This section describes the detailed explanation of the proposed system operation which deliberates the controlled switching device principles & interruption of inductive current.
A. Controlled switching device
Controlled switching refers to the circuit breaker operation at the predefined instant with respect to the reference signal typically the busbar voltage or load current with the use of IED (intelligent electronic device). This is also termed as the “point-on-wave control” or else the “synchronous switching”. During controlled opening process, the intentional delay (Ttotal) is added to random generated tripping command for the purpose of synchronization with reference (Voltage or current). After getting synchronized with reference signal and considering the CB rated mechanical opening time alongwith calculated arcing time, the DC command is given to trip coil of CB [22]. Total delay in turn is composed of two parts as shown in Fig. 1 as:-
Ttotal = Tw+ Tcont (1)
Tcont =N1 Tzero – Tarc – Topening (2)
N1 indicated in Eq. (2) indicates the number of power cycles considered in calculating the delay in releasing command to CB and is limited to one or two cycles for normal controlled operation. In EHV CBs, there are two sets of contacts, one termed as main contacts and others known as arcing contacts. The Tarc (arcing time) is then defined as the time span between the instant of current interruption and the instant of contact separation. Once after the contact separation, an arc strikes among the contacts and was interrupted in the current zero vicinity and might lead to current chopping. Arc conduction during arcing time is done through arcing contact and its healthiness is of utmost importance for any switching operation of CB.
The electrical targets for opening of reactors (without NGR and magnetically independent) shall be the peak of respective phase voltage. The objective of reactor controlled opening operation is to issue tripping command such that contact parting leads to arc extinguishing within safer time zone; and thus, reduces re-ignition/breakdown probability post their natural zero current and decrease imposed stresses on the CB, power system and the reactor. Constraints in thermal capability of arcing chamber of CB will arise if the arc prolonged for longer period (more than one cycle) after contact parting. There is high possibility of re-ignition within CB interrupter for the opening operation with shorter arcing time. Typically, there were two significant technical issues for enhancing the accuracy of the controlled switching technology. At first, the precise time at which the reactor current level reached zero should be attained for the controlled switching device like an operation time benchmark. Next, the controlled switching device could predict the time of operation exactly with the influence of some external circumstances like ambient temperature, contact wear, control voltage, aging mechanism and so on).
B. Interruption of inductive current
Interruption of inductive current by EHV CB is a special duty and need attention for the successful de-energization process. During the interruption process, the electromagnetic energy stored in inductance of reactor interact with electrostatic energy of capacitance within its vicinity. The energy stored in inductance can be evaluated by Eq. (1) as:-
$${\mathbf{E}}_{\mathbf{L}}= \frac{1}{2}. \mathbf{L}.{\mathbf{I}}^{2}$$
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Whereas electrostatic energy stored by capacitance is given by Eq. (2) as:-
$${\text{E}}_{\text{C}}= \frac{1}{2}. \text{C}.{\text{V}}^{2}$$
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During interruption process, there will be exchange of energy from inductance to capacitive elements for maintaining the energy conservation as given by Eq. (5):-
$${{\text{E}}_{\text{L}}= \text{E}}_{\text{C}}=>\frac{1}{2}. \text{L}.{\text{I}}^{2}= \frac{1}{2}. \text{C}.{\text{V}}^{2}$$
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During interruption, current chopping effect does happen and CB interrupt the load current prior to natural current zero. In Eq. (5), I represent the chopping current at the instant of interruption and V is the voltage developed across capacitor which in turn is in parallel to CB. Due to pre-mature current interruption, it builds up high frequency oscillating voltage across CB. Magnitude of peak recovery voltage (TRV) and Rate of Rise of recovery voltage are the limiting factors for deciding the capability of CB. Further, the magnitude of TRV can be managed by installing surge arrester at the terminals of reactor but surge arrester did not have the property to reduce the steepness of recovery voltage.
Subsequent to current chopping there will be an arc voltage drop (Kin) and peak suppression voltage (Va) appears across the CB contacts. The magnitude of Va is proportional with chopping current. Larger arcing times gives more peak suppression voltage. After the parting of CB contacts, the built up of dielectric strength of insulating medium (SF6) grows with certain rate (kV/ms) known as Rate of rise of dielectric strength (RRDS). If the steepness of recovery voltage exceeds the RRDS of CB, then breakdown of insulation medium occurs and results in re-ignition as shown in Fig. 2.
The possible outcome of re-ignition is the un-successful interruption of current and it will travel for another half cycle. It is also possible that multiple reignition occurs during the interruption process and escalate the voltage even upto 3-4pu (voltage escalation). Re-ignition process is a high frequency process that can degrade the interrupter capability, deteriorate grade the nozzles and even degrade the dielectric integrity of shunt reactor.
The system under study is effectively earthed and value of first pole to clear factor of 1.3 has been considered in the evaluation process. For a 400kV system, design voltage of reactor is 420kV and therefore, Single phase peak voltage (Vo) and RRDS of CB is given by Eqs. (6) and (7) respectively:-
$${\text{V}}_{\text{o}}= \frac{{\text{V}}_{\text{r}}}{\sqrt{3}}. \sqrt{2 }=\frac{420}{1.732} \text{x} 1.414=343\text{k}\text{V}$$
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$$\text{R}\text{R}\text{D}\text{S}= \frac{\text{T}\text{R}\text{V}}{\text{M}\text{i}\text{n}\text{i}\text{m}\text{u}\text{m} \text{a}\text{r}\text{c}\text{i}\text{n}\text{g} \text{t}\text{i}\text{m}\text{e}} \text{k}\text{V}/\text{m}\text{s}$$
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Value of RRDS for a typical 400kV double chamber CB lies in range of 140–150 kV/ms. Considering the system operating frequency of 50Hz, the maximum rate of change of system voltage and RRDS in per unit convention for a CB with typical RRDS of 148kV/ms can been computed by Eqs. (8) and (9) as:-
$$\frac{\text{d}\text{v}}{\text{d}\text{t}}= \frac{{\text{V}}_{\text{o} } {\omega }}{1000}=\frac{343 \text{x} 314}{1000}=108 \text{k}\text{V}/\text{m}\text{s}$$
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$${\text{R}\text{R}\text{D}\text{S}}_{\text{p}\text{u}}=\frac{\text{R}\text{D}\text{D}\text{S} \left(\text{a}\text{b}\text{s}\text{o}\text{l}\text{u}\text{t}\text{e} \text{o}\text{f} \text{C}\text{B}\right)}{\text{S}\text{y}\text{s}\text{t}\text{e}\text{m} \text{v}\text{o}\text{l}\text{t}\text{a}\text{g}\text{e}\text{max}\text{d}\text{v}/\text{d}\text{t}}=\frac{148}{108}=1.37\text{p}\text{u}$$
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For an 80Mvar shunt reactor, the inductance at rated voltage value for 50Hz system can be computed by Eq. (10) :-
$$\text{Q}= \frac{{{\text{V}}_{\text{r}}}^{2}}{{\omega } \text{L}}=> \text{L}= \frac{{\left(420\right)}^{2}}{314 \text{x} 80}=7.02\text{H}$$
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Testing of CB pole for reactor switching test is done on individual phase (single phase basis) as per IEC 62271-110 and accordingly two test circuits as per following details has been proposed to carry out the reactor switching test:-
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Load circuit-I for 315A (± 20%)
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Load circuit-II for 100A (± 20%)
Number of tests are needed to establish the characteristics of reactor CB during interruption process. The test duties have been classified into four major heads for single phase test above 100kV as:-
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Test duty-1-20 operations on load circuit-1 at the interval of 18º electrical for scanning of complete cycle for calculating TRV, minimum arcing time and re-ignition (if occur)
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Test duty-2-20 operations on load circuit-2 at the interval of 18º electrical for scanning of complete cycle for calculating TRV, minimum arcing time and re-ignition (if occur)
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Test duty-3-18 operations with tripping timing set to arcing time with highest chopping over voltage.
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Test duty-4- 20 operations with lock out condition every 18º electrical
The 1-Φ equivalent circuit for understanding the current chopping and re-ignition effect is depicted in Fig. 3. Ct is the effective capacitance seen from CB end and can be computed as:-
$${\text{C}}_{\text{t}}= {\text{C}}_{\text{P}}+\frac{{\text{C}}_{\text{S}}+ {\text{C}}_{\text{L}}}{{\text{C}}_{\text{S}}{\text{C}}_{\text{L}}}$$
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CP is the capacitance available across CB terminals by means of grading capacitor. 400kV CB under study is having 2-chambers with grading capacitor of 700pF each value resulting in equivalent capacitance of 350pF per pole. CL is the total capacitance offered by load circuit i.e reactor. In a shunt reactor major capacitance is composed of bushing capacitance and winding to earth capacitance. The bushing capacitance considered in this study is 425pF [23] and winding to earth capacitance lies in the range of 3.27nF. Cs is the total capacitance seen from source side and as per IEC 62271-110, the source side capacitance can be considered 10 times the load side capacitance if actual not available. Therefore, value of 32.7nF has been considered.
Vs is the voltage source emulating the short circuit capacity of Grid at that point, source resistance and inductance (Ls) are given in appendix-1. LP1 and LP2 are the CB inductances and is of negligible magnitude therefore not been considered in this study. LB is the lead/cable inductance between CB and reactor; owing to small length, the factor LB has been neglected.
The theoretical computation and analysis were needed for analyzing the characteristics of interrupting CBs at the capacitance influence in the short circuit condition (SC). The TRV ratings and interrupting current of HV CBs needs to be matched in terms of SC. The interrupting characteristics of CB in the extreme condition (that is, the angle of interrupting φ is 180°) are estimated as shown. The short-circuit current (Is) of the CB could be approximately given through Eq. (12):-
Is = \(\frac{{\text{V}}_{0}}{\sqrt{{{\text{R}}_{\text{s}}^{2}+\left({{\omega }\text{L}}_{\text{s}}\right)}^{2}}}\) (12)
The effect of the Earth branch neglecting, the value of effective interrupting current Iop is given by Eq. (13):-
Iop = \(\frac{2|\text{s}\text{i}\text{n}(\frac{{\phi }}{2}\left)\right|{\text{V}}_{0}}{\sqrt{4{{\text{R}}_{\text{s}}^{2}+({2{\omega }\text{L}}_{\text{s}}-\left(\frac{2}{{\omega }{\text{C}}_{\text{c}}}\right)+{\omega }{\text{L}}_{0})}^{2}}}\) (13)
On integrating Eq. (12) and Eq. (13), p is distinct as below:-
p = Iop/Is = \(\frac{2|\text{s}\text{i}\text{n}(\frac{{\phi }}{2}\left)\right|\sqrt{{{\text{R}}_{\text{s}}^{2}+\left({{\omega }\text{L}}_{\text{s}}\right)}^{2}}}{\sqrt{4{{\text{R}}_{\text{s}}^{2}+({2{\omega }\text{L}}_{\text{s}}-\left(\frac{2}{{\omega }{\text{C}}_{\text{c}}}\right)+{\omega }{\text{L}}_{0})}^{2}}}\) (14)
For HV AC CBs, the current interruption rating i.e short-circuit current rating shall be only limited to 25% during out-of-phase fault conditions. In this position, the p computation is inferior to the value rated that implies that the current can be normally interrupted.