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Page 121

| Liquid Insulation Tests |

| This usually calls for a cup of oil or askarel in which two insulated terminals are set, usually 0.1 inch apart, and across which a voltage of 20,000 volts or more is applied. Good oil or askarel should not break down at voltages under this value. (The process is described in Chap. 4, under Maintenance.) |

| Bushing Power-Factor Test |

| In addition to the resistance and high-voltage tests, the measurement of power factor of a bushing is used to determine the condition of the insulating materials in a bushing. The bushing may or may not be removed from the transformer, but it must be disconnected from other apparatus and the connecting bus. The power factor is obtained by reading volts, amperes, and watts in the circuit (Fig. 7-13). The instruments and instrument transformers are sometimes contained in a package, with only a power factor indicating scale and voltage control device mounted on the package. Power factor values are in the range of 5 to 10 percent. It is not the single reading that necessarily determines the condition of the bushing insulation. As it deteriorates, more current will leak through the insulation and the power factor, therefore, becomes increasingly greater, (approaching 100 percent). Power-factor readings are taken periodically, as part of maintenance programs. These readings are compared, not with readings of other bushings, but with previous readings on the same bushing. To what extent power factor increases may be acceptable is a matter of judgement. High values may depend on the importance of the |

| Figure 7-13 Connection for power factor test. |

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