In medium voltage (3-36 kV) power system applications, recent trends in both international and Indian markets show that conventional types like oil, air and SF6 circuit breakers are being mostly replaced by vacuum technologies. This technology will completely dominate the industry in coming years due to the fact that a vacuum circuit breaker represents an ideal solution to the widely varying demands of switching functions in medium voltage applications. The following section discusses in depth the usage of vacuum technology to meet the switching requirements of MV applications, in distribution, in the industrial and power segments of the market. 3.6.1 Vacuum as an Interrupting Medium The performance of a circuit breaker depends upon the properties of the dielectric medium and the design of the arc-quenching system. The superior performance of the vacuum circuit breaker is purely due to the inherent characteristics of vacuum as an interrupting medium. This is due to certain important features, which are detailed below. For a given contact gap, the dielectric strength of high vacuum is approximately eight times that of air or four times that of SF6 (one bar). This higher dielectric strength makes it possible to quench an arc with a very small contact gap. Vacuum has the fastest recovery strength after full arc interruption to its full dielectric value at current zero. This makes it ideally suitable for capacitor switching. The arc energy dissipated in vacuum for a given interrupting current is approximately one-tenth of that in oil or one-fourth of that in SF6. This is due to the short clearance times and low arc length in vacuum. The low arc energy keeps the contact erosion to a minimum, thereby giving a maintenance-free interruption system. For a given breaking current, the drive energy required in vacuum switchgear is the least as compared to that for other types of breakers (Fig. 3.13).  Fig. 3.13: Comparison of energy requirement This low drive energy is due to the low contact mass, short contact travel and the fact that no compression of medium is required in vacuum switchgear. This low drive energy facilitates a simple mechanism with minimum linkages requiring little maintenance, and gives a high mechanical life of upto 60,000 operations in properly designed mechanisms. When the contacts open in vacuum, the current to be interrupted initiates a metal vapour arc discharge and continuous flowing through this plasma until the next zero. The arc is extinguished near the current zero and the conductive metal vapour re-condenses on the contact surfaces in a matter of micro-seconds. Only one-hundredth condenses in the arc chamber wall provided for this purpose. As a result, the dielectric strength of the breaker recovers very rapidly and contact erosion is almost negligible. There are two types of arc shapes. Up to about 10 kA, the arc remains diffused. It takes the form of vapour discharge and covers the entire contact surface. Above 10 kA, the diffused arc is constricted considerably by its own magnetic field and it contracts. The phenomenon gives rise to over-heating of contacts at its centre. In order to prevent this, the design of the contacts should be such that the arc does not remain stationary but keeps travelling by its own magnetic field. Various manufacturers have used different designs to achieve this. Specially designed contact configurations make the constricted stationary arc travel along the surface of the contacts, thereby causing minimum and uniform contact erosion. 3.6.2 Demands Made on Circuit Breakers The demands made on MV power systems, irrespective of the place of use of the circuit breakers, vary widely in nature. All these functions cannot obviously be performed with the same efficiency. Thus, finally any circuit breaker design can only be a compromise. The main function of circuit breakers is to interrupt short-circuit currents and protect their loads against the effect of such faults. This function must be carried out with a high degree of safety and reliability. 3.6.2.1 Switching Functions In an MV power system, apart from the normal ON/OFF operations, the switching operations can be divided into the following: - Short-circuit Interruptions;
- Switching of Capacitive Currents;
- Switching of Inductive Currents; and
- Special Applications.
3.6.2.1.1 Short-circuit Interruptions All circuit breakers are designed to interrupt rated short-circuit currents. The number of short-circuit interruptions seen in the life of a circuit breaker depends on its location in the system, quality of the system design, environmental conditions, etc. In countries like India, where most of the distribution is by means of overhead lines (except in the case of large urban cities where underground cable networks are used), the distribution switchgear is subjected to a large number of short-circuit interruptions. This is due to bird hits, snapping of lines, thunderstorms, etc. Vacuum switchgear, which can interrupt rated short-circuit current up to 25 kA 100 times without requiring maintenance, is the obvious choice for this application as compared to other circuit breakers which can do so for only 15-20 operations. The other important criterion to be considered in distribution through overhead lines is the auto-reclosing feature required due to the transient nature of the faults. The VCBs are ideally suited for auto-reclosing duties because they act fast and can be reclosed with a minimum time interval between two closings as cooling of the dielectric is not involved. Most of the sub-stations in rural areas are outdoor and situated in far-off places. This necessitates the setting up of outdoor maintenance-free circuit breakers. Porcelain-clad vacuum circuit breakers meet this demand reliably as against the conventional indoor BOCBs used in outdoor kiosks. Moreover, the availability of vacuum technology for auto-reclosers and sectionalisers has given birth to highly economical/unattended sub-stations for rural application. 3.6.2.1.2 Switching of Capacitive Currents This includes switching of capacitors, unloaded cables and overhead lines as well as single and parallel capacitor banks. Most of the electricity utilities use capacitors in their systems to improve the power factor and to tackle voltage drops. Vacuum circuit breakers disconnect these loads safely without re-ignition and thus without the associated over-voltages. When switching in capacitor bank, and especially when paralleling them, very high making currents and high rates of rise of current occur. Conventional circuit breakers with liquid-quenching medium and tulip contacts may suffer from contact pin retardation. Thus additional measures are usually needed for these breakers to reduce such effects. Very low energy loss in the contact gap owing to low arcing during the short pre-arcing time, and flat contact surfaces of vacuum breakers obviate the necessity for additional measures. VCB's are suitable for switching in single capacitor bank as well as paralleling of Multi-Capacitor banks at much higher currents and rates of rise than MOCBs and SF6 circuit breakers, without the need for damping reactors. 3.6.2.1.3 Switching of Inductive Currents The following values are likely to be encountered in various cases of switching inductive currents.  Table 3.5 Typical Values of Inductive Current Switched by Medium Voltage Switchgear Vacuum circuit breakers have proved to be adequately suitable for these applications. Switching of Very Low Inductive Currents Older vacuum circuit breaker designs had current chopping levels that were as high as 20 A. When these breakers were used to switch transformers, special surge protection devices were required. Other types of circuit breakers also have high chopping currents of, say, 20 A. Modern Vacuum Circuit breakers with very low chopping currents of 2 to 4 A are capable of switching unloaded transformers (most difficult inductive loads) with very low over-voltage, thereby requiring no surge limiters. Switching of Low Inductive Currents Two main applications fall in this category. These are switching of compensating reactors and switching of motors under stalled rotor starting conditions. In these special cases, high over-voltages may occur due to multiple re-ignitions. Vacuum interrupter manufacturers have systematically investigated the switching of motors during starting and made the following observations: - High voltage motors may be safely operated under start-stop conditions by vacuum circuit breakers where surge suppressors are provided to limit associated switching over-voltages.
- The front time of the switching impulses is determined by the system configurations mainly by the surge impedance of busbars and cables, and current transformer inductances. These tests were conducted to meet the most severe conditions and it was observed that the occurrence of a front time of less than one micro-second is most unlikely in service. This ensures the safe operation of high voltage motors which are tested with a standard lightning surge wave of 1.2/50 micro-seconds.
- Switching off of motors having starting currents of more than 600 A during starting generates low switching over-voltages and the motors do not require protection by surge suppressors even in 11 kV cable systems.
It has also been observed that when other types of breakers are used to handle such switching operations, high over-voltages may occur due to multiple re-ignition. Moreover, it has now been found that when motors are switched 'ON', high over-voltage surges can occur which are independent of the arc-quenching medium including in SF6 circuit breakers. Thus, vacuum circuit breakers, which are provided with surge limiters, offer protection to the motor even when it is switched 'ON.' 3.6.2.1.4 Special Applications Special applications like arc furnaces involve a large number of operations per day. Track-side sub-stations require the switchgear to operate in varying set of conditions from switching the charging current of catenary systems to transformer magnetising currents to a whole range of load currents and fault currents varying from 2 kA to 12 kA. These applications also require the switchgear to have the capability to withstand a range of voltages of various waveforms from sinusoidal to steep fronted surges throughout its useful life. Vacuum circuit breakers have been found to be superior to all conventional and SF6 circuit breakers for such applications. | (a) | Arc Furnace Applications Electric arc furnaces with ratings up to 100 MVA generally employ special circuit breakers, for which purpose air blast circuit breakers have been used till now. In this application, the falling scrap in the furnace causes short-circuits between the electrodes during the melting process, and the currents to be switched lie between zero and 1-8 times the rated current of the furnace transformer. The frequency of switching can be as high as 100 operations per day with rated currents up to 2000 A. Normal oil, air and SF6 circuit breakers are found to be unsuitable for this application. Special air circuit breakers are very expensive due to stringent requirements for relatively small quantities. The standard vacuum circuit breaker offers an economical and reliable solution for this application. | | (b) | Use in Traction System The system used for electric traction generally has voltages between 15 and 25 kV and frequency of 16-2/3, 50 and 60 Hz. The main function of the single-phase traction circuit breakers is quick interruption of short-circuits on the overhead catenary system, which occur frequently and are usually transient in nature. Since VCBs have short contact travel and shorter arcing times, the total break time is quite less and thus meets the special requirements of short breaking times easily. |
In case of 16-2/3 Hz traction systems, the current zero occurs in every 30 milli-seconds resulting in an arcing time of around 33 milli-seconds. Although the arc energy in the contact gap for a single-phase breaker is much greater than that for three-phase circuit breakers, it is still much lower in a vacuum circuit breaker than in conventional circuit breakers owing to the low arc voltage. Figure 3.14 shows vacuum circuit breakers for use in traction systems.  Fig. 3.14: 25 kV outdoor track-side vacuum switchgear The number of short-circuits occurring on an overhead catenary system is much higher than those occurring on transmission lines. Thus the higher permissible cumulative current of vacuum circuit breakers, i.e. up to 100 operations at rated short-circuit current or 30,000 operations at rated normal current, makes them especially suitable for this application. (c) Use on Ships Circuit breakers meant for use on ships have to fulfil the following special requirements: - They must remain operational even in an inclined position; and
- Circuit breakers used on ships are subjected to vibrations in actual use.
Circuit breaker manufacturers are able to comply with these requirements more easily in the case of vacuum circuit breakers. References - IEC: 62271-100, "High Voltage Alternating Current Circuit Breakers".
- IEC: 62271-200, "Metal Enclosed Smitchgear and Control Gear for Rated Voltage upto and including 38kV".
- IEC: 60694, "Common Specifications".
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