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