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ISSN 0040-6015, Thermal Engineering, 2018, Vol. 65, No. 4, pp. 200–207. © Pleiades Publishing, Inc., 2018. Original Russian Text © K. Venna, Yu.P. Gusev, E.P. Oknin, G.Ch. Cho, 2018, published in Teploenergetika. ELECTRICAL PART OF THERMAL AND NUCLEAR POWER PLANTS Extending the Application Field of Vacuum Circuit Breakers to Generators for Capacities up to 400 MW K. Vennaa, *, Yu. P. Gusevb, **, E. P. Okninb, and G. Ch. Chob aSiemens, bNational Berlin, 13629 Germany Research University Moscow Power Engineering Institute (NRU MPEI), Moscow, 111250 Russia *e-mail: [email protected] **e-mail: [email protected] Received September 7, 2017; in final form, September 27, 2017 Abstract⎯The progress that has been achieved in the development and manufacture of vacuum circuit breakers opens the possibility of using them for a wider range of applications at power plants, including as generator circuit breakers. Such characteristics of modern vacuum circuit breakers as increased breaking capacity and high switching life are factors that make them closer in competitiveness to SF6 circuit breakers for generators with capacities up to 400 MW. The article considers problem aspects relating to clearing of short-circuit faults in the generator voltage circuits and interruption of out-of-phase making currents and no-load currents of generator transformers. Conditions leading to a longer period of time to the moment at which the switched current crosses zero are considered. It is pointed out that, unlike the IEC/IEEE Standard 62271-37-013, GOST (State Standard) R 52565-2006 does not specify the requirements for generator circuit breakers in full. The article gives the voltage drop values across the arc for different design versions of vacuum circuit breaker contacts and shows the effect the arc in a vacuum circuit breaker has on the time delay to the moment at which the current crosses zero. The standardized parameters of transient recovery voltage across the generator circuit breaker contacts are estimated along with the contact gap electric strength recovery rates ensured by modern arc quenching chambers. The switching overvoltages arising when vacuum circuit breakers interrupt short-circuit currents and no-load currents of generator transformers are analyzed. The article considers the most probable factors causing the occurrence of switching overvoltages, including current chopping, repeated breakdowns of the circuit breaker contact gap, and virtual current chopping. It is found that, unlike repeated breakdowns and virtual current chopping, an actual current chopping does not give rise to dangerous switching overvoltages. The article also determines the vacuum circuit breaker application field boundaries in which dangerous switching overvoltages may occur that would require additional measures for limiting them. Keywords: vacuum generator circuit breaker, switching overvoltage, transient recovery voltage, current chopping, time delay to the current zero crossing moment, voltage drop across the arc DOI: 10.1134/S0040601518040092 The conditions under which generator circuit breakers operate are more intense than those of circuit breakers used in medium voltage electric networks. Significantly more stringent requirements in terms of rated current, rated breaking current, DC component in the breaking current, and transient recovery voltage parameters are posed to generator circuit breakers. However, standard [1] that is currently in force for circuit breakers stipulates the need of agreeing the requirements for circuit breakers between the manufacturer and customer on a number of parameters. Sometimes, lack of information available to the customer results in the fact that the requirements for generator circuit breakers (primarily those produced by foreign manufacturers) made according to international standards, in particular, standard [2], are specified incorrectly. Previously, power plants in Russia used MGG and VGM oil generator circuit breakers and VVG and VVOA air-blast generator circuit breakers for rated currents ranging from 2000 to 3000 A and rated breaking currents ranging from 45 to 160 kA [3]. Type KAG load interrupters were installed in the circuits of the largest generators. Later on, minimum-oil and air-blast circuit breakers were replaced by SF6 generator circuit breakers and SF6 generator systems, which are presently a standard solution used in upgrading or new construction. Vacuum circuit breakers are used in the circuits of small-capacity generators; however, the application of these circuit breakers for large-capacity generators is restrained due to the following drawbacks that were inherent in the firstgeneration vacuum circuit breakers: (1) increased levels of switching overvoltages, (2) insufficiently high rated currents and rated breaking currents of these circuit breakers, and 200 EXTENDING THE APPLICATION FIELD OF VACUUM CIRCUIT BREAKERS 201 Is.c SC current DC component Time delay interval to the current zero crossing moment τ Fig. 1. Generator short circuit current Is.c versus time τ. (3) a low breaking capacity when there is an increased content of the DC component in the switched current. Modern vacuum circuit breakers are already free from these drawbacks. It was as far back as in 1988 that vacuum generator circuit breakers successfully passed independent short-circuit (SC) current breaking tests with a delay to the moment at which the current crosses zero caused by the DC component at a level of 120%. Good progress has been achieved in control of switching overvoltages. The vacuum circuit breakers proposed by Siemens can be used in the circuits of generators for capacities up to 400 MW. Modern vacuum circuit breakers have become free from the bottlenecks they had at the early stage of their development and have advantages over their SF6 counterparts: they permit a larger number of load current switching cycles, they feature a faster recovery of contact gap electric strength, they need a smaller amount of maintenance work, they are friendly to the environment, and have a low cost [4, 5]. At present, vacuum circuit breakers for rated currents up to 12500 A and rated breaking currents up to 100 kA are serially produced [5]. They have successfully passed tests for conformity to the requirements of international standard [2]. The use of vacuum circuit breakers allows more efficient and reliable operation of upgraded and newly constructed power plants to be secured. Below, the characteristics of modern vacuum generator circuit breakers and the conditions of using them at generator–transformer-type power plants are considered in more detail. Interrupting an SC fault current in the generator voltage circuit is the heaviest mode of circuit breaker operation. Depending on the SC fault location, the current from the generator or power unit transformer due to the feed from the grid and from the power THERMAL ENGINEERING Vol. 65 No. 4 2018 plant’s other generators will flow through the circuit breaker for a short period of time. Each of these cases has its specific features that are of interest in considering the SC current interruption process by a vacuum circuit breaker. Owing to a high content of the DC component in the SC current from the generator in the case of a shortcircuit fault between the generator and transformer, the resulting instantaneous current curve shifts with respect to the horizontal line corresponding to the zero value of current. Due to low resistance and high inductance of the generator stator winding, the SC current’s DC component decays quite slowly. The periodic component of the SC current from the generator, which is characterized at the SC fault initial stage by the direct-axis subtransient time constant, can decay more rapidly than the DC component, which is characterized by the time constant of the SC current DC component. The initial value of the DC component in each phase depends on the SC occurrence phase angle and can reach the periodic component’s amplitude value. Thus, by the instant at which the circuit breaker contacts begin moving apart, the SC current’s slowly decaying DC component may exceed the amplitude of the periodic component, and the SC current will not cross zero for a few cycles (Fig. 1), during which the circuit breaker will not interrupt the current. The resistance of electric arc between the circuit breaker contacts facilitates more rapid decaying of the switched current DC component. As the SC current DC component decays, the resulting current shifting amplitude decreases, and the circuit breaker interrupts the SC current during its first crossing of zero value. The time delay between the moment the circuit breaker contacts start moving apart to the moment the instantaneous current crosses zero results in the fact that the arc burns for a longer period of time, and that 202 VENNA et al. Is.c, rel.units 8 Circuit breaker contacts parting moment 2 1 6 4 2 0 –2 20 40 60 80 100 120 140 τ, ms –4 Fig. 2. Interrupted SC current value versus time [2]. 1—DC component of the interrupted SC current; 2—interrupted SC current. the arc chute and circuit breaker contacts experience additional heating. Standard [1] does not specify requirements for circuit breakers in regard to disconnecting the circuits under the conditions in which the instantaneous current crosses zero after a time delay. Standard [2] specifies two standardized values of the DC component in the SC current from the generator by the moment the circuit breaker contacts start moving apart, which are equal to 110 and 130%. The electric arc that ignites after the circuit breaker contacts are parted from each other adds external resistance to the SC circuit, as a result of which the time constant becomes smaller, the DC component decays more rapidly, and the time delay to the moment the instantaneous current crosses zero becomes shorter. Despite the fact that the voltage drop across the arc is small in comparison with the circuit breaker operating voltage, the former has a noticeable effect on the SC current interruption process (Fig. 2). The extent to which the circuit breaker arc affects the shift of the resulting instantaneous SC current depends on the voltage drop across the arc. Vacuum circuit breakers are characterized by the shortest distance between the contacts and by the lowest voltage drop across the arc in comparison with circuit breakers of other types. The results obtained from tests of vacuum generator circuit breakers have shown that the average voltage drop across the arc does not exceed 100 V for Type AMF contacts, which generate an axial magnetic field [6], and 120–150 V for Type TMF contacts, which generate a transverse magnetic field [7]. In [5], the results obtained from simulating the process of interrupting the SC current from a generator are presented for estimating the effect the arc in a vacuum circuit breaker has on the SC current interruption process. The arc burning time between the contacts of two circuit breaker poles is equal to 80 and 29 ms for the voltage drop across the arc equal to zero and 100 V, respectively, after which the instantaneous current reaches zero value and the SC fault is cleared (Fig. 3). With an SC fault between the generator and circuit breaker, the SC current periodic component shows hardly any decay due to remoteness of the SC fault location from the power sources. The DC component in the SC current does not exceed the amplitude of its periodic component by the time the circuit breaker contacts part from each other, due to which the instantaneous current will cross zero without time delay (Fig. 4). As a rule, the current from the transformer that flows through the circuit breaker is higher than the SC current from the generator in the case of an SC fault on the generator voltage side due to feed from the grid and from the power plant’s other generators. The SC currents from the transformer produce significant electrodynamic impacts on the generator circuit breaker and determine the circuit breaker rated breaking currents. The ability of vacuum generator circuit breakers to interrupt SC currents up to 100 kA [5] is a factor that extends their application field at power plants. The quenching of the arc in a circuit breaker pole is accompanied by the growth of the transient recovery voltage (TRV) across its contacts. The TRV growth rate depends on the frequency of the TRV high-frequency component, which is proportional to the natural oscillation frequency of the generator or power unit transformer loops. Due to the difference between the capacitances of the generator and transformer phases (with the commensurable inductances of their windings), the transformer loop has a higher frequency of its natural oscillations. In view of this circumstance, the TRV in the case of an SC fault between the generator and circuit breaker has a higher growth rate. Thus, standard [2] specifies the required TRV parameters separately for SC currents from the generator and from the transformer. In the first case, the required voltage recovery rate does not exceed 2 kV/μs, whereas it may reach 5 kV/μs in the second case. At the moment in which the arc in the circuit breaker pole is extinguished, the voltage is close to its amplitude value, because the switched circuit is inductive in nature and because there is a low content of DC component in the breaking current. Superposition of amplitude voltage on the high-frequency component results in higher peak values of the TRV in interrupting the SC current from the transformer. Out-of-step connection of the generator done as a result of errors committed in arranging the secondary circuits gives rise to high equalizing currents. Interruption of equalizing currents entails the occurrence of a TRV across the generator circuit breaker contacts, the parameters of which are essentially higher than those of the TRV that occurs in interrupting SC currents. In accordance with [2], the circuit breaker shall be designed for an increased voltage under the conditions of out-of-step connection; it shall ensure the possibility to perform out-of-step connection with subsequent disconnection of the generator. In the most unfavorTHERMAL ENGINEERING Vol. 65 No. 4 2018 EXTENDING THE APPLICATION FIELD OF VACUUM CIRCUIT BREAKERS 203 (a) I, kA 100 3 1 50 0 0.04 0.06 0.08 0.10 0.16 τ, s 0.14 0.12 −50 −100 2 4 6 5 −150 (b) I, kA 3 100 1 50 0 0.04 0.06 0.08 0.10 0.12 τ, s −50 −100 2 4 5 6 −150 Fig. 3. Interruption of the generator current I in the case of a three-phase SC fault and voltage drop across the arc equal to (a) 0 V and (b) 100 V. Phases: 1—А; 2—В; 3—С; 4—SC fault occurrence instant; 5—parting the circuit breaker contacts; 6—clearing the SC fault. able case, the TRV peak value resulting from circuit breaker disconnection under out-of-phase conditions is produced when the high-frequency voltage component is superimposed on twice the generator phase voltage. Clearly, the peak voltages produced in this mode will exceed their values in the case of clearing an SC fault in the generator voltage circuit. Thus, standard [2] specifies the standard TRV peak value with respect to the circuit breaker’s highest operating voltage in the out-of-phase switching mode equal to 2.6 relative units and equal to 1.84 relative units in the SC clearing mode with the commensurable TRV THERMAL ENGINEERING Vol. 65 No. 4 2018 growth rate in both cases. The standardized TRV parameters are given for the standardized breaking current, the value of which in the out-of-phase mode is determined by the difference between the voltage phasors at the generator terminals and at the terminals of the transformer low voltage winding and between the grid equivalent impedance and the generator subtransient reactance. The grid impedance is commensurable with the generator subtransient reactance, and the current resulting from out-of-phase connection of the generator will not be significantly higher than the SC current from the generator even if the generator is 204 VENNA et al. Is.c SC current DC component τ Fig. 4. Short-circuit current from the transformer versus time. connected in the opposite-phase mode [8]. In accordance with [1, 2], the standardized breaking current shall be no less than 50% of the circuit breaker rated breaking current, which corresponds to a phase mismatch angle equal to 90 el. deg. The actual breaking current may be significantly higher than its standardized value. Thus, with the phase mismatch angle equal to 90 el. deg, the breaking current can make 63% of the circuit breaker rated breaking current [9]. Interruption of current in a circuit breaker that occurs before the expected moment at which the instantaneous power frequency current crosses zero, called current chopping, is characteristic for all types of circuit breakers. Instability of arc discharge at low current values and intense dissipation of the arc column energy in the circuit breaker chute under the effect of electromagnetic or gas blasting are factors causing the current chopping phenomenon [10, 11]. The current chopping phenomenon can initiate repeated ignitions of the arc in the circuit breaker and the occurrence of a high-frequency current component. The current chopping phenomenon causes the energy stored in the switched circuit inductances to be discharged onto the capacitance of live parts with respect to ground and gives rise to overvoltages. The overvoltage amplitude is directly proportional to the chopped current Ich and inversely proportional to the capacitance. The chopping current of vacuum circuit breakers depends on the material of their contacts [12], because the arc burns in the vapors of metal vaporized from the cathode surface under the effect of high temperature. Figure 5 shows the possible chopping current values for different media and the distribution of its occurrence probability P for vacuum circuit breakers. It can be seen from Fig. 5a that the vacuum generator circuit breakers with contacts made of chro- mium–copper compositions [5, 13] ensure the lowest chopping current in comparison with that in using other media. A low chopping current allows switching voltages to be limited to a level that does not pose danger for electrical machines provided that no repeated arc ignition and voltage escalation occur [14, 15]. In what follows, we consider the conditions under which repeated arc ignitions occur in a vacuum circuit breaker. A vacuum differs advantageously from other arc quenching media in the steepness of the impulse breakdown voltage growth curve. In comparison with SF6 generator circuit breakers operating at a pressure of 0.5 MPa, the vacuum circuit breaker impulse breakdown voltage is higher by more than a factor of two than that of SF6 circuit breakers at the distances characteristic for the initial stage of parting the circuit breaker contacts. Thus, the vacuum circuit breakers produced by Siemens that comply with the requirements of standard [2] ensure the contact gap electrical strength recovery rate more than 10 kV/μs. Nonetheless, repeated breakdowns in disconnecting a circuit occur even in vacuum circuit breakers. Cases in which the circuit breaker pole (phase С) contacts begin to part at less than 0.5 ms before the current zero crossing moment (Fig. 6a) are the most dangerous ones. After the current has been interrupted, the circuit breaker contacts still continue to part, and the contact gap electrical strength continues to grow. This period of time involves a danger of repeated breakdowns to occur with generation of a high-frequency current component. Repeated arc ignitions entail voltage escalation and a growth of the high-frequency current amplitude. Due to the inductive and conductive couplings between the phases, a high-frequency current arises also in phases A and В, which may also be a factor causing virtual chopping of current in them (see THERMAL ENGINEERING Vol. 65 No. 4 2018 EXTENDING THE APPLICATION FIELD OF VACUUM CIRCUIT BREAKERS (а) 205 (b) Air-blast circuit breakers 20 Ich, A Vacuum circuit breakers (with Cu–Bi contacts) 15 Minimum-oil circuit breakers P 0.4 10 Bulk-oil circuit breakers 5 0.3 SF6 circuit Vacuum circuit breakers breakers (with Cu–Cr contacts) 0.2 0.1 0 0 1 2 3 4 Ich, A Fig. 5. (a) Chopping current for different arc quenching media and (b) probabilistic distribution of chopping current in vacuum circuit breakers with contacts made of Cu–Cr composition [16]. Iint Ic (а) Ic (b) Ic A fnom = 50 Hz I B B C τ Ic High-frequency current zero crossing moment IA Fig. 6. Induced virtual current chopping [16]. (a) Interrupted currents of phases А, В, and С versus time and (b) switching state of the disconnected circuit at the virtual chopping current occurrence instant. Fig. 6a). This may give rise to an overvoltage with a ratio of up to six phase voltages applied to the turn and main insulations of the generator and transformer. Based on the results from tests of medium voltage vacuum circuit breakers produced by Siemens that involved interruption of inductive currents, which were reported in [16], the conditions under which dangerous overvoltages may occur due to repeated contact gap breakdowns entailing virtual current chopping have been determined. Figure 7 shows the voltage frequency fa across the circuit breaker contacts as a function of the interrupted current rms value Iint for 12 kV electrical installations. Overvoltages may occur if the interrupted current parameters lie in the separated zone of the diagram. Cases involving disTHERMAL ENGINEERING Vol. 65 No. 4 2018 connection of neutral grounding transformers with arc suppression coils, motors in their starting mode, and shunting reactors are dangerous in terms of switching overvoltages. The dangerous range of interrupted current rms values is from 20 to 600 A. Thus, interruption of an SC current by a vacuum circuit breaker does not entail the occurrence of dangerous switching overvoltages. This statement is also valid for disconnection of idle generator transformers with magnetization currents below 20 A (which corresponds to transformers with capacities equal to or less than 100 MVA): repeated arc ignitions and dangerous switching overvoltages do not occur. For power units of a larger capacity, vacuum circuit breakers with several parallelconnected arc chutes per pole are produced. 206 VENNA et al. fa, kHz 8 100 50 6 10 5 5 3 1 0.5 0 1 0.5 5 10 7 4 2 50 102 5 × 102 5 × 103 5 × 104 Iint, A Fig. 7. Diagram showing interruption of inductive currents in 12 kV electrical installations [16]. Transformers: 1—idle, 2—of electric arc furnaces, 3—for neutral grounding with arc suppression coils, and 6—with short-circuited secondary windings; 4—motors in the starting mode; 5—shunting reactors; 7, 8—short-circuit faults in the network and in connections equipped with reactors. Overvoltages caused by repeated breakdowns and virtual current chopping were observed in disconnecting the idle transformer of the Kama hydroelectric power plant’s unit [14]. An analysis of the test conditions indicates that there are additional factors that affect the interruption process. The domestically produced vacuum generator circuit breakers were equipped with arc chutes produced by Siemens. Poor technical state of the vacuum circuit breaker drive may become a factor causing a degraded gap strength recovery rate at the initial stage of interruption and giving rise to dangerous switching overvoltages. Disconnections of the power plant auxiliary electric motor performed before finishing its startup with a startup current lower than 600 A were accompanied by repeated breakdowns and entailed the occurrence of overvoltages with ratios equal to 3.0 and 2.7 of the amplitude phase voltage with the availability of overvoltage limiters [15]. Dangerous switching overvoltages caused by the operation of vacuum generator circuit breakers may occur in disconnecting the power unit’s idle transformer if the current interrupted by the arc chute exceeds a certain threshold value. The threshold current value for the vacuum circuit breakers produced by Siemens is 20 A. Owing to the use of several parallelconnected arc chutes in circuit breakers for rated currents up to 14 kA, the threshold current value does not occur as a rule, and repeated arc ignitions and dangerous overvoltages do not occur in the course of disconnection. Hence, there is no need to mandatorily provide protection from overvoltages by installing overvoltage limiters or R-C circuits. CONCLUSIONS (1) Vacuum generator circuit breakers outperform the currently used SF6 circuit breakers in the majority of their characteristics. Owing to the state-of-the-art technologies for manufacturing vacuum arc chutes and low chopping current values, protection from the overvoltages caused by the operation of vacuum circuit breakers is not a must. The progress achieved in the vacuum circuit breaker manufacturing technology opens the possibility to use these circuit breakers at power plants in the circuits of generators with capacities up to 400 MW. (2) Wide-scale application of vacuum generator circuit breakers is hindered by the preconceived attitude of some specialists to their manufacturing technology stemming from the negative experience gained from using first-generation vacuum circuit breakers and incorrect choice of circuit breakers. To secure reliable operation of vacuum generator circuit breakers, they should be selected with due regard to the results from calculations of the periodic and DC short-circuit current components and the parameters of transient recovery voltage. REFERENCES 1. GOST R 52565-2006. Alternating-Current CircuitBreakers for Voltages from 3 to 750 kV. General Specifications (Standartinform, Moscow, 2007). 2. IEC/IEEE 62271-37-013 High-Voltage Switchgear and Controlgear — Part 37-013: Alternating-Current Generator Circuit-Breakers (Int. Electrotechnical Commission, 2015). THERMAL ENGINEERING Vol. 65 No. 4 2018 EXTENDING THE APPLICATION FIELD OF VACUUM CIRCUIT BREAKERS 3. P. A. Sheiko, “6–24 kV generator circuit breakers. The problem of choice and application,” Nov. Elektrotekh., No. 2, 70–72 (2006). 4. A. N. Nazarychev, “Analysis of main advantages of the use of vacuum circuit breakers,” Energoekspert, No. 4– 5, 58–63 (2007). 5. K. Venna, N. Anger, and T. 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