Maintenance Strategies, Dielectric Theory, Insulating Materials, Failure Modes, and Maintenance Impact on Arc-Flash Hazards:Overview of Testing and Test Methods

Overview of Testing and Test Methods

Testing of electrical equipment is usually performed in the field on new equipment after installation and on existing equipment to assess its condition. The manufacturer conducts electrical tests on equipment before it leaves the factory; these tests, known as factory tests, are outside the scope of this text and therefore will not be discussed. Field tests are conducted to see whether newly installed equipment has been damaged, to indicate whether any correc- tive maintenance or replacement is necessary on existing equipment, to indi- cate if the equipment can continue to perform its design functions safely and adequately, to chart the gradual deterioration of the equipment over its service life, and to check new equipment before energization. In view of these objec- tives, the electrical testing of equipment can be divided into the following:

• Types of tests

• Types of testing methods

Types of Tests

The types of field tests are acceptance tests, routine maintenance tests, and special maintenance tests that are conducted for specific purposes.

Acceptance Tests

These tests are known as start-up or commissioning tests and are performed on new equipment, usually after installation and prior to energization. When these tests are repeated within a year, that is before the warranty period expires, then these tests are referred to as proof tests. Tests of this type are made at 80% of the final factory test voltage value. They are run to determine the following:

Whether the equipment is in compliance with the specification To establish a benchmark for future tests

To determine that the equipment has been installed without damage To verify whether the equipment meets its design intent and limit

Routine Maintenance Tests

These tests are performed at regular intervals over the service life of the equipment. They are made concurrently with PM and at 60% of the final factory test voltage value. In the course of routine maintenance tests, it is

very helpful to record the information as it is found on the equipment and to also record the condition in which the equipment is left. Therefore, these tests can be further subdivided into the following:

Asfound tests: These tests are performed on equipment on receipt or after it has been taken out of service for maintenance, but before any maintenance work is done.

Asleft tests: These tests are performed after maintenance has been performed and just before reenergization. They can indicate the degree of improvement in the equipment and service as a benchmark for comparison for future tests.

Special Maintenance Tests

These tests are performed on equipment that is known to be defective or has been subjected to adverse conditions that may affect its operating char- acteristics. An example might be the fault interruption by a circuit breaker, which requires inspection, maintenance, and tests before it can be put back into service.

Types of Testing Methods

The testing of electrical power system equipment involves checking the insu- lation system, electrical properties, and other factors as they relate to the overall operation of the power system. Therefore, testing of electrical equip- ment can be divided into the following types:

• Solid insulation testing

• Insulating liquid testing

• Relay and protective device testing

• Circuit breaker time–travel analysis

• Grounding electrode resistance testing

• Fault gas analysis testing

• Infrared inspection testing

Solid Insulation Testing

Insulation can be either solid, liquid, or gaseous dielectric materials that prevent the flow of electricity between points of different potential. Insulation testing is done to determine the integrity of the insulating medium. This usually consists of applying a high potential (hi-pot) voltage to the sample under test and determining the leakage current that may flow under test conditions. Excessive leakage current flows may indicate a deteriorated condition or impending failure of the insulation. Insulation testing can be performed by applying either direct current (DC) voltage or alternating current (AC) voltage. The testing of solid insulation with these voltages can be categorized as nondestructive testing and destructive testing, respec- tively. The destructive test may cause equipment under test to fail or render it unsuitable for further service. Nondestructive tests are performed at low- voltage stress, and the equipment under test is rarely damaged.

The AC hi-pot test is primarily a “go” or “no-go” test. The voltage is raised to a specified level. If the equipment fails or shows excessive leakage current, the equipment under test is unusable. If the equipment does not fail, it has passed the test. This test can only indicate whether the equipment is good or bad. It cannot indicate with what safety margin the test was passed. However, there are nondestructive tests that can be performed with AC voltage, such as power factor (PF), dissipation factor (DF), capacitance, etc., which are dis- cussed in greater detail in Chapter 3.

The DC hi-pot test can indicate more than a “go” or “no-go” condition. It can indicate that equipment is all right at the present time but may fail in the future. DC testing is done to obtain information for comparative analysis on a periodic basis. With dc testing, the leakage current is measured during the progress of the test and compared to leakage current values of previous tests. However, the DC hi-pot test is considered to be a destructive test if the test voltage is not applied in a predetermined control-voltage steps. The DC voltage tests can be performed at lower voltages, which are nondestructive tests, such as insulation resistance, dielectric absorption ratio, and polariza- tion index. These tests are discussed in more detail in Chapter 2.

Insulating Liquid Testing

Insulating liquids used in transformers or other electrical apparatus are sub- ject to deterioration and contamination over a period of time. These contami- nants have a detrimental effect on the insulating properties of the fluid, as well as on the solid insulation system of the transformer winding. Basically, the elements that cause the deterioration of the insulating fluids are mois- ture, heat, oxygen, and other catalysts that result in a chemical reaction that produces acid and sludge, which in turn attack the insulating fluids. The main insulating fluids that are in use today for transformers are oil, silicone, and RTemp and Wecosol. Askarel was used in the past, but its use was banned by federal regulations owing to its high toxicity; however, there may be installations that still may have this fluid at their plant sites. Regular tests are recommended to monitor the condition of the insulating liquid. Samples should be taken from the transformers on periodic basis to perform various tests in accordance with American Society of Testing Materials (ASTM) methods, which are discussed in detail in Chapter 4.

Protective Device Testing

Protective device testing involves the testing and maintenance of protective relays, low-voltage draw out power circuit breakers, low-voltage molded-case

breakers, and associated equipment such as instrument transformers and wiring. The function of protective relays and devices maintenance and test- ing is to assure that a particular breaker or protective relay is able to perform its basic protective function under actual operating conditions. The tests on relays, protective trip devices, and circuit breakers can be classified as com- missioning tests, routine maintenance testing, and verification testing. These tests are discussed in more detail in Chapters 7 through 9.

Circuit Breaker Time–Travel Analysis

The circuit breaker time–travel analysis test is performed to determine if the operating mechanism of the circuit breaker is operating properly. This test is usually performed on medium- and high-voltage circuit breakers and depicts the position of breaker contacts with relation to time. This relationship can then be used to determine the operating speed of the circuit breaker for opening and closing and contact bounce, and the interval time for closing and tripping. The breaker operating time data can be used to evaluate the condition of mechanical parts of breakers, such as closing mechanism, springs, and shock absorbers. Circuit breaker time–travel analysis test is described in greater detail in Chapter 7.

Grounding Electrode Resistance Testing

The integrity of the grounding system is very important in an electrical power system for the following reasons:

To maintain a reference point of potential (ground) for equipment and personnel safety To provide a discharge point for traveling waves due to lightning To prevent excessive high voltage due to induced voltages on the power system Therefore, to maintain ground potential effectiveness, periodic testing of grounding electrodes and the grounding system is required. Electrical power system grounding and ground resistance measurements are discussed in greater detail in Chapter 11.

Fault Gas Analysis Testing

Fault gas analysis testing comprises of dissolved gas analysis and total com- bustible gas tests. The dissolved-gas analysis provides information on the individual combustible gases dissolved in the insulating oil. The total com- bustible fault gas analysis test provides information on incipient faults in oil- filled transformers by measuring the total combustible gases present in the nitrogen cap of the transformer. Because of excessive heat due to loading of the transformer, or arcing and sparking inside the transformer insulating oil, some of the oil in the transformer decomposes and generates combustible gases, which then are dissolved in the oil, and eventually become liberated where they mix with the nitrogen above the top oil. The dissolved oil gas and total combustible gas test methods are discussed in more detail in Chapter 4.

Infrared Inspection Testing

There are many different devices available using infrared technology to check hot spots in switchgear and other energized parts of the power system. They are very useful in routine maintenance and inspection for finding bad connections and joints and overloaded terminals or lines. The infrared inspection testing is discussed in greater detail in Chapter 8.

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