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5. SURGE PROTECTION
5.1 Rod or Spark Gaps
Rod or spark gaps are easy and cheap to install. They are usually installed in parallel with insulators between the live equipment terminal and earth. The gap distance setting is arranged such that the sparkover occurs at overvoltages well below the breakdown insulation level of the plant the gaps are protecting.
Gaps have the following disadvantages:
_ When they operate a short circuit fault is created which will cause protection to operate and isolate the circuit. However, the alternative of insulation failure of the plant being protected is much more serious.
_ Sudden reduction in voltage during gap operation places high stress on transformer interturn insulation.
_ The breakdown of plant insulation varies with the duration of the over voltage. A gap has a relatively slow response to fast rise time overvoltage surges and performance is influenced by polarity and atmospheric conditions.
_ Short distance gaps applicable at the lower distribution voltages are vulnerable to mal-operation due to wind-borne debris, birds, etc.
Notwithstanding these disadvantages the rod gap is widely used for the protection of small distribution transformers and as a back-up protection for transformers protected by surge arresters. In the UK, the present National Grid Company practice is to use rod gaps in preference to surge arresters at all voltage levels. However, internationally, because of the disadvantages, surge arresters are used as the principal form of substation plant overvoltage protection. Air gaps are used across insulators on overhead lines up to several kilometers from substations in order to protect the substation plant from surges emanating from the overhead lines. The gap settings are reduced as the overhead line approaches the substation. Gaps may also be used as back up protection to surge arresters at cable sealing ends and transformer bushings. The gaps are arranged so that the distance can be easily adjusted. The rods are angled such that the power arc is directed away from the associated insulator sheds in order to avoid possible damage during flashover.
Typical back-up transformer spark gap settings are given in TBL. 3.
Normally the rod gap characteristic should lie just above the surge arrester characteristic by, say, 20% so that the rod gap will protect the transformer or other plant against all but the steepest surges (rise times less than 1 or 2 µs), if the surge arrester fails. This philosophy also applies in the absence of surge arresters when the minimum gap setting for flashover should be at least 20% above the highest possible power frequency system voltage. For example, on a 132 kV system with a highest phase-to-earth voltage under transient fault conditions of 1323110%=145 kV, the rod gaps should be set to operate at 1453120%=174 kV.
Under impulse conditions the breakdown characteristics of the equipment to be protected are normally not known and only a BIL figure will be avail able. In such cases the rod gaps may be set to give a flashover on impulse, with a 1.2/50 µs wave, of 80% of the BIL of the protected equipment with a 50% probability. Thus a 132 kV system designed to a BIL of 550 kV might be given a rod gap setting on surge impulse of 440 kV. The gap setting may be taken from graphs giving both positive and negative surge impulse and power frequency gaps. In this particular case a minimum gap setting of 560 mm (22 inches) using 1/2 inch (1.27 cm) square rod gaps would be suitable ( FIG. 7).
At the higher transmission voltage levels rod gaps are not used because of the corona discharge effect and radio frequency interference associated with high electric fields around pointed objects. Loops are therefore used instead with a radius sufficient to reduce these effects.
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TBL. 3 Typical Spark Gap Settings
Transformer Basic Impulse Insulation Level (BIL, kV Peak) | Spark Gap Setting (mm)
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FIG. 7 Flashover voltage of 1 /2 inch square rod gaps.
Flashover voltage in kV effective or peak 50% Negative 1/50 impulse 50% Positive 1/50 impulse 50 Cycle/s effective
Gap spacing in inches
5.2 Surge Arresters
5.2.1 Zinc Oxide Types
Modern surge arresters (also and perhaps more accurately known as surge diverters) are of the gapless zinc oxide (ZnO) type. Under nominal system operating voltages the leakage current is of the order of a few milliamperes.
When a surge reaches the arrester only that current necessary to limit the overvoltage needs to be conducted to earth. ZnO has a more non-linear resistance characteristic than the previously used silicon carbide (SiC) surge arrester material. It’s therefore possible to eliminate the series of gaps between the individual ZnO blocks making up the arrester. A change in cur rent by a factor of some 10^5 will result in a change of voltage across the ZnO arrester of only about 56% thus yielding a high but finite energy dis charge capability. IEC 60099 details the standards applicable to both gapped SiC and ZnO non-spark-gapped surge arresters. Typical ZnO surge arrester characteristics are shown in FIG. 8. The devices have a particularly good response to fast rise time overvoltage impulses.
The construction of ZnO surge arresters is relatively simple. It’s essential that good quality control is employed when manufacturing the non-linear resistor blocks since the characteristics are very dependent upon the tempera ture firing range. Good electrical contact must be maintained between the non-linear resistor blocks by well-proven clamping techniques. SiC arresters employing series spark gaps must ensure equal voltage division between the gaps under all operating and environmental conditions. The power frequency sparkover of such arresters should be greater than 1.5 times the rated arrester voltage. FIG. 9 shows a selection of typical surge arresters together with the individual ZnO elements. Pressure relief diaphragms are fitted to the porcelain housings in order to prevent shattering of the units should the arrester fail.
5.2.2 Selection Procedure
The principles for the application of surge arresters to allow a sufficient mar gin between the plant breakdown insulation level and surge arrester protection capability are shown in FIG. 10. Withstand voltages as a function of the operating voltage within the two phase-to-phase insulation level ranges are shown in FIG. 11.
The application process is described below:
1. Determine the continuous operating arrester voltage _ normally the sys tem-rated voltage.
2. Select a rated voltage for the arrester (IEC 60099).
3. Determine the nominal lightning discharge current. At distribution volt age levels below 36 kV when it’s necessary to keep costs to a minimum 5 kA ratings are often specified. In most circumstances 10 kA surge arresters should be considered. For insulation greater than 420 kV 20 kA rating may be appropriate.
4. Determine the required long-duration discharge capability. At system rated voltages of 36 kV and below light duty surge arresters may be specified unless the duty is particularly onerous (e.g. surge arresters connected adjacent to large capacitor banks). At rated voltage levels between 36 kV and 245 kV and where there is a risk of high switching, long-duration fault currents (discharge of long lines or cable circuits) heavy duty surge arresters are normally specified. If any doubt exists the network parameters should be discussed with the surge arrester manufacturer. At rated voltages above 245 kV (IEC range II insulation level) long-duration discharge capabilities may be important.
5. Determine the maximum prospective fault current and the protection trip ping times at the location of the surge arresters and match with the surge arrester duty (including pressure relief class per IEC 60099).
6. Select the surge arrester housing porcelain creepage distance in accordance with the environmental conditions and state to the manufacturer if live line washing is electricity supply company practice.
7. Determine the surge arrester protective level and match with standard IEC 60099 recommendations. Typical protective levels are given in TBL. 4.
In order to assist specifying surge arresters, details of typical technical particulars and guarantees are given in TBL. 5.
FIG. 9 Individual ZnO elements.
Rated Voltage
The power frequency voltage across an arrester must never exceed its rated voltage otherwise the arrester may not reseal and may catastrophically fail after absorbing the energy of a surge. As a rule of thumb if the system is effectively earthed the maximum phase-to-earth voltage is 80% of the maxi mum line voltage. For a non-effectively earthed system the maximum phase to-earth voltage is equal to the maximum line voltage.
Consider a 132 kV system with a maximum line or phase-to-phase voltage 110% of the nominal system voltage 1. effectively earthed and 2. not effectively earthed:
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FIG. 8 Typical ZnO surge arrester characteristics.
MB Series: Voltage ratings from 3 to 150 kV
IEC Arrester classification: 37 (co) 38
10,000 A line discharge classes 1 and 2
ANSI Arrester classification
10,000 A station class MC Series: Voltage ratings from 3 to 288 kV
IEC Arrester classification: 37 (co) 38
10,000 A and 20,000 A line discharge class 3
ANSI Arrester classification
10,000 A station class
Ratio of minimum power frequency withstand voltage/rated voltage Ratio of minimum power frequency withstand voltage/power frequency rated voltage vs. time curve for surge arrester-type MB MC The curve refers to tests carried out with the pro-rated sample at a temperature of 60 °C Time in seconds
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1. Arrester voltage rating .0.8313231.15116 kV and 120 kV arresters are usually selected.
2. Arrester voltage rating .13231.15145 kV.
FIG. 10 Plant breakdown insulation level and surge arrester protection
capability.
FIG. 11 Withstand voltages as a function of operating voltage for insulation
ranges I and II.
Rated Current
Arresters are tested with 8/20 µs discharge current waves of varying magnitude: 1.5 kA, 2.5 kA, 5 kA, 10 kA and 20 kA yielding increasing values of residual discharge voltage. Maximum residual discharge voltages are detailed in IEC 60099-1 and this parameter is usually taken care of in the manufacturer's design specification. For areas with high isokeraunic levels (e.g. the tropics) or at locations near to generators or for unshielded lines 10 kA arresters should be specified. Lower-rated arresters can be selected for well screened systems if it can be demonstrated that the surge discharge current is less than 10 kA. However, the cost of arresters is small compared to the overall system cost and therefore if some doubt exists regarding the discharge current it’s safer to specify the higher-rated heavy duty type of arrester.
Although lightning strikes have impressive voltage and current values (typically hundreds to thousands of kV and 10_100 kA) the energy content of the discharge is relatively low and most of the damage to power plant is caused by the 'power follow-through current'. The lightning simply provides a suitable ionized discharge path. The likelihood of power follow-through current after a lightning discharge is statistical in nature and depends in a complicated way on the point on the wave of lightning discharge relative to the faulted phase voltage.
5.2.3 Location
Surge arrester and spark gap devices are installed in parallel with the plant to be protected between phase and earth. They should always be located as close as possible to the items of plant they are protecting consistent with maintenance requirements. This is to avoid back flashovers caused by any surge impedance between the surge arrester and the plant. The earth terminals should be connected directly and separately to earth as well as to the tank or frame of the plant being protected. Dedicated earth rods will provide the necessary low inductive path together with additional connections to the substation earth grid.
Note that generator windings have a low impulse strength, typically 50 kV for the 1.2/50 µs wave. Arresters for generators should therefore be heavy duty (10 kA station-type) which may be shunted by 0.1_0.25 µF capacitors which absorb very fast surges with rise times less than 1 µs. Surge protection of generators becomes particularly important when they feed directly onto an overhead line without the benefit of an interposing generator trans former. Shunting capacitors may be essential in such applications.
Probably the best way to understand insulation co-ordination is by way of worked examples.
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TBL. 5 Surge Arrester Technical Particulars and Guarantees
Characteristic | Requirement or Manufacturer's Guarantee
System highest voltage (kV) Insulation levels of protected systems:
_ Transformers (kV)
_ Switchgear (kV)
Manufacturer Type no.
Class of surge arrester (IEC 60099):
_ Duty
_ Long-duration discharge class
_ Pressure relief class
Rated voltage (kV rms) Nominal discharge current (kA) Number of separate units per arrester Discharge residual voltage based on 8/20 wave at:
(a) 5 kA (kV peak) (b) 10 kA (kV peak) (c) 20 kA (kV peak) Power frequency voltage capability for:
(a) 1 s (kV rms) (b) 3 s (kV rms) (c) 10 s (kV rms) (d) continuous (kV rms) Switching impulse residual voltage for wave shape (kV) Total height of arrester (mm) Total weight of arrester (kg) Minimum creepage distance per unit:
_ Specified (mm)
_ Guaranteed (mm)
Porcelain housing cantilever strength (kN) Surge monitor required (yes/no) Surge monitor type reference
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TBL. 4 Surge Arrester Ratings and Protective Characteristics Arrester Rating, Ur (kV) Front of Wave (kV) Lightning/Discharge Voltage (kV)
FIG. 12 132 kV system insulation co-ordination.
Insulation Co-ordination Example 1
The example is for co-ordination for a typical 132 kV substation on an effectively earthed system having transformers of 550 kV impulse withstand level and the other apparatus having an impulse level of 650 kV. It’s assumed that the altitude is below 1,000 m and that the pollution level is not unduly heavy. The positive voltage/time breakdown curves for the various devices have been plotted as shown in FIG. 12 to demonstrate the co-ordination which can be obtained. Normally it’s not necessary to plot curves in this way since a simple tabulation of figures is usually adequate. The curves could also include breakdown characteristics for substation post type insulators and overhead line cap and pin insulator strings with different numbers of units for completeness.
For the protection of the transformers and other equipment either a surge arrester or a rod gap system may be used. Since this is an effectively earthed system an '80% arrester' would be used; that is one rated at 120 kV (see Section 5.2.2 above). If the particular arrester chosen has a maximum residual impulse discharge voltage of 350 kV when discharging a 10 kA surge then using the 20% safety margin the capability is 350x120%= 420 kV. This is well below the transformer impulse withstand level of 550 kV, assuming the arrester is located within about 20 m of the trans former terminals.
If a rod gap is to be used for protection, then from FIG. 7, a value of 560 mm (22 inches) could initially be thought of as suitable. This gap also gives protection to the transformer even for waves with rise times as short as 1 µs. However, on longer duration surges (possible switching surges) that are below the impulse strength of the transformer such a setting could give the occasional flashover. The gap setting could therefore be increased to 660 mm (26 inches) in order to reduce such a possibility. This larger setting does not, however, give adequate transformer protection against very fast rise time waves. A degree of judgment and experience is therefore required in order to determine the final rod gap setting. Such experience should also take into account whether or not the substation or incoming overhead lines have overhead earth wire screens.
Insulation Co-ordination Example 2
This is an example of insulation co-ordination carried out for a large gas turbine power station in the UK, feeding directly into the National Grid at 275 kV. The Grid is insulated to the highest (BIL) level whereas, for economic reasons, the power station equipment has been specified to lower levels and is protected by surge arresters. The system itself is shown in FIG. 14 and the system data is listed in FIG. 13. This example stresses the need for engineers to question the reliability or meaning of system data presented to them. It also shows how a technical understanding of a subject can lead to innovative and cost effective design solutions.
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GENERATOR Nominal voltage 11 kV continuous Maximum nominal voltage 12.1 kV continuous Maximum transient 50 Hz overvoltage on loss of load Generator BIL 44.9 kV Generator winding capacitance 0.46 F The generator is resistance earthed Not effectively earthed 11 kV SYSTEM Nominal voltage 11 kV Maximum nominal voltage 12.1 kV (10%) Maximum transient 50 Hz overvoltage 14.4 kV rms System BIL 60 kV GENERATOR TRANSFORMER Ratio 295/11 kV Winding Star/delta (Ynd11)
Winding capacitances: Primary/earth Cp 0.0029 F
Secondary/earth Cs 0.0102 F
Primary/secondary Ct 0.0032 F 275 kV GRID SYSTEM Nominal voltage range Max 302.5 kV (10%) Min 247.5 kV ( 10%) Temporary 50 Hz overvoltage 385 kV for a few seconds Design BIL 1050 kV Typical lighting overvoltage 850 kV Typical switching overvoltage 550 kV Surge protection policy Gaps coordinated for probability at 835 kV Chopped wave 1200 kV chopped on wave front rising at 100 kV/ s 14.4 kV for a few seconds (131%)
FIG. 13 System data.
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HV Arrester Selection
The Grid is insulated to the highest BIL for 300 kV, namely 1,050 kV.
Below this level two other standard BIL ratings are possible in accordance with IEC 60071-1 at 950 kV and 850 kV. The 950 kV level has been specified in the transformer enquiry documents as a compromise between cost and closeness to the typical 275 kV nominal voltage grid system overvoltage level. In accordance with IEC 60071, a surge arrester is required that will intercept surges 20% below the rated equipment BIL. That is, with a maxi mum 'impulse protective level' (IPL) voltage:
We now encounter a conflict. If we require the surge arrester to intercept the switching surge, then for the arrester:
However, the peak temporary 50 Hz overvoltage is:
These figures are too close. With an adequate transformer BIL of 950 kV the solution in this case could be to accept that the switching surges may not be intercepted by the HV transformer surge arresters. Such surges will be transferred through the transformer without damage if correctly intercepted by arresters on the 11 kV side. From manufacturers' catalogue data available at the time a 275 kV nominal system voltage 10 kA heavy duty (Bowthorpe 2 MC 240) arrester could be chosen with the following characteristics:
Rated voltage 240 kV rms Maximum continuous operating voltage 180 kV rms One second temporary overvoltage 288 kV rms Maximum residual voltage (MRV at 10 kA, 8/20 µs wave) 718 kV Steep current residual voltage (1.2/50 µs wave) 790 kV The maximum continuous 275 kV system voltage is 302:5= and we note that the maximum residual arrester voltage of 718 kV . 790/ 1.15 kV so we choose the former maximum residual voltage for the arrester impulse protective level.
The 50 Hz temporary overvoltage under earth fault conditions for an effectively earthed 275 kV nominal voltage system would be expected to be 302.530.85242 kV. The Grid Company data, however, includes a figure for temporary overvoltage of 385 kV and under earth fault conditions this implies that the phase-to-earth voltage on the healthy phases could reach 38530.85308 kV. Should a higher-rated arrester (such as a Bowthorpe 3 MC 264 unit) therefore be chosen?
Let us look at some other figures. According to IEC 60099-1 the BIL/ IPL . 1.2. From manufacturer's data:
2 MC 240 arrester BIL=IPL5950=71851:32
3 MC 264 arrester BIL=IPL5950=79051:20
In addition, it should be noted that the 2 MC 240 arrester may give some protection against the switching surge whereas the 3 MC 264 unit would not.
The final selection must therefore be made on the basis of the foregoing figures and engineering judgment. The possibility of the 275 kV Grid actually reaching the quoted 385 kV rms level for several seconds should be technically researched with Grid company systems engineers. Consider, For example, if the Grid voltage approached 385 kV (140%) all Grid transformers would be driven hard into saturation. A huge reactive power demand would result making it difficult, if not impossible, to actually achieve such a voltage level sustained for the several seconds specified. Such a figure should therefore be questioned as possibly incorrect data.
LV Arrester Selection
Transferred surges will pass through the transformer.
For fast transients using the transformer capacitance figures and assessing an equivalent pi circuit the surge
Alternatively, using the IEC 60071-2 formula which is of the same order of magnitude. In addition, it should be noted that these figures don’t allow for a load or generator connected to the 11 kV side of the transformer, which would otherwise reduce the value of the surge.
For electromagnetically transferred surges IEC 60071-2 gives the equation:
For the 295/11 kV star/delta transformer:
518:4kV --again assuming no connected load-- The 11 kV system is not effectively earthed and the maximum nominal volt age will be 12.1 kV. The temporary overvoltage could be 14.4 kV from the generator and 385311/295514.35 kV from the Grid (assuming the unrealistic case of no transformer saturation). The 11 kV system BIL is 60 kV.
However, the generator BIL is only 44.9 kV and this figure will be used for protection purposes. From manufacturers' data a heavy duty (station-type) arrester rated at 12 kV is chosen (Bowthorpe type 1 MC 12). IEC 60099-1 recommends BIL/IPL, $1.2. Therefore arrester IPL # BIL/1.2544.9/ 1.2537.4 kV.
The 12 kV 1 MC 12 arrester has an MRV535.9 kV at 10 kA for a 8/20 µs wave and 39.5/1.15534.3 kV at 10 kA for the steep-fronted 1.2/50 µs wave.
Again in accordance with IEC 60099-1 BIL/IPL51.25 . 1.2 which is judged to be satisfactory for the generator since the rise time of the transferred surge would be much greater than 1.2 µs and is certainly satisfactory for the general 11 kV system. Such an arrester could therefore be applied to both the 11 kV terminals of the 295/11 kV transformer and to the generator terminals for further added protection.
FIG. 14 Barking Reach power station. MECH=turbine generator mechanical
protection; EXC=excitation control system; INS5instruction circuits; MET=metering
circuits; SYUN=synchronizing scheme; AVC=Trans. tap change control.
The system diagram shown in FIG. 14 also shows an 11/6.9 kV Dzn0 station transformer connected to the 11 kV busbar. This transformer will also be subjected to electromagnetically transferred surges but the 11 kV incident surge will be limited to the residual voltage of the 11 kV surge arrester.
Again, a check can be made as to the transferred surge value appearing on the 6.9 kV side of this transformer in a similar way to that indicated above:
Up 535.9 kV (the limited switching surge)
513:5kV --again assuming no connected load-- This is only twice the normal 50 Hz line voltage and therefore constitutes a temporary overvoltage lasting for a few milliseconds at most. Furthermore, the actual voltage appearing on the 6.9 kV side would be significantly reduced by cables and loads. Since the 6.9 kV system should have a BIL of 40 kV no additional surge protection is required.
5.2.4 Monitoring
Surge counters are often specified for plant rated voltages of 145 kV and above. In such cases the base of the surge arrester is supported on small insulators and the surge counter fitted at the earthy end of the surge arrester in the lead to earth. The counters should be located so that they may be easily read from ground level.
5.2.5 Testing
As for all substation or overhead line plant type test certificates should be obtained from the manufacturer. ZnO surge arrester type tests include:
_ residual voltage test;
_ current impulse withstand test;
_ operating duty tests;
_ power frequency voltage vs time curve;
_ pressure relief tests;
_ tests on arrester disconnectors (if applicable).
Routine tests include:
1. On all arrester sections:
_ radio interference tests;
_ test to check sealing or gas leakage from completed housing.
2. On SiC gapped arrester sections:
_ power frequency sparkover test.
3. On a sample number of surge arresters to be supplied:
_ lightning voltage impulse sparkover on the complete arrester (SiC types) or time voltage characteristic (ZnO types);
_ residual voltage at nominal discharge current on complete arrester or section;
_ leakage current with 40% to 100% of rated voltage applied.
4. On all gapless arrester sections:
_ measurement of grading current when energized at maximum continuous operating voltage;
_ measurement of power frequency voltage at a resistive current level to be determined between manufacturer and purchaser (1_10 mA peak);
_ residual voltage at a discharge current level to be determined between manufacturer and purchaser.
REFERENCES:
1. IEC 60038 _ IEC standard voltages.
2. IEC 60060 _ High voltage test techniques.
Part 1 _ General definitions and test requirements Part 2 _ Measuring systems Part 3 _ Definitions and requirements for on-site testing.
3. IEC 60071 _ Insulation Co-ordination.
Part 1 _ Terms, definitions, principles and rules Part 4 _ Computational guide to insulation co-ordination and modeling of electrical networks.
4. IEC 60099 _ Surge Arresters.
Part 1 _ Non-linear resistor type gapped surge arresters for AC systems Part 3 _ Artificial pollution testing of surge arresters Part 4 _ Metal oxide surge arresters without gaps for AC systems Part 5 _ Selection and application principles Part 6 _ Surge arresters containing both series and parallel gapped structures rated 52 kV and less.
5. IEC 61643 _ Low voltage surge protective devices.
6. IEC 62272-203 _ Gas insulated metal enclosed switchgear for rated voltages of 72.5 kV and above.
7. White LJH, Tufnell DHA, Gosling GG. A review of insulation coordination practice on AC systems. CEPSI; 1980.
8. Tufnell DHA. Insulation coordination _ a review of present practices and problems. In: IEC Symposium. Jakarta: Indonesian Institute of Sciences; 1983.
9. Berger U. Insulation coordination and selection of surge arresters. Brown Boveri Rev 1979;6(4).