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The Power Factor Correction

The Power Factor Correction

The power factor of a load, which may be a single power-consuming item, or a number of items (for example an entire installation), is given by the ratio of P/S i.e. kW divided by kVA at any given moment.

The value of a power factor will range from 0 to 1. If currents and voltages are perfectly sinusoidal signals, power factor equals cos ϕ.

A power factor close to unity means that the reactive energy is small compared with the active energy, while a low value of power factor indicates the opposite condition.

Power vector diagram
  • Active power P (in kW)
    • Single phase (1 phase and neutral): P = V x I x cos ϕ
    • Single phase (phase to phase): P = U x I x cos ϕ
    • Three phase (3 wires or 3 wires + neutral): P = √3 x U x I x cos ϕ
  • Reactive power Q (in kvar)
    • Single phase (1 phase and neutral): P = V x I x sin ϕ
    • Single phase (phase to phase): Q = UI sin ϕ
    • Three phase (3 wires or 3 wires + neutral): P = √3 x U x I x sin ϕ
  • Apparent power S (in kVA)
    • Single phase (1 phase and neutral): S = VI
    • Single phase (phase to phase): S = UI
    • Three phase (3 wires or 3 wires + neutral): P = √3 x U x I

where:

V = Voltage between phase and neutral
U = Voltage between phases

  • For balanced and near-balanced loads on 4-wire systems

The power factor is the ratio of kW to kVA. The closer the power factor approaches its maximum possible value of 1, the greater the benefit to consumer and supplier.
PF = P (kW) / S (kVA)
P = Active power
S = Apparent power

Current and voltage vectors, and derivation of the power diagram

The power vector diagram is a useful artifice, derived directly from the true rotating vector diagram of currents and voltage, as follows:

The power-system voltages are taken as the reference quantities, and one phase only is considered on the assumption of balanced 3-phase loading. The reference phase voltage (V) is co-incident with the horizontal axis, and the current (I) of that phase will, for practically all power-system loads, lag the voltage by an angle ϕ. The component of I which is in phase with V is the wattful component of I and is equal to I cos ϕ, while VI cos ϕ equals the active power (in kV) in the circuit, if V is expressed in kV.

The component of I which lags 90 degrees behind V is the wattless component of I and is equal to I sin ϕ, while VI sin ϕ equals the reactive power (in kvar) in the circuit, if V is expressed in kV.

If the vector I is multiplied by V, expressed in kV, then VI equals the apparent power (in kVA) for the circuit. The above kW, kvar and kVA values per phase, when multiplied by 3, can therefore conveniently represent the relationships of kVA, kW, kvar and power factor for a total 3-phase load, as shown in Figure K3 .

SOURCE: Schneider Electric

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The Benefits of VFDs In HVAC Systems

The Benefits of VFDs In HVAC Systems

One of the most successful energy management tools ever applied to building HVAC systems is the variable frequency drive (VFD). For more than 20 years, VFDs have successfully been installed on fan and pump motors in a range of variable load applications. Energy savings vary from 35 to 50 percent over conventional constant speed applications, resulting in a return on investment of six months to two years.

While the number of applications suitable for early generation drives was limited based on the horsepower of the motor, today’s drives can be installed in practically any HVAC application found in commercial and institutional buildings. Systems can be operated at higher voltages than those used by earlier generations, resulting in off the shelf systems for motors up to 500 horsepower.

Early generation systems also suffered from low power factor. Low power factor robs the facility of electrical distribution capacity and can result in cost penalties imposed by electrical utility companies. Today’s systems operate at a nearly constant power factor over the entire speed range of the motor.

Another problem that has been corrected by today’s systems is operational noise. As the output frequency of the drives decreased in response to the load, vibrations induced in the motor laminations generated noise that was easily transmitted through the motor mounts to the building interior. Today’s drives operate at higher frequencies, resulting in the associated noise being above the audible range.

And VFDs continue to evolve. From numerous system benefits to an increasing range of available applications, VFDs are proving to be ever more useful and powerful.

The Heart of VFDs

Most conventional building HVAC applications are designed to operate fans and pumps at a constant speed. Building loads, however, are anything but constant. In a conventional system, some form of mechanical throttling can be used to reduce water or air flow in the system. The drive motor, however, continues to operate at full speed, using nearly the same amount of energy regardless of the heating or cooling load on the system. While mechanical throttling can provide a good level of control, it is not very efficient. VFDs offer an effective and efficient alternative.

Three factors work together to improve operating efficiency with VFDs:

1. Operating at less than full load. Building systems are sized for peak load conditions. In typical applications, peak load conditions occur between 1 and 5 percent of the annual operating hours. This means that pump and fan motors are using more energy than necessary 95 to 99 percent of their operating hours.

2. Oversized system designs. Designing for peak load oversizes the system for most operating hours. This condition is further compounded by the practice of oversizing the system design to allow for underestimated and unexpected loads as well as future loads that might result from changes in how the building space is used.

3. Motor energy use is a function of speed. The most commonly used motor in building HVAC systems is the induction motor. With induction motors, the power drawn by the motor varies with the cube of the motor’s speed. This means that if the motor can be slowed by 25 percent of its normal operating speed, its energy use is reduced by nearly 60 percent. At a 50 percent reduction in speed, energy use is reduced by nearly 90 percent.

The installation of a VFD in an HVAC application addresses the inefficiencies introduced by the first two factors, while producing the energy savings made possible by the third. The VFD accomplishes this by converting 60 cycle line current to direct current, then to an output that varies in voltage and frequency based on the load placed on the system. As the system load decreases, the VFD’s controller reduces the motor’s operating speed so that the flow rate through the system meets but does not exceed the load requirements.

VFD Benefits

The most significant benefit to using a VFD is energy savings. By matching system capacity to the actual load throughout the entire year, major savings in system motor energy use are achieved.

Another benefit of the units is reduced wear and tear on the motors. When an induction motor is started, it draws a much higher current than during normal operation. This inrush current can be three to ten times the full-load operating current for the motor, generating both heat and stress in the motor’s windings and other components. In motors that start and stop frequently, this contributes to early motor failures.

In contrast, when a motor connected to a VFD is started, the VFD applies a very low frequency and low voltage to the motor. Both are gradually ramped up at a controlled rate to normal operating conditions, extending motor life.

VFDs also provide more precise levels of control of applications. For example, high-rise buildings use a booster pump system on the domestic water supply to maintain adequate water pressure at all levels within the building. Conventional pump controls in this type of application can maintain the pressure within a certain range, but a VFD-based system can maintain more precise control over a wider range of flow rates, while reducing energy requirements and pump wear.

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SOURCE: facilitiesnet

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Short Circuit Parameters in Low Voltage AC Circuits

Low-voltage equipment standards IEC60947 and IEC60439 currently include short-circuit ratings for products and assemblies respectively, defined in terms of the ability of the equipment to operate at a level of peak current, an RMS current for a specified time and/or a level of current conditional upon a short-circuit protective device in series. In practice the correct application of the various short-circuit ratings needs to be fully understood by the circuit designer to avoid leaving a circuit or equipment with inadequate short-circuit protection. It is also useful to take full advantage of the capability of devices and systems to avoid over-engineering, with the consequent unnecessary additional cost. This guide does not concern itself with the issue of selectivity between devices in series, which must be considered separately.

Principles of Application

The Installation

In order to ensure the capability of equipment under short-circuits conditions the circuit designer must firstly have available the prospective fault level at the point of installation of each item of equipment. This is produced by a system protection study. IEC60781 provides an application guide for calculation of short-circuit currents in lowvoltage radial systems. Short-circuit parameters are defined by this guide in terms, which include the following:

  • Prospective (available) short-circuit current:
    The current that would flow if the short-circuit were replaced by an ideal connection of negligible impedance without any change of the supply.
  • Peak short-circuit current Ip
    The maximum possible instantaneous value of the prospective (available) short-circuit current.
  • Symmetrical short-circuit breaking current Ib
    The r.m.s. value of an integral cycle of the symmetrical a.c. component of the prospective (available) shortcircuit current at the instant of contact separation of the first pole of a switching device.
  • Steady-state short-circuit current Ik
    The r.m.s. value of the short-circuit current which remains after the decay of the transient phenomena.
    - unlimited
    - limited by an SCPD (short-circuit protective device)
LV Assemblies (switchboard, distribution board etc.)

An assembly will have a short-circuit rating, assigned by the manufacturer, defined in terms of the maximum prospective fault level applicable at the point it is connected into the system.

This will have been determined by test and/or design calculations as specified in the assembly standard, IEC60439-1, or applicable part thereof.

The terminology to define the short-circuit rating of an assembly is given in the standard as follows:

  • Rated short-time current (Icw) (of a circuit of an assembly)
    Summarised as: The r.m.s value of short-time current that a circuit of an assembly can carry without damage under specified test conditions, defined in terms of a current and time e.g. 20kA, 0,2s.
  • Rated peak withstand current (Ipk) (of a circuit of an assembly)
    Summarised as: The value of peak current that a circuit can withstand satisfactorily under specified test conditions.
  • Rated conditional short-circuit current (Icc) (of a circuit of an assembly)
    Summarised as: The value of prospective short-circuit current that a circuit, protected by a specified shortcircuit protective device (SCPD), can withstand satisfactorily for the operating time of that device, under specified test conditions. Note: the short-circuit protective device may form an integral part of the assembly or may be a separate unit.An assembly may be assigned a value of Icc alone.

- An assembly may be assigned values of Icw and Ipk (but cannot be assigned a value of Icw or Ipk alone).
- An assembly may be assigned values of Icw, Ipk and Icc.
- An assembly may be assigned different values of Icc for different circuit protective devices and/or system voltages.
- An assembly may be assigned different values of Icw for different short-time periods e.g. 0.2s, 1s, 3 s.

Switchgear

In terms of short-circuit capability switchgear must be considered in respect of it’s function in the particular application. A switching device is considered in two respects, self-protection and use as a short-circuit protective device (SCPD) where applicable.

Switchgear – Self Protection Against Short Circuit

Two cases are considered:

  • Load and overload switching alone, without any short-circuit switching capability:
    In this case the switching device will be short-circuit rated on a similar basis to a circuit of an assembly (see above), with a rating of Icw and/or a conditional short-circuit rating, but will in addition have a rated short-circuit making capacity Icm.
  • Load, overload and short-circuit switching capability:
    • Fused switchgear – in this case the short-circuit breaking function is provided by the integral fuses and the device will have a conditional short-circuit rating
    • Circuit breakers – the circuit-breaker will be self-protecting up to its breaking capacity rating (see later). At fault levels above the breaking capacity rating a circuit-breaker may be capable of operating with ‘back-up’ protection by an SCPD (this is in effect a conditional rating, but the term is not generally used in this context).
Switchgear – Application as SCPD
  • Fused Switchgear and Fuses as SCPD
    Since the short-circuit breaking function in fused switchgear is provided by the fuses it is the fuse characteristics that are considered. These are given in IEC60269-1 as follows:

    • Breaking capacity of a fuselink
      - value (for a.c. the r.m.s. value of the a.c. component) of prospective current that a fuselink is capable of breaking at a stated voltage under prescribed conditions.
    • Cut-off current
      Summarised as: maximum instantaneous value reached by the current during the breaking operation of a fuselink when it operates to prevent the current reaching the prospective peak.
    • Operating I²t (Joule integral)
      Summarised as: Integral of the square of the current over the operating time of the fuse.
      Sometimes referred to as ‘energy let-through’. When expressed in A²t gives the energy dissipated per ohm and thus represents the thermal effect on the circuit.
  • Circuit-breakers as SCPD
    • Moulded-case circuit-breakers (MCCBs) and air circuit-breakers (ACBs) are rated according to IEC60947-2 as follows
      • Rated short-circuit making capacity (Icm)
        Summarised as: The maximum peak prospective current that the circuit-breaker can make on to satisfactorily.
    • Rated short-circuit breaking capacities:
      • Rated ultimate short-circuit breaking capacity (Icu)
        Summarised as: The r.m.s prospective current that the circuit breaker is capable of breaking at a specified voltage under defined test conditions, which include one break and one make/break operations.
      • Rated service short-circuit breaking capacity (Ics)
        Summarised as: The r.m.s prospective current that the circuit breaker is capable of breaking at a specified voltage under defined test conditions, which include one break and two make/break operations. The standard specifies fixed relationships to Icu of 25, 50, 75 or 100%.
      • Rated short-time withstand current (Icw)
        Summarised as: The r.m.s value of short-time current assigned by the manufacturer based on specified test conditions. Minimum values are given in the standard.

A circuit-breaker can only be assigned a rated short-time withstand current Icw if it is equipped with a time-delay overcurrent release.

All circuit-breakers to IEC60947-2 will have values of Icu and Ics.

Characteristics of circuit-breakers not mandated in IEC60947-2 but having application to short-circuit protection:

  • Cut-off current
    The maximum instantaneous value reached by the current during the breaking operation of a circuit-breaker when it operates to prevent the current reaching the prospective peak.
  • Operating I²t (Joule integral)
    Integral of the square of the current over the operating time of the circuit-breaker on a short-circuit. Sometimes referred to as ‘energy let-through’. When expressed in A²t gives the energy dissipated per ohm and thus represents the thermal effect on the circuit.

Examples of the Practical Application of the Product Characteristics

In simple studies only the r.m.s value of steady-state short-circuit current (Ik) is quoted. The peak current is assumed to be in a standard relationship to the r.m.s current, determined by the overall power factor, and taken into account in the rating of SCPDs to the respective IEC standards.

Circuit Protection

The application of short-circuit protective devices (SCPD) to circuit protection i.e. the protection of cables, is detailed in the installation rules, IEC364. In general it is accepted that selection of the protective device on the basis of thermal protection of a cable automatically provides short-circuit protection up to the breaking capacity of the SCPD, in the case of non-time-delayed devices.

Short-Circuit Protection for LV assemblies
Switchboard/Motor-Control Centre

The prospective short-circuit current at the input to the switchboard is obtained from a system protection study.
This will be given as an r.m.s value.

  • If the switchboard has an Icw current value higher than the prospective current level then the only requirement is to limit the time for which a short-circuit could persist to within the short-time value. This is achieved by the setting of releases upstream or at the incomer to the switchboard.
  • If the switchboard has an Icc rating higher than the prospective current level then the only requirement is to include the specified SCPD in the circuit. This may be added in the circuit upstream or may already be included as an incomer to the switchboard.
Busbar Trunking (BBT)

The prospective short-circuit current at the input to the switchboard is obtained from a system protection study.
This will be given as an r.m.s value.

  • If the BBT has an Icw current value higher than the prospective short circuit current level then the only requirement is to limit the time for which a short-circuit could persist to within the short-time value. This is achieved by the time-delay setting of overcurrent releases upstream.
  • If the BBT has an Icw lower than the prospective short circuit current level Ik but has an Icc rating higher than Ik then the only requirement is to include the specified SCPD in the circuit upstream or in the end-feed unit. The suitability of any given SCPD may be derived from the cut-off current and Joule-integral characteristics by comparison with proof-test parameters.
Motor Control Gear (MCG)

Motor starters and contactors are not generally self-protecting against the effects of short-circuit and therefore need to be associated with an SCPD. In this particular case test procedures to IEC60947-4-1 recognise the difficulty of protecting sensitive devices from damage under heavy short-circuit conditions. Thus a special case of conditional rating is obtained which allows two classes of co-ordination with an SCPD:

Type 1 – in which a certain amount of damage to the MCG is accepted.
Type 2 – in which the MCG is capable of further use.

These ratings can only be obtained by type-testing and thus the data must be obtained from the manufacturer of the SCPD or the MCG.

Miniature Circuit Breakers (MCBs)

When applied in other than domestic (household) situations the short-circuit capability of MCBs to IEC60898 is often inadequate and they need to be ‘backed-up’ by another SCPD. Details of how the appropriate SCPD is determined are given, for circuit-breakers, in Appendix A of IEC60947-2. Basically this shows that only testing of the required combination is satisfactory and thus the data must be obtained from the manufacturer of the SCPD or the MCB. The same applies to fuses used as SCPD.

Izvor: www.voltimum.co.uk

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What to use? Vacuum or SF6 circuit breaker?

What CB to use? Vacuum or SF6 circuit breaker?

Until recently oil circuit breakers were used in large numbers for Medium voltage Distribution system in many medium voltage switchgears. There are number of disadvantages of using oil as quenching media in circuit breakers. Flammability and high maintenance cost are two such disadvantages! Manufacturers and Users were forced to search for different medium of quenching. Air blast and Magnetic air circuit breakers were developed but could not sustain in the market due to other disadvantages associated with such circuit breakers. These new types of breakers are bulky and cumbersome. Further research were done and simultaneously two types of breakers were developed with SF6 as quenching media in one type and Vacuum as quenching media in the other. These two new types of breakgasers will ultimately replace the other previous types completely shortly. There are a few disadvantages in this type of breakers also. One major problem is that the user of the breakers are biased in favour of old fashioned oil circuit breakers and many of the users always have a step motherly attitude to the new generations of the breakers. However in due course of time this attitude will disappear and the new  type of breakers will get its acceptance among the users and ultimately they will completely replace the oil circuit breakers. An attempt is made to make a comparison between the SF6 type and vacuum type circuit breakers with a view to find out as to which of the two types is superior to the other. We will now study in detail each type separately before we compare them directly.

Vacuum Circuit Breaker
Evolis Circuit Breaker

Evolis MV Circuit Breaker

In a Vacuum circuit breaker, vacuum interrupters are used for breaking and making load and fault currents. When the contacts in vacuum interrupter separate, the current to be interrupted  initiates a metal vapour arc discharge and flows through the plasma until the next current zero. The arc is then extinguished and the conductive metal vapour condenses on the metal surfaces within a matter of micro seconds. As a result the dielectric strength in the breaker builds up very rapidly.

The properties of a vacuum interrupter depend largely on the material and form of the contacts. Over the period of their development, various types of contact material have been used. At the moment it is accepted that an oxygen free copper chromium alloy is the best material for High voltage circuit breaker. In this alloy , chromium is distributed through copper in the form of fine grains. This material combines good arc extinguishing characteristic with a reduced tendency to contact welding and low chopping current when switching inductive current. The use of this special material is that the current chopping is limited to 4 to 5 Amps.

At current under 10KA, the Vacuum arc burns as a diffuse discharge. At high values of current the arc changes to a constricted form with an anode spot. A  constricted arc that remain on one spot for too long can thermically over stress the contacts to such a degree that the deionization of the contact zone at current zero can no longer be guaranteed . To overcome this problem the arc root must be made to move over the contact surface. In order to achieve this, contacts are so shaped that the current flow through them results in a magnetic field being established which is at right angles to the arc axis. This radial field causes the arc root to rotate rapidly around the contact resulting in a uniform distribution of the heat over its surface. Contacts of this type are called radial magnetic field electrodes and they are used in the majority of circuit breakers for medium voltage application.

A new design has come in Vacuum interrupter, in which switching over the arc from diffusion to constricted state by subjecting the arc to an axial magnetic field. Such a field can be provided by leading the arc current through a coil suitably arranged outside the vacuum chamber. Alternatively the field can be provided by designing the contact to give the required contact path. Such contacts are called axial magnetic field electrodes. This principle has advantages when the short circuit current is in excess of 31.5 KA.

SF6 Gas Circuit Breaker
SF6 circuit breakers

SF6 circuit breakers

In an SF6 circuit-breaker, the current continues to flow after contact separation through the arc whose plasma consists of ionized SF6 gas. For, as long as it is burning, the arc is subjected to a constant flow of gas which extracts heat from it. The arc is extinguished at a current zero, when the heat is extracted by the falling current. The continuing flow of gas finally de-ionises the contact gap and establishes the dielectric strength required to prevent a re-strike.

The direction of the gas flow, i.e., whether it is parallel to or across the axis of the arc, has a decisive influence on the efficiency of the arc interruption process. Research has shown that an axial flow of gas creates a turbulence which causes an intensive and continuous interaction between the gas and the plasma as the current approaches zero. Cross-gas-flow cooling of the arc is generally achieved in practice by making the arc move in the stationary gas. This interruption process can however, lead to arc instability and resulting great fluctuations in the interrupting capability of the circuit breaker.

In order to achieve a flow of gas axially to the arc a pressure differential must be created along the arc. The first generation of the SF6 circuit breakers used the two-pressure principle of the air-blast circuit-breaker. Here a certain quantity of gas was kept stored at a high pressure and released into the arcing chamber. At the moment high pressure gas and the associated compressor was eliminated by the second generation design. Here the pressure differential was created by a piston attached to the moving contacts which compresses the gas in a small cylinder as the contact opens. A disadvantage is that this puffer system requires a relatively powerful operating mechanism.

Neither of the two types of circuit breakers described was able to compete with the oil circuit breakers price wise. A major cost component of the puffer circuit-breaker is the operating mechanism; consequently developments followed which were aimed at reducing or eliminating this additional cost factor. These developments concentrated on employing the arc energy itself to create directly the pressure-differential needed. This research led to the development of the self-pressuring circuit-breaker in which the over – pressure is created by using the arc energy to heat the gas under controlled conditions. During the initial stages of development, an auxiliary piston was included in the interrupting mechanism, in order to ensure the satisfactory breaking of small currents. Subsequent improvements in this technology have eliminated this requirement and in the latest designs the operating mechanism must only provide the energy needed to move the contacts.

Parallel to the development of the self-pressuring design, other work resulted in the rotating – arc SF6 gas circuit breaker. In this design the arc is caused to move through, in effect the stationery gas. The relative movement between the arc and the gas is no longer axial but radial, i.e., it is a cross-flow mechanism. The operating energy required by circuit breakers of this design is also minimal.

Table 1. Characteristics of the SF6 and vacuum current interrupting technologies.

SF6 Circuit BreakersVacuum Circuit Breakers
CriteriaPuffer Circuit BreakerSelf-pressuring circuit-breakerContact material-Chrome-Copper
Operating energy requirementsOperating Energy requirements are high, because the mechanism must supply the energy needed to compress the gas.Operating Energy requirements are low, because the mechanism must move only relatively small masses at moderate speed, over short distances. The mechanism does not have to provide the energy to create the gas flowOperating energy requirements are low, because the mechanism must move only relatively small masses at moderate speed, over very short distances.
Arc EnergyBecause of the high conductivity of the arc in the SF6 gas, the arc energy is low. (arc voltage is between 150 and 200V.)Because of the very low voltage across the metal vapour arc, energy is very low. (Arc voltage is between 50 and 100V.)
Contact ErosionDue to the low energy the contact erosion is small.Due to the very low arc energy, the rapid movement of the arc root over the contact and to the fact that most of the metal vapour re-condenses on the contact, contact erosion is extremely small.
Arc extinguishing mediaThe gaseous medium SF6 possesses excellent dielectric and arc quenching properties. After arc extinction, the dissociated gas molecules recombine almost completely to reform SF6. This means that practically no loss/consumption of the quenching medium occurs. The gas pressure can be very simply and permanently supervised. This function is not needed where the interrupters are sealed for life.No additional extinguishing medium is required. A vacuum at a pressure of 10-7 bar or less is an almost ideal extinguishing medium. The interrupters are ‘sealed for life’ so that supervision of the vacuum is not required.
Switching behavior in relation to current choppingThe pressure build-up and therefore the flow of gas is independent of the value of the current. Large or small currents are cooled with the same intensity. Only small values of high frequency, transient currents, if any, will be interrupted. The de-ionization of the contact gap proceeds very rapidly, due to the electro-negative characteristic of the SF6 gas and the arc products.The pressure build-up and therefore the flow of gas is dependent upon the value of the current to be interrupted. Large currents are cooled intensely, small currents gently. High frequency transient currents will not, in general, be interrupted. The de-ionization of the contact gap proceeds very rapidly due to the electro-negative characteristic of the SF6 gas and the products.No flow of an ‘extinguishing’ medium needed to extinguish the vacuum arc. An extremely rapid de-ionization of the contact gap, ensures the interruption of all currents whether large or small. High frequency transient currents can be interrupted. The value of the chopped current is determined by the type of contact material used. The presence of chrome in the contact alloy with vacuum also.
No. of short-circuit operation10—5010—5030—100
No. full load operation5000—100005000—1000010000—20000
No. of mechanical operation5000—200005000—2000010000—30000

Comparison of the SF6 And Vacuum Technologies

The most important characteristics of the SF6 gas and vacuum-circuit breakers, i.e., of SF6 gas and vacuum as arc-extinguishing media are summarized in Table-1.

In the case of the SF6 circuit-breaker, interrupters which have reached the limiting number of operations can be overhauled and restored to ‘as new’ condition. However, practical experience has shown that under normal service conditions the SF6 interrupter never requires servicing throughout its lifetime. For this reason, some manufacturers no longer provide facilities for the user to overhaul the circuit-breaker, but have adopted a ‘sealed for life’ design as for the vacuum-circuit breaker.

The operating mechanisms of all types of circuit-breakers require servicing, some more frequently than others depending mainly on the amount of energy they have to provide. For the vacuum-circuit breaker the service interval lies between 10,000 and 20,000 operations. For the SF6 designs the value varies between 5,000 and 20,000 whereby, the lower value applies to the puffer circuit-breaker for whose operation, the mechanism must deliver much more energy.

The actual maintenance requirements of the circuit-breaker depend upon its service duty, i.e. on the number of operations over a given period of time and the value of current interrupted. Based on the number of operations given in the previous section, it is obvious that SF6 and vacuum circuit-breakers used in public supply and /or industrial distribution systems will, under normal circumstances, never reach the limits of their summated breaking current value. Therefore, the need for the repair or replacement of an interrupter will be a rare exception and in this sense these circuit-breakers can be considered maintenance-free. Service or maintenance requirements are therefore restricted to routine cleaning of external surfaces and the checking and lubrication of the mechanism, including the trip-linkages and auxiliary switches. In applications which require a very high number of circuit-breaker operations e.g. for arc furnace duty or frequently over the SF6 design, due to its higher summated-breaking current capability. In such cases it is to be recommended that the estimation of circuit-breaker maintenance costs be given some consideration and that these be included in the evaluation along with the initial, capital costs.

Reliability

In practice, an aspect of the utmost importance in the choice of a circuit-breaker is reliability.

The reliability of a piece of equipment is defined by its mean time to failure (MTF), i.e. the average interval of time between failures. Today, the SF6 and vacuum circuit-breakers made use of the same operating mechanisms, so in this regard they can be considered identical.

However, in relation to their interrupters the two circuit breakers exhibit a marked difference. The number of moving parts is higher for the SF6 circuit-breaker than that for the vacuum unit. However, a reliability comparison of the two technologies on the basis of an analysis of the number of components are completely different in regards design, material and function due to the different media. Reliability is dependent upon far too many factors, amongst others, dimensioning, design, base material, manufacturing methods, testing and quality control procedures, that it can be so simply analyzed.

In the meantime, sufficient service experience is available for both types of circuit-breakers to allow a valid practical comparison to be made. A review of the available data on failure rates confirms that there is no discernible difference in reliability between the two circuit-breaker types. More over, the data shows that both technologies exhibit a very high degree of reliability under normal and abnormal conditions.

Switching of fault currents

Today, all circuit-breakers from reputable manufacturers are designed and type-tested in conformance with recognized national or international standards (IEC56). This provides the assurance that these circuit-breakers will reliably interrupt all fault currents up to their maximum rating. Further, both types of circuit-breakers are basically capable of interrupting currents with high DC components; such currents can arise when short circuits occur close to a generator. Corresponding tests have indeed shown that individual circuit-breakers of both types are in fact, capable of interrupting fault currents with missing current zeros i.e. having a DC component greater than 100 per cent. Where such application is envisaged, it is always to be recommended that the manufacturer be contacted and given the information needed for a professional opinion.

As regards the recovery voltage which appears after the interruption of a fault current the vacuum-circuit breaker can, in general, handle voltages with RRV values of up to 5KV. SF6 circuit-breakers are more limited, the values being in the range from 1 to 2 KV. In individual applications, e.g. in installations with current limiting chokes or reactors, etc., With SF6 circuit-breakers it may be advisable or necessary to take steps to reduce that rate of rise of the transient recovery voltage.

Switching small inductive currents

The term, small inductive currents is here defined as those small values of almost pure inductive currents, such as occur with unloaded transformers, motor during the starting phase or running unloaded and reactor coils. When considering the behavior of a circuit-breaker interrupting such currents, it is necessary to distinguish between high frequency and medium frequency transient phenomena.

Medium frequency transients arise from, amongst other causes, the interruption of a current before it reaches its natural zero. All circuit-breakers can, when switching currents of the order of a few hundred amperes and, due to instability in the arc, chop the current immediately prior to a current zero.

This phenomenon is termed real current chopping. When it occurs, the energy stored in the load side inductances oscillates through the system line to earth capacitances (winding and cable capacitances) and causes an increase in the voltage. This amplitude of the resulting over voltage is a function of the value of the current chopped. The smaller the chopped current, the lower the value of the over voltage.

In addition to the type of circuit – breaker, the system parameters at the point of installation are factors which determine the height of the chopping current, in particular the system capacitance parallel to the circuit breaker is of importance. The chopping current of SF6 circuit-breakers is essentially determined by the type of circuit-breaker. The value of chopping current varies from 0.5A to 15A, whereby the behavior of the self – pressuring circuit-breaker is particularly good, its chopping current being less than 3A.This ‘soft’

Switching feature is attributable to the particular characteristics of the interrupting mechanism of the self-pressuring design and to the properties of the SF6 gas itself.

In the early years of the development of the vacuum circuit-breaker the switching of small inductive currents posed a major problem, largely due to the contact material in use at that time. The introduction of the chrome copper contacts brought a reduction of the chopping current to between 2 to 5A.The possibility of impermissible over voltages arising due to current chopping has been reduced to a negligible level.

High frequency transients arise due to pre- or re-striking of the arc across the open contact gap. If, during an opening operation, the rising voltage across the opening contacts, exceed the dielectric strength of the contact gap , a re-strike occurs. The high-frequency transient current arising from such a re-strike can create high frequency current zeros causing the circuit-breaker to, interrupt again. This process can cause a further rise in voltage and further re-strikes. Such an occurrence is termed as multiple restriking.

With circuit- breakers that can interrupt high frequency transient currents, re-striking can give rise to the phenomenon of virtual current chopping. Such an occurrence is possible when a re-strike in the first-phase-to-clear, induces high frequency transients in the other two phases, which are still carrying service frequency currents. The superimposition of this high frequency oscillation on the load current can cause an apparent current zero and an interruption by the circuit-breaker, although the value of load current may be quite high. This phenomenon is called virtual current chopping and can result in a circuit breaker ‘chopping’ very much higher values of current than it would under normal conditions. The results of virtual current chopping are over-voltages of very high values.

This phenomenon is termed real current chopping. When it occurs, the energy Stored in the load side inductances oscillates through the system line to earth capacitances (winding and cable capacitances) and causes an increase in the voltage. This amplitude of the resulting over voltage is a function of the value of the current chopped. The smaller the chopped current, the lower the value of the over voltage.

In addition to the type of circuit – breaker, the system parameters at the point of installation are factors which determine the height of the chopping current, in particular the system capacitance parallel to the circuit breaker is of importance. The chopping current of SF6 circuit-breakers is essentially determined by the type of circuit-breaker. The value of chopping current varies from 0.5A to 15A, whereby the behaviour of the self – pressuring circuit-breaker is particularly good, its chopping current being less than 3A.This ‘soft’   Switching feature is attributable to the particular characteristics of the interrupting mechanism of the self-pressuring design and to the properties of the SF6 gas itself.

In the early years of the development of the vacuum circuit-breaker the switching of small inductive currents posed a major problem, largely due to the contact material in use at that time. The introduction of the chrome copper contacts brought a reduction of the chopping current to between 2 to 5A.The possibility of impermissible over voltages arising due to current chopping has been reduced to a negligible level.

High frequency transients arise due to pre- or re-striking of the arc across the open contact gap. If, during an opening operation, the rising voltage across the opening contacts exceeds the dielectric strength of the contact gap, a re-strike occurs. The high-frequency transient current arising from such a re-strike can create high frequency current zeros causing the circuit-breaker to, interrupt again. This process can cause a further rise in voltage and further re-strikes. Such an occurrence is termed as multiple re-striking.

With circuit- breakers that can interrupt high frequency transient currents, re-striking can give rise to the phenomenon of virtual current chopping. Such an occurrence is possible when a re-strike in the first-phase-to-clear, induces high frequency transients in the other two phases, which are still carrying service frequency currents. The superimposition of this high frequency oscillation on the load current can cause an apparent current zero and an interruption by the circuit-breaker, although the value of load current may be quite high. This phenomenon is called virtual current chopping and can result in a circuit breaker ‘chopping’ very much higher values of current than it would under normal conditions. The results of virtual current chopping are over-voltages of very high values

Table2. Comparison of the SF6 And Vacuum Technologies In Relation To Operational Aspects

CriteriaSF6 BreakerVacuum Circuit Breaker
Summated current cumulative10-50 times rated short circuit current30-100 times rated short circuit current
Breaking current capacity of interrupter5000-10000 times10000-20000 times
Mechanical operating life5000-20000 C-O operations10000-30000 C-O operations
No operation before maintenance5000-20000 C-O operations10000-30000 C-O operations
Time interval between servicing Mechanism5-10 years5-10 years
Outlay for maintenanceLabour cost High, Material cost LowLabour cost Low, Material cost High
ReliabilityHighHigh
Dielectric withstand strength of the contact gapHighVery high

Very extensive testing has shown that, because of its special characteristics the SF6 self-pressuring circuit-breaker possesses considerable advantages in handling high frequency transient phenomena, in comparison with both the puffer type SF6 and the vacuum circuit breakers. The past few years have seen a thorough investigation of the characteristics of vacuum circuit breakers in relation to phenomena such as multiple re-striking and virtual current chopping. These investigations have shown that the vacuum circuit-breaker can indeed cause more intense re-striking and hence more acute over voltages than other types. However, these arise only in quite special switching duties such as the tripping of motors during starting and even then only with a very low statistical probability. The over-voltages which are created in such cases can be reduced to safe levels by the use of metal oxide surge diverters.

Table3. Comparison of the SF6 And Vacuum Switching Technologies In Relation To Switching Applications

CriteriaSF6 Circuit BreakerVacuum Circuit Breaker
Switching of Short circuit current with High DC componentWell suitedWell suited
Switching of Short circuit current with High RRVWell suited under certain conditions (RRV>1-2 kV per Milli secondsVery well suited
Switching of transformersWell suited.Well suited
Switching of reactorsWell suitedWell suited. Steps to be taken when current <600A. to avoid over voltage due to current chopping
Switching of capacitorsWell suited. Re-strike freeWell suited. Re-strike free
Switching of capacitors back to backSuited. In some cases current limiting reactors required to limit inrush currentSuited. In some cases current limiting reactors required to limit inrush current
Switching of arc furnaceSuitable for limited operationWell suited. Steps to be taken to limit over voltage.

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