CVT Capacitance and Tan Delta Limits (IEC 61869-5)

Short answer

The capacitive divider is the most delicate part of a capacitor voltage transformer (CVT). IEC 61869-5 controls its condition with two measurements: capacitance and dielectric dissipation factor, tan δ. The capacitance must stay within a band around the rated value. The tan δ must stay below a maximum that depends on the dielectric type. Both are measured at the factory, before and after the voltage tests, and they prove that the unit was built uniformly and without punctured elements.

What each test measures

• Capacitance. This is the value of the capacitor. If an internal element punctures, capacitance rises slightly. The divider keeps its transformation ratio from the capacitances C1 and C2, so a change alters accuracy.
• Tan δ. This measures dielectric losses. The standard explains it this way: the losses of the capacitor are an indication of the drying and impregnation process.

This guide covers the limits of the product standard. Reading trends in the field is covered in the guide on power factor and tan delta testing.

Capacitance limits

Clause 5.3.3.3.501 of IEC 61869-5 sets two requirements.

1. The capacitance of a unit, a stack or the complete divider, measured at rated primary voltage and ambient temperature, must not differ from the rated capacitance by more than -5 % to +10 %.
2. In a stack made up of several units, the capacitance ratio of any two units must not differ by more than 5 % from the reciprocal ratio of their rated voltages.

The second rule shares the voltage proportionally among the units. If one unit has twice the rated voltage of another, its capacitance must be half, within that tolerance.

Tan δ limits

Clause 5.3.3.3.502 gives the permitted tan δ values measured at rated primary voltage.

The values refer to dielectrics impregnated with mineral or synthetic oil and to a temperature of 20 °C. The standard adds that tan δ depends on the insulation design, the voltage, the temperature and the measuring frequency. In certain dielectrics it also depends on how long the unit was energized before measuring.

How capacitance is tested

Clause 7.2.501 describes the test in general terms.

• It can be measured on the complete divider, on a stack or on separate units. The electromagnetic unit is disconnected.
• The method must exclude errors from harmonics and from accessories in the measuring circuit. The uncertainty of the method is stated in the test report.
• The final measurement is made at rated primary voltage, with a tolerance of ±10 %, after the dielectric tests and at rated frequency.
• Before the dielectric tests, a preliminary measurement is made at low voltage, below 15 % of the rated value. It ensures that no element is punctured and gives a reference for comparison.
• The capacitance change during the tests must not exceed a limit that the standard relates to the number of elements in series.

Tan δ is measured at the same voltage and at the same time as capacitance (7.2.501.2). The standard makes the purpose clear: to check the uniformity of production. The permissible variation limits can be agreed between manufacturer and purchaser.

IEC 61869-1, clause 7.4.3, treats this measurement as a special test. It requires it after the power-frequency voltage withstand test on the primary terminals and, for a CVT, also before. The values refer to rated frequency and a voltage between 10 kV and Um/√3, where Um is the highest voltage for equipment.

Routine test of the divider

Under clause 7.3.1.502, every divider, stack and unit undergoes an AC voltage test with capacitance and tan δ measurement. There must be no puncture or flashover. The reference capacitance is measured before and after with a voltage below 15 % of the rated value. The test voltage of a unit or stack that is part of an assembly is calculated as a proportional fraction of the assembly voltage, increased by 5 %.

In addition, the chopped impulse is a mandatory type test for a CVT (5.3.3.2). Its purpose is to check the design of the internal series connections of the capacitor elements.

Insulation of the electromagnetic unit and the low-voltage terminal

The divider delivers an intermediate voltage to the electromagnetic unit. That is why the standard sets its insulation from the voltage distribution (5.3.501).

• Lightning impulse. The rated withstand voltage of the electromagnetic unit is the CVT test impulse voltage multiplied by C1/(C1 + C2).
• Power frequency. The rated short-duration withstand voltage is the rated primary voltage, multiplied by 3.3 and by C1/(C1 + C2). The standard explains that the factor 3.3 is set for all values of Um and covers the worst case.

The low-voltage terminal of the divider is also tested. If present, an AC voltage of 4 kV is applied for 1 minute between that terminal and earth (5.3.3.501). If the terminal is exposed to the weather, the voltage is 10 kV for 1 minute, with the electromagnetic unit connected (5.3.3.502). The power line carrier accessories are disconnected during these tests, and the terminal surge arrester must not operate.

What to ask the manufacturer for

• The capacitance and tan δ report of the divider, with the preliminary and final measurements.
• The declared dielectric type, to compare tan δ with the limit that applies to it.
• The uncertainty of the measuring method.
• The rated capacitance of each unit and of the stack.

What not to do

• Do not compare factory tan δ with a field value without considering the voltage, temperature and frequency of each measurement.
• Do not accept a capacitance report without the preliminary low-voltage measurement: without it, a puncture cannot be ruled out.
• Do not accept a capacitance measured with a method that does not exclude the effect of harmonics and of the accessories in the measuring circuit.

Related topics

The divider capacitance also determines the accuracy class and frequency range of the CVT. Its effects in transients appear in the transient response and in ferroresonance. In the field it is verified with power factor and tan delta and partial discharge tests.

When to call us

At Tiza Group we measure capacitance and tan δ on coupling capacitors and capacitor voltage transformers on receipt and during maintenance, and we deliver the trend against the nameplate value. Learn about our service of testing and commissioning of current and voltage transformers, available in Colombia, or elsewhere on request. Write to us and we will send you a quote.

Related guides

• Accuracy classes of a CVT and its frequency range
• Ferroresonance of the capacitor voltage transformer
• Transient response of a CVT: classes T1, T2 and T3
• Capacitor voltage transformer (CVT) in HV and EHV
• Inductive and capacitor voltage transformer: tests

References

• IEC 61869-5, Instrument transformers - Part 5: Additional requirements for capacitor voltage transformers, 2011 edition, clauses 5.3.3.2, 5.3.3.3.501, 5.3.3.3.502, 5.3.3.501, 5.3.3.502, 5.3.501, 7.2.501 and 7.3.1.502.
• IEC 61869-1, Instrument transformers - Part 1: General requirements, 2007 edition, clause 7.4.3.

Frequently asked questions