High voltage test: types, methods and rules for conducting

2024 Author: Howard Calhoun | [email protected]. Last modified: 2023-12-17 10:16

Today, people actively use a variety of electrical equipment, power cables, electrical connections and more. Since in some equipment the voltage can reach huge values \u200b\u200bthat can cause serious damage to human he alth, periodic monitoring is required. High voltage testing is one of the methods for detecting insulation defects.

What is the verification and why is it carried out

The main purpose of such tests is insulation testing. By increasing the voltage, local defects can be detected. Moreover, some of the problems can be determined only by this method and no more. In addition, overvoltage testing of the insulation allows you to check its ability to withstand overvoltage and, if successful, gives some confidence in the quality of the winding. The essence of the test is quite simple. applied to the insulationvoltage that exceeds the rated operating voltage and is considered an overvoltage. A normal insulating winding will withstand, but a defective one will pierce.

It is worth noting here that with the help of high voltage tests, you can check the ability of the insulation to work until the next repair, control, change, etc. However, this type of test allows you to only indirectly determine this parameter. The main task of this method is to reveal the absence of gross local winding defects.

Further, it is worth noting that the insulation test with increased voltage for some power devices is carried out only in the case of a rated operating voltage not higher than 35 kV. If this parameter is exceeded, the installations themselves are usually too cumbersome. Today, there are three main types of surge testing.

These include power frequency overvoltage test, rectified DC voltage and impulse overvoltage test (standard lightning impulse simulation).

Types of tests. Power frequency and DC

The first and main type of test is increased power frequency voltage. In this case, an overvoltage is applied to the insulation for 1 minute. The winding is considered to have passed the test if no breakdowns were observed during this time, and the insulation itself remained intact. For some cases, the overvoltage frequency may be 100 or 250 Hz.

In the event that the capacitance of the tested insulation willmore, then you will need to take test equipment with more power. In this case, we are talking about testing cable lines with increased voltage. For such cases, the second method is more often used, using increased DC voltage. However, it must be taken into account here that when using a direct voltage, the dielectric losses in the insulation, which, in fact, lead to heating, will be significantly lower than when using an alternating voltage with the same values. In addition, the intensity of partial discharges will be reduced. All this leads to the fact that when testing cable lines with increased voltage using the direct current method, the load on the insulation will be significantly less. For this reason, the power of the applied overvoltage should be increased to ensure the quality of the insulation and the absence of breakdowns.

Among other things, it should be added here that during DC tests, one more parameter should be taken into account, such as the leakage current through the insulation. As for the overvoltage application time, it is from 5 to 15 minutes. Insulation will be considered of high quality not only on the condition that no breakdown was detected, but also on the condition that the leakage current has not changed or decreased by the end of the test period.

When comparing the two methods, it can be clearly seen that the power frequency overvoltage test is much more convenient, but this method cannot always be applied.

In addition, there is another disadvantage of direct current. During the test, the voltage will be distributed overinsulating winding in accordance with the resistance of the layers, and not their capacitance. Although at operating voltage or normal overvoltage, the current will diverge through the thickness of the insulation precisely according to this principle. Because of this, it often happens that the value of the test voltage and the working voltage differ too much.

Lightning Impulse Test

Testing electrical equipment with increased voltage of the third type is the use of standard lightning impulses. The voltage in this case is characterized by a front of 1.2 μs and a duration of up to a half-decay of 50 μs. The need to check the insulation with such an impulse voltage is due to the fact that during operation the winding will inevitably be subjected to lightning overvoltage with similar parameters.

Here it is important to know that the effect of a lightning impulse is very different from a voltage with a frequency of 50 Hz in that the rate of change of voltage is much faster. Due to the higher rate of voltage change, it will be distributed differently over the insulating winding of complex devices, for example, transformers. An overvoltage test with such characteristics is also important because the insulation breakdown process itself with a small amount of time will differ from breakdown at a frequency of 50 Hz. You can understand this in more detail if you look at the volt-second characteristic.

Due to all these conditions, it often happens that testing a transformer with increased voltage according to the first method is not enough - it is necessary to resort toverification also by the third method.

Cut pulses, outer and inner windings

In the event of a lightning surge in most equipment, a surge arrester is triggered, which, after a few microseconds, will cut off the wave of the incoming pulse. For this reason, when testing a transformer with increased voltage, for example, such pulses are used that are specially cut off after 2-3 μs. They are called clipped standard lightning impulses.

Such pulses have certain characteristics, such as amplitude.

This pulse value will be selected based on the capabilities of the device that will protect the equipment from overvoltage, with a certain margin. In addition, when choosing, one should proceed from such a factor as the possibility of accumulation of latent defects with numerous pulses. As for the choice of specific values, the selection rules are described in a special government document 1516.1-76.

High-voltage testing of the equipment for the internal winding will be carried out according to the principle of the three-shock method. The bottom line is that three pulses of positive and three pulses of negative polarity will be applied to the winding. First, voltages that are complete in terms of the nature of the flow of the pulse will be applied, and then cut off. It is also important to know that at least 1 minute must elapse between each successive pulse. The insulation will be deemed to have passed the test if no faults are found and the winding itself receives nodamage. It is worth saying that such a verification technique is quite complicated and is most often carried out using oscillographic control methods.

As for the outer insulation, the 15-strike method is used here. The essence of the test remains the same. At intervals of at least 1 minute, 15 pulses will be applied to the winding, first of one polarity, then the opposite. Both full and chopped pulses are applied. The tests are considered passed if there were no more than two complete overlaps in each series of 15 blows.

How the verification process works

The AC or DC overvoltage test must be carried out in strict accordance with the regulations. The procedure is as follows.

• Before proceeding with the test, the inspector must ensure that the test equipment is in good condition.
• Next, you should start assembling the test circuit. The first step is to provide protective and working grounding for the equipment under test. In some cases, if required, a protective earth connection is also provided for the case of the device under test.

Connect equipment

Before proceeding to connect the equipment to a 380 or 220 V network, grounding should also be applied to the high voltage input of the installation. Here it is important to comply with the following requirement - the cross section of the copper wire applied to the input as grounding must be at least 4 squaremillimeters. The assembly of the circuit is carried out by the personnel of the brigade, which will conduct the tests themselves.

• Connection of the unit under test to the 380 or 220 V circuit should be done through a special switching device with a visible open circuit or a plug, which should be located at the control point of this unit.
• Next, the wire is connected to the phase, pole of the equipment under test or to the cable core. Disconnect the wire only with the permission of the person in charge of the test, and after grounding.

However, before applying current to the installation under test, the worker must do the following:

• It is necessary to make sure that all members of the checking personnel have taken their places, all unauthorized persons have been removed and that the device can be energized.
• Before applying voltage, be sure to notify all the testing personnel about this, and only after making sure that all employees have heard this, you can remove the ground from the output of the equipment under test and apply voltage of 380 or 220 V.
• Immediately after grounding is removed, all equipment involved in testing electrical equipment with increased voltage is considered to be energized. This means that any changes to the circuit or cable connections or other changes are strictly prohibited.
• After the tests are carried out, the manager must reduce the voltage to 0, disconnect all equipment from the network, ground it yourself or give an order to ground the output of the installation. Oboall this must be reported to the work team. Only after that it is allowed to disconnect the wires if the tests are completed or reconnect them if further work is required. Guardrails are also removed only after the plant is completely shut down and work is completed.

The test protocol for increased voltage of any equipment must also be drawn up by the head of the work group.

Cable testing

Cable tests are also carried out according to a specific plan.

1. First, you need to equip the ground for the equipment and the manual arrester. It happens that a high-voltage transformer installation and a kenotron attachment are moved outside the apparatus. In this case, they should also be grounded.
2. After that, you need to fold the door, which is located on the back of the top of the machine, and install it on the bracket. Next, the lower door leans back, a kenotron attachment is mounted on it, and its paws are wound under the bracket and door extrusion.
3. The top door has a hole where you can insert the limit switching handle. Using a key, the handle is connected with a microammeter. Handle must be grounded.
4. A special spring must be kept in spare parts when carrying out such work. At one end, it is connected to a high-voltage step-up transformer, and at its other end, to the output of a high-voltage type kenotron prefix. The output is located in the middle of the console.
5. Next, insert the plug of the prefix intocontrol panel socket. There is a special handle marked "Protection", it must be rearranged to the "Sensitive" position.
6. Use a cable to connect the equipment under test to the attachment. In this case, you need to throw the cable sleeve onto the output of the microammeter until it stops, after which a protective fence is installed.
7. The equipment plug can then be connected to the network, and after the employee stands on the rubber stand, the device itself can be turned on. At this time, the green diode will light up, and after pressing the power button - red.
8. The equipment has a handle that rotates clockwise, thereby increasing the voltage. Thus, it should be rotated until the test voltage is reached. The reading is usually carried out on the kV scale, which is calibrated in maximum kilovolts.
9. Leakage current can be changed by switching the limit handle by pressing the button in the center of this handle.
10. After all the tests, it is necessary to reduce the supplied voltage to 0, and then press the button to turn off the device.

The protocol for testing the cable with increased voltage is also drawn up after all the work of the main testing group has been completed.

Testing with power frequency RU

In the following order, tests are carried out for switchgear switchgears together with their switching devices.

First you need to prepare the equipment for work. To do this, you need to disableswitchgear, all voltage transformers and other devices connected to it, which are short-circuited or earthed. All equipment is cleaned of dust, moisture, and any other contaminants. After that, according to the rules for testing insulation with increased voltage of increased frequency, it is necessary to measure and record the resistance of the winding of the equipment under test. For this, a megohmmeter with a voltage of 2.5 kV is taken. After that, the entire installation is prepared for subsequent work as described earlier.

After that, all test measurements of the switchgear are carried out using increased voltage.

Testing with the most common instruments

One of the common devices for testing is AII-70. Also quite often used installation marked UPU-1M.

Before proceeding with any tests, it is necessary that the arrows of all devices are at zero, the circuit breakers are turned off. The voltage regulator knob must be turned fully counterclockwise. As for the position of the fuses, it must correspond to the mains voltage. If transportation of a high-voltage transformer is required, then it must be very securely fixed inside the apparatus, the regulator handle must be recessed in this case, and the doors must be tightly closed. The kenotron attachment should also be securely fixed if the cable is tested, and you should also removecontainer with liquid dielectric from the unit.

Using a probe during transportation, periodically check the distance between the electrodes of the jar. It should be equal to 2.5 mm. The probe should pass between the electrodes not too tight, but also without pitching.

Safety rules for testing

As for the safety rules and high voltage test standards, they are as follows.

Firstly, before starting any work, you should equip the ground with a copper wire with a cross section of at least 4.2 square millimeters, such devices as the apparatus itself, a manual spark gap, a high-voltage transformer and a kenotron attachment.

Any work without grounding is strictly prohibited.

Second, be sure to install a protective fence. It should be fixed from the side of the insulating pipes to the kenotron attachment. Warning notices must be on the guardrail. The fence should also be fixed from the side of the metal rods. Here it connects to the swivel lugs of the control box frame.

As for any switching of high-voltage and low-voltage parts of the apparatus, they are carried out only when the voltage is completely turned off, as well as in the presence of a connected and reliable ground.

Both the cable and any other object that has been tested with significant capacitance must be grounded after testing. This is due to the fact that even after the completion of the tests, the object is able to retain a sufficiently powerful charge that can harm human he alth.

As can be seen from the above, the test methods for increased voltage are quite similar to each other. But there are also significant differences, due to which sometimes it is necessary to check the same equipment in different ways.