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Thursday, November 27, 2014
Testing of of electrical power system equipment
The testing of electrical power system equipment involves checking the insulation system, electrical properties, and other factors as they relate to the overall operation of the power system. Therefore, testing of electrical equipment 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 fl ow under test conditions. Excessive leakage current fl ows 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 30 Electrical Power Equipment Maintenance and Testing current (AC) voltage. The testing of solid insulation with these voltages can be categorized as nondestructive testing and destructive testing, respectively.
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.,.
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 polarization index.
Insulating Liquid Testing
Insulating liquids used in transformers or other electrical apparatus are subject to deterioration and contamination over a period of time. These contaminants 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 fl uids are moisture, heat, oxygen, and other catalysts that result in a chemical reaction that produces acid and sludge, which in turn attack the insulating fl uids. The main insulating fl uids 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 fl uid 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.
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 Maintenance Strategies, Dielectric Theory breakers, and associated equipment such as instrument transformers and wiring. The function of protective relays and devices maintenance and testing 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 commissioning tests, routine maintenance testing, and verification testing.
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.
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.
Fault Gas Analysis Testing
Fault gas analysis testing comprises of dissolved gas analysis and total combustible gas tests. The dissolved-gas analysis provides information on the individual combustible gases dissolved in the insulating oil. The total combustible fault gas analysis test provides information on incipient faults in oilfilled transformers by measuring the total combustible gases present in the nitrogen cap of the transformer. Because of excessive heat due to loading of Electrical Power Equipment Maintenance and Testing 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.
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.