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Line Protection With Distance Relays
Distance relaying should be considered when overcurrent relaying is too slow or is not selective. Distance relays are generally used for phase-fault primary and back-up protection on subtransmission lines, and on transmission lines where high-speed automatic reclosing is not necessary to maintain stability and where the short time delay for end-zone faults can be tolerated.
Overcurrent relays have been used generally for ground-fault primary and back-up protection, but there is a growing trend toward distance relays for ground faults also. Single-step distance relays are used for phase-fault back-up protection at the terminals of generators. Also, single-step distance relays might be used with advantage for back-up protection at power-transformer banks, but at the present such protection is generally provided by inverse-time overcurrent relays. Distance relays are preferred to overcurrent reIays because they are not nearly so much affected by changes in short-circuit-current magnitude as overcurrent relays are, and, hence, are much less affected by changes in generating capacity and in system configuration.
This is because, distance relays achieve selectivity on the basis of impedance rather than current.
The choice between impedance, reactance, or MHO
Because ground resistance can be so variable, a ground distance relay must be practically unaffected by large variations in fault resistance. Consequently, reactance relays are generally preferred for ground relaying. For phase-fault relaying, each type has certain advantages and disadvantages. For very short line sections, the reactance type is preferred for the reason that more of the line can be protected at high speed. This is because the reactance relay is practically unaffected by arc resistance which may be large compared with the line impedance, as described elsewhere in this chapter. On the other hand, reactance-type distance relays at certain locations in a system are the most likely to operate undesirably on severe synchronizing power surges unless additional relay equipment is provided to prevent such operation.
The mho type is best suited for phase-fault relaying for longer lines, and particularly where severe synchronizing-power surges may occur. It is the least likely to require additional equipment to prevent tripping on synchronizing-power surges. When mho relaying is adjusted to protect any given line section, its operating characteristic encloses the least space on the R-X diagram, which means that it will be least affected by abnormal system conditions other than line faults; in other words, it is the most selective of all distance relays.
Because the mho relay is affected by arc resistance more than any other type, it is applied to longer lines. The fact that it combines both the directional and the distancemeasuring functions in one unit with one contact makes it very reliable.
The impedance relay is better suited for phase-fault relaying for lines of moderate length than for either very short or very long lines. Arcs affect an impedance relay more than a reactance relay but less than a mho relay. Synchronizing-power surges affect an impedance relay less than a reactance relay but more than a mho relay. If an impedance-relay characteristic is offset, so as to make it a modified relay, it can be made to resemble either a reactance relay or a mho relay but it will always require a separate directional unit.
There is no sharp dividing line between areas of application where one or another type of distance relay is best suited. Actually, there is much overlapping of these areas. Also, changes that are made in systems, such as the addition of terminals to a line, can change the type of relay best suited to a particular location. Consequently, to realize the fullest capabilities of distance relaying, one should use the type best suited for each application. In some cases much better selectivity can be obtained between relays of the same type, but, if relays are used that are best suited to each line, different types on adjacent lines have no appreciable adverse effect on selectivity.
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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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Dry-Type disc wound transformers in MV applications
Medium voltage, dry-type transformers may have their high voltage windings constructed using either the layer winding technique or the disc winding technique.
Both winding techniques provide the same result in terms of electrical performance parameters, i.e. turns ratio, impedance etc.
However, the use of transformers employing disc wound high voltage windings can result in increased reliability and therefore reduced downtime.
Introduction
The basic purpose of a transformer is to convert electricity at one voltage to electricity at another voltage, either of higher or lower value. In order to achieve this voltage conversion, coils are wound on a laminated silicon steel core which provides a path for the magnetic flux. The coils comprise a number of turns of conductor, either copper or aluminum, wound as two electrically separate windings, called the primary winding and the secondary winding. The primary winding is connected to the source of voltage while the secondary winding is connected to the load. The ratio of primary to secondary turns is the same as the required ratio of primary to secondary voltages.
The turns of conductor forming the primary and secondary windings must be insulated from one another, while the primary winding must be insulated from the secondary winding and both the primary and secondary windings must be insulated from ground. The insulation of turns and windings is collectively called the insulation system of the transformer. The insulation system must be designed to withstand the effects of lightning strikes and switching surges to which the transformer is subjected, in addition to the normal operating voltages. A further requirement of the insulation system is that it must withstand the environmental conditions to which it is exposed, such as moisture, dust etc. A variety of techniques and materials are employed to achieve the necessary performance characteristics of the insulation system.
Layer winding
Fig.1 Layer winding
For low voltage, i.e. 600 Volt class windings, the winding technique used almost exclusively is the layer winding technique, also sometimes called helical winding or barrel winding. In this technique, the turns required for the winding are wound in one or more concentric layers connected in series, with the turns of each layer being wound side by side along the axial length of the coil until the layer is full. The conductors of the winding are insulated and so between turns there will be a minimum of two thicknesses of insulation. Between each pair of layers there will be layers of insulation material and/or an air duct.
Low voltage windings will generally be wound top to bottom, bottom to top etc. using a continuous conductor, until all layers are complete. High voltage windings, i.e. above 600 Volt class, may be wound in the same way, provided the voltage between layers is not too great.
To reduce the voltage stress between layers, high voltage windings are often wound in only one direction, for example, top to bottom. When the first layer of winding is complete, the winding conductor is laid across the completed layer from bottom to top and then the next layer is wound, again from top to bottom. In this way, the voltage stress between layers is halved.
The conductor must, of course, have additional insulation where it crosses the winding from bottom to top.
Fig.2 Transformer with layer wound coils
Disc winding
In the disc winding, the required number of turns are wound in a number of horizontal discs spaced along the axial length of the coil. The conductor is usually rectangular in cross-section and the turns are wound in a radial direction, one on top of the other i.e. one turn per layer, until the required number of turns per disc has been wound.
Fig.3 Disc winding
The conductor is then moved to the next disc and the process repeated until all turns have been wound. There is an air space, or duct, between each pair of discs. The disc winding requires insulation only on the conductor itself, no additional insulation is required between layers, as in the layer winding.
The disc wound high voltage winding is usually wound in two halves, in order that the required voltage adjustment taps may be positioned at the electrical center of the winding. In this way the magnetic, or effective length of the winding is maintained, irrespective of which tap is used, and therefore the magnetic balance between primary and secondary windings is always close to its optimum.
This is essential to maintain the short circuit strength of the winding, and reduces the axial electromagnetic forces which arise when the windings are not perfectly balanced.
Fig.4 Transformer with disc wound coils
Characteristics of Layer wound coils
As stated previously, the layer wound coil requires insulation between layers, in addition to the conductor insulation. The thickness of insulation required will depend upon the voltage stress between layers, and comprises one or more thicknesses of the appropriate insulation material. In practice, due to the nature of the construction of a layer wound coil, the finished coil will have several unavoidable small air pockets between turns and between layers. Many of these air pockets will become filled with resin during vacuum pressure impregnation of the coil.
Fig. 5 Equivalent circuit for Impulse voltage distribution
However, it sometimes happens that some air pockets remain and it is in these air pockets that partial discharges can occur, greatly increasing the possibility of premature aging of the insulation and eventual failure.
Catastrophic failure can occur within a few months of energization. Under short circuit conditions, the electromagnetic forces developed cause transformer windings to attempt to telescope. At the same time the coil end blocking is trying to prevent movement. The result is often that the turns of the winding have a tendency to slip over one another, causing turn-to turn failure, due to abrasion of the insulation as the turns rub together. A further disadvantage of the layer wound coil is its poor impulse voltage distribution between the first few turns of the winding, due to the high ground capacitance and the low series capacitance.
A transformer winding forms a complex network of resistance, inductance and capacitance. As far as the impulse voltage distribution is concerned, the resistance can be ignored and at the instant of application of the impulse wave, when very high frequencies are predominant, the inductive elements become effectively infinite impedances. The whole structure therefore reduces to a capacitive network (see fig.5). Each turn of a transformer winding is insulated with a dielectric material and can be thought of as one plate of a multiple plate capacitor. In addition, the combination of dielectric material and air between each turn and ground forms further capacitive elements.
Characteristics of Disc wound coils
The major advantage of the disc wound coil lies in its open construction and relative lack of insulation. For a 15kV class transformer employing a disc wound primary winding, the number of discs will typically be in the range 36 to 48, resulting in a relatively low voltage per disc. Since each disc is separated from the next by an air space, the voltage stress between discs can easily be handled by the combination of conductor insulation and air, no additional insulation being necessary.
Each disc comprises a number of turns with each turn occupying one layer, i.e. one turn per layer: the voltage stress between layers is therefore the same as the voltage stress between turns and again, can easily be handled by the conductor insulation. The turns of each disc, being wound tightly together provide almost no possibility of air pockets being present within the disc.
Due to the open construction of the discs, any small air pockets which may be present are readily filled with resin during vacuum pressure impregnation of the coil. A properly designed and manufactured dry-type transformer disc winding therefore displays very low values of partial discharge, typically in the range 10 to 20 picocoulombs.
Unlike the layer wound coil, the disc wound coil provides good impulse voltage distribution, due to its inherently low value of ground capacitance and high series capacitance. The disc wound coil also displays excellent short circuit strength. Each disc by itself is mechanically very strong and the complete assembly of discs are held very securely in place. While the electromagnetic forces resulting from a short circuit result in a tendency, for the windings to telescope, the high voltage turns usually remain intact relative to each other. Instead, the complete disc has a tendency to distort as an assembly, with all the turns distorting by the same amount. The transformer can often continue to function, despite the distortion, until a convenient time arises for repair.
Losses/heat
The flow of electric current through the turns of a transformer winding causes power losses which manifest themselves in the form of heat. These losses are called ‘’load losses’’ and are proportional to the square of the current. Obviously, it is necessary to dissipate this heat, to prevent overheating of the transformer, and in a dry-type transformer, this is achieved by the use of air spaces, or ducts, within the winding. The layer wound coil relies on vertical air ducts between layers and between windings, for cooling. Cool air enters the air ducts at the bottom of the coil and by natural convection, rises through the ducts, collecting heat on its way, then exits the coil at the top. It is essential for proper operation of the transformer that these air ducts are kept clear at all times.
The insulation required between the layers of a layer wound coil has a tendency to thermally lag the winding, impeding the dissipation of heat. The greater the operating voltage of the winding, the greater is the amount of insulation required and the greater is the lagging effect of the insulation. Some radiation also takes place from the outer surfaces of the coils. The open nature of the disc wound coil greatly improves the transfer of heat from the winding to the surrounding air. The thermal lagging effect of insulation is removed and the multiple horizontal air spaces between discs provide a large surface area for cooling by both radiation and convection.
Conclusions
The combination of layer wound low voltage winding, disc wound high voltage winding, NOMEX insulation and vacuum pressure impregnation of the windings with a solventless epoxy resin, results in a very reliable transformer with a long life expectancy. Transformers constructed in this way will be relatively free from partial discharge and will provide excellent impulse strength and short circuit strength, vital requirements for reliable operation in the most demanding of applications.
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Author: Derek Foster, Olsun Electrics Corporation
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Maintenance Of Molded Case Circuit Breakers (MCCB)
The maintenance of circuit breakers deserves special consideration because of their importance for routine switching and for protection of other equipment.
Electric transmission system breakups and equipment destruction can occur if a circuit breaker fails to operate because of a lack of preventive maintenance.
The need for maintenance of circuit breakers is often not obvious as circuit breakers may remain idle, either open or closed, for long periods of time. Breakers that remain idle for 6 months or more should be made to open and close several times in succession to verify proper operation and remove any accumulation of dust or foreign material on moving parts and contacts.
Frequency Of Maintenance
Molded case circuit breakers are designed to require little or no routine maintenance throughout their normal life time. Therefore, the need for preventive maintenance will vary depending on operating conditions. As an accumulation of dust on the latch surfaces may affect the operation of the breaker, molded case circuit breakers should be exercised at least once per year.
Routine trip testing should be performed every 3 to 5 years.
Routine Maintenance Tests
Routine maintenance tests enable personnel to determine if breakers are able to perform their basic circuit protective functions. The following tests may be performed during routine maintenance and are aimed at assuring that the breakers are functionally operable. The following tests are to be made only on breakers and equipment that are deenergized.
Insulation Resistance Test
A megohmmeter may be used to make tests between phases of opposite polarity and from current-carrying parts of the circuit breaker to ground. A test should also be made between the line and load terminals with the breaker in the open position. Load and line conductors should be dis connected from the breaker under insulation resistance tests to prevent test mesurements from also showing resistance of the attached circuit.
Resistance values below 1 megohm are considered unsafe and the breaker should be inspected for pos sible contamination on its surfaces.
Milivolt Drop Test
A millivolt drop test can disclose several abnor mal conditions inside a breaker such as eroded contacts, contaminated contacts, or loose internal connec tions. The millivolt drop test should be made at a nominal direct-current volt age at 50 amperes or 100 amperes for large breakers, and at or below rating for smaller breakers. The millivolt drop is compared against manufacturer’s data for the breaker being tested.
Connections Test
The connections to the circuit breaker should be inspected to determine that a good joint is present and that overheating is not occurring. If overheating is indi cated by discoloration or signs of arcing, the connections should be re moved and the connecting surfaces cleaned.
Overload tripping test
The proper action of the overload tripping components of the circuit breaker can be verified by applying 300 percent of the breaker rated continuous current to each pole. The significant part of this test is the automatic opening of the circuit breaker and not tripping times as these can be greatly affected by ambient conditions and test condi tions.
Mechanical operation
The mechanical operation of the breaker should be checked by turning the breaker on and off several times.
SOURCE: HYDROELECTRIC RESEARCH AND TECHNICAL SERVICES GROUP
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