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Busbar Technical Specification
Copper busbars are normally part of a larger generation or transmission system. The continuous rating of the main components such as generators, transformers, rectifiers, etc., therefore determine the nominal current carried by the busbars but in most power systems a one to four second short-circuit current has to be accommodated.
The value of these currents is calculated from the inductive reactances of the power system components and gives rise to different maximum short-circuit currents in the various system sections.
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Performance under Short-circuit Conditions
Busbar trunking systems to BS EN 60439-2 are designed to withstand the effects of short-circuit currents resulting from a fault at any load point in the system, e.g. at a tap off point or at the end of a feeder run.
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Rating under Short-circuit Conditions
The withstand ability will be expressed in one or more of the following ways:
- short-time withstand rating (current and time)
- peak current withstand rating
- conditional short-circuit rating when protected by a short-circuit protective device (s.c.p.d.)
These ratings are explained in more detail:
1. Short-time Withstand Rating
This is an expression of the value of rms current that the system can withstand for a specified period of time without being adversely affected such as to prevent further service. Typically the period of time associated with a short-circuit fault current will be 1 second, however, other time periods may be applicable.
The rated value of current may be anywhere from about 10kA up to 50kA or more according to the construction and thermal rating of the system.
2. Peak Current Withstand Rating
This defines the peak current, occurring virtually instantaneously, that the system can withstand, this being the value that exerts the maximum stress on the supporting insulation.
In an A.C. system rated in terms of short-time withstand current the peak current rating must be at least equal to the peak current produced by the natural asymmetry occurring at the initiation of a fault current in an inductive circuit. This peak is dependent on the power-factor of the circuit under fault conditions and can exceed the value of the steady state fault current by a factor of up to 2.2 times.
3. Conditional Short-circuit Rating
Short-circuit protective devices (s.c.p.ds) are commonly current-limiting devices; that is they are able to respond to a fault current within the first few milliseconds and prevent the current rising to its prospective peak value. This applies to HRC fuses and many circuit breakers in the instantaneous tripping mode. Advantage is taken of these current limiting properties in the rating of busbar trunking for high prospective fault levels. The condition is that the specified s.c.p.d. (fuse or circuit breaker) is installed up stream of the trunking. Each of the ratings above takes into account the two major effects of a fault current, these being heat and electromagnetic force.
The heating effect needs to be limited to avoid damage to supporting insulation. The electromagnetic effect produces forces between the busbars which stress the supporting mechanical structure, including vibrational forces on A.C. The only way to verify the quoted ratings satisfactorily is by means of type tests to the British Standard.
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Type Testing
Busbar trunking systems are tested in accordance with BS EN 60439-2 to establish one or more of the short circuit withstand ratings defined above. In the case of short-time rating the specified current is applied for the quoted time. A separate test may be required to establish the peak withstand current if the quoted value is not obtained during the short-time test. In the case of a conditional rating with a specified s.c.p.d. the test is conducted with the full prospective current value at the trunking feeder unit and not less than 105% rated voltage, since the s.c.p.d. (fuse or circuit breaker) will be voltage dependent in terms of let through energy.
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Application
It is necessary for the system designer to determine the prospective fault current at every relevant point in the installation by calculation, measurement or based on information provided e.g. by the supply authority. The method for this is well established, in general terms being the source voltage divided by the circuit impedance to each point. The designer will then select protective devices at each point where a circuit change occurs e.g. between a feeder and a distribution run of a lower current rating. The device selected must operate within the limits of the busbar trunking short-circuit withstand.
The time delay settings of any circuit breaker must be within the specified short time quoted for the prospective fault current. Any s.c.p.d. used against a conditional short-circuit rating must have energy limitation not exceeding that of the quoted s.c.p.d. For preference the s.c.p.d. recommended by the trunking manufacturer should be used.
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Voltage Drop
The requirements for voltage-drop are given in BS 7671: Regulation 525-01-02. For busbar trunking systems the method of calculating voltage drop is given in BS EN 60439-2 from which the following guidance notes have been prepared.
Voltage Drop
Figures for voltage drop for busbar trunking systems are given in the manufacturer’s literature.
The figures are expressed in volts or milli-volts per metre or 100 metres, allowing a simple calculation for a given length of run.
The figures are usually given as line-to-line voltage drop for a 3 phase balanced load.
The figures take into account resistance to joints and temperature of conductors and assume the system is fully loaded.
Standard Data
BS EN 60439-2 requires the manufacturer to provide the following data for the purposes of calculation, where necessary:
R20 the mean ohmic resistance of the system, unloaded, at 20ºC per metre per phase
X the mean reactance of the system, per metre per phase
For systems rated over 630A:
RT the mean ohmic resistance when loaded at rated current per metre per phase
Application
In general the voltage drop figures provided by the manufacturer are used directly to establish the total voltage drop on a given system; however this will give a pessimistic result in the majority of cases.
Where a more precise calculation is required (e.g. for a very long run or where the voltage level is more critical) advantage may be taken of the basic data to obtain a more exact figure.
- Resistance – the actual current is usually lower than the rated current and hence the resistance of the conductors will be lower due to the reduced operating temperature.
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Rx = R20 [1+0.004(Tc - 20)] ohms/metre and Tc is approximately Ta + Trwhere Rx is the actual conductor resistance
Ta is the ambient temperature
Tr is the full load temperature rise in ºC (assume say 55ºC)
- Power factor – the load power factor will influence the voltage drop according to the resistance and reactance of the busbar trunking itself.
The voltage drop line-to-line ( Δv) is calculated as follows:Δv = √ 3 I (R x cos Φ + X sin Φ) volts/metre
where I is the load current
Rx is the actual conductor resistance (Ω/m)
X is the conductor reactance (Ω/m)
Cos Φ is the load power factor
sin Φ = sin (cos-1 Φ )
- Distributed Load – where the load is tapped off the busbar trunking along its length this may also be taken into account by calculating the voltage drop for each section. As a rule of thumb the full load voltage drop may be divided by 2 to give the approximate voltage drop at the end of a system with distributed load.
. - Frequency – the manufacturers data will generally give reactance (X) at 50Hz for mains supply in the UK. At any other frequency the reactance should be re-calculated.
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Xf = x F/50
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where Xf is the reactance at frequency F in Hz
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Source: Siemens Barduct Busbar Specification
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Power quality
The term power quality seeks to quantify the condition of the electrical supply. It not only relates largely to voltage, but also deals with current and it is largely the corrupting effect of current disturbances upon voltage. Power quality can be quantified by a very broad range of parameters, some of which have been recognized and studied for as long as electrical power has been utilized. However, the advent of the term itself is more modern and it has created a useful vehicle for discussing and quantifying all factors that can describe supply quality. Power quality is yet another means of analysing and expressing electromagnetic compatibility (EMC), but in terms of the frequency spectrum, power quality charac- terizes mainly low-frequency phenomena. Perhaps because of this and because of the manner in which it affects electrical equipment, power quality has largely been dealt with by engineers with electrical power experience rather than those with an EMC expertise. In reality, resolving power problems can benefit from all available expertise, particularly since power quality disorders and higher frequency emissions can produce similar effects.
In 1989, the European Community defined the supply of electricity as a product, and it is therefore closely related to the provisions and protection of the EMC Directive (89/336/EEC), but in drawing a comparison between electricity and other manufactured product it is essential to recall a significant difference.
Electricity is probably unique in being a product which is manufactured, delivered and used at the same time. An electricity manufacturer cannot institute a batch testing process for example and pull substandard products out of the supply chain. By the time electricity is tested it will have been delivered and used by the customer whether it was of good quality or not.
Key parameters
The parameters that are commonly used to characterize supplies are listed in Table 1 together with the typical tolerance limits which define acceptable norms. Within Europe these power quality limits are defined by the EN 61000 series of standards in order to be compatible with the susceptibility limits set for equipment.
| Table 1: Summary of power quality levels defined by EN 50160 | |
| .Power frequency (50Hz) | .Interconnected systems .±1% (95% of week) .+4% (absolute level) .−6% (absolute level) |
| .Supply voltage variations on 230V nominal | .±10% (95% of week based on .10 min samples, rms) |
| .Rapid voltage changes | .±5% Frequent .±10% Infrequent |
| .Flicker | .Pk=1.0 (95% of week) |
| .Supply voltage dips | .Majority .Few 10s .Duration <1s .Depth <60% .Some locations .Few 1000 per year of <15% depth |
| .Short interruptions | .20–500 per year .Duration 1s of 100% depth |
| .Long interruptions | .10–50 per year .Duration >180s of 100% depth |
| .Temporary power frequency overvoltage | .<1.5kV |
| .Transient overvoltages | .Majority .<6kV .Exceptionally .>6kV |
| .Supply voltage unbalance | .Majority .<2%(95% of the week) .Exceptionally .>2%, <3%(95% of the week) |
| .Harmonic voltage distortion | .THD <8%(95% of the week) |
| .Interharmonic voltage distortion | .Under consideration |
| .Mains signalling | .95 to 148.5kHz at up to 1.4Vrms (not in MV) |
The more a supply deviates from these limits, the more likely it is that malfunction could be experienced in terminating equipment. However, individual items of equipment will have particular sensitivity to certain power quality parameters while having a wider tolerance to others. Table 2 provides examples of equipment and the power quality parameters to which they are particularly sensitive. Table 2 shows a preponderance of examples with a vulnerability to voltage dips. Of all the power quality parameters, this is probably the most troublesome to the manufacturing industry; and in the early 1970s, as the industry moved towards a reliance on electronic rather than electromagnetic controls, it was commonly observed how much more vulnerable the industrial processes were to supply disturbances.
Supply distortion (characterized by harmonics) is another power quality parameter that has received enormous attention, with many articles, textbooks and papers written on the subject. However, the modern practices that will be discussed later have reduced the degree to which this currently presents a problem. Other parameters tend to be much less problematic in reality, although that is not to say that perceptions sometimes suggest otherwise. Voltage surge and tran- sient overvoltage in particular are often blamed for a wide range of problems.
| Table 2: Examples of sensitivity to particular power quality parameters | |||
| Equipment type | Vulnerable power quality parameter | Effect if exceeded | Rang |
| .Induction motor | .Voltage unbalance | .Excessive rotor heating | .<3% |
| .Power factor correction .capacitors | .Spectral frequency .content.This is usually .defined .in terms of harmonic .distortion | .Capacitor failure due to .excessive current flow or .voltage.Most sensitive if .resonance occurs | .In resonant .conditions |
| .PLCs | .Voltage dips | .Disruption to the programmed .functionality | .V tr |
| .Computing systems | .Voltage dips | .Disruption to the programmed .functionality | .V |
| .Variable speed drives, .motor starters and .attracted .armature control .relays | .Voltage dips | .Disruption to the control system .causing shutdown | . V |
| .Power transformers | .Spectral frequency content of .load current.This is usually .defined in terms of harmonic .distortion | .Increased losses leading to excessive temperature rise | .At full load |
| .Devices employing .phase .control, such as .light .dimmers and .generator .automatic .voltage regulator .(AVRs) | .Alteration in waveform zero .crossing due to waveform .distortion, causing multiple .crossing or phase asymmetry | .Instability | .Will depend .upon the r |
| .Motor driven .speed-.sensitive plant | .Induction and synchronous .motor shaft speed are .proportional to supply .frequency. Some driven loads .are .sensitive to even small .speed variations | .The motors themselves are .tolerant of small speed .variations.At high supply .frequencies (>10%) shaft .stresses may be excessive due .to high running speeds | .Limits .depend on .the .sensitivity |
However, very often this is a scapegoat when the actual cause cannot be identified. Even when correlation with switching voltage transients is correctly observed, the coupling introduced by poor wiring installations or bad earth bonding practices can be the real problem. Unlike the other power quality parameters, voltage transients have a high frequency content and will couple readily through stray capacitance and mutual inductance into neighbouring circuits. Coupling into closed conductor loops that interface with sensitive circuits such as screens and drain wires can easily lead to spurious events.
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