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1. INTRODUCTION
Insulation co-ordination is the technique used to ensure that the electrical strengths of the various items of plant making up the transmission and distribution system and their associated protective devices are correlated to match the system characteristics and expected range of voltages. The objective of the analysis and application of its conclusions is to reduce the probability of plant failure and supply interruptions caused by insulation breakdown to an operationally and economically acceptable level.
IEC 60071 covers the subject of insulation co-ordination as indicated in TBL. 1. The standard recognizes that insulation may occasionally fail since it’s not economically feasible to eliminate failure completely. A pro posed order of priorities for an insulation co-ordination policy is to:
_ Ensure safety to public and operating personnel.
_ Avoid permanent damage to plant.
_ Minimize interruption of supplies to consumers.
_ Minimize circuit interruption.
2. SYSTEM VOLTAGES
2.1 Power Frequency Voltage
It should be noted that insulation levels are dependent upon the highest sys tem operating voltage and not the nominal voltage. IEC 60038 gives details of standard transmission and distribution voltage levels. Thus for a 132 kV system, the highest voltage is 145 kV. Plant may be subjected to the normal power frequency voltages which don’t exceed the highest rated voltage for which the equipment has been designed. Obviously the insulation must be able to withstand these steady state power frequency voltages and plant must be specified accordingly. Breakdown does, however, occur due to pollution, heavy rain, etc. Section 6 describes how insulators should be specified to minimize this risk and how adequate insulator creepage distances may be determined to match the environmental conditions.
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TBL. 1 IEC 60071 Insulation Co-ordination
IEC 60071 Insulation Co-ordination
60071-1 _ Part 1 Terms, definitions, principles and rules
Specifies the insulation for the various items of plant used in a given installation.
Applies to plant for AC systems having a higher voltage for plant above 1 kV, and covers phase-to-earth insulation.
60071-2 _ Part 2 Application guide
Provides guidance on the selection of the electric strength of plant, of surge arresters or protective spark gaps, and on the extent for which it will be useful to control switching overvoltages. Indicates the lines to be followed to obtain rational and economic solutions. Deals with phase-to-phase and phase-to-earth insulation co ordination, completing the principles and rules laid down in IEC 60071-1 Having specified the general principles gives the standard insulation levels for the ranges I (1_245 kV) and II (above 245 kV).
TR 60071-4 _ Part 4 Computational guide to insulation co-ordination and modeling of electrical networks
Gives guidance on conducting insulation co-ordination studies. Gives information in terms of methods, modeling and examples, allowing for the application of the approaches presented in IEC 60071-2 and for the selection of insulation levels of equipment or installations, as defined in IEC 60071-1.
60071-5 _ Part 5 Procedures for high-voltage direct current (HVDC) converter stations
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2.2 Overvoltages
2.2.1 Internal Overvoltages
As well as steady state power frequency overvoltages it’s also necessary to ensure that plant is able to withstand short-duration power frequency over voltages or other types of weakly damped oscillatory voltages with harmonic content which may last in the worst cases for tens of seconds. Such phenomena can occur during transformer saturation. Also, distribution systems with lightly loaded large cable networks involving high capacitance when fed from a source rich in harmonics can greatly magnify the voltage distortion during switching operations. In general, the principal causes of temporary power frequency overvoltages are:
_ Phase-to-earth faults _ on normal systems it may be assumed that the temporary overvoltages won’t exceed:
1.4 per unit for solidly earthed networks, 1.7 per unit for resistance earthed networks, 2.0 per unit for reactance earthed networks.
_ Load rejection _ (supplying capacitive current through a large inductive reactance, e.g. a small generator connected to a long cable or overhead line).
_ Ferro resonance _ (interchange of stored energy for series or parallel combinations of inductive and capacitive reactance).
_ Ferranti effect _ (receiving end voltage greater than sending end voltage under no load or for lightly loaded lines).
Sustained overvoltages involving resonance and arcing ground faults are normally eliminated by careful system design and correct neutral earthing. At distribution voltage levels (below 145 kV) the method of earthing will normally determine the level of temporary overvoltage.
2.2.2 Switching Surges
Switching surges are of short duration, of irregular or impulse form and highly damped. A typical switching impulse standard form is the 250/2,500 microsecond, time-to-crest/time-to-half value wave. Overvoltages due to switching phenomena become important at the higher transmission voltage levels (above 245 kV). Section 13 describes the effect of various types of switching surges which are well understood. The magnitude of internally generated switching surges is related to the system operating voltage. On a system where the circuit breakers are not subject to multiple restriking the switching surges will rarely exceed 3 per unit and 2.5 per unit would be a typical maximum upon which the discharge duty for surge arresters may be assessed. On range II installations (above 245 kV) it may be necessary to suppress maximum switching surges to 2 per unit or less by the installation of shunt reactors and/or closing resistors on the circuit breakers.
At voltage levels below 245 kV some practical aspects of switching surges found on networks are listed below:
_ Resonance effects when switching transformer feeders or combinations of cable and overhead line. Resonance can occur between the lumped reactive and capacitive elements and the overhead line. If the frequency of the reflections of the travelling waves along the line approximates to the natural frequency of the lumped elements, high voltages can be generated.
_ Ferro resonance encountered on transformer feeder circuits greater than 5 to 10 km in length when one feeder/transformer on a double circuit is switched out but the parallel feeder remains energized. The dead circuit draws energy by capacitive coupling from the parallel live circuit which resonates with the transformer impedance at a subharmonic frequency.
Operational procedures such as opening the line isolator at the transformer end on the disconnected circuit will eliminate the problem.
_ In addition to the transformer feeder energization cases listed above, line energization can also create large switching surges particularly at the remote end of the line being energized. Such circumstances include:
_ Very long lines particularly if there is no shunt reactor compensation.
_ Lines already energized with a standing charge such as might occur from auto-reclose conditions.
_ Current chopping during shunt reactor, transformer and motor switching. Nowadays modern circuit breakers should be restrike free, or virtually so. This was a particular problem with early vacuum circuit breaker designs and air blast circuit breakers where the current may be broken before the natural cyclic current zero. Overvoltages due to these sudden interruptions may be of the order of 2.5 to 3 times the normal voltage. When a circuit breaker interrupts reactive current any magnetic energy in the reactor is exchanged with electrical energy according to the relationship:
L=inductance
Ic =chopped current level
C=shunt capacitance
V=voltage created by current chopping
_ Existing reactor switching installations may have this phenomenon resolved by installation of suitable surge arresters.
_ The possibility of circuit breaker arc restriking when switching large capacitive currents. It’s therefore very important to specify the correct capacitive current which the circuit breaker may have normally to switch and to match the circuit breaker manufacturer guarantees for restrike-free operations with the network application. Early low oil volume (LOV) circuit breaker designs were vulnerable to this phenomenon when low surge impedances (cables or capacitor banks) were connected to both sets of switchgear terminals.
2.2.3 External Overvoltages/Lightning Surges
On power systems operating at 145 kV and below overvoltages due to lightning will predominate rather than overvoltages generated by internal phenomena (fault conditions, resonance, etc.) or switching operations. Such overvoltages arise from lightning discharges which are usually of very short duration, unidirectional and of a form similar to the standard impulse wave shape 1.2/50 microsecond, front time/time-to-half value wave.
The point of insulation flashover in the system depends upon a number of independent variables:
_ The geographical position of the stroke.
_ The magnitude of the stroke.
_ The rise time of the voltage wave.
_ The system insulation levels.
_ The system electrical characteristics.
_ The local atmospheric or ambient conditions.
The damaging part of the lightning flash is the 'return stroke', where a charged cell in a thunder cloud is discharged to earth. The current in the return stroke varies from about 2 kA to 200 kA, in accordance with a log normal distribution:
1% . 200 kA 10% . 80 kA 50% . 28 kA 90% . 8kA 99% . 3kA
Impulse rise times are of the order of 10 microseconds for the more common negative flow from cloud to ground (and considerably longer for strikes from a positive part of the cloud), together with a relatively slow decay time of approximately 100 µs or less. For design purposes, the most severe peak lightning current and rate of rise of 200 kA and 200 kA/µs may be considered.
The cloud potential is of the order of 100 MV and therefore high enough to ensure that the potential of the object struck is controlled by the current flow and impedance to ground. When a lightning strike takes place on an overhead line support structure the potentials along the current path will rise to very high values due to even the smallest inductive and resistive impedance to true earth. If the effective impedance to true earth is high enough to break down the insulation then a flashover will take place either from the earth wire or tower to the phase conductor(s), usually across the insulator strings. This type of lightning fault is known as a 'back flashover'. A reduction in lightning outages requires adequate overhead line shielding angles and low tower footing resistances of less than 10 to 20 ohms. An unearthed woodpole structure offers superior lightning performance and hence higher reliability through the reduced risk of back flashover because of the inherent insulating properties of wood.
The short duration of a lightning strike is usually insufficient to present temperature rise problems to the earthing and shielding conductors. A mini mum cross-sectional area of 50 mm^2 is recommended in order to reduce surge impedance and temperature rise. In contrast, the conductivity of an arc path through air is high and with the large currents involved the air adjacent to the flash will experience a rapid temperature rise with a resulting explosive expansion. Large mechanical forces will also be present for parallel conductors or conductors with sharp bends.
The lightning flash density Ng is the number of flashes to ground per year per km^2 and maps are available with this or the number of thunderstorm days per year data. The relationship between such data is given in TBL. 2.
The effective collection area Ac is a function of a structure's dimensions.
The probability, P, of the number of strikes to a structure per year is given by P=Ac _ Ng _ 1026, to which weighting factors based on experience are applied to cover different types of structure, construction, contents, degree of local isolation and profile of the surrounding country. For buildings risks less than 1025 don’t generally require lightning protection.
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TBL. 2 Relationship Between Thunderstorm Days per Year and Lightning Flashes per km^2 per Year (Lightning Parameters for Engineering
Application. Electra 1980;69:65_102)
Thunderstorm Days per Year Flashes per km^2 per Year (Mean) Flashes per km^2 per Year (Limits)
5 0.2 0.1 to 0.5 10 0.5 0.15 to 1 20 1.1 0.3 to 3 30 1.9 0.6 to 5 40 2.8 0.8 to 8 50 3.7 1.2 to 10 60 4.7 1.8 to 12 80 6.9 3 to 17 100 9.2 4 to 20
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FIG. 1a Lightning protection using shielding towers. Top: Zones of protection
from vertical conductor (VC) shielding towers according to British Standards.
Bottom: German research association for high voltage and current technology
(FGH) equivalent. Plan view of zone of protection at ground level Plan
view of zone of protection at ground level; One vertical conductor; Four
vertical conductors showing protective angles and associated zones of protection
2.2.4 Substation Lightning Shield Protection
Outdoor substations may be shielded by overhead earthwire screens strung across the substation site or by the use of shielding towers. The zone of protection provided by an earthed structure is the volume within which it gives protection against a direct and/or attracted lightning strike. British and German Standards differ as to the extent of the coverage offered (Figs. 9.1a and 9.1b). The function of the overhead earthwire shield or shielding towers is to divert to itself a lightning discharge which might otherwise strike the phase conductors or substation plant. The use of shielding towers alone tends to require high structures in order to give adequate coverage. The shielding wire system allows lower height structures for a given coverage and the lightning current will be attenuated by increasing the number of paths to earth and thereby reducing the risk of back flashover. Often substation overhead line termination towers act as suitable support points for the shielding wire earth screen. Some electricity supply companies in areas with low lightning activity believe that the risk of an overhead earth wire screen falling onto the substation and causing a major outage is greater than an outage due to a lightning strike.
FIG. 1b Lightning protection using aerial earth wires. Top: Zone of protection
(ZP) from aerial earth wire according to British Standards. Bottom: German
(FGH) equivalent.
Electro-geometric lightning theory considers that the lightning arc stroke distance, rsc, is a function of the lightning stroke leader current:
rsc =8:=I 2=3 c where Ic is the critical stroke current which is the peak value of impulse cur rent which will cause failure of the insulation. Then:
Ic = Vi
0:5Z where
Vi =impulse voltage withstand for the insulation Z=surge impedance of the conductor
By knowing Vi and ZIc may be determined and hence the strike distance, rsc. A series of arcs is drawn around the substation phase conductors with radius rsc and around the earth wire screen with radius rse. Similarly a line is drawn at a height rsg parallel to the ground with:
rsc =rse _ rsg
If the lightning arc stroke distance cuts either the line above the earth or one of the earth wire radius arcs before it cuts an arc whose centre is the phase conductor perfect shielding will be obtained. Examples are shown in FIG. 2. In practice electricity supply companies and engineering consultants tend to adopt specific shield designs similar to those shown in FIG. 1.
2.2.5 Surges in Transformers
The winding of a transformer can be represented as a distributed capacitance to steep-fronted waves as shown in FIG. 3.
As the steep-fronted surge Up travels down the winding it can be shown that the voltage U at any point in the winding is given by:
1. for an earthed winding
2. for an open circuited winding
...where:
Co =capacitance to earth
C =interturn capacitance
The presence of capacitance to earth causes a non-uniform distribution of voltage in the winding and the greater the value of the greater will be the concentration of voltage at the line end of the winding and the larger the interturn insulation stress on the first few turns of the transformer winding. Such a phenomenon has been responsible for many unprotected distribution transformer insulation failures.
After the surge has travelled down the winding the picture becomes complicated by multiple reflections and natural frequency oscillations in the winding ( FIG. 4). In high-voltage (HV) transformer design, the value of the interturn capacitance can be artificially increased by screening and by winding interconnections. These measures improve the transformer surge response and reduce the stressing of the line end turns.
Another factor to bear in mind is the near voltage doubling effect that occurs when a surge travelling down a line encounters the high surge impedance of a transformer. This effect can be virtually eliminated by the presence of a short length of cable of low surge impedance between the transformer and overhead line. However, because of improvements in transformer insulation and the high cost of such cable and fixings this practice is diminishing, certainly in the UK.
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Strike distance to ground Strike distance to phase conductor Strike distance to earth shield wire Maximum height of phase conductor Overhead earth shield wire Overhead phase conductor
FIG. 2 Electrogeomagnetic model for lightning screen.
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2.2.6 Transferred Surges
FIG. 3 Representation of a transformer winding with distributed capacitance
undergoing a voltage surge.
Waves in one part of a circuit can be transferred to other circuits by inductive and capacitive coupling. As indicated above a transformer appears to a steep-fronted wave as a distributed capacitance which can be crudely represented by a simple pi network ( FIG. 5). From the figure, Cp and Cs are the lumped capacitances to earth and Ct is the lumped interturn capacitance so that the transferred wave Us is given by:
The values of these capacitances are not easily obtainable so IEC 60071-2 and the identical EN 60071-2 give various formulae for transferred waves. It’s considered that the initial voltage on the secondary side of the transformer is given by:
...where s can range from 0 to 0.4 and is typically about 0.2 and p for a star/ delta or delta/star is about 1.15 and for a star/star or delta/delta transformer is typically about 1.05.
Consider an 800 kV steep fronted lightning surge impinging upon the high voltage side of a 295/11 kV star/delta transformer. The transferred surge is:
...which will appear on the 11 kV side.
Alternatively if capacitance values are available:
In reality the transferred wave is complicated by multiple reflections inside the transformer and is attenuated by the transformer and any connected load. Nevertheless, the problem of wave transference should be recognized and low voltage equipment should be protected by surge arresters if such an event is likely to occur. The presence of external cables and loads further modifies the voltage wave appearing at the transformer terminals.
Slow surges such as switching surges that have rise times of the order of a few tens of microseconds or with an effective frequency of the order of 5_10 kHz will transfer through transformers electromagnetically. IEC 60071-2 gives an equation:
... where again p depends upon the winding configuration and is 1.15 for a star/ delta transformer, q is a response factor for the lower voltage circuit with a value of 0.9 to 1.3, r is a correction factor and N is the transformer phase-to phase voltage ratio.
Consider a star/delta 295/11 kV transformer with a 500 kV incident wave on the HV side:
The magnitude of this surge would be modified, more or less, by whatever is connected to the 11 kV side of the transformer.
3. CLEARANCES
3.1 Air
Recommendations for insulation clearances are given in IEC 60071-2. For system nominal voltages up to 245 kV it implies use of the same insulation levels and electrical clearances for phase-to-phase as phase-to-earth cases although it warns against use of the lowest insulation levels without great caution and very careful study. Before publication of this standard, electricity supply companies developed their own policies regarding insulation levels and clearances. In the UK it was assumed that the phase-to-phase insulation should be able to withstand a full lightning impulse on one phase simultaneously with a peak power frequency voltage of opposite polarity on the adjacent phase. This policy has resulted in a satisfactory, reliable and possibly conservative design with phase-to-phase insulation levels 15% to 25% higher than the phase-to-earth level.
For the higher voltages, including 500 kV, when air clearances are deter mined by the level of switching surges, IEC 60071-2 recommends withstand voltages between 1.5 and 1.8 per unit greater than the phase-to-earth level.
The recommendations in the standard give a choice of two clearances depending on the conductor-to-conductor symmetrical or unsymmetrical con figuration. Thus for a 525 kV system with a rated switching surge withstand level of 1,175 kV, the IEC document recommends the adoption of a phase to-phase switching surge withstand of 1,800 kV, and clearances of either 4.2 meters or 5.0 meters depending on the gap configuration. It should be possible to avoid the use of unsymmetrical gaps between phases, and therefore permit the use of the reduced clearances. In the UK, where the main transmission nominal voltage level is 400 kV, the reduced phase-to-phase clearance of 3.56 m has been used without any reliability problems. The IEC document recommends for such a system clearances of either 3.6 or 4.2 m.
TBL. 2a shows the choice of impulse insulation strengths for systems operating at some typical rated voltages in accordance with the recommendations of IEC 60071.
3.2 SF6
The use of SF6 as an insulating medium requires special insulation co ordination attention. Insulation failure in gas insulated switchgear (GIS) is not self-restoring and long repair times are likely to be involved. The withstand level of SF6 for various impulse wave fronts and polarity varies significantly from air. As with air for very fast wave fronts, the negative break down voltages are higher than the positive. For wave fronts slower than 1 microsecond the SF6 positive voltage withstand level is greater than the negative. Also the SF6 voltage withstand level does not reduce so markedly for the longer switching surge voltage wave fronts as does air insulation.
GIS disconnectors often have to break small magnitude capacitive charging currents. This can cause high frequency discharges across the contacts as the disconnector commences opening. The resulting overvoltages of 3 to 4 per unit must not be allowed to cause flashovers from the phase contacts to earth. Considerable design effort has been involved in reducing this problem since the mid-1970s. Surge arresters must be located very close to any open ended busbar if they are to be effective in attenuating such high frequency surges.
Where possible GIS switchgear should be transported to site in pre assembled and pre-impulse tested sections. Guidance on selection on a test procedure to be adopted for a particular equipment is provided in IEC 60060.
4. PROCEDURES FOR CO-ORDINATION
4.1 The IEC Standard Approach
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TBL. 2a IEC Insulation Levels for Some Standard System Rated Voltages
System Highest Voltage (kV) Range Standard Short Duration Power Frequency Withstand Voltage (kV rms) Standard Lightning Impulse Withstand Voltage a (kV peak) Standard Switching Impulse Withstand Voltage Minimum Clearance (mm) Based on Standard Lightning Impulse Withstand b Minimum Clearance Phase-to-Earth/Phase to-Phase (mm) Based on Standard Switching Impulse Withstand Longitudinal Insulation (kV Peak) d Phase to-Earth (kV peak) Phase to-Phase (kV peak) Rod Structure Conductor Structure Rod structure/ Rod Conductor Structure/ Conductor c
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The detailed procedure for insulation co-ordination set out in IEC 60071-1 (European standard EN60071-1 is identical) consists of the selection of a set of standard withstand voltages which characterize the insulation of the equipment of the system. This set of withstands correspond to each of the different stresses to which the system may be subject:
_ Continuous power frequency voltage (the highest voltage of the system for the life of the system).
_ Slow-front overvoltage (a standard switching impulse).
_ Fast-front overvoltage (a standard lightning impulse).
_ Very fast-front overvoltage (depends on the characteristics of the connected apparatus).
_ Longitudinal overvoltage (a voltage between terminals combining a power frequency voltage at one end with a switching (or lightning) impulse at the other).
These voltages and overvoltages need to be determined in amplitude, shape and duration by system study. For each class of overvoltage, the analysis then determines a 'representative overvoltage', taking account of the characteristics of the insulation. The representative overvoltage may be characterized by one of:
_ an assumed maximum;
_ a set of peak values;
_ a complete statistical distribution of peak values.
The next step is the determination of 'co-ordination withstand' voltages _ the lowest values of the withstand voltages of the insulation in use which meet the system or equipment performance criteria when subjected to the 'representative overvoltages' under service conditions. Factors are then applied to compensate for:
_ the differences in equipment assembly;
_ the dispersion of the quality of the products within the system;
_ the quality of installation;
_ the ageing of installation during its lifetime;
_ atmospheric conditions;
_ contingency for other factors.
This results in so-called 'required withstand voltages' _ test voltages that must be withstood in a standard withstand test. In specifying equipment the next step is to specify a standard test withstand voltage (a set of specific test voltages is provided in IEC 60071-1) which is the next above the required withstand voltage, assuming the same shape of test voltage.
A test conversion factor must be applied to the required withstand voltage if the test voltage is of a different shape to the class of overvoltage in question.
FIG. 6 sets this procedure out in diagrammatic form, and full details of what is involved with each step is provided in IEC 60071.
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FIG. 6 Flow chart for the determination of rated or standard insulation
level (from IEC 60071-1).
Origin and classification of stressing voltages Protective level of overvoltage limiting devices Insulation characteristics Insulation characteristics Atmospheric correction factor, Ka Test conditions Test conversion factor, Kt Standard withstand voltages Ranges of Um System analysis Selection of the insulation meeting the performance criterion Application of factors to account for the differences between type test conditions and actual service conditions Selection of standard withstand voltages, Uw Sided boxes refer to required input.
Sided boxes refer to performed actions.
Sided boxes refer to obtained results.
Representative voltages and overvoltages, Urp Coordination withstand voltages, Ucw Required withstand voltages, Urw Rated or standard insulation level: set of Uw (_) Effects combined in a coordination factor, Kc Performance criterion Statistical distribution (_) Inaccuracy of input data (_) Equipment test assembly Dispersion in production Quality of installation Aging in service Other unknown factors*)
Effects combined in a safety factor, Ks
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4.2 Statistical Approach
The statistical approach is especially valuable where there is an economic incentive for reducing insulation levels and where switching overvoltages are a problem. The method is therefore particularly applicable at the higher volt age range II installations above 245 kV.
The risk of insulation failure, R, may be expressed by the formula:
fo (U)=the overvoltage probability density
Pt(U)=the probability of insulation failure in service at voltage U Since it’s difficult to determine fo(U) and Pt(U) in practice IEC 60071 recommends a simplified method of assessment taking a 90% withstand level for a given insulation system equated with a 2% probability of an overvoltage being exceeded. From this different safety factors, ?, may be applied and the risk of failure determined. Modeling of the network on a computer may also be used to determine possible overvoltage conditions although obviously the accuracy of such simulations is only as good as the input data.
Laboratory tests on insulation will give an assessment of withstand capability. If the insulation is to be installed in outdoor conditions then the effects of rain and pollution on insulation strength must also be simulated. For a given state of the insulation there is a statistical spread in the breakdown voltage coupled with time effects and variations in environmental conditions.
This may be expressed as:
st =standard deviation at a given instance in time
sn =standard deviation due to environmental conditions
Some standards suggest that st may be assumed to be equal to 0.06 for switching surges and 0.03 for lightning impulses. The 50% breakdown volt age UT50 is related to the required withstand voltage URW by the relationship UT=0 5k _URW/(1_1.3st). Constant k is dependent upon weather but typically may be made equal to 1 and sn is associated with pollution levels and may be made equal to 0.6.
4.3 Non-statistical Approach
The conventional procedure is based on adopting an adequate margin between produced overvoltages and the withstand strength of the plant. The margin determines the safety factor, and is to some extent provided by the various factors mentioned in Section 9.4.1. However, it should never be less than the value found to be adequate from experience. This method is generally applied to the lower transmission (upper end of range I: 52 kV to 245 kV) installations because of the practical difficulty of determining fo(U) and Pt(U) with any degree of accuracy. Computer simulations are recommended for range II system voltage levels above 245 kV.
Transient overvoltages are limited to a protective level established by the use of surge arresters and/or co-ordinating spark gaps. The insulation requirements of the various items of plant are selected to be above this protective level by a safe margin of 15% to 25%. Overhead lines are generally regarded as the main collectors of lightning surges on a system, and transformers, cables and switchgear associated with overhead lines will require protection.