Today, people are actively using a variety of electrical equipment, power cables, electrical connections and more. Since in some equipment the voltage can reach enormous values ββthat can cause serious damage to human health, periodic monitoring is required. High voltage testing is one of the methods for detecting insulation defects.
What is it and why is the check carried out
The main purpose of such tests is to check insulation. 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, tests with increased insulation voltage make it possible to test its ability to withstand overvoltage and, if successful, give a certain confidence in the quality of the winding. The essence of the test is quite simple. A voltage is applied to the insulation that exceeds the rated operating voltage and is considered an overvoltage. The normal insulation winding will withstand, but the defective one will break through.
It is worth noting here that with the help of high-voltage tests, it is possible to check the possibility of insulation to work out until the next repair, inspection, change, etc. However, this type of test allows you to only indirectly determine this parameter. The main objective of this method is to identify 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 of not higher than 35 kV. If this parameter is exceeded, the settings themselves are usually too bulky. Today, there are three main types of overvoltage tests.
These include industrial frequency overvoltage tests, rectified direct voltage and pulse test overvoltages (simulation of a standard lightning impulse).
Types of tests. Industrial Frequency and DC
The first and main type of test is an increased voltage of industrial frequency. In this case, an overvoltage of 1 minute is applied to the insulation. It is believed that the winding passed the test if no breakdowns were observed during this time, and the insulation itself remained intact. For some cases, the overvoltage frequency can be 100 or 250 Hz.
In the event that the capacity of the insulation tested is greater, then it will be necessary to take test equipment with more power. In this case, we are talking about testing cable lines with high voltage. For such cases, the second method is often used, using increased direct voltage. However, it must be borne in mind that when applying a constant voltage, the dielectric loss in insulation, which, in fact, leads to heating, will be significantly lower than when using an alternating voltage with the same values. In addition, the intensity of the partial discharge will be reduced. All this leads to the fact that when tested with high voltage cable lines using the direct current method, the load on the insulation will be significantly less. For this reason, it is necessary to increase the power of the supplied overvoltage in order to verify the quality of insulation and the absence of breakdowns.
Among other things, it must be added here that during DC tests, one more parameter, such as the leakage current through the insulation, should be taken into account. As for the application time of the overvoltage, it is from 5 to 15 minutes. The insulation will be considered high-quality not only provided that no breakdown has been detected, but also provided that the leakage current has not changed or decreased by the end of the test period.
When comparing the two methods, it is clearly seen that testing with an increased voltage of industrial frequency is much more convenient, however, this method cannot always be applied.
In addition, there is another disadvantage of direct current. During testing, the voltage will be distributed over the insulating winding in accordance with the resistances of the layers, and not their capacitance. Although, at operating voltage or normal overvoltage, the current will diverge in 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 worker 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 ΞΌs and a duration of up to half-life of 50 ΞΌs. The need to check the insulation with such a pulsed voltage is due to the fact that during operation the winding will inevitably be subjected to lightning overvoltage with similar parameters.
It is important to know that the effect of a lightning pulse is very different from a voltage with a frequency of 50 Hz in that the rate of change of voltage is much greater. Due to the higher rate of voltage change, it will be distributed differently along the insulating winding of complex devices, for example, transformers. An overvoltage test with such characteristics is also important because the very process of breakdown of insulation with a small amount of time will differ from the breakdown at a frequency of 50 Hz. This can be dealt with in more detail if you look at the volt-second characteristic.
Due to all these conditions, quite often it happens that testing a transformer with an increased voltage according to the first method is not enough - it is necessary to resort to testing by the third method as well.
Pulse cut off, external and internal winding
In the event of a lightning overvoltage, most equipment triggers an arrester that will cut off the incoming pulse wave in a few microseconds. For this reason, when testing transformer with high voltage, for example, such pulses are used that are specially cut off after 2-3 ΞΌs. They are called cut off standard lightning pulses.
Such pulses have certain characteristics, for example, amplitude.
This pulse value will be selected based on the capabilities of the device, which 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 quantities, the selection rules are described in special state document 1516.1-76.
Testing equipment with high voltage for the internal winding will be carried out according to the principle of a three-shock method. The bottom line is that three pulses of positive and three pulses of negative polarity will be fed to the winding. First, voltage pulses that are complete in nature will be supplied, and then cut off. It is also important to know that at least 1 minute must elapse between each subsequent pulse. The insulation will be considered tested if no breakdowns are detected and the winding itself does not receive any damage. It is worth saying that such a verification technique is quite complicated and is most often carried out using oscillographic control methods.
As for external insulation, the 15-stroke method is used here. The essence of the verification remains the same. 15 pulses will be applied to the winding with an interval of at least 1 minute, first of one polarity, then the opposite. Both full and cut pulses are applied. Tests are considered successful if each series of 15 hits had no more than two complete overlaps.
How does the verification process go?
AC or DC type overvoltage tests shall be carried out in strict accordance with the regulations. The procedure is as follows.
- Before proceeding with the inspection, the inspector must verify that the test equipment is in good working order.
- Next, proceed with the assembly of the test circuit. The first step is to provide protective and operational grounding for the equipment under test. In some cases, if required, protective earth is also provided for the housing of the device under test.
Equipment connection
Before proceeding to connect the equipment to the 380 or 220 V network, grounding should also be applied to the input of the high-voltage installation. Here it is important to observe the following requirement - the cross-section of the copper wire superimposed on the input as grounding must be at least 4 square millimeters. The assembly of the circuit is carried out by the personnel of that team, which will conduct the tests themselves.
- The connection of the test installation to the circuit of 380 or 220 V should be done through a special switching device having a visible open circuit or the plug, which should be located at the control site of this installation.
- Then 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 who supervises the testing, as well as after grounding.
However, before applying current to the installation under test, the employee must do the following:
- It is necessary to make sure that all members of the inspection staff took their places, all unauthorized persons were removed and whether it is possible to apply voltage to the device.
- Before applying voltage, it is imperative 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 removing the ground, all equipment involved in the testing of electrical equipment with high voltage is considered to be live. 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 is obliged to lower the voltage to 0, disconnect all equipment from the network, ground it independently, or give an order to ground the unit output. All this must be reported to the workers' brigade. Only after that it is allowed to disconnect the wires if the tests are completed or to reconnect them if further work is required. Fencing is also removed only after a complete shutdown of the installation and completion of work.
The test report of the increased voltage of any equipment should also be drawn up by the leader of the group of workers.
Cable Testing
Cable tests are also carried out according to a specific plan.
- First you need to equip the grounding for the equipment and manual arrester. It happens that a transformer high-voltage installation and a kenotron prefix are moved outside the apparatus. In this case, they should also be grounded.
- After that, you need to swing down the door, which is located at the top back of the device, and install it on the bracket. Next, the lower door leans back, a kenotron attachment is mounted on it, and its paws are brought under the bracket and the door extrusions.
- The upper door has an opening where the limit switching handle should be inserted. Using a key, the handle is connected with a microammeter. The handle is grounded.
- When carrying out such work, a special spring must be stored in spare parts. At one end, it connects to a high-voltage transformer of a step-up type, and with its second end to the output of a high-voltage type kenotron attachment. The conclusion is located in the middle of the console.
- Next, insert the plug of the console into the socket of the control panel. There is a special handle marked "Protection", it must be moved to the "Sensitive" position.
- Use the cable to connect the equipment under test to the set-top box. In this case, you need to put the cable sleeve on the microammeter output until it stops, after which a protective fence is installed.
- After that, the equipment plug can be connected to the network, and after the employee stands on the rubber stand, you can turn on the device itself. At this time, the green diode will light up, and after pressing the power button - red.
- The equipment has a handle that rotates clockwise, thereby increasing the voltage. Therefore, it should be rotated until the test voltage is reached. The reading is usually carried out on a kV scale, which is calibrated in maximum kilovolts.
- The leakage current can be changed by switching the limit handle by pressing the button in the center of this handle.
- After carrying out all the tests, it is necessary to reduce the supplied voltage to 0, and then press the button to turn off the device.
The cable test report for high voltage is also compiled after all the work of the main inspection team.
Testing with industrial frequency of RU
In the following order, tests are carried out for switchgear switchgear together with their switching devices.
First you need to prepare the equipment for work. This requires disconnecting from the switchgear all voltage transformers and other devices connected to it that are shorted or grounded. All equipment is cleaned of dust, moisture, and any other contaminants. After that, according to the rules of insulation testing with increased voltage of increased frequency, the resistance of the winding of the equipment under test should be measured and recorded. 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 previously.
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 checking is AII-70. Also quite often used installation labeled UPU-1M.
Before starting 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 device, the handle of the regulator in this case must be recessed, and the doors tightly closed. The kenotron attachment should be firmly fixed if the cable is to be tested, and the container with the liquid dielectric should be removed from the unit.
Using the probe during transport, periodically check the distance between the electrodes of the can. It should be equal to 2.5 mm. The probe should not pass between the electrodes too tight, but also without pitching.
Test Safety
As for the safety rules and overvoltage test standards, they are as follows.
First, before starting any work, you should arrange grounding with a copper wire with a cross section of at least 4.2 square millimeters such devices as the device itself, a hand discharger, a high voltage transformer and a kenotron prefix.
Any work without grounding is strictly prohibited.
Secondly, a protective fence must be installed. It should be fastened from the side of the insulating pipes to the kenotron attachment. Safety guards must bear warning labels. The fence should also be fixed from the side of the metal rods. Here it connects to the rotary ears of the control panel frame.
As for any switching of high-voltage and low-voltage parts of the device, they are made 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 capacity should be grounded after testing. This is due to the fact that even at the end of the tests, the object is able to retain a sufficiently powerful charge that can harm human health.
As can be seen from the foregoing, the test methods of increased voltage are quite similar to each other. But there are significant differences, because of which sometimes you have to check the same equipment in different ways.