Title of Invention

A SOLID STATE TAP CHANGER FOR PHASE SHIFTING TRANSFORMER

Abstract A solid state tap-changer for phase shifting transformers with a plurality of tap changing positions (0, 1,2 ) for smooth changeover between tap positions under non-zero current condition, the tap-changer comprising: a plurality of thyristor valves (TH1, TH2 ), each valve consisting of a pair of thyristors in antiparallel combination and assigned for each tap position; at least one polarity changeover switch (SW1) for reversing the polarity from positive (+90°) to negative - (90°); a bypass switch (SW2) with a resistor (R) disposed in series for bypassing the thyristor valves during starting and/or emergency conditions, and a controller (C) for automatic changeover between the upper and lower tap positions, or vice versa, depending on current/power requirement, characterized in that the controller is enabled to maintain the bypass switch in closed position during initial starting; build-up a voltage across the bias resistor with respect to the ground voltage to enable the thyristor valves to trigger and changeover from bypass to tap operation; position the polarity changeover switch (SW1) to one of a position "A" and "B" for selecting a polarity for one of positive and negative phase shifting respectively; release trigger pulses to T+ and T- alternatively to position the thyristor valve (TH4) to a tap position (Tap-0); open the bypass switch to transfer the total current to the thyristor valve (TH4) and allow the phase-shifting transformer to attain the state of readiness for changeover from tap position (Tap-0) to higher tap position; and allow the transformer to changeover to a higher tap position corresponding to instruction from the controller. FIGS. 2 AND 3
Full Text FIELD OF INVENTION:-
The present invention relates generally to a solid state tap
changer specifically meant for phase shifting transformers. In
particular it uses a new triggering strategy of thyristors in a
solid-state tap change for phase shifting transformer.
BACKGROUND ART:-
In electrical power transmission network, the control of power
needs special attention. Fast acting control strategies can
enhance the power handling capabilities of the transmission
lines in the. network.
In order to enhance the power handling capabilities of the
network under different operating condition including emergency
situations, flexible AC transmission system (FACTS) technology
has been introduced. A number of flexible AC transmission system
devices like thyristor controlled series capacitors (TCSC),
controlled shunt reactors (CSR) , static synchronous compensator
(STATCOM), static VAR compensator (SVC) and solid state phase
shifting transformers have been evolved. These devices are fast
acting and they employ solid state switching devices like
thyristors, IGBTs etc.
One of the requirements is to avoid unnecessary circulating
currents in the network to avoid losses. Another requirement is
to quickly adjust the distribution of currents between the lines
to sail through emergency situations in the network to maintain
the required availability of the network to the consumers.

In order to meet this requirement, phase shifting transformers
(PST) with manually operated mechanical taps, positioned at
strategic locations of the network, are presently being
employed.
Phase shifting transformers with mechanical tap changer however
are sluggish and it is difficult to meet the speed of response
demanded by the network under emergency situation.
Thus there was a need for faster acting phase shifting
transformer. However, quick acting fully solid state tap changer
for phase shifting transformers (SSPST) could not be used due to
the commutation problems of thyristors used during changeover of
taps. Therefore hybrid varieties of tap changers are in use.
In order to make the phase shifting transformers fast acting,
the present invention proposes fully solid-state tap changer
specifically for PST, with a novel triggering strategy of
thyristors.
For overcoming any commutation problems of thyristors used in
solid state tap changer during changeover of taps a novel
triggering strategy of the thyristor has been developed for the
present invention to ensure smooth changeover of tap positions.
As desired, the speed of response achieved is fast (less than 10
ms) .
SUMMARY OF THE INVENTION:-
Therefore, one object of the present invention is to provide a

solid state tap changer for phase shifting transformer with a
response time of less than 10 ms.
Another object of the invention is to make the solid-state tap
changer free from commutation problems during changeover. These
and other objects are achieved in the present invention by using
a thyristor based solid state tap changer with a novel
triggering strategy to ensure a smooth changeover during lower
to upper or upper to lower change of the taps.
The thyristors based solid-state tap changer is introduced in
the secondary of the shunt transformer and is provided with a
controller and a mechanical switch. The controller is a digital
unit which can automatically change the taps to the required
level both in upward and downward direction. To accomplish a
smooth changeover, the controller withdraw the trigger pulses to
the thyristor of the present tap and issue the trigger pulses to
the correct thyristor of the upper or lower tap, as dictated by
the triggering strategy. A mechanical switch is used to reverse
the polarity of the voltage applied in series with the line
through primary of the series transformer.
The triggering strategy followed in the present invention is
that the changeover, either lower tap to upper tap or upper to
lower, unlike in the case of voltage regulator where the
commutation problem is encountered when the changeover is
required form lower to upper tap at current zero, is at current
non-zero.
The smooth changeover is proven by simulation in electro
magnetic transient programme (EMTDC).

Thus the present invention provides a solid state tap-changer
specifically for phase shifting transformers with a plurality of
tap changing positions comprising a plurality of thyristor
valves one for each tap position; and a controller for automatic
changeover from a lower to an upper tap, or vice versa,
depending on current/power requirement.
BRIEF DESCRIPTION OF THE ACCOMPAYING DRAWINGS: -
Figure 1 shows a phase shifting transformer with mechanical
tap-changer.
Figure 2 shows a phase shifting transformer with solid-state
tap changer.
Figure 3 shows a phase shifting transformer with solid-state
tap changer in bridge configuration.
Figure 4a shows changeover from Tap:0 to Tap:2 for SSPST scheme
of Figure-2.
Figure 4b shows changeover from Tap:2 to Tap:0 for SSPST scheme
of Figure-2.
Figure 5a shows tap change in bridge type SSPST.
Figure 5b shows tap change in bridge type SSPST.
Figure 5c shows tap change in bridge type SSPST.
Figure 5d shows tap change in bridge type SSPST.

DETAILED DESCRIPTION:-
Figure 1 shows the schematic of phase shifting transformer with
mechanical taps.
Figure 2 shows the schematic diagram of phase shifting
transformer with solid-state tap-changer of the present
invention. In this configuration, a polarity reversal mechanical
switch (SW1) is required.
Figure 3 shows the bridge configuration of a solid state tap
changer 1. It was found that the triggering strategy developed
in the present invention could also be extended to the bridge
circuit based solid state tap changer. In this case the polarity
reversal switch SW1 (in figure 2) is not required.
Figures 4a and 4b show the changeover from tap 0 to tap 2 and
from tap 2 to tap 0 respectively in the scheme of Figure 2. Both
the changeover are of the same polarity.
Figures 5a to 5d show various changeover of taps of both
polarities in the bridge type solid state phase shifting
transformer of figure-3.
The phase shifting transformer of Figure 2 consists of two
transformers: one shunt transformer T1 and one series
transformer T2. The secondary of transformer T1 is equipped with
mechanical taps. The secondary voltage of transformer T1 is
applied to the secondary of transformer T2 and in turn to the
primary of transformer T2, which is connected in series to the
transmission line as shown in Figure two. The voltage across the

secondary winding of T2 is in phase quadrature with the line to
ground voltage of the transmission line. The mechanical tap
arrangement in the secondary of transform T1 also has the
provision to reverse the polarity and thereby make it possible
to add + or - 90° voltage in series with the line voltage
through the secondary transformer T2 . The magnitude of this
voltage is adjustable in steps.
In order to make PST fast acting, a thyristor based solid state-
tap changer (SSTC) is introduced in the secondary of the shunt
transformer T1 as shown in Figure 2 . The digital controller C
issues commands to solid state-tap changer by blocking the
triggering pulses to the conducting thyristors and issuing
trigger pulses to the correct thyristors of required tap, either
up or down, as decided by the controller. The details of
changeover from one tap to another are explained in the next
section. The present invention included the solid state tap
changer as shown in Figure 2. It is economical and simple. The
number of taps considered in figure 2 is 3.
The applicability of the evolved triggering strategy for bridge
configuration (Figure - 3) was also checked in electro magnetic
transient DC. The operation of the solid-state tap changer in
bridge configuration was tested in the field and the performance
is shown in figure 5a (Tap 0 to + 1) , figure-5b (Tap 0 to 1),
figure-5c (Tap + 1 to - 1) and figure 5d (Tap - 1 to + 1).
The solid state tap changer-based phase shifting transformer of
Fig-2 shows four taps in the secondary, namely, 0, 1, 2, and 3.
This can be increased to a larger number depending on the system
requirements. For reversing the polarity, switch SW1 is used.
Each of the anti-parallel connected thyristor valves TH1, TH2,

TH3 and TH4 are used for each tap as shown in fig-2 . Each
thyristor valve denoted by TH1 to TH4 consists of two thyristors
connected in anti-parallel to allow current flow in both
directions. The anti-parallel thyristors are denoted as T+ and T-
depending on their direction.
The SW2 (vacuum contactor) is used to bypass the taps during
initial starting and also during emergency situations to provide
path for the secondary current transformer of T2. A biasing
resistor R in series with the switch SW2 is used to build up the
required voltage to trigger the thyristor valve and bring the
tap changer into the system.
Switch SWl will be in position 'A' for positive phase shifting
and in position 'B' for negative phase shifts.
The apparatus in accordance with the present invention consists
of thyristor valves (TH1, TH2, TH3 and TH4), polarity changeover
switch SWl with position A for positive polarity (+90°) and
position B for negative polarity (-90°) , bypass switch SW2 for
bypassing the thyristor valves during initial starting and
during abnormal operation including emergency situations and
resistor (R) in series with the switch SW2 to develop required
voltage with respect to ground in order to enable the valves to
trigger during changeover from bypass to tap operation. The
scheme is shown in Fig-2.
Depending upon the line current/power requirement, the
controller (C) chooses the right tap position and execute the
tap changeover as per the strategy developed in this invention.
For a changeover from a lower tap to a higher tap (for example
tap 0 to tap 2) ,

the positive thyristor T+ (TH2) is triggered when T+ (TH4) is
conducting. Further trigger pulses to T+ (TH4) is stopped. The
successful changeover is clear from the waveforms shown in Fig-
4a.
A changeover from an upper tap to a lower tap (for example tap 2
to tap 0) is achieved by triggering T+ (TH4) after current in T+
(TH2) is established. Further trigger pulses to T+ (TH2) is
stopped. In this case also the changeover was successful without
commutation problem as shown in Fig-4b. The above changeovers
can also be carried out using the negative thyristors instead of
the positive thyristors as described above.
The strategy was also demonstrated for the bridge type SSTC of
fig-3 using only one winding. The number of taps in this
strategy can be extended by adding similar bridge circuits and
windings in series. The winding voltages can be in the ratio of
1:2:4:8 or 1:3:9:27 to achieve a large number of steps using
fewer windings. The changeover waveforms are shown in Fig-5a to
Fig-5d.
The major problem with thyristor based tap changer is that there
are chances of commutation failure during changeover, if the
changeover is carried out at current zero crossing.
The problem has been overcome in the present invention. Whenever
a changeover is required either from a lower voltage tap to a
higher voltage tap or from a higher voltage tap to a lower
voltage tap, the changeover is carried out at current non-zero.
Changeover to a higher tap is carried out by triggering T+ (n+1)
when T+ (n) is conducting or by triggering T- (n+1) when T- (n)
is conducting.

Whenever a changeover is required from a higher voltage tap to a
lower voltage tap, triggering T+ (n) when T+ (n+1) is conducting
or triggering T- (n)when T- (n+1) is conducting, after current
zero, can ensure smooth changeover. However, the choice of
changeover strategy i.e. positive to positive thyristor or
negative to negative thyristor is determined by the line
voltage. This strategy has been verified first using Electro
magnetic Transient Programme (EMTDC) before implementing in the
prototype Solid State phase shift transformer (SSPST). The
strategy is found to be working satisfactory. This strategy is
suitable up to a line power factor as low as 0.5 (inductive),
which is much below the normal power factor of a transmission
line. Facility to disable changeover is incorporated in the
controller whenever a power factor of below 0.5 is sensed.

WE CLAIM:
1. A solid state tap-changer for phase shifting transformers with a
plurality of tap changing positions (0, 1,2 ) for smooth
changeover between tap positions under non-zero current condition,
the tap-changer comprising:
a plurality of thyristor valves (TH1, TH2 ), each valve consisting
of a pair of thyristors in antiparallel combination and assigned for each
tap position; at least one polarity changeover switch (SW1) for
reversing the polarity from positive (+90°) to negative - (- 90°); a
bypass switch (SW2) with a resistor (R) disposed in series for
bypassing the thyristor valves during starting and/or emergency
conditions, and a controller (C) for automatic changeover between the
upper and lower tap positions, or vice versa, depending on
current/power requirement, characterized in that the controller is
enabled to:

maintain the bypass switch (SW2) in closed position during initial
starting;
build-up a voltage across the bias resistor with respect to the
ground voltage to enable the thyristor valves to trigger and
changeover from bypass to tap operation;
position the polarity changeover switch (SW1) to one of a position
"A" and "B" for selecting a polarity for one of positive and negative
phase shifting respectively;
release trigger pulses to T+ and T- alternatively to position the
thyristor valve (TH4) to a tap position (Tap-0);
open the bypass switch to transfer the total current to the thyristor
valve (TH4) and allow the phase-shifting transformer to attain the
state of readiness for changeover from tap position (Tap-0) to
higher tap position; and
allow the transformer to changeover to a higher tap position
corresponding to instruction from the controller.

2. A solid state tap-changer for phase shifting transformer (PST) as
claimed in claim 1, wherein said thyristors are triggered by said
controller (C) for implementing smooth changeover in non zero
condition.
3. The solid state tap changer as claimed in claim 1, wherein the bypass
switch is closed to develop the required voltage across thyristor valves
to allow the thyristor valve to trigger on receipt of trigger pulses from
said controller during changeover from bypass to tap operation.
4. The solid state tap-changer as claimed in claim 1, wherein said switch
(SW2) is a vacuum contactor.
5. The solid state tap-changer as claimed in claim 1, wherein said solid
state tap changer is constructed in a bridge type configuration with a
single winding or multiple winding of equal or unequal turns.
6. The solid state tap-changer as claimed in claim 1, wherein the winding
voltage is in the ratio of 1:2:4:8.

7. The solid state tap-changer as claimed in claim 1, wherein the winding
voltage is in the ratio of 1:3:9:27.
8. The solid state tap-changer as claimed in claim 1 wherein a pair of
thyristors is connected in antiparallel combination for each section and
marked positive or negative according to flow direction of the current.
9. A method of operation of a solid state tap changer for phase shifting
transformers with a plurality of tap changing positions as claimed in
any of the preceding claims, the method comprising the steps of:
keeping the bypass switch SW2 closed during initial starting,
building up required voltage with respect to ground across bias
resistor (R) in order to enable the valves to trigger during
changeover from bypass to tap operation,
switching the polarity changeover switch (SW1) in position 'A' or
'B' for selection of polarity for positive or negative phase shifting
respectively,

bringing the thyristor valve TH4 (Tap-0) into circuit by releasing
trigger pulses to T+ and T- alternatively,
opening the bypass switch SW2 to transfer full current to the
thyristor valve TH4, while the solid state phase shifting
transformer operating at Tap-0 and ready to changeover to
higher tap,
changing over to any higher tap, if decided by the controller
(C), is carried out by implementing the steps of:
triggering T+ (n+1) when T+ (n) is conducting and stopping
trigger to T+ (n), or triggering T - (n+1) when T - (n) is
conducting and stopping trigger to T - (n) when changing over
from higher voltage to lower voltage tap in current non-zero
condition, and
triggering T+ (n) when T+ (n+1) is conducting and stopping
trigger to T+ (n+1) or triggering T - (n) when T - (n+1)
conducting and stopping trigger to T- (n+1) in current non zero
condition, when changing over from lower voltage tap to higher
voltage tap.

10 A solid state tap-changer for phase shifting transformer substantially
as herein described and illustrated in the accompanying drawings.


A solid state tap-changer for phase shifting transformers with a plurality of tap
changing positions (0, 1,2 ) for smooth changeover between tap positions
under non-zero current condition, the tap-changer comprising: a plurality of
thyristor valves (TH1, TH2 ), each valve consisting of a pair of thyristors in
antiparallel combination and assigned for each tap position; at least one polarity
changeover switch (SW1) for reversing the polarity from positive (+90°) to
negative - (90°); a bypass switch (SW2) with a resistor (R) disposed in series for
bypassing the thyristor valves during starting and/or emergency conditions, and
a controller (C) for automatic changeover between the upper and lower tap
positions, or vice versa, depending on current/power requirement, characterized
in that the controller is enabled to maintain the bypass switch in closed position
during initial starting; build-up a voltage across the bias resistor with respect to
the ground voltage to enable the thyristor valves to trigger and changeover from
bypass to tap operation; position the polarity changeover switch (SW1) to one of
a position "A" and "B" for selecting a polarity for one of positive and negative
phase shifting respectively; release trigger pulses to T+ and T- alternatively to
position the thyristor valve (TH4) to a tap position (Tap-0); open the bypass
switch to transfer the total current to the thyristor valve (TH4) and allow the
phase-shifting transformer to attain the state of readiness for changeover from
tap position (Tap-0) to higher tap position; and allow the transformer to
changeover to a higher tap position corresponding to instruction from the
controller.

FIGS. 2 AND 3

Documents:

88-KOL-2005-(02-02-2012)-CORRESPONDENCE.pdf

88-KOL-2005-(03-11-2011)-ABSTRACT.pdf

88-KOL-2005-(03-11-2011)-AMANDED CLAIMS.pdf

88-KOL-2005-(03-11-2011)-CORRESPONDENCE.pdf

88-KOL-2005-(03-11-2011)-DRAWINGS.pdf

88-KOL-2005-(03-11-2011)-FORM 1.pdf

88-KOL-2005-(03-11-2011)-PA.pdf

88-KOL-2005-ABSTRACT 1.1.pdf

88-kol-2005-abstract.pdf

88-KOL-2005-CLAIMS.pdf

88-KOL-2005-CORRESPONDENCE 1.2.pdf

88-KOL-2005-CORRESPONDENCE-1.1.pdf

88-kol-2005-correspondence.pdf

88-KOL-2005-DESCRIPTION (COMPLETE) 1.1.pdf

88-kol-2005-description (complete).pdf

88-KOL-2005-DRAWINGS 1.1.pdf

88-kol-2005-drawings.pdf

88-KOL-2005-EXAMINATION REPORT REPLY RECIEVED.pdf

88-KOL-2005-EXAMINATION REPORT.pdf

88-KOL-2005-FORM 1 1.1.pdf

88-kol-2005-form 1.pdf

88-KOL-2005-FORM 18 1.1.pdf

88-kol-2005-form 18.pdf

88-KOL-2005-FORM 2 1.1.pdf

88-kol-2005-form 2.pdf

88-KOL-2005-FORM 3 1.1.pdf

88-kol-2005-form 3.pdf

88-KOL-2005-GPA 1.1.pdf

88-kol-2005-gpa.pdf

88-KOL-2005-GRANTED-ABSTRACT.pdf

88-KOL-2005-GRANTED-CLAIMS.pdf

88-KOL-2005-GRANTED-DESCRIPTION (COMPLETE).pdf

88-KOL-2005-GRANTED-FORM 1.pdf

88-KOL-2005-GRANTED-FORM 2.pdf

88-KOL-2005-GRANTED-SPECIFICATION.pdf

88-KOL-2005-OTHERS 1.1.pdf

88-kol-2005-others.pdf

88-KOL-2005-PETITION UNDER RULE 137.pdf

88-KOL-2005-REPLY TO EXAMINATION REPORT 1.1.pdf

88-kol-2005-specification.pdf


Patent Number 252702
Indian Patent Application Number 88/KOL/2005
PG Journal Number 22/2012
Publication Date 01-Jun-2012
Grant Date 28-May-2012
Date of Filing 10-Feb-2005
Name of Patentee BHARAT HEAVY ELECTRICALS LIMITED
Applicant Address PLOT NO. 9/1, D-J BLOCK 3RD FLOOR, KARUNAMOYE, SALT LAKE CITY-700091, INDIA
Inventors:
# Inventor's Name Inventor's Address
1 ARUN ACHALAM MALAIYANDI POST BOX NO. 2606, MYSORE ROAD, BANGALORE,-560026, INDIA
2 KHODAY CHANDANAND DAMODAR POST BOX NO. 2606, MYSORE ROAD, BANGALORE,-560026, INDIA
3 JITHIN SUNDAR SISHTLA VENKATA NATARAJ CORPORATE RESEARCH AND DEVELOPMENT, VIKASNAGAR, HYDERABAD 500093, INDIA
4 DUTTA DIPAK POST BOX NO. 2606, MYSORE ROAD, BANGALORE,-560026, INDIA
PCT International Classification Number H03K17/10
PCT International Application Number N/A
PCT International Filing date
PCT Conventions:
# PCT Application Number Date of Convention Priority Country
1 NA