with the number of turns in winding 2. The test should be performed for each tap position for transformers equipped with tap changers. The TTR test can also verify the polarity of the transformer. The TTR test value for acceptance should not be greater than 0.5% as com- pared to calculated values (see Section 5.8.2).
Hi-pot test: The hi-pot test (also called the overpotential test) should be made on all high- and low-voltage windings of the transformer to ground. Either AC or DC voltage can be used. However, the accepted practice is to apply either an AC or DC hi-pot test to transformers up to 34 kV. For transformers above 34 kV, only the AC hi-pot test is used. For acceptance of the transformer, the AC hi-pot test can be applied at rated transformer voltage for 3 min. This is a go or no-go test. If the hi-pot voltage is held without any failure or malfunction of the transformer, the transformer is considered to have passed the test (see Section 5.8.1).
PF (dissipation factor) test: This test should be performed on important and/or large transformers. This test stresses the insulation in pro- portion to the stresses produced in normal service because it is an AC voltage test. The PF tests are discussed in great detail in Chapter 3 and the reader is urged to refer it (see Section 3.6.1).
Frequency response analysis (FRA): FRA is performed on large power transformers to assess mechanical properties of the windings and core. The purpose of the test is to detect changes in the physical char- acteristics of the transformer caused by through faults, shipment, repair, or other forces. A voltage signal is applied to the transformer terminals over a wide frequency range and the reflected response is measured. Various techniques for this test are currently being studied. Even so, FRA is beginning to gain wide acceptance in the industry. Refer to Section 5.8.5 for additional information on FRA.
5.7.3.4 Maintenance
The objective of transformer maintenance is to safeguard against breakdowns by detecting potential causes and eliminating them. Therefore, periodic trans- former maintenance will ensure many years of trouble-free operation. The transformer is a very simple, rugged device and is often ignored and forgot- ten until transformer failure occurs. However, transformers are a vital link in the electrical distribution system and should be given proper care and atten- tion. Transformer maintenance schedules should be determined according to the critical or noncritical nature of the transformer and the load that is connected to it. Large power transformers are obviously more important than small lighting and distribution transformers; thus they warrant more atten- tion and care. Proper maintenance of the transformer should include routine inspection and repair, transformer liquid maintenance and testing, trans- former winding insulation maintenance and testing, and any other special
maintenance that is recommended by the manufacturer of the transformer. A power transformer maintenance and testing guide with recommended frequency is given in Table 5.7.
Routine inspection and repair: Routine inspection and repair of the trans- former involve the visual observation of the operating conditions of the transformer and necessary repair. The frequency of these observations depends upon the critical importance of the transformer, the environmental
TABLE 5.7
Transformer Inspection and Maintenance Checklist
General inspection items Frequency
Load current Hourly or use recording meters Voltage Hourly or use recording meters Liquid level Hourly or use recording meters Temperature Hourly or use recording meters Protective devices Yearly
Protective alarms Monthly Ground connections Every 6 months Tap changer Every 6 months Lightning arresters Every 6 months Pressure-relief devices Every 3 months Breather Monthly Auxiliary equipment Annually External inspection Every 6 months Internal inspection 5 to 10 years
Insulating liquid Frequency Dielectric strength Annually Color Annually Neutralization number Annually Interfacial tension Annually PF test Annually Moisture content Annually Gas-analysis test Annually
Solid insulation (winding) Frequency IR Annually PF Annually FRA Annually PI Annually
Hi-pot (AC or DC) Five years or more Induced voltage Five years or more Polarization recovery voltage Annually
DC winding resistance Annually
with the number of turns in winding 2. The test should be performed for each tap position for transformers equipped with tap changers. The TTR test can also verify the polarity of the transformer. The TTR test value for acceptance should not be greater than 0.5% as com- pared to calculated values (see Section 5.8.2).
Hi-pot test: The hi-pot test (also called the overpotential test) should be made on all high- and low-voltage windings of the transformer to ground. Either AC or DC voltage can be used. However, the accepted practice is to apply either an AC or DC hi-pot test to transformers up to 34 kV. For transformers above 34 kV, only the AC hi-pot test is used. For acceptance of the transformer, the AC hi-pot test can be applied at rated transformer voltage for 3 min. This is a go or no-go test. If the hi-pot voltage is held without any failure or malfunction of the transformer, the transformer is considered to have passed the test (see Section 5.8.1).
PF (dissipation factor) test: This test should be performed on important and/or large transformers. This test stresses the insulation in pro- portion to the stresses produced in normal service because it is an AC voltage test. The PF tests are discussed in great detail in Chapter 3 and the reader is urged to refer it (see Section 3.6.1).
Frequency response analysis (FRA): FRA is performed on large power transformers to assess mechanical properties of the windings and core. The purpose of the test is to detect changes in the physical char- acteristics of the transformer caused by through faults, shipment, repair, or other forces. A voltage signal is applied to the transformer terminals over a wide frequency range and the reflected response is measured. Various techniques for this test are currently being studied. Even so, FRA is beginning to gain wide acceptance in the industry. Refer to Section 5.8.5 for additional information on FRA.
5.7.3.4 Maintenance
The objective of transformer maintenance is to safeguard against breakdowns by detecting potential causes and eliminating them. Therefore, periodic trans- former maintenance will ensure many years of trouble-free operation. The transformer is a very simple, rugged device and is often ignored and forgot- ten until transformer failure occurs. However, transformers are a vital link in the electrical distribution system and should be given proper care and atten- tion. Transformer maintenance schedules should be determined according to the critical or noncritical nature of the transformer and the load that is connected to it. Large power transformers are obviously more important than small lighting and distribution transformers; thus they warrant more atten- tion and care. Proper maintenance of the transformer should include routine inspection and repair, transformer liquid maintenance and testing, trans- former winding insulation maintenance and testing, and any other special
maintenance that is recommended by the manufacturer of the transformer. A power transformer maintenance and testing guide with recommended frequency is given in Table 5.7.
Routine inspection and repair: Routine inspection and repair of the trans- former involve the visual observation of the operating conditions of the transformer and necessary repair. The frequency of these observations depends upon the critical importance of the transformer, the environmental
TABLE 5.7
Transformer Inspection and Maintenance Checklist
General inspection items Frequency
Load current Hourly or use recording meters Voltage Hourly or use recording meters Liquid level Hourly or use recording meters Temperature Hourly or use recording meters Protective devices Yearly
Protective alarms Monthly Ground connections Every 6 months Tap changer Every 6 months Lightning arresters Every 6 months Pressure-relief devices Every 3 months Breather Monthly Auxiliary equipment Annually External inspection Every 6 months Internal inspection 5 to 10 years
Insulating liquid Frequency Dielectric strength Annually Color Annually Neutralization number Annually Interfacial tension Annually PF test Annually Moisture content Annually Gas-analysis test Annually
Solid insulation (winding) Frequency IR Annually PF Annually FRA Annually PI Annually
Hi-pot (AC or DC) Five years or more Induced voltage Five years or more Polarization recovery voltage Annually
DC winding resistance Annually
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