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38 años de experiencia realizando pruebas estudios y mantenimientos a subestaciones eléctricas.
Correct use of tests can help identify condition, performance and detect future failures, avoiding large financial losses and personnel accidents. In ISIEM we take care of the correct maintenance with highly trained and certified equipment:
Transformer ratio measurement
The transformer ratio test, or TTR test, of a confirmed transformer that that transformer has the correct ratio of turns between primary and secondary. Proper use of this test can help identify tap changer performance, short circuit turns, open windings, incorrect winding connections, and other FAULTS WITHIN TRANSFORMERS.
First, torque ratio tests serve to confirm the transformer ratio and polarity of new and used transformers and to identify deviations in torque ratio readings, indicating problems in one or both windings or in the core magnetic circuit.
For transformers that have tap changers to modify their voltage ratio, the transformer ratio is based on a comparison between the nominal reference voltage of the respective winding and the operating voltage or percentage of nominal voltage to which it refers. The transformation ratio of these transformers will be necessary for all faucets and for the entire winding.
The tolerance for the transformer ratio, measured when the transformer is discharged, must be ± 0.5% on all its taps.
TYPES OF TTR
Currently, TTRs are divided into two groups: single-phase and three-phase. Some manufacturers offer single-phase TTRs that are capable of measuring per phase the ratio of turns, excitation current, phase deviation, resistance of the “X” and “H” windings and polarity of the connection of “X” and “H” distribution windings and current transformers, as well as voltage regulators.
Similarly, automatic three-phase TTRs are related to measure the relationship between the number of secondary and primary spies simultaneously in the three phases of power transformers, instrumentation and distribution in substations or factories.
INSULATION RESISTANCE (MEGGER)
The meaning of the insulation resistance test refers to the opposition of an insulator to the application of a given DC voltage (direct current) for a given time and which is measured from the application of the same.
Insulation resistance and electrical resistance produce products from units of the International System of Measurement Units (SI) and the unit for this measurement is the ohm and is represented by the Greek letter omega (Ω).
The INSULATION RESISTANCE test consists of applying voltage between the electrodes and measuring the current flowing through the circuit. The test kit consists of a direct current source and a current meter circulating in the circuit, as illustrated in the diagram. In other words, when we perform the insulation resistance test, what we are doing is measuring the voltage and current circulating in the circuit and, using Ohm’s Law, we determine the RESISTANCE of the element under test.
CAPACITIVE CURRENT: corresponds to the load of the capacity of the insulation under test, has a relatively high magnitude, but short duration because after about 15 seconds is negligible, decreases exponentially to a value close to zero, as the insulation is loaded and is due to the conductor behaves like a capacitor. This component is responsible for the low initial value of the insulation resistance.
DIELECTRIC ABSORPTION CURRENT: this current is the one that takes up the insulation when polarized as a consequence of the increase in the intensity of the electric field. It is high at the beginning of the test, decreases much slower than the capacitive load current and requires more minutes to reach a value close to zero. For test purposes you can neglect the change that occurs after 10 min.
FUSE CURRENT: is the current that circulates through the insulation and is proportional to the applied voltage, remains constant over time and is the main factor in determining the conditions of an insulation.
Electrical absorption: – The insulation resistance is directly proportional to the thickness of the insulation and inversely to the area of the insulation; when a direct current voltage is applied to an insulation, the resistance starts with a low value and gradually goes over time until it stabilizes.
We also mentioned three other components that have relevance in this test:
By graphing the values of insulation resistance against time, it obtains a dielectric absorption curve; indicating its slope the relative degree of drying and cleaning or dirt of the insulation. If the insulation is wet or dirty, a stable value will be reached in one or two minutes after starting the test and the result will be a curve with a low slope.
The slope of the curve can be expressed by the ratio of two insulation resistance readings, taken at different time intervals, during the same test to the ratio of 60 to 30 seconds is known as: “ABSORPTION RATIO”, and the ratio of 10 to 1 minute as “POLARIZATION RATIO”.
The determination of this test will be made using an Ohmmeter, among some brands that manufacture this equipment we can find MEGGER, AEMC, DUCTER, DOUBLE, KYORITSU among others.
TEST RESISTANCE MEASUREMENT WINDINGS
The resistance measurements of transformer windings, tap changers or motors present a particular challenge within the resistance measurements of electrical equipment, due to the highly magnetic nature of the windings.
Winding resistance measurement equipment must cope with the strong inductive load in order to apply the stable DC needed to magnetize the winding, in addition to taking into account the connection group and temperature.
Winding resistance measurements are used for both factory and field verification. In the factory, they ensure proper manufacturing to the design, they also serve to calculate losses, and as a variable magnitude in thermal tests.
In the field, winding resistance measurements are performed to evaluate possible problems, such as short windings, open windings, internal connection problems, hot spots and tap changer condition. After the supply of new transformers, winding resistance measurements are used to detect any damage during transport, and as the transformer’s initial reference for future measurements.
Winding resistance measurements are also part of periodic maintenance programs to find problems that impact SYSTEM PERFORMANCE and can lead to unexpected outages.
DIELECTRIC RIGIDITY OF THE OIL, like all insulating materials, degrades over time. If it is also exposed to extreme operating conditions, it can suffer accelerated degradation, reducing its electrical properties prematurely.Moisture is the main cause that significantly reduces the electrical properties of oils. Hence the importance of keeping them always dry through oil preservation systems.In the case of circuit breakers, there is also the degradation due to the effect of the electric arc that occurs between the contacts when the switching operations are completed.In order to be able to measure the degradation of the electrical properties of the oil, among other tests, we have the measurement of the dielectric rigidity.
The dielectric rigidity test of the oil (also known as the electrical breakdown voltage of the oil) allows the capacity of this insulating liquid to withstand an electrical stress without producing an arc to be measured.
This test reveals the amount of water, dust, sludge or any conductive particle that may be contaminating the oil and thus preventing it from continuing to perform its insulating function. In order to do this, it is necessary to obtain the oil samples from the switch or transformer to be evaluated.
In order to do this, the following steps must be followed:
Clean the sampling valve and then drain some oil before collecting the sample.
Make sure that the container is clean and rinse it at least once with the oil to be collected.
Avoid contact of the container with the sampling valve, fingers or any other foreign body.
Close the container hermetically to transport it to the laboratory where the test will be carried out.
A liquid dielectric stiffness tester is used to perform the test. This tester consists of a voltage regulator, an elevating transformer, a voltage divider, a voltmeter and a test cup. The schematic diagram of the equipment is illustrated in the figure:
The voltage regulator serves to increase the voltage gradually and automatically.
The transformer is used to obtain the high voltage necessary to cause the electric arc in the oil. Its output should be 0 to 60 kV and the speed at which the voltage should be increased depends on the Standard used to perform the test:
For ASTM D-877, the rate of voltage increase is 3 kV per second.
For ASTM D-1816, the rate of voltage increase is 0.5 kV per second.
The voltage divider serves to reduce the high voltage to values manageable by a low voltage voltmeter in order to measure the voltage at which the electric arc is produced.
The test cup is used to contain the oil to be tested. It has an integrated pair of electrodes to which the high voltage is applied. The electric arc occurs specifically in the space between the electrodes.
The characteristics of the test cup also depend on the standard used:
For ASTM D-877 the electrodes are flat and separated by a distance of 2.5 mm.
For ASTM D-1816 the electrodes are hemispherical and separated by a distance of 1 mm. In this case, the cup also has a stirrer that provides slow circulation of the oil, making it more representative of the conditions under which the oil operates.
PHYSICAL-ELECTROCHEMICAL ANALYSIS OF THE INSULATING OIL
IN ORDER TO VERIFY THE CONDITION OF THE OIL AND PROGRAM PREVENTIVE (RECONDITIONING) OR CORRECTIVE (CHANGE) MEASURES, AN ANALYSIS OF THE PHYSICAL, ELECTRICAL AND CHEMICAL CHARACTERISTICS OF THE OIL IS PERFORMED.
THE SERVICE CONSISTS OF OBTAINING A SAMPLE OF INSULATING OIL FROM THE TRANSFORMER AND SENDING IT TO AN ACCREDITED LABORATORY FOR ANALYSIS.
OIL SAMPLING AND DIAGNOSIS IS PERFORMED IN ACCORDANCE WITH NMX-J-308 AND LABORATORY TEST METHODS IN ACCORDANCE WITH NMX-J-123.
THE ANALYSIS TO ASSESS THE QUALITY OF THE DIELECTRIC OIL INCLUDES THE FOLLOWING TESTS:
DIELECTRIC BREAKDOWN VOLTAGE
POWER FACTOR AT 25 ° C (60HZ)
RELATIVE DENSITY A 15.6 C /15.6C
IGNITION TEMPERATURE 101.3 KPA
Insulation resistance tests on power circuit breakers are important to know the conditions of your insulation. In high volume oil circuit breakers there are insulating elements of hygroscopic materials, such as oil, the operation bar and some others that intervene in the support of the arcing chambers; also the carbonization caused by the operations of the circuit breaker causes contamination of these elements and by a reduction in the insulation resistance. The insulation resistance test applies to other types of circuit breakers, such as small volume oil, vacuum and SF6 circuit breakers in which porcelain is normally used as insulation.
Hi-pot means high voltage or high power, this test verifies the insulation of an electrical product for the sea capable of protecting the user from an electric shock and ensures the safety and reliability of finished accessories such as cables, circuits and motors. During this test, an extreme high voltage load (this load is much higher than the load of a normal operation) is applied between the conductors and their insulation. The Hit-pot or high voltage test is the device shown in the photo that monitors the current flowing through the insulation or the leakage current. The test attempts to demonstrate whether the product is safe to use or not, even when it is sometimes a high voltage load. If there is a leak in the insulation,
This test can help uncover design defects, defects, small gaps between conductive parts and ground, loose cables, contaminated conductors, terminal problems, tolerance errors in IDC cables, etc.
There are 3 types of high voltage tests (Hi-pot Test)
Dielectric Break Test – In this test the voltage is increased until the dielectric fails or breaks to determine the highest voltage load “). The dielectric is usually destroyed with this test so this test is used on random samples.
Dielectric Strength Voltage Test – During this test a standard voltage load is applied, the leakage current is controlled and must be below a preset limit for a certain amount of time. This test does not destroy the dielectric so the inspector can check 100% of the products.
Insulation Resistance Test – This test is performed to determine the resistance value of insolated products. Voltage and current are used to calculate insulation resistance.
Considering the NOM-001-SEDE in its article 4.4.2 indicates that “the electrical installations must be tested before being put in service and after any important modification, to verify the suitable execution of the works”.
VLF are very low frequency initial letters. Consider that VLF is 0.1 Hz or less; that is, the full cycle of the sine wave is 10 seconds.
a) DC tests.
b) Tests in alternating current (60 Hz)
c ) Low frequency alternating current (LFV) tests.
Potential tests applied to VLF are carried out in accordance with the IEEE Std 400.2 standard, which establishes the test voltage and the corresponding test time.
To verify correct installation of cables and terminals prior to commissioning, insulation resistance (megohmmeter) and applied potential (Hi-Pot) tests are performed on CD in accordance with IEEE Std. 400.1 and ANSI / NETA ATS.
38 años de experiencia realizando pruebas estudios y mantenimientos a subestaciones eléctricas.