Testing of line insulators, surge arresters & bushings

Condition Monitoring of Power System Equipment

(EEC6440/EEE6440)

M.Tech II Sem. (HV Engg and Inst. & Control)

Course Incharge: Prof. Asfar Ali Khan

Introduction

Overhead line and substation insulators in their different forms are the simplest capacitive-type components of the supply system .These include string (cap-and-pin – Type B; long-rod – Type A), pin, line post and station post insulators as specified in the various Standards. All of these components have low capacitance values.

Because of the developments in requirements and materials, the updating of Standards is a continual process.

The electrical assessment procedures are slightly different for the various classes of insulator but one or more of the following tests are specified as appropriate. The basic test procedures and wave shapes are defined in IEC 60060 [S7/1].

Tests

Tests that are specified in the standards are follows:

• dry power-frequency withstand voltage;

• dry power-frequency voltage with sporadic flashovers;

• high-frequency (100–500 kHz) flashover voltage;

• dry lightning-impulse withstand voltage;

• 50 per cent lightning flashover voltage;

• dry switching-impulse withstand voltage;

• wet power-frequency withstand voltage;

• wet lightning-impulse withstand voltage;

• wet switching-impulse withstand voltage;

• power-frequency puncture withstand voltage (and breakdown value if required);

• lightning-impulse puncture withstand voltage

Tests(Contd.)

Type tests are carried out on one or more insulators in order to confirm the characteristics of the design.

Sample tests are applied to samples in a group of, for example, four and three items selected from a batch (lot) of 2000 after completion of the routine tests. The sample tests are specified for insulators where the solid insulation puncture distance is ≤ 50 per cent of the flashover or creep path (Type B insulators). For station post insulators 1 per cent may be chosen from a lot of 100 to 500.

Routine tests are for checking each insulator on completion.

The physical arrangements for testing the different insulator configurations are outlined in the relevant Standard.

Porcelain and glass insulators (overhead lines) Testing

These insulators include cap-and-pin and long-rod string types, pin type and line post (Australia) type.

Type tests include dry impulse (1.2/50μs) withstand voltage and wet power frequency withstand voltage applications. The impulse tests are carried out according to IEC 60060-1, preferably using the ‘up and down’ procedure for determination of the flashover voltage. Both polarities are applied unless it is evident that one gives a lower withstand voltage. A wet withstand-voltage test is applied and if flashover occurs a retest is allowed.

Sample tests consist of power-frequency puncture withstand voltage applications with the insulator immersed in a dielectric fluid.

Routine electrical tests are required only for Type B insulators. Power-frequency or high-frequency sources may be used. For these tests the testing transformer or high-frequency source must be stable and designed to withstand the sudden current and voltage surges produced during the many routine tests carried out.

Ceramic and glass insulators (post type – indoor and outdoor) testing

Post insulators are designated according to whether their construction includes a solid cylindrical core (puncture-proof), a cavity core with internal integral ceramic barrier or a cylindrical core and internal metal fittings with the solid insulating material of at least half the length of the external flashover path (puncture-proof). For units with the solid length less than half of the total, and for the pedestal-post type in which the thickness of the solid is small compared with the external distance, electrical sample and routine tests are specified. Type tests are applied to one insulator only. For indoor applications, dry lightning impulse, switching impulse and power frequency withstand-voltage tests may be required. For outdoor applications, wet switching impulse and power-frequency tests may be required. Because of the variability in the basic insulation levels the dry lightning-impulse value is chosen as the reference and not the system-highest voltage. Note that switching impulses (250/2500μs) would usually be specified for units intended for operation in systems with highest voltages of 300 kV and above. Sample puncture tests may be required on units where the solid insulating material distance is half or less than that of the flashover path. Routine electrical tests are specified for the same designs as the sample tests.

Composite insulators for overhead lines (string & post units) testing

Extensive design tests are called for in the assessment of composite insulators. These involve dry power-frequency flashover tests for both the line and post insulators, in addition to steep fronted impulse (1 000 kV/μs) applications for the special conditions specified. Tracking tests are also required. The comprehensive sequence of mechanical, thermal and moisture tests (including leakage current measurements) may be found in standards. Type tests include dry lightning impulses, wet switching impulses and wet power-frequency withstand voltages. These are applied to one insulator or insulator unit only. No electrical tests are specified for sample checks and for routine inspections. Within the Standard, voltage levels are given for an optional radio-interference voltage test (RIV). No RIV values are quoted but the test voltage indicates the value at which corona would be expected to extinguish under dry conditions. Acceptable levels are agreed between the purchaser and manufacturer – a guide for establishing limit values is given in CISPR 18-2, Amendment 1. Detailed procedures for obtaining RI characteristics for dry high-voltage insulators are given in IEC 60437 [S7/8]. The specified reference frequency given in CISPR 18-2 is (0.5 +0.05) MHz but others in the range 0.5–2MHz may be selected based on previous practice. It is expected that RI disturbances from insulators will not normally affect television reception.

Overhead line and substation hardware

Although the Standards recommend air clearances for the design and operation of overhead line systems and substations, it is often necessary to prove the performance of a particular configuration, especially at the higher voltage levels, where switching-impulses or unusual pollution conditions exist. Such testing includes the determination of corona inception levels in order to ensure that unexpected flashovers do not occur and that interference does not affect local radio transmissions and receptions. This interference can invalidate some of the monitoring systems being used on site for the detection and measurement of partial discharges. In the laboratory, levels can be quantified by PD measurements and the Radio Interference Voltage methods described in CISPR 18-2. Usually, insulators and the HV hardware will be checked simultaneously. The measurements are made utilizing a standard CISPR test set as specified in CISPR 16-1:1993 . The technique has been applied for many years and is sometimes used as a partial-discharge test (see IEC 60270). The diameters of overhead conductors for a given voltage and condition are well defined. Sometimes, however, it is necessary to determine the self-screening efficacy of multiple conductors and the satisfactory performance of a new design of spacer (or spreaders), as in quad arrangements. Proposed electrode configurations at the terminals of bushings, switchgear and arresters as well as insulators will require checking in the high-voltage laboratory at above operating voltages requiring appropriate corona-free supply sources.

Surge arresters

Surge arresters are a vital part of the power system. They must be shown to be able to withstand the steady-state voltages without deterioration, to respond correctly by reducing an excessive lightning or switching surge to a safe value and then to recover their previous power-frequency strength. The earlier units included spark gaps and silicon carbide elements requiring tests as detailed in IEC 60099-1 [S7/9]. The tests for the later designs of metal-oxide gapless units are described in Part 4 of the Standard. The testing procedures are complex and extensive. In the following are summarized some of the type and routine tests for proving of the insulation requirements and V-I performance. Type tests on the arrester housings include a lightning-impulse dry withstand voltage test and a similar switching-impulse test both above the corresponding protection level. In the latter case a wet test is required for outdoor units. The power frequency withstand test is applied for one minute with a peak value of just below the lightning-impulse protection level. Residual-voltage-type tests on three complete sample arresters determine the highest residual voltage at the rated discharge current. From the results of other tests the switching impulse protection level for specified currents may be defined.

Surge arresters(Contd.)

Type tests are carried out in order to simulate current flow from a recharged transmission line. Long-duration current-impulse withstands with virtual lengths of the wave peak within the range 500–3 000μs are applied. A simplified circuit is described in Appendix J of IEC 600994. Operating-duty-type tests incorporating the simultaneous application of impulse and power-frequency voltages are also included in the Standard. A major aim is to prove that thermal runaway does not occur and that the unit can cool down during the power-frequency periods. A special multi-pulse operating-duty test representing multiple lightning strokes has been proposed [S7/10]. This has been shown to fail some units that would have passed the normal requirements. Routine reference voltage tests require determination of the power-frequency voltage necessary to produce the reference current as measured at the bottom of the unit. The voltage value must be within the range specified by the manufacturer. This is a parameter that may be used for monitoring in service. Routine lightning-impulse residual-voltage tests are carried out at, preferably, the nominal discharge current. The voltage must not exceed the appropriate specified values for a complete arrester – typically of the order of 3 × the rated line voltage. As part of the routine tests a partial-discharge contact noise measurement is made. The internal partial discharges, including disturbances due to bad contacts, should not exceed the equivalent of 10 pC. It may be practical to monitor the condition of suspect units in service by means of PD measurements.

Bushings testing

The insulation systems of high-voltage bushings vary according to the materials and configuration adopted. Withstand-voltage tests are required as routine tests together with DDF and PD measurements. Such data are used for quality control and for future reference in service. The test voltage magnitudes are determined from IEC 60071-1, depending on the particular application. Refer to IEC 60137 [S7/18] for details. For outdoor bushings of lower voltage ratings a wet power-frequency voltage withstand- type test is applied for one minute. Dry lightning-impulse voltage withstand- type tests are carried out on all bushings with 15 impulses of both polarities. Dry-switching impulse-voltage-type tests are required for indoor units of the higher voltage ratings and similar wet tests for outdoor bushings of these ratings. The condition of the insulation is monitored by DDF, capacitance and PD measurements after testing and, also, by recording of impulse wave shapes during tests if considered necessary. DDF measurements are made as routine for voltages up to 1.5× rated voltage to ground. Values at 2.5 kV and 10 kV are recorded as being useful during later service maintenance procedures. Maximum allowable DDF values at 1.05×rated voltage are quoted according to the various materials, i.e. for oil-impregnated paper, resin-bonded paper, resin-impregnated paper, cast resin and insulating gas in capacitance-graded bushings and cast resin in non-capacitance-graded units. Allowable changes in DDF and capacitance magnitudes are specified.

Bushings testing(Contd.)

Routine dry, power-frequency, one-minute, withstand-voltage tests are applied to all bushings and partial-discharge tests at just above rated voltage. If required for a transformer installation the voltage is increased to 1.5×rated voltage. PD acceptance values are 10 pC for these conditions except for resin-bonded and cast-resin insulation systems, where levels of 100/300 pC and 100 pC respectively are usually taken as reasonable limits. The insulation of the bushing test tap must be checked by an applied voltage of 2 kV and the DDF and capacitance values measured. These values should be 100mR and 5000 pF or less. The lower the tap capacitance the more sensitive is the PD test as carried out according to IEC 60270 for transformers and for some switchgear. When testing for special applications it is important to use correct terminations (e.g. for re-entrant types) and to ensure that clearances within the test tank are typical of those in the equipment in which the bushing is to be installed. Air terminations may need to be screened, rod gaps removed and attention be given to the method of mounting/clamping the earthed flange to the test tank. Temporary clamps can be a source of corona. The measurement of low-value PDs requires the usual ‘good housekeeping’ and preferably, for routine tests, a dedicated area where the PD circuit layout can be permanent. Guidance is given in IEC 61464 [S7/19] regarding the interpretation of DGA results for in-service oil-impregnated-paper bushings. Normal site tests include DDF and IR measurements.