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.,.
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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.

Determining load efficiency and electric motors

The engines are responsible for a large portion of our electric bill so energy efficiency deserve preferential attention. Too often the engines are misaligned or over-sized for the load plan to serve, or have been rewound multiple times.

Many times we considered the need to replace engines as a way to save energy but do not have clear operating costs and get real savings by applying this measure. To find out we need to determine the hours of operation , the values ??of improved efficiency , and load. The partial charge is a term used to describe the actual load served by the engine compared to the rated motor capacity . in this article we briefly describe the techniques we estimate loads .

Reasons for determining the load of a motor

Most electric motors are designed to operate at 50 - 100 % of rated load. The maximum efficiency is usually about 75 % of rated load . Thus, an engine 10 H.P. works acceptably in a range of cargo ranging from 5 to 10 hp , and peak efficiency occurs at 7.5 HP Engine efficiency tends to decrease dramatically below 50 % load . An engine is considered working with an insufficient charge when working in the range where the efficiency drops significantly with decreasing load .

Overloaded motors can overheat and lose efficiency. Many engines have been designed with a service factor that allows occasional overload. The service factor is a multiplier that indicates how much can be overloaded under ideal environment conditions . For example, a 10 -hp motor with a 1.15 service factor can handle a load of 11.5 hp for short periods of time without significant damage. Although many motors have service factors of 1.15 , run the motor continuously above rated capacity reduces engine life and efficiency. Never operated an overloaded motor when the voltage is below the nominal or when cooling is malfunctioning for altitude, high ambient temperature , engine or dirty surfaces .

If we use in our operation teams with engines that operate extended periods at loads below 50% , we should consider making changes . Sometimes motors are over sized because they must accommodate peaks , such as those occurring conditions when a pumping system must occasionally meet high demands. The options available to suit varying loads include two-speed motors , adjustable speed drives and load management strategies to keep them within acceptable ranges.

DETERMINATION OF CHARGE ENGINES

Measuring the power input

Determining if our engines are loaded properly can make decisions about when to replace them and what is the best way to replace them. Measuring motor loads is relatively easy and allows us to analyze the efficiency of the motor operation .

It is advisable to inspect all engines operating above 1000 hours a year, dividing engines in the following categories :

    Motors that are significantly oversized or low-load need to be replaced .
    Engines that are moderately oversized or working at low loads but moderately efficient engines will be replaced with adequate size for models when they fail .
    Appropriately sized but standard efficiency motors will be replaced with energy-efficient models when they fail .

The easiest way to test is to use engines direct power meter with the engine loaded for which there are many fairly inexpensive multifunctional teams .

Measurement of the line current

The estimation method of load current is recommended only when the measurement is disponiblemente ampere only . The motor that drives ampere varies approximately linearly with respect to the load , down to approximately 50% at full load. Below about 50% load due to the reactive requirements of magnetizing current , the power factor is degraded and the curve is nonlinear ampere progressively. In the region of low load current measurements are not a useful indicator of load.

The current is indicated on the nameplate of the motor is the value of current at full load and only occurs when the engine runs at its rated voltage. The effective value of the current is always correct for the voltage. If the supply voltage is below or on the rating plate of the motor, the measured ampedaje correpondientemente is higher than expected under nominal condition and must be adjusted downward . The reverse also applies.

Method sliding ( slip )

Deslizamineto method for estimating an engine load is recommended only when the speed of operation is available . The synchronous speed of an induction motor is dependent on the power supply frequency and the number of poles in the motor coil. The higher the frequency, the motor rotates faster . The more poles have a motor rotates more slowly .

The actual motor speed is less than its synchronous speed and the difference between synchronous and actual speed is called slip or deslizamineo . The amount of this slip is proportional to the load on the engine by the team.

Using a tachometer to measure the speed of the current engine , you can calculate the motor loads .

DETERMINING THE EFFICIENCY OF MOTOR

The energy efficiency of a motor is the ratio between the useful output power and total power input and is usually expressed in percentage.

By definition, a bike with a number of H. P. expected given the amount of power transmitted mechanically to the motor shaft. The motor losses are the difference between the input power and output . Once you have determined the engine efficiency and input power is known , we can calculate the output power.

Most analyzes of saving energy conservation Motor assume that the existing engine is operating at the efficiency of your plate. This assumption is reasonable above 50 % load point and that the peak efficiency occurs about 3/4 charging. Larger engines powers exhibit relatively planapor efficiency curve below 25% of full load.

It is more difficult to determine the efficiency of an engine that has been in service for a long time . It is difficult for plaque has been lost and it is impossible to locate information efficiently. The engine may have been rewound , and it is likely that the efficiency is reduced .


When information is not available on the efficiency , we can determianrla operand at the point of engine load. These data we can determine the plate or engine looking for technical details .

We can also estimate the efficiency tables in Attachment C of that article in the literature.

 

Beat Noise

Beat noise is the result of two vibratory forces having very close, but not identical, driving frequencies (vibratory forces generally produce noise). These frequencies alternately add and subtract from each other. The end result is an audible beat with a maximum amplitude
equivalent to the sum of the individual maximum amplitudes, and a beat frequency which occurs at the difference in the two individual frequencies.
Beat noise occurs most commonly in two pole motors. The primary reason for this is the closeness of the twice line and twice rotational frequencies (i.e. 2X and 2f). The two graphs on the next page should clarify the addition and subtraction the 2X and 2f frequencies. This beat frequency is essentially twice slip frequency. In the case of two pole, 50Hz ANEMA motors, typical full load speed is 2985 rpm. The 2X frequency is 5970 RPM, or 99.50 Hz, and the twice line frequency is obviously 100 Hz. The beat frequency would then be 0.5 Hz. Another way to state this is to say that the beat noise will have a period of 2 seconds, or, that the noise will go from zero, to some maximum noise level, and back to zero, in two seconds. While two polemachines are more susceptible to beat noise than slower speed machines, 50 Hz machines are not impacted any more or less than 60 Hz motors. The existence of an audible beat is common in these machines, and in and of itself, does not detract from performance or reliability in any way.
In addition to two pole motors being susceptible to beat noise, when similar machines operate in close proximity to each other, beat noise can occur as well. A twin engine turboprop plane
should be a familiar example of this phenomenon.