design (example)
L
S
L
S
L3
S
87-1
87-2
Figure 11
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7 Special tests for PST’s
7.1 Special lightning impulse test
The reason to require a special lightning impulse test with connected source and load terminals is described in
[1]. IEC Standard 62032 clause 10.2.1. [2] mentions that if a bypass switch is installed this test may be
specified by the user.
For a PST according Figure 10 at the connecting point of series and exciting winding (crossover) peak
voltages of approximately 140% of the applied impulse level can arise. In PST’s of the 400kV and 500kV
class this leads to special insulation solutions with a tremendous effect on costs and increased losses.
High costs can be avoided if this test will be made with a lower voltage level but a lower safety margin to the
residual voltage of an arrester very close to the line terminals.
This was sometimes agreed with users because the probability is very low that such an event can happen
when the time with a closed bypass is minimized to a shortest possible extent.
In each case if nothing is mentioned in a specification no such test is included.
Standard LI test:
S2
Special LI test:
L2
Figure 13
S2
L2
Figure 14
7.2 Temperature rise test and load loss test
For a classical PST according Figure 10 with two tanks it is recommended that the temperature rise and load
loss test should be done with the two tanks connected together, like the service condition, because in the
series unit there exists a longitudinal part with ampere-turn balance between the primary and secondary circuit
but also a transversal part, which is specific to a PST, with ampere-turn balance between the two sections of
the series winding.
This means that there exists a transversal stray flux component which causes additional losses that are not
covered by a separate measurement on each unit.
In conclusion, it can be seen that there are important differences to standard substation transformers and not
all of them are well known and not for all special PST conditions recommendations exist.
Therefore a good cooperation between all involved parties from the planning engineers to the operation
peoples is necessary to get the required performance, a good understanding of all demands from the tender
stage to the finals design stage, testing and operating of the units.
BIBLIOGRAPHY
[1] Walter Seitlinger "Phase Shifting Transformers – Discussion of Specific Characteristics" (CIGRÉ Session 1998,
Paris, 30th August – 5th September 1998, paper 12-306)
[2] "Guide for the application, specification and testing of phase-shifting transformers", (IEEE Standard C57.135,
First edition, 2001, IEC Standard 62032, First Edition, 2005)
[3] "Tap-changers – Part 2: Application Guide", (IEC Standard 60214-2 , First edition, 2004)
[4] A. Krämer, D. Dohnal, B. Herrmann „Special Considerations on the Selection of On-load Tap-changers for
Phase-shifting Transformers” (A2-205 CIGRE 2006)
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