HV and EHV Bushings: Types, C1, C2 and Tests

What a bushing is and where it is located

A bushing is the insulator that lets a high-voltage conductor pass through a grounded wall. That wall can be a transformer or reactor tank, the tank of a dead-tank circuit breaker, the enclosure of a GIS or the wall of a building. IEC 60137 defines it this way and includes the mounting flange as part of the bushing.

In a transformer, the bushing connects the busbar, on the air side, to the winding, on the oil side. Around its base, inside the tank, there are usually bushing-type current transformers (CT) that feed the 87T transformer differential protection and the thermal image. That is why a bushing failure usually ends as a failure of the power transformer.

Main parts

• Central conductor: tube or rod. In draw-lead bushings the winding lead is pulled up through the inside of the tube; in fixed-conductor bushings the connection is made from below.
• Condenser core: paper or fabric wound on the tube, with aluminum foils interleaved at defined intervals.
• Outer housing: porcelain or silicone composite on the air side; porcelain, resin or the oil itself on the lower side.
• Flange: fixes the bushing to the tank and is grounded. It carries the test tap or voltage tap.
• Head: in oil-filled bushings, with the level indicator.
• Nameplate with the factory data, including C1, C2 and the power factor.

How capacitive grading works

Without field control, the stress would concentrate next to the conductor and at the edge of the flange. The condenser core prevents this: each aluminum foil forms, with its neighbor, a cylindrical capacitor, and all of them are in series between the conductor (at full voltage) and the flange (at ground). The lengths and radii of the foils are stepped so that the voltage divides evenly, both radially (through the paper) and along the surface.

The last foil, or one of the last, is brought out to the flange tap. Between the conductor and the tap is the main capacitance C1; between the tap and the flange, C2. In service, a cap or a monitoring adapter grounds the tap, so C2 is short-circuited. If the tap is left open, C1 and C2 form a capacitive divider and the tap rises to a floating voltage far above what its insulation withstands.

Depending on the manufacturer and the standard, the tap can be a test tap (on lower-voltage bushings) or a voltage tap (on higher-voltage ones), the latter also suitable for taking a voltage signal. Its insulation is tested at the factory as a routine test.

Types and differences by voltage level

• OIP (oil-impregnated paper): kraft paper impregnated with mineral oil, normally with porcelain on both sides. It is the oldest technology and is still widely used.
• RIP (resin-impregnated paper): crepe paper impregnated with curable resin. It is a dry bushing, with no oil, and a silicone or porcelain housing. It is used in transformers, reactors and circuit breakers.
• RIS (resin-impregnated synthetic): like RIP, but with synthetic fabric instead of paper, so the core does not absorb moisture and is easier to store. IEC 60137 added it in its 2017 edition.
• Gas bushings: connect a GIS, a gas-insulated line or a dead-tank circuit breaker to the overhead line; the gas in the bushing is shared with that of the equipment.

At medium voltage (up to 52 kV) the bushing without a condenser core, in porcelain or resin, is common, because the field is controlled by geometry alone. At high voltage the OIP or RIP condenser bushing dominates, in the high-voltage power transformer and in the reactor. At extra-high voltage the core is longer, creepage distances grow and the tests become more demanding; IEC 60137 includes bushings up to 1,200 kV. The same applies to the power autotransformer.

The direct connection of a transformer to a GIS is explained in GIS interfaces with cables and transformers.

Rated characteristics and nameplate

IEC 60137 defines, among others, these characteristics:

• Highest voltage for equipment (Um) and insulation levels (lightning impulse and, at the higher levels, switching impulse).
• Rated current (Ir), short-time thermal current (Ith) and dynamic current (Id).
• Cantilever load it withstands, mounting angle and minimum creepage distance.
• Temperature and temperature-rise limits.
• Characteristics of the test tap on transformer bushings.

The nameplate of a condenser bushing shows, depending on the manufacturer and the standard, this data: identification and serial number, voltage and impulse level, and the currents for draw-lead and for fixed-conductor use. It also shows the length below the flange, the weight and the date of the final tests. It also carries the C1, C2 and power factor values measured at the factory.

Integration with protection and control

The bushing has no relay of its own. An internal fault is seen by the protections of the equipment that contains it: 87T differential, overcurrent, and Buchholz and pressure relief on the transformer (see mechanical protections 63 and 26). Its monitoring comes from online monitoring: an adapter on the tap measures the capacitive current of each bushing and calculates the capacitance and tan δ. In IEC 61850, the logical node ZBSH represents the bushing, and the monitoring IED reports its alarms to the substation control system.

Commissioning and maintenance tests

At the factory, IEC 60137 requires as routine tests, among others, measurement of tan δ and capacitance, the power-frequency voltage withstand test, partial discharge measurement and the tap insulation test. For transformer bushings with Um above 72.5 kV it also requires the dry lightning impulse test.

In the field, the usual tests are:

• C1 and tan δ in UST mode: high voltage on the central conductor and measurement at the tap, with the flange grounded; the test voltage is 10 kV or the one given by the bushing manufacturer.
• C2 and its tan δ at reduced voltage, the one the manufacturer allows.
• Comparison with the nameplate values and with earlier measurements, corrected for temperature where applicable.
• Inspection: oil level, leaks, porcelain or silicone, connections and the condition of the tap cap.
• Thermography of the upper terminal and the head.

These tests are part of the power transformer tests. When finished, verify that every tap has been closed and grounded. Capacitance and tan delta are measured with equipment such as the CP TD accessory, which works connected to a CPC 100 or a TESTRANO 600.

Common failures and what to check first

• Increase in C1: according to Megger, it usually indicates short-circuited core layers. Compare with the nameplate and with the other phases.
• Decrease in C1: often a connection problem at the tap, not an insulation problem.
• Rising tan δ: moisture or paper ageing, especially in OIP with damaged seals.
• Low oil level or leaks in OIP bushings.
• Open or poorly closed tap, with discharges and damage to the tap.
• Hot spots at the terminal from loose connections, and flashover from contamination.

If you suspect the bushing after a trip, inspect and measure C1 and tan δ of all three bushings before re-energizing.

Testing and commissioning with Tiza Group

At Tiza Group we measure bushing capacitance and tan δ on high and extra-high voltage transformers and reactors, in substations and industry, in Colombia or elsewhere on request, and we deliver the report compared with the nameplate values. Write to us and we will propose a test plan.

Related guides

• GIS interfaces with cables and transformers

References

• IEC 60137, Insulated bushings for alternating voltages above 1,000 V.
• IEEE C57.19.00, General requirements and test procedure for power apparatus bushings.
• IEEE C57.19.01, Performance characteristics and dimensions for outdoor power apparatus bushings.
• IEC 61850-7-4, Compatible logical node classes (ZBSH).

Frequently asked questions