FIELD OF THE INVENTION

[0001] The present invention relates to controlled switching applications in electrical power systems. More specifically, the present invention relates to controlling operation of switching devices for controlled switching applications in such power systems.

BACKGROUND OF THE INVENTION

[0002] Controlled switching techniques are well known in electrical power systems for limiting voltage or current surges, protecting equipment such as power transformers, capacitor banks etc. Switching devices such as circuit breakers, disconnectors etc. are used for performing the controlled switching functions.

[0003] Typically, voltage and current measurements are utilized to optimize the operations of the switching device such as, but not limited to, controlled closing and opening. Controlled closing operation is one of the most significant requirement for controlled switching devices. The closing operation needs to be optimized based on voltage and current measurements. This also requires using various device characteristics such as Rate of Decrease of Dielectric Strength (RDDS) of the device, mechanical closing times, electrical scatter, mechanical scatter etc. These characteristics, specifically the electrical and mechanical scatter have effect on the making voltages amongst others. The complete details of the effect of such characteristics such as effect of electrical and mechanical scatter can be found in CIGRE WG 13.07 (1998).

[0004] Due to device characteristics such as electrical and mechanical scatter, the making voltages can vary between two voltage levels. Accordingly, there is a limitation in the angles at which closing can be done. The limits on the current making angles are generally determined based on the encountered gap voltage, RDDS, electrical scatter and mechanical scatter of the breaker. Trying to energize the circuit beyond these angles can lead to pre-intended or post- intended energization and can lead to generation of unwanted transients.

[0005] Prior art techniques for controlled switching try to limit the switching transients by utilizing measurements of voltages and currents at different terminals. Specifically, gap voltages are determined from voltages measured at source and load sides. When measurements from source and load sides are utilized, there can be an impact in the determinations due to measurements errors. [0006] Measurements and synchronization errors can easily occur in systems due to instrument limitations (e.g. hardware), field effects (e.g. magnetic fields) etc. Also, in different power systems, voltage measurements may not be available at both source and load sides, specifically at the load side. For example, there can be certain source load configurations where the load voltages are not possible to measure due to limitations of the electrical equipment. Consider a case of a star-delta transformer as a load, or a three-phase reactor as a load. In such a case, extra sensors (e.g. VTs) need to be provided at the transformer or reactor side to have the measurements at the load side. This may need three or six extra sensors. Usually, such extra sensors are not available, and it may not be feasible to have extra sensors provided at the load side, due to cost / space constraints.

[0007] Additionally, in loads where there is coupling between two or more phases, e.g. due to connection between two or more windings, reactors etc. the effect of such coupling needs to be taken, even when measurements are available. Taking the example of a three-phase reactor connected at the load side. Here, there is an effect on gap voltages in different phases due to coupling between the reactors. This coupling affect adds to complexity in determining optimal operating requirements when measurements are available only at the source side.

[0008] In order to have optimized controlled switching for such power system configurations where there is coupling between two or more phases at the load side, and where the voltage measurements are available only at the source side, there is a need for an improved method and device which can be used for controlled switching, while minimizing the possibility of switching transients during closing.

SUMMARY OF THE INVENTION

[0009] An aspect of the invention relates to a method for controlling operation of a switching device in a power system. The power system comprises a power source connected with a load in a three-phase configuration. At the load side, there is coupling between at least two phases of the load (coupled load). In an embodiment, the load is a three-phase reactor with a neutral grounding reactor, wherein there is coupling between the two or more phases of the load based on the connection between two or more reactors of the three-phase reactor. [0010] The method comprises obtaining measurements of voltages for each phase of the three phases at a source side (or source end). Source side refers to measurements at a charging or a power supply side (or end) of the switching device.

[0011] The method further comprises estimating a closing angle of a gap voltage for each pole of the switching device according to a switching criteria, a switching sequence and the voltages measured in each phase at the source side. The switching criteria and the switching sequence may be based on the configuration of the load.

[0012] The switching criteria defines a phase angle of a gap voltage at which the pole or connection between the power source and the load is to be closed (in one or more phases). Here, the gap voltage refers to the voltages across the terminals of contacts (poles) of the switching device.

[0013] The switching sequence defines a sequence for closing the connection (pole) between the power source and the load in the three phases. In the switching sequence the connection in a lead phase is closed first, followed by closing the connection in a first following phase, and further followed by closing the connection in a second following phase.

[0014] The closing angle of the gap voltage for the pole in the lead phase is estimated based on phase angles of the voltages measured in the lead phase at the source side. In an embodiment, the closing angle of the gap voltage for the pole in the lead phase is estimated from a closing angle of the source side voltage in the lead phase.

[0015] The closing angle of the pole in the first following phase is estimated based on phase angles of the voltages measured in the first following phase at the source side, a phase difference between the voltages measured in the lead phase and the first following phase at the source side, and at least one of a magnitude of gap voltage in the first following phase or a multiplication factor associated with the gap voltage in the first following phase. The magnitude of the gap voltage or the multiplication factor may be based on configuration of the load.

[0016] In an embodiment, the closing angle of the gap voltage for the pole in the first following phase is estimated based on a closing angle of the source side voltage in the first following phase, and a phase difference between the gap voltage and a phase-to-ground voltage for the pole in the first following phase. This phase difference may be estimated based on:

• a phase shift of the source side phase-to-ground voltage of the switching device for the first following phase pole; • a phase shift of the source side phase-to-ground voltage of the switching device for the lead phase pole;

• a root mean square (RMS) (or other suitable magnitude) value of the gap voltage across contacts (terminals) of the switching device for the pole in the first following phase

• a root mean square (RMS) (or other suitable magnitude) value of the voltage in the first following phase at the source side.

[0017] The closing angle of the pole in the second following phase is determined based on phase angles of the voltages measured in the second following phase at the source side. In an embodiment, the closing angle of the gap voltage for the pole in the second following phase is estimated from a closing angle of the source side voltage in the second following phase.

[0018] In an embodiment, the step of estimating the closing angles comprises transforming closing angles of the voltages at the source side to the closing angles of the gap voltages. Here, the gap voltages are in phase-to-phase and the voltages at the source-side are phase-to-ground. Further, the step comprises optimizing the closing angles of the gap voltages and mapping back the closing angles of the gap voltages to the closing angles at the source side.

[0019] The method further comprises generating a signal for operation of the switching device based on the closing angles estimated for each pole of the switching device. The signal may be generated such that each pole of the switching device operates in accordance with the switching criteria and the switching sequence, to minimize switching transients based on the estimated closing angle for the pole. This signal may be generated according to the mapping of the closing angles of the gap voltages to the closing angles of the voltages at the source side. [0020] The method may be implemented with a device of the power system, which has the measurements in the three phases at the source side. The information of the switching criteria, and the switching sequence are also available at the device. The source side measurements can be obtained with one or more measurement equipment provided at the source side. In accordance with an embodiment, the device is a relay which is operatively coupled with a circuit breaker (switching device). Also, the measurement equipment can be potential transformers, and the relay receives voltage measurements from the potential transformers.

[0021] In accordance with an aspect, the device comprises a measurement unit, an estimator, and a control unit, which perform one or more steps, or parts thereof, of the method. The device can be used for controlling operation of the switching device, where the load is a coupled load.

BRIEF DESCRIPTION OF DRAWINGS

[0022] The subject matter of the invention will be explained in more detail in the following text with reference to exemplary embodiments which are illustrated in the attached drawings in which:

[0023] Fig. 1 is a single line diagram of a power system, in accordance with various embodiments of the present invention;

[0024] Fig. 2 is a three-phase source load configuration of the power system, in accordance with an embodiment of the invention;

[0025] Fig. 3 shows measurement equipment and devices for controlled switching in the power system, in accordance with an embodiment of the invention;

[0026] Fig. 4 is a flowchart of a method for controlling operation of a switching device, in accordance with an embodiment of the invention;

[0027] Fig. 5 is a three-phase source load configuration of the power system, in accordance with various embodiments of the invention;

[0028] Fig. 6 is a flowchart of a step of the method for controlling operation of the switching device, in accordance with an embodiment of the invention;

[0029] Fig. 7 shows relationship between gap voltage and source voltage, in accordance with various embodiments of the invention;

[0030] Figs. 8 and 9 show normalization for RDDS > 1 pu/rad and RDDS < 1 pu/rad, in accordance with different embodiments of the invention;

[0031] Fig. 10 is a block diagram of a device for controlling operation of the switching device, in accordance with an embodiment of the invention;

[0032] Fig. 11 shows a power system with a solid grounded load, in accordance with an embodiment of the invention;

[0033] Fig. 12 shows a plot of various voltages for the power system of Fig. 11, in accordance with an embodiment of the invention;

[0034] Fig. 13 shows a plot of various voltages for a power system with a star grounded load with neutral reactor, in accordance with an embodiment of the invention; [0035] Fig. 14 shows a power system with an ungrounded or delta load, in accordance with an embodiment of the invention;

[0036] Fig. 15 shows a plot of various voltages for the power system of Fig. 14, in accordance with an embodiment of the invention; and

[0037] Fig. 16 shows a plot of various voltages for a power system with a star grounded coupled reactor, in accordance with an embodiment of the invention.

DETAILED DESCRIPTION

[0038] The invention relates to power systems such as power transmission or distribution systems, where there are switching devices involved in power system applications such as controlled switching. As is generally known, controlled switching is used to eliminate or minimize harmful electrical transients, by performing a planned switching of loads such as capacitors, reactors, power transformers or other power system equipment. In such applications, switching devices such as circuit breakers, are typically used for connecting the power supply with the load (or for charging or energizing the load). A controller, relay or Intelligent Electronic Device (IED) can be used for controlling the operation of the circuit breaker and switching of the load.

[0039] The operation of the circuit breaker (or breaker) is controlled by optimizing opening and closing operations at the breaker. As an example, opening and closing times of the poles of the breaker are controlled. There are different power system configurations due to different types of loads and load configurations. Also, measurement equipment may be available only at specific locations. This can have significant impact on the closing and opening time estimations. In certain power systems, voltage measurements are available only at the source side. Also, as per the load and its connection / configuration, there is coupling between different phases at the load side, which impacts the controlled switching.

[0040] The present invention provides optimized controlled switching for such power system configurations where the voltage measurements are available only at the source side, and where there is coupling between two or more phases at the load side. The present invention will be described taking a power system (100) shown in Fig. 1 as an example.

[0041] The power system shown in Fig. 1, includes a power source (102), for providing power to a load (104). The power source can be a three-phase alternating source, and the load is a coupled load. For example, the power source can be an AC grid, while the load can include capacitor banks, shunt reactors, power transformers, three-limbed reactors. The load can be magnetically and / or electrically coupled. For example, the load can be inductive, capacitive, resistive or a combination of thereof, with different design and / or connection configuration. [0042] The power source is connected to the load through a switching device (106). The switching device can be a breaker, a disconnector or a combination thereof, like disconnecting circuit breakers or other similar switching device based on power electronics technology. The switching device has one or more poles, each of which is operated (i.e. connected or disconnected) for energizing or de-energizing a corresponding phase in which the load is connected.

[0043] In accordance with different embodiments, the load is a coupled load. In other words, there is coupling between two or more phases at the load side due to the load and its connection configuration. Consider the embodiment of Fig. 2. As shown, a three-phase reactor with neutral grounding reactor is a load (204). In this case, there are three reactors (L _p ), one for each phase, and a neutral grounding reactor (L _n ) connecting the neutral point to the ground. As a result of this configuration, whenever a voltage is provided in any one phase, a proportion of the voltage is induced in another phase. For example, when the power source (202) is connected to the load in the lead phase of Fig. 2, a voltage is induced in the first following phase at the load side due to the connection between the reactors at the load side.

[0044] Other examples of loads where there is coupling between two or more phases at the load side include, but are not limited to, a solid grounded load, a star grounded load with neutral resistor or reactor, an ungrounded / delta load, and a star grounded coupled reactor. This coupling as per the load and its connection / configuration has effect on the gap voltages and needs to be considered to accurately estimate the closing times for each pole in each phase. Due to the coupling at the load side, after the closing of the pole in the lead phase, the gap voltage across the terminals of the switching device in the following phase(s) changes. This affects the closing time estimations.

[0045] In order to estimate the closing times, measurements of voltages (voltage measurements) at the source side are required. The measurements are performed with measurement equipment provided for performing measurements at different line locations. For example, the measurement equipment can include a potential transformer, a sensor-based measurement equipment (e.g. Rogowski coils, non-conventional instrument transformers etc.) and/or the like, which provides a signal corresponding to voltage, as sensed from the line. For example, a voltage transformer provides single/multiple phase voltage signal.

[0046] Consider the embodiment shown in Fig. 3, where there is a potential transformer (302) at the source side. The potential transformer measures voltages in each phase at the source side. It should be noted that a measurement equipment may be provided for each line / phase for performing measurements associated with the corresponding line / phase. Accordingly, there would be three potential transformers for the three lines providing the power at the source side. Alternately, there can be three wirings fed for such measurements.

[0047] The measurements obtained with the measurement equipment are provided to a device (304). For example, a relay or an intelligent electronic device (IED) receives a signal(s) from the measurement equipment and obtains the measurements from the signal. Alternately, the measurement equipment publishes the measurements over a bus (e.g. process bus), and the IED (e.g. subscribed to receive data from such bus) receives the measurements. It is to be noted that the voltage signals can be processed in one or more steps, including pre-filtering as needed. Such processing can be done either by using wiring and / or filtering circuitry and the output provided to the device (304). Alternately, the signals can be processed internally at the device to obtain the required measurements of required electrical parameters such as voltages in different phases. [0048] The operation of the switching device such as 106, can be optimally controlled using a device such as 304, which has the source side voltage measurements. Moving now to Fig. 4, which is a flowchart of a method for controlling operation of a switching device (e.g. 106) in a power system (e.g. 100), in accordance with various embodiments of the present invention. [0049] At 402, the method comprises obtaining measurements of voltages for each phase of the three phases at a source side. Consider a general three phase source-load configuration shown in Fig. 5. Here, V _sa , V _sb and V _sc represents the phase to ground voltage of the respective phases on the source side (end) of the switching device. In the example, the switching device is a breaker. Further, V _la , V _lb and V _lc represents the phase to ground voltage of the respective phases on the load side (end) of the breaker.

[0050] In accordance with such power system, voltages V _sa , V _sb and V _sc are measured with the measurement equipment (e.g. 302). The measurements are available with the device (e.g. 304) for estimating the closing angles. The load side voltages V _la , V _lb and V _lc are not available and needs to be determined while estimating the closing angles.

[0051] At 404, the method comprises estimating a closing angle for each pole of the switching device according to a switching criteria, a switching sequence and the voltages measured in each phase at the source side. The switching criteria and the switching sequence may be based on the configuration of the load.

[0052] The switching criteria is determined considering the characteristics of the switching device such as, but not limited to, electrical and mechanical scatter. The switching criteria defines a phase angle of a gap voltage at which the connection (pole) between the power source and the load is to be closed. Here, the gap voltage refers to the voltages across the terminals of the switching device. The closing angles are generally estimated such that the closing happens at a peak of gap voltage, a zero crossing etc. For example, the closing angle should be such that the closing happens at the peak of the gap voltage in each phase when the load is inductive in nature (e.g. an inductor, a transformer etc.). Taking another example, the closing angle should be such that the closing happens at the zero crossing of the gap voltage in each phase when the load is capacitive in nature (e.g. a capacitor).

[0053] The switching sequence defines a sequence for closing the connection between the power source and the load in the three phases. In the switching sequence the connection in a lead phase is closed first, followed by closing the connection in a first following phase, and further followed by closing the connection in a second following phase. As an example, the pole in phase A can be closed first, then the pole in phase C can be closed and eventually the pole in phase B can be closed. Taking another example, the connection in phase B can be closed first, followed by the connections in phases A and C respectively. Such sequence depends on the connections between the power source and the load, and on the load type, and is defined beforehand.

[0054] In order to determine the closing angles (or energizing angles), the source voltage parameters (e.g. angle) needs to be transformed into gap voltage parameters. The need for transformation and potential optimization is explained taking the power system of Fig. 5 as example. In the configuration of Fig. 5, gap voltages across the breaker contacts (switching device contacts) can be evaluated from equation (1), (2) and (3) for phases A, B and C respectively. V _CBa = V _sa - V _la (1)

V _CBb = V _sb - V _lb (2)

V _CBc = V _sc - V _lc (3)

[0055] Now, re-defining the whole network in per phase basis, the relationship between the gap voltage and the source voltage can be visualized from Fig. 7, where:

• δ = Phase difference between the gap voltage and the phase-to-ground voltage;

• β = Intended energizing (closing) angle with respect to phase-to-ground voltage; and

• = Intended energizing (closing) angle mapped onto the gap voltage of the breaker. [0056] From Fig. 7, it can be inferred that: = β - δ (4)

[0057] Typically, the energizing angle with respect to phase-to-ground voltage (β) is available. However, physically in a system the breaker switches on the gap voltage. Therefore, transforming the phase-to-ground energizing angle (β) to the gap voltage energizing angle (α) is a necessary step to achieve an optimized switching. Additionally, all the scatter optimization needs to be performed on the gap voltage energizing angle (α).

[0058] Accordingly, as shown in Fig. 6, estimating the closing angles can involve transforming phase-to-ground energizing angle to gap voltage energizing angle at 602. In an embodiment, to transform the energizing angles, a transformation function that maps phase-to- ground energizing angle (β) to the gap voltage energizing angle (α) is estimated.

[0059] The transformation function that maps phase-to-ground energizing angle (β) to the gap voltage energizing angle (α) requires the information about breaker switching sequence and the magnitude of gap voltage. As given in equation (4), the transformation function depends on the phase difference between the gap voltage and the phase-to-ground voltage (δ).

[0060] Based on the switching sequence, the three poles of the breaker can be referred as:

• Lead phase pole (notated with a subscript of _lp )

• First following phase pole (notated with a subscript of _ffp )

• Second following phase pole (notated with a subscript of _sfp )

[0061] The superscript of the above notations can be filled with the appropriate parameter notation. For example, V _{s_ffp} represents the source voltage of the first following phase pole. [0062] Lead phase pole:

[0063] In case of the lead phase pole, which means that the phase difference between the phase-to-ground voltage and gap voltage (δ _lp ) is zero. This is because in a de- energized load, the gap voltage of the lead phase breaker pole ( V _{CB_lp} ) is equal to the source voltage of the breaker pole (V _{s_lp} ).

[0064] First following phase pole:

[0065] In case of first following phase pole, after the energization of the lead phase, due to the electrical coupling with the lead phase, there is an induced voltage at the load end of the breaker pole. As a result of the connections at the load side, the phase shift of the induced voltage at the load end of the breaker pole is same as the phase shift of the energized lead phase. Due to the electrical or magnetic coupling of the phases, the induced voltage will be a scaled version of the energized lead phase voltage without any phase difference. The gap voltage equation of the first following phase pole can be re-defined by equation (5).

V _{CB_ffp} = V _{s_ffp} - V _{l_ffp} (5)

[0066] In equation 5,

• V _{CB_ffp} = Gap voltage across breaker contacts for the first following phase breaker pole;

• V _s _ _ffp ⁼ Source end phase to ground voltage of the breaker for the first following phase pole; and

• V _{l_ffp} = Load end phase to ground voltage of the breaker for the first following phase pole.

[0067] The bold lettered components above represent vector quantity (i.e. magnitude and angle).

[0068] Considering that the phase of the induced voltage at the load end of the breaker pole is the same as the phase of the energized lead phase and Fig. 7, equation (5) can be re-defined in polar form, which is given by equation (6).

[0069] In equation 6, • θ _ffp = Phase shift of the source end phase to ground voltage of the breaker for the first following phase pole;

• θ _lp = Phase shift of the source end phase to ground voltage of the breaker for the lead phase pole;

• δ _ffp = Phase difference between the gap voltage and the phase to ground voltage for the first following phase pole;

• V _{CB_ffp} ⁼ RMS of the gap voltage across breaker contacts for the first following phase pole breaker;

• V _{CB_ff} ⁼ RMS of the source end phase to ground voltage of the breaker for the first following phase pole; and

• V _{l_ffp} = RMS of the load end phase to ground voltage of the breaker for the first following phase pole breaker.

[0070] With the known switching sequence and the gap voltage of each breaker pole, the known parameters in the equation are:

• θ _ffp

• θ _lp

• V _{CB_ffp}

• V _{s_ffp}

[0071] Comparing and solving real part of the equation (6), we get equation (7).

[0072] From equation (7), V _{l_ffp} can be solved as shown in equation (8).

[0073] With the calculated value of V _{l_ffp} , equation (6) can be solved to obtain δ _ffp as shown in equation (9). 0074] Therefore, the gap voltage energizing angle can be obtained from the phase- to-ground energizing angle (β _ffp ) from equation (10). (10)

[0075] Second following phase pole:

[0076] In case of the second following phase pole, due to the energization of the lead phase and the first following phase, the induced voltage at the load end of the breaker second following phase pole is either zero or in phase with the source end of the breaker. This is because in case of every balanced system, the summation of all three phase voltages or all three fluxes will equate to zero. Thereby, ensuring that if two of the breaker poles are already energized, the induced voltage at the load end of the breaker second following phase pole is pre-determined. So mathematically if the voltage at the load end of the breaker is zero or in phase with source end, the gap voltage is also in phase with source voltage. Considering this, it can be inferred that,

[0077] Thus, the closing angle for the pole in the lead phase is determined based on phase angles of the voltages measured in the lead phase at the source side, i.e. β _lp .

[0078] Similarly, the closing angle for the pole in the first following phase is determined based on phase angles of the voltages measured in the first following phase at the source side (β _{ff p} ), a phase difference between the voltages measured in the lead phase and the first following phase at the source side, and at least one of a magnitude of gap voltage in the first following phase or a multiplication factor associated with the gap voltage in the first following phase (refer δ _ffp - equation 9).

[0079] The closing angle for the pole in the second following phase is determined based on phase angles of the voltages measured in the second following phase at the source side, i- ^e - β _sfp .

[0080] As can be noted, the magnitude of the gap voltage or the multiplication factor may be based on configuration of the load.

[0081] The gap voltage index has been used in the equations to determine the result. As seen in the equations above, the term such as V _{CB_ffp} is the gap voltage magnitude in the first following phase. This is evaluated as: (gap voltage index * measured source voltage magnitude).

[0082] In the above, the measured source voltage magnitude is the base value.

[0083] For different loads, the gap voltage index is different. For example, if the load is star grounded, the gap voltage index is 1. Taking another example, if the load is star ungrounded or delta, the gap voltage index is 1.732.

[0084] A similar approach can be used for neutral reactor grounded reactor load. The impedance ratio of neutral grounding reactor and load reactor is taken as 0.3, the gap voltage index is 1.13.

[0085] It is to be noted that in place of the gap voltage index, the magnitude of the gap voltage may be directly used, and suitable changes to the equations above can be made to arrive at the result.

[0086] The gap voltage energizing angles (closing angles) as obtained with the transformation (602), needs to be optimized and mapped back to phase-to-ground. Accordingly, as shown in Fig. 6, estimating the closing angles can involve optimizing the gap voltage energizing angles and mapping the energizing angles to phase-to-ground source voltage angles at 604. Optimum switching angle while closing depends on the RODS and both the scatter values (electrical and mechanical). There are two methodologies for optimizing switching angles depending whether the RODS is less than lpu/rad or more than lpu/rad, as given in CIGRE WG 13.07 (1998). Here, base value is taken as the peak of gap voltage.

[0087] The methodologies for normalizing are explained below.

[0088] RDDS>lpu/rad

[0089] Normalization for RDDS > lpu/rad is shown in Fig. 8. Fig. 7 shows the gap voltage across circuit breaker ( V _CB ) and phase to ground voltage (V _s ). The phase difference between V _CB and V _s i.e. δ has been evaluated as explained in relation to transformation step above.

[0090] For zero voltage switching, the switching angle is shifted to the right so that the voltage levels at points A and B becomes equal to minimize the effect of scatters. Therefore, the optimized angle for zero switching is . Similarly, for voltage peak switching the switching angle is shifted to the left of 90° so that the voltage levels at A’ and B’ become equal. The optimized angle for voltage peak switching is a _u . If the switching angle is between α _L and α _u no normalization is possible because of the rising slope.

[0091] RDDS<lnu/rad [0092] Normalization for RDDS < lpu/rad is shown in Fig. 9. Fig. 7 shows the gap voltage across circuit breaker ( V _CB ) and phase to ground voltage (V _s ). The phase difference between V _CB and V _s i.e. δ has been evaluated in previous section.

[0093] For zero voltage switching, the switching angle is shifted to the right of 0° so that the leftmost RDDS scatter line recaches to the point on the gap voltage wave where the slope of the gap voltage wave is equal to its slope(A). The corresponding optimized zero switching angle is α _L . Similarly, for voltage peak switching, switching angle is shifted towards left of 90° so that the right most RDDS scatter line reaches to the point on the gap voltage wave where the slope of the gap voltage wave is equal to its slope(A’). The corresponding optimized zero switching angle is α _u .

[0094] After finding the lower and upper limits of switching angles i.e. α _L and α _u respectively, these must be mapped to the phase to ground voltage. The mapping relation is given by equation (13). β _L = α _L + δ (13)

[0095] In the above,

• δ = phase difference between gap voltage and phase to ground voltage; and

• β _L = Switching angle on phase to ground voltage corresponding to α _L( shown in Figs. 8 and 9).

[0096] Similarly, β _u can be evaluated from equation (14). βu = α _u + δ (14)

[0097] The final phase to ground mapped angle can be used to report to an operating personnel or for any subsequent adaptive controlled switching.

[0098] The closing angles are used in the switching operations. Accordingly, at 406 (refer Fig. 4), the method comprises generating a signal for operation of the switching device based on the closing angles estimated for each pole of the switching device. The signal(s) may be generated such that each pole of the switching device operates in accordance with the switching criteria and the switching sequence, to minimize switching transients based on the estimated closing angle for the pole. This signal may be generated with the closing angles (β _L , β _u ) as estimated above. [0099] The method may be implemented with a device of the power system, such as device 304 which has the measurements in the three phases at the source side. These measurements can be obtained with one or more measurement equipment at the source side. In accordance with an embodiment, the device is a relay which is operatively coupled with a circuit breaker (e.g. as shown in Fig. 3). Also, the measurement equipment can be potential transformers, and the relay receives voltage measurements from the potential transformers.

[0100] In accordance with an embodiment, the device comprises a plurality of components for performing the method or steps thereof. The components or modules of the device can be implemented with hardware such as a processor, I/O’s etc., and configured to execute various steps of the method. In the embodiment shown in Fig. 10, the device comprises a measurement unit (1002), an estimator (1004), a control unit (1006) and an output interface (1008). The measurement unit is configured to obtain the measurements from the measurement equipment. The measurement unit can be configured to perform certain signal processing to remove noise. In case the voltage signals are processed at another device, the measurement unit can be configured to receive the processed signals/measurements for performing the various steps of the methods.

[0101] The estimator is configured to estimate the closing angles for the switching device for the closing operation. The control unit is configured to utilize the energizing angles for generating the signal for the switching device. The output interface is configured to provide the signal(s) as output for example output commands to the switching to utilize the information for subsequent switching. The optional memory (1010) can store the required information of previous operations and the information needed for performing the different steps. For example, the memory can store measurements, information of switching instants etc.

[0102] The above modules can be implemented in device (304), which can be a relay, an intelligent electronic device or other power system devices for controlled switching.

[0103] The above method and device can enable controlled switching at a switching device with only voltage measurements at source side, and where there is a coupled load by estimating closing angles according to the load and its configuration. Accordingly, the method and device disclosed herein assists to avoid any erroneous switching due to statistical behavior of breaker characteristics. The method establishes an upper and lower limit of gap voltage angles for switching. Beyond these limits, there are chances that the switching device might switch in the previous half cycle or the next half cycle of the gap voltage and at a very different angle. [0104] There is dynamic and adaptive alignment to any major or minor change of the source bus voltage. As the source side voltage is a real time measured quantity which is being used for the evaluation, so any fluctuation in the source side (or bus) voltage can be accommodated in the method.

[0105] The following description provides results of simulation for different load and its configurations.

[0106] Solid grounded load

[0107] In a solid grounded load, each phase is independent of each other. The switching sequence considered is a-c-b. The gap voltage across the circuit breaker will always be lpu (per unit) in this configuration. After closing of phase A, the load side voltage V _lc of the circuit breaker will be 0 as the neutral point is grounded. From equation (9), δ will be 120° which is same as the phase angle of source voltage V _sc .

[0108] Fig. 11 shows the circuit diagram of a star grounded load along with source side and load side voltages. The source voltage taken here is 100 V peak. After closing of phase A, the gap voltages across phase B (VSB-VLB) and phase C (VSC-VLC) are shown in Fig. 12. The gap voltage across phase C (VSC-VLC) is 120° apart from phase A source voltage which is same as the results obtained from equation (9).

[0109] Star grounded load with neutral reactor

[0110] Here a case has been taken where grounding has been done through a reactor referred as neutral grounding reactor. The impedance ratio of neutral grounding reactor and load reactor is taken as 0.3. The switching sequence considered is a-c-b.

[0111] After closing of lead phase(A) the gap voltage across the phase C (first following phase) is 1.13 pu which is the function of impedance ratio. Here, the base value is taken as the peak of source end voltage. Putting V _{CB_ffp} = 1.13, θ _ffp = 120° and θ _lp = 0° in equation (8) we get,

V _{l_ffp} = 0.2306 pu.

[0112] After evaluating V _{l_ffp} , from equation (9) we get δ = 130.15°.

[0113] The same has been simulated and results are as shown in Fig. 13. From Fig. 13, it can be observed that after closing of lead phase, the magnitude of load end phase to ground voltage of the breaker for the first following phase pole ( V _{l_ffp} ) is 0.23074 pu which is equal to the calculated value. Further, δ is coming as 130.28° which approximately matches with the calculated value.

[0114] Ungrounded/Delta load

[0115] The switching sequence considered is ac-b. Fig. 14 shows the circuit diagram of an ungrounded load. After the energization of lead and first following phase, the phase B can be closed after 90°. The gap voltage across B(VSB-VLB) phase is 1.5 pu and it is in phase with phase B source voltage (VSB). Here, the base value is taken as the peak of source end voltage. Same can be observed from Fig. 15.

[0116] Once the phase difference between phase to ground voltage and gap voltage is known, the normalization of RDDS that is evaluated on the gap voltage can be easily mapped to phase to ground voltage.

[0117] Star grounded coupled reactor

[0118] After closing of the first phase, due to the coupling effect of first phase on to the other two phases there will be some induced voltage on the other phases. Therefore, the gap voltage across the other two phases will be the difference of phase voltage and induced voltage. Therefore, by putting V _{CB_ffp} = 0.866 pu (considering equal flux distribution in other de- energized phases) in equation (9) we get δ = 90°, which can be also verified from simulation as shown in Fig. 16.