FIELD OF THE INVENTION

The present invention relates generally to controlled switching in power networks and more particularly to an intelligent electronic device used for controlled switching of a magnetically coupled reactor.

BACKGROUND OF THE INVENTION

Controlled switching of circuit breakers is a convenient and economical way to minimize electrical transients in a power system. The controlled switching of circuit breakers is carried out depending upon the behavior of current flowing through or voltage across the breaker contacts during switching operation. In order to regulate system voltage to its rated value, shunt reactors are installed on transmission lines or cables to absorb the excess reactive power. Any random de- energization of a magnetically coupled reactor can cause re-ignition thereby adversely affecting the life of the circuit breaker. Thus controlled switching of the circuit breaker is important for the reactors.

Controlled switching of uncoupled reactors is a well-known practice in utilities. In a reactor (coupled or uncoupled), the circuit breaker contact is generally switched such that there is a condition of reactor de-energization at the zero-crossing of current flowing through the circuit breaker. In case of a coupled reactor, the phase relationship between the phase voltage and the phase current also depends upon the mutual coupling between the phases influenced by the structure of the magnetic core. The magnetic core structure is either three limb or five limb based on the application requirement. Due to the mutual coupling between the phases, the phase relationship between the phase voltage and the phase current cannot be accurately predicted.

It is desired for controlled switching to take into account the shift in load current for each phase in a multiphase system resulting from mutual coupling between the phases to avoid transients and accordingly ascertain the instant for controlled switching. Hence, for a coupled reactor, there is a need to perform controlled switching that takes into account the impact of opening of the circuit breaker associated with a first phase onto the other phase load currents and also to take into account the impact of closing of the circuit breaker associated with a first phase on the other phase load currents.

SUMMARY

The above-mentioned shortcomings, disadvantages and problems are addressed herein which will be understood by reading and understanding the following specification.

In one aspect, the present invention provides a method of operating one or more circuit breakers with an intelligent electronic device (IED), wherein each phase of a multiphase power source is connected to a multiphase coupled reactor through a circuit breaker. The multiphase coupled reactor is having windings for each phase wound on a magnetic core. The IED is connected to at least one voltage transformer and at least one current transformer for receiving measurement values of voltage and current. The method comprising, the IED: receiving measured values of voltage and current flowing in each phase of the windings of the multiphase coupled reactor; computing values of self-inductance and values of mutual inductance between pairs of the windings based on the measured values of current and voltage; estimating an instant of time for opening and closing (switching) of a second circuit breaker on opening (switching) of a first circuit breaker as a result of at least one of: (a) computed values of self-inductance and value of mutual inductance between pairs of the windings, and (b) change in effect of mutual inductance in the windings of the multiphase coupled reactor; and operating the second circuit breaker at the estimated instant of time. In an embodiment of the method mentioned herein above, the opening of the first circuit breaker is carried out to have a condition of de-energization at around zero-crossing of the current flowing in the first circuit breaker.

In an embodiment, estimating an instant of time for opening of a second circuit breaker on opening of a first circuit breaker comprises estimation of an arcing time. In an embodiment, the estimated arcing time is utilized to open the second circuit breaker with reference to a zero-crossing of the current flowing through the second circuit breaker.

In an embodiment, the multiphase power source is a three-phase power source. In an embodiment, the multiphase reactor is having at least one of a three limb core structure and a five limb core structure.

In another aspect, the present invention provides an Intelligent Electronic Device (IED) for operating one or more circuit breakers connected to a multiphase power source. A multiphase coupled reactor is connected to the multiphase power source using a circuit breaker from the one or more circuit breakers. The multiphase coupled reactor is having windings for each phase wound on a magnetic core. The IED is connected to at least one voltage transformer and at least one current transformer for receiving measurement values of voltage and current, the IED configured to: receive measured values of voltage and current flowing in each phase of the windings of the multiphase coupled reactor; compute or receive values of self-inductance of the windings individually and values of mutual inductance between pairs of the windings based on the measured values of current and voltage; estimate an instant of time for opening of a second circuit breaker on opening of a first circuit breaker as a result of change in effect of self- inductance and mutual inductance in the windings of the multiphase coupled reactor; and operate the second circuit breaker at the estimated instant of time.

In an embodiment of the IED mentioned herein above, the IED receives the computed values from at least one of a remote device and a remote server based locally or in a cloud network.

BRIEF DESCRIPTION OF DRAWINGS

The appended drawings illustrate exemplary embodiments as disclosed herein, and are not to be considered limiting in scope. In the drawings:

Figure 1 shows a single line diagram of a multiphase power system with an Intelligent Electronic Device;

Figure 2a shows three limb coupled reactor;

Figure 2b shows a five limb coupled reactor; Figure 3 shows comparison between current waveforms along with a voltage waveform in a three phase power system;

Figure 4a shows a circuit representing a three-phase star connected magnetically coupled reactor with solid grounding; Figure 4b shows the circuit representing the three-phase star connected magnetically coupled reactor after opening of the circuit breaker associated with phase R;

Figure 4c shows the circuit representing the three-phase star connected magnetically coupled reactor after opening of the circuit breakers associated with phase R and phase B; and

Figure 5 shows a flow diagram of the method used for operating the circuit breakers associated with each phase of the multiphase system.

DETAILED DESCRIPTION

The present invention provides a method for operating circuit breakers connected between a multiphase power source and a multiphase magnetically coupled reactor. In order to regulate system voltage to its rated value, shunt reactors (coupled reactors) are installed on transmission lines or cables to absorb the excess reactive power. Any random de-energization of a reactor can cause re-ignition thereby adversely affecting the life of the circuit breaker. Thus controlled switching of the breaker is quite important for the reactors.

Figure 1 shows a single line diagram of a multiphase power system 100 (three-phase) in accordance to an embodiment of the present invention. Figure 1 shows an Intelligent Electronic Device (IED) 110 connected to a circuit breaker 120, a multiphase power source 130 supplying power to a reactor being a coupled reactor 140 through the circuit breaker 120. The IED 110 controls the operation of the circuit breaker 120 and hence the IED 110 controls the energization and de-energization of the coupled reactor 140. As shown in Figure 1, the IED 110 is connected to a voltage transformer 150 and receives measured values of voltage at the source from the voltage transformer 150. The IED 110 receives measured values of current from a current transformer 160 connected to the coupled reactor 140.

Figure 2a shows an exemplary construction (structure) of coupled reactor with three limbs with an inter-connecting top and bottom yoke. As shown in Figure 2a, in a three limb coupled reactor there is a middle limb 200 and two side limbs 200a and 200b, and the reactance path between the middle limb 200 and the side limbs (200a and 200b) is same on both the sides. Assuming that the number of turns in each limb windings are same, the self-inductance of three limbs are same. Due to equal reactance path for both the side limbs, the mutual inductance of the side limbs to the middle limb are also expected to be the same. Similarly, Figure 2b shows a five limb coupled reactor wherein the self-inductances and mutual inductances can be assumed to be the same between the side limbs and the middle limb.

Figure 3 shows exemplary current waveforms in the various phases referenced with respect to a voltage waveform measured for a phase (Phase R) in the exemplary power system connecting a source to coupled reactor banks. In coupled reactor banks, behavior of current and voltage differs from that of an uncoupled reactor bank. Application of controlled switching requires to take into account the effects of mutual coupling between the multiple phases which leads to a modification in current wave shape and also influences the value of the arcing time (time interval between parting of the arcing contacts of a circuit breaker and extension of the arc), as will be observed in Figure 3.

Figure 3 specifically shows current waveforms for each phase (Phase R, Phase Y and Phase B) and voltage waveform for a single phase (Phase R) in the three-phase power system. A comparison is shown in Figure 3 between phase currents of the circuit breaker at a closed state and phase currents in the circuit breaker during opening (switching) of the contacts of the circuit breaker. In an exemplary scenario with three phases R, Y and B, the voltage waveform for phase R 300 is shown and corresponding to voltage waveform for phase R the current waveform for phase R 310, the current waveform for phase Y 320, and the current waveform of phase B 330 are also shown. As shown in Figure 3, 310 is a depiction of the electric current in the breaker for phase R during breaker closed or steady state condition, 320 is the electric current in the breaker for phase Y during breaker closed or steady state condition and 330 is the electric current through the breaker for phase B during breaker closed or steady state condition. The breaker corresponding to phase R is shown to be opened to have a de-energized condition at an instant 340 and causes current waveforms in the other phases to get affected because of opening of the circuit breaker in the phase R. As can be seen from the figures, opening of the circuit breaker for the phase R can distort the current waveforms in the phase Y. Similarly, for the phase B, opening of the circuit breaker in the phase R and in the phase Y, both, would affect the electric current values (waveform) in phase B. The figure provides the current waveforms for Phase Y and phase B depicting the distortions (380, 390a, 390b) together with the continuous (otherwise regular when circuit breaker remains in closed condition) sinusoidal waveforms of the electric currents in phase Y and phase B to show the influence of opening of a circuit breaker in a first phase on the electric current flowing through the other phases. As a result of these distortions due to opening of circuit breaker in phase R (first phase), the breaker corresponding to phase Y (second phase) needs to be opened such that the condition of de-energization is at an instant 350 (new current zero instance in view of the distortion 380) and the breaker corresponding to phase B needs to be opened at instance 360.

Breaker current in phase R (first phase) after opening of the circuit breaker (first circuit breaker) is of zero value indicated as 370. The electric current in Phase Y is affected due to opening of the circuit breaker connected to phase R (current zero in the winding of the reactor and thereby no effect of mutual coupling due to the zero current in the winding) and due to this effect in the electric current flowing in the phase Y, the IED operating the circuit breaker needs to estimate and account the shift in the zero crossing of the electric current value (de-energization) and accordingly provide command for opening of the corresponding circuit breaker at the instant 350 for the circuit breaker connected to phase Y in view of the distortion in electric current values indicated as 380. Thus, the effect of opening of a circuit breaker (first circuit breaker) in a first phase on the electric current flowing in the other phases is illustrated together with the effect on the switching time for the other circuit breaker (second circuit breaker).

Similarly the effect on the breaker current in Phase B after opening of the circuit breakers in phase R at the instant 340 and phase Y at the instant 350 are indicated as 390a and 390b respectively. Thus, the current flowing through the circuit breaker associated with phase B (one phase) gets modified after the opening of the circuit breaker corresponding to phase R and phase Y (other phase/phases). The change in the current waveform is expected to also affect arcing time.

The change in arcing time can be estimated in view of the phasor relationship between the voltage and the current in a coupled reactor and influenced by the mutual coupling between the phases and the self-inductance of the individual phase. Correctly estimating the values of mutual and self-inductance and using it to determine the effect on the current waveforms and also in modified arcing time is a key to a successful switching strategy of the circuit breakers. As mentioned before, for successful controlled switching, the current zero condition (de- energization) is desired and the instant of switching can also be determined considering the estimated arcing time with reference to expected zero crossing (de-energization). In an exemplary case, the time instant for opening of the circuit breakers can be estimated as the time instant before the estimated zero crossing by the value of estimated/computed arcing time. Further, the calculated self and mutual inductances can also be used as a monitoring parameter to deduce the health of the coupled reactor. The ageing and the mechanical properties of the core can be predicted observing the rate of change of the inductances. The IED concerned with the switching of the circuit breakers connected to the coupled reactor can monitor the values of mutual and self-inductance by either computing these values from the measurement of voltage and current signals in the three phase electric network or can also receive these computed values from any other electronic device or a remote server (locally or in a cloud network) communicating with the IED associated with switching of the circuit breakers. The subsequent paragraphs provide for a technique to calculate the value of mutual and self-inductances based on measurements that can be made by an IED.

Figure 4a shows a three-phase star connected coupled reactor with solid grounding (resistance neglected). Referring to Figure 4a, for calculating the values of mutual inductance and self- inductance for the three phases R, Y and B the following mathematical approach is taken. Mathematically, a coupled reactor can be represented by the Equation (1), (2) and (3), where:

Wherein: and are the source voltages for phase R, Y and B respectively, andi _1, i ₂ and

i ₃ are values of load currents flowing in each winding of the coupled reactor. Where:

If the resistances are neglected and the equations 1, 2 and 3

are re-written as follows:

The vector form representation of these equations is given as below:

On the other hand, the eqn. (7) - (9) can be further simplified considering the physical structure of the reactor.

The core structure of a three limb coupled reactor (as shown in Figure 2a), consists of 3 separate limbs with an inter-connecting top and bottom yoke. The reactance path between the middle limb 200 and the side limbs (200a and 200b) is same on both the sides. Assuming that the number of turns in each limb windings are same, the self-inductance of three limbs are same. Due to equal reactance path for both the side limbs, the mutual inductance of the side limbs to the middle limb are also same.

Similarly for a 5 limb couple reactor (as shown in Error! Reference source not found.b), the self-inductances are same and the mutual inductance of the side limbs to the middle limb are also same. Mathematically,

And

Thus, the equations further reduce to:

Considering the eqns. (10)-(11), if the source voltages and the load currents flowing through the system are known, the values of self-inductance (L) and mutual

inductances ( M and M ₁₃ ) can be calculated.

The magnitude and the phase angle of source voltage and the currents flowing in the breaker can be estimated in a steady state closed condition. An electronic relay or IED (Intelligent electronic device) which receives the discrete samples/values of the source voltages and the load currents, can calculate the RMS value and phase angle of the signal by processing the input samples. Theoretically, if the estimated source voltage can be assumed as (considering the system to be balanced):

Similarly, the load currents can be assumed as:

So fitting these values into eqns. (10)-(12), the equations can be re-written as:

On converting the polar quantities into Cartesian quantities and considering the imaginary (or real) part of the equations, Eqns. (10)-(12) can be represented as:

As will be known to a person skilled in the art, the above equation (13) may be solved to calculate the values of L, M and M ₁₃ . The equation may be solved using Cramer’s rule to calculate the value of self-inductance and mutual coupling coefficients.

The above method can be implemented in a computing device such as an intelligent electronic device (IED). The IED (also referred as a numerical relay) is used to calculate load specifications (for example, values of self and mutual inductances) for controlled switching application.

The calculated self and mutual inductance is applied to achieve the controlled switching of the coupled reactor and also for the monitoring purpose. The IED computes values of self- inductance and mutual inductances every five power frequency cycles. This will ensure the change in frequency reflects onto the computed values of self- inductance and mutual inductance as well. The subsequent paragraphs provide a description of how the values of self and mutual inductances can be applied for controlled switching by estimating distortions in the electric currents (in other phases) caused by opening of a circuit breaker in a phase. Also, method to estimate a modified arcing time, in particular change in arcing time resulting from the values of self and mutual inductances is provided. In an exemplary scenario, the current waveform in the one or more phases can also be estimated using values of impedance and voltage as shown below:

Further the estimated value of current in a phase can be compared to at least one measured value of the current in that phase using a current transformer.

For an initial action of opening the circuit breaker corresponding to phase R phase, the circuit breaker can be opened taking the phase shift of the current ίc in phase R breaker. The circuit breaker associated with phase R is opened and a condition of de-energization results when the current through the circuit breaker is going through its current zero. After opening of the circuit breaker associated with phase R, the circuit to be analyzed can be modified to as shown in Figure 4b. As shown in Figure 4b, when circuit breaker associated with phase R is opened and the corresponding winding is de-energized, the current / ₂ vanishes i.e. ii = 0. As the R phase has been de-energized at current zero condition, the rate of change of current

Thus, the Eq. (5) and (6) can be also reduced to:

Similarly the change in i ₃ , can be deduced as shown below: To find i and (5))

Figure 4c shows a circuit corresponding to a state when the circuit breaker associated with phase Y is opened. As shown in Figure 4c, when circuit breaker associated with phase B is opened, the mutual inductance between the two lines will no longer exist and thus it becomes a circuit with only self-inductance taken into consideration. The equation can be given as follows:

Thus, the effect of mutual inductance on the electric currents in the reactor windings as a result of opening of circuit breaker in a particular phase can be estimated and accounted by the IED. The estimated change in phase angle values are converted to the corresponding change in time instant for switching with the help of time period/frequency information of the AC power cycle.

The switching sequence can either be R-Y-B or R-B-Y. The invention supports a provision for both with the IED that estimates the instant of opening of the circuit breakers. In brief, the IED receives the measured values of voltage and current flowing in each phase of the windings of the multiphase coupled reactor; receives/computes values of self-inductance and mutual inductance; estimates the instant of opening of the first pole breaker based on the phase difference between the measured first phase voltage and first phase current; estimating the instant of opening the second pole circuit breaker based on the opening of first pole circuit breaker and computed values of self-inductance and mutual inductance; estimates the instant of opening the third pole breaker based on the opening of first and second pole breaker. For the further analysis, the switching sequence to be considered is R-Y-B. Mathematically, the modification in the current zero instance for phase Y after de-energization of phase R can be deduced from:

Where, is the computed value (change in the phase angle of current)

and

The equation for modified arcing time for phase Y is given as:

Where, f is the system frequency

Similarly, after de-energizing of Phase R and Phase Y, the modification in the current zero instance for phase B can be deduced as:

Where. Æ _B = 30° is change in the phase angle of current and q _B measured phase difference between phase B current and phase R voltage. The equation for modified arcing time for phase B is given as: where, f is the system frequency (AC cycle power frequency).

As an example to illustrate the working of the method to estimate change in values of current and for estimation of change in arcing time associated with a breaker, the specification of the coupled reactor considered (pre-calculated by the IED) are:

In the context of the invention, as provided earlier, the assumptions in relation to structure for three-phase system for computing by the IED are:

And

These assumptions can be applied to simplify computations by the IED to estimate the shift in zero crossing values and also for estimating values of arcing time.

After opening the Phase R breaker, the phase shift of current in breaker Y is as shown in Eqn.

(21). Fitting the values of Error! Reference source not found, in Eqn. (21):

Further, the arcing time of the circuit breaker corresponding to phase Y is expected to be modified after the opening of the circuit breaker associated with phase R. Considering a 50 Hz system, change in the arcing time is:

After opening the phase R and phase Y breaker, the phase angle of current in breaker B is as shown in Eqn. (23)

Also, the change in the arcing time of Phase B breaker is given as:

The equations can be computed using the IED/numerical relay which in turn controls the circuit breakers connected to the coupled reactor, and can be precomputed for application to operate (controlled switching) circuit breakers connected to a coupled reactor. The computed values can also be made available to the IED operating the circuit breakers. As may be known to the persons skilled in the art, the specifications of the coupled reactor and the circuit breakers can be set with actual values as per the commissioned device. The IED can take the source end voltage as an input via a voltage transformer and also measure electric current with help of a current transformer. The IED issues the opening commands to the circuit breakers based on the source voltage received as input by estimating the effect due to opening of circuit breaker for a phase in other phases and accordingly operating the circuit breakers accounting such effects (pre- computed).

The controlled opening of the coupled reactor can also be achieved by adhering to the modified arcing times as calculated by the IED. Dependence of self-inductance and mutual inductance of each phase of the coupled reactor are considered before operating the circuit breaker by the IED as per methods disclosed in this invention.

As mentioned before, the sequence of opening the circuit breakers can be carried out either in the sequence R-Y-B or in the sequence R-B-Y. For the purpose illustration of the method of the invention the opening of the two subsequent circuit breakers has been considered, for example opening in the sequence R-Y and Y-B, however the opening of the circuit breaker can take place in the sequence R-B, B-Y as well. This method accounts the effect resulting from opening of one circuit breaker (first circuit breaker) into the considerations for opening of the other circuit breaker (second circuit breaker).

Figure 5 is a flow diagram of the method used for operating the circuit breakers associated with each phase of the multiphase system. The method is performed with the IED connected in the multiphase system where each phase of a multiphase power source is connected to a multiphase coupled reactor through a circuit breaker. The multiphase magnetically coupled reactor is having windings for each phase wound on a magnetic core and the IED is connected a voltage transformer and a current transformer for receiving measured values of voltage and current. As a first step, shown in 510, measured values of current and voltage signal flowing in each phase of the windings of the multiphase coupled reactor is received from the voltage transformer. Secondly, as shown in step 520 values of self-inductance of the windings individually and values of mutual inductance between pairs of the windings are computed based on the measured values of current and voltage. Thirdly, as shown in step 530, an instant for opening (switching) of a second circuit breaker on opening (switching) of a first circuit breaker as a result of change in effect of self-inductance and mutual inductance in the windings of the multiphase coupled reactor is estimated. And finally, as shown in 540 the second circuit breaker is operated at the estimated instance of time.

In an embodiment, the application of the method of the invention is also illustrated for a closing operation (switching) of the circuit breakers to provide power to (energize) the magnetically couple reactor. Here, the initial state is the state where circuit breakers are open for all the phases. For example, the first phase (phase R) is closed (energized) and hence it acts as a self- inductive circuit (neglecting resistances and there is no mutual inductance). The Phase R is preferably closed at a voltage peak condition and the instant for closing can be estimated in advance computationally with equations relating voltage and current values with coupled reactor parameters (self-inductance and mutual inductance values).

The equations are given below for estimating the voltage and current values, and switching can be made for the circuit breaker connecting the Phase R with the corresponding winding of the reactor:

Following which, the second closing operation can be controlled to take place with either Phase Y or Phase B being closed at the controlled instant where the effect of mutual coupling is minimal i.e. the resulting current that would begin to flow in the winding smoothly (without abrupt rise) from current zero (at around zero phase). Considering that Phase Y is closed after Phase R, since two phases are now closed, there will be a mutual inductance generated between them due to the coupling effect. Accordingly the voltage and current relationship (provided in the equation below) can be solved to estimate the instant for switching to have an expected current zero condition for the phase Y by accounting the coupling effect and in consideration of the voltage applied to the winding (estimated instant of voltage peak along with corrections required accounting the effect of mutual coupling).

The corresponding shifts in peak and effect in voltage and current waveforms can be estimated and considered to calculate instant of switching i.e. instant for closing of the circuit breaker.

The closing of Phase R and Phase Y, results in phase shift off currents i± andi ₂ , due to presence of mutual coupling and accordingly affects the estimation of closing instant for the circuit breaker i.e. closing on the instant of measured voltage peak along with a phase correction (estimated instant for switching) to have a phase zero (current zero) condition on closing of the circuit breaker. The phase shift angles for correction to the instant of voltage peak can also be estimated computationally and relevant equations are shown below and converting the estimated phase correction to corresponding time instant:

Similarly, the closing of the third circuit breaker for the third phase can be controlled. The source voltages and its relationship with the winding currents along with coupling effects are given below:

The phase shifts for correction with respect to instant of (estimated) voltage peak for closing of circuit breaker for Phase B can also be estimated with the following equations:

As described before, the phase shift corrections and corresponding corrections in instant of time for switching the circuit breakers are calculated and applied on the estimated voltage peaks by the IED. As these correction values are constants, these values can be predetermined and used by the IED for switching the circuit breakers. Thus the IED accounts for the effect of mutual coupling between the windings of the coupled reactor to determine the instance for switching (opening and closing) of circuit breaker.

This written description uses examples to describe the subject matter herein, including the best mode, and also to enable any person skilled in the art to make and use the subject matter. The patentable scope of the subject matter is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal language of the claims.