High voltage insulation tests make it possible to identify local defects that cannot be detected by other methods; In addition, this test method is a direct way to control the ability of the insulation to withstand the effects of overvoltages and provides some confidence in the quality of the insulation. A test voltage greater than the operating voltage is applied to the insulation, and normal insulation passes the test, while defective insulation breaks through.
When testing with increased voltage, three main types of test voltages are used: increased power frequency voltage, rectified direct voltage and impulse test voltage (standard lightning impulses).
The main type of test voltage is power frequency voltage. Application time such voltage – 1 min, and insulation is considered to have passed the test, if during this time no breakdown or partial damage to the insulation was observed. In some cases, tests are carried out at higher frequency voltages (usually 100 or 250 Hz).
With a large capacity of the insulation being tested (when testing cables, capacitors), the use of testing equipment is required high power, therefore such objects are most often tested with increased direct voltage. As a rule, at a constant voltage, dielectric losses in the insulation, leading to its heating, are several orders of magnitude lower than at an alternating voltage of the same effective value; in addition, the intensity of partial discharges is much lower. In such tests, the load on the insulation is significantly less than in AC tests, so a higher DC voltage than the AC test voltage is required to break down the defective insulation.
When testing with constant voltage, the leakage current through the insulation is additionally monitored. The time for applying a constant test voltage is from 5 to 15 minutes. The insulation is considered to have passed the test if it has not broken through and the leakage current value has not changed or decreased by the end of the test.
The third type of test voltage is standard lightning voltage pulses with a rise time of 1.2 μs and a half-time duration of 50 μs. Impulse voltage tests are carried out because the insulation during operation is exposed to lightning overvoltages with similar characteristics. The effect of lightning impulses on insulation differs from that of 50 Hz voltage due to the much greater rate of voltage change, resulting in a different voltage distribution across complex insulation such as transformer insulation; in addition, the breakdown process itself at short times differs from the breakdown process at a frequency of 50 Hz, which is described by volt-second characteristics. For these reasons, testing with power frequency voltage is in some cases not enough.
The impact of lightning overvoltages on insulation is often accompanied by the operation of protective arresters that cut off the overvoltage wave a few microseconds after its onset; therefore, during testing, pulses cut off 2–3 μs after the start of the pulse (cut off standard lightning pulses) are used. The pulse amplitude is selected based on the capabilities of the equipment protecting the insulation from overvoltages, with some reserves and based on the possibility of accumulation of hidden defects during repeated exposure impulse voltage. Specific values of test pulses are determined according to GOST 1516.1-76.
Internal insulation tests are carried out using the three-impact method. Three pulses of positive and negative polarity are applied to the object, first full and then cut. The time interval between pulses is at least 1 minute. The insulation is considered to have passed the test if no breakdowns occurred during the test and no damage was detected. The damage detection technique is quite complex and is usually carried out using oscillographic methods.
External insulation of equipment is tested by the shock method, when fifteen pulses of both polarities, both full and cut, are applied to the object at intervals of at least one minute. The insulation is considered to have passed the test if in each series of fifteen pulses there were no more than two full discharges (overlaps).
All types of tests can be divided into three main groups, differing in purpose and, accordingly, in scope and standards:
· testing new products at the manufacturing plant;
· tests after laying or installing new equipment, tests after major repairs;
· periodic preventive tests.
During preventive or post-repair tests, the ability of the insulation to work without failure until the next regular tests is checked. Insulation testing with increased voltage provides only an indirect assessment of the long-term electrical strength of the insulation, and its main task is to check the absence of gross concentrated defects.
Test voltages for new equipment at manufacturing plants are determined by GOST 1516.2-97, and during preventive tests, test voltage values are accepted to be 10–15% lower than factory standards. This reduction takes into account the aging of the insulation and reduces the risk of accumulation of defects that arise during testing.
High voltage insulation monitoring under operating conditions is carried out for some types of equipment (rotating machines, power cables) with a rated voltage not higher than 35 kV, since at higher voltages the test setups are too bulky.
Cables.Test voltages for cables are set in accordance with the expected level of internal and lightning overvoltages.
At manufacturing plants, oil-filled cables and cables with low-viscosity impregnation are tested with increased industrial frequency voltage (about 2.5 U nom). To prevent damage to the insulation, cables with viscous impregnation and gas cables are tested with a rectified voltage of the order of (3.5..4) U nom, where U nom is the linear voltage at operating voltages of 35 kV or less.
In addition, the insulation resistance is measured, and at operating voltages of 6 kV or more, the insulation resistance and tgδ.
After laying the cable, after major repairs and during preventive tests, the cable insulation is tested with increased rectified voltage. The test time for cables with a voltage of 3–35 kV is 10 minutes for cables after installation and 5 minutes after major repairs and during maintenance tests. The frequency of preventive tests ranges from twice a year to once every three years for different cables. During the tests, the leakage current is monitored, the values of which range from 150 to 800 μA/km for normal insulation. The insulation resistance is measured before and after the tests.
Power transformers . At the manufacturing plant, internal and external insulation is tested with full and chopped standard lightning impulses, as well as with increased alternating power frequency voltage. Detection of damage to longitudinal insulation is most often carried out by oscillography of the current in the neutral of the transformer and comparison of the oscillogram with a standard one.
If the insulation of the neutral and line terminal is the same, then when testing with increased alternating voltage, both ends of the winding under test are insulated and voltage is applied to the winding from an external source. If the neutral insulation level is reduced, then tests are carried out with an induced voltage of higher frequency (up to 400 Hz) so that a voltage of the order of 2 U nom can be applied. The neutral is grounded or an extraneous voltage of the same frequency is applied to it. Since the self-induction EMF in the winding is proportional to the frequency, then at the same maximum induction it is possible to apply an increased test voltage compared to the operating voltage.
When testing insulation, each electrically independent circuit or parallel branch must be tested in turn (in the latter case, if there is complete insulation between the branches), and the test voltage is applied between the terminal and the grounded frame, all other windings are grounded. Insulation resistance measurements are carried out before and after high voltage tests.
Before turning on the newly installed transformer for the first time, measure the breakdown voltage of the transformer oil, insulation resistance and absorption coefficient, ratio C 2 /C 50 , tgδ(the value of which is compared with the results of factory tests).
During periodic maintenance tests, the same tests are carried out as before the first start-up, but the permissible values tgδ at the same time increased. Insulation tests with increased voltage during preventive tests are assumed for windings with voltages up to 35 kV, the values of the test voltages are reduced to 0.85-0.9 of the factory test voltage.
The frequency of preventive tests for different transformers ranges from once a year to once every four years.
High voltage bushings . The main type of control is periodic inspection (from once every three days to once every six months); the insulation resistance between the special measuring plate of the input and the connecting sleeve is also measured. The frequency of such tests varies for different bushings, but at least once every 4 years.
5.1. Standardized values
Tests of electrical equipment with increased voltage are carried out before acceptance into operation within the time limits provided for by the schedule of scheduled maintenance and preventive testing of electrical equipment.
The standards, test conditions and procedure for conducting them are presented in Table 1.
Table 1. Standards, conditions for high voltage testing and instructions for their implementation
|
Test object |
Test standards |
Directions |
|
1. Insulation of the windings and current-carrying parts of the cable of a hand-held power tool relative to the housing and external metal parts |
For power tools with voltage up to 50 V, the test voltage is 550 V, for power tools with voltage above 50 V, power up to 1 kW - 900 V, power over 1 kW - 1350 V. Test time - 1 min. |
The body of the power tool and the parts connected to it, made of dielectric material, must be wrapped in metal foil and connected to a grounding electrode. If the insulation resistance is at least 10 MΩ, then the high voltage insulation test can be replaced by a one-minute insulation resistance measurement with a mega-ohmmeter, voltage 2500 V |
|
2. Insulation of the windings of step-down transformers |
At a rated voltage of the primary winding of the transformer 127 - 220V, the test voltage is 1350 V, at a rated voltage of the primary winding 380 - 440 V, the test voltage is 1800 V. Test duration - 1 min. |
The test voltage is applied alternately to each of the windings. In this case, the remaining windings must be connected to a grounded case and magnetic core |
|
3. Insulation distribution devices, drive elements of switches, short circuiters, separators, devices, as well as secondary control circuits, protection, automation, telemechanics, measurements with all connecting devices, voltage above 60V, not containing devices with microelectronic elements |
Instead of testing with industrial frequency voltage, it is allowed to take a one-minute measurement of insulation resistance with a megohmmeter, voltage 2500 V, except for relay protection and automation circuits |
|
|
4. Insulation of power and lighting electrical wiring |
Test voltage 1000 V. Test duration – 1 min. |
Performed if the measured insulation resistance is less than 1 MOhm |
|
5. Cables with voltage up to 10 kV |
Test voltage depending on the rated operating voltage, kV, for cables: |
5.2. Instruments and installations for testing electrical equipment with high voltage
To test electrical equipment with increased voltage, the following devices and installations can be used:
· universal breakdown installation UPU-5M;
· apparatus for testing the insulation of power cables and solid dielectrics AID 70/50;
· small-sized testing unit MIU-60;
· installation for testing cable insulation UI-70;
· megohmmeters type F4100, F4101, F4102 and ESO202/2 (G) with an output voltage of 2500 V.
Descriptions and diagrams for connecting megohmmeters to the equipment being tested are given in laboratory work №3.
5.2.1. Universal punching device UPU-5M
Designed to measure the electrical strength of insulation when testing with direct or alternating voltage up to 6 kV.
The installation (Fig. 1) is available in two versions:
· “U” - universal (AC and DC voltage);
· “P” - only alternating voltage;
Rice. 1. Universal breakdown device UPU-5M
Basic specifications UPU-5M are given in Table 2.
Table 2. Technical characteristics of the universal breakdown device UPU-5M
|
Parameter |
Magnitude |
|
Output voltage setting range: |
|
|
– constant, kV (only for option “U”) |
0,2 – 6 |
|
– AC, kV |
0,2 - 6 |
|
Leakage current measurement, mA |
0,1 - 100 |
|
Threshold setting range |
|
|
– voltage, kV |
0,2 – 6 |
|
– leakage current, mA |
1 - 99 |
|
Maximum output power, not less, kVA |
5.2.2. Apparatus for testing the insulation of power cables and solid dielectrics AID 70/50
The testing device AID-70/50 (Figure 5.2) is designed for testing the insulation of power cables and solid dielectrics with rectified electrical voltage, as well as for testing solid dielectrics with a sinusoidal electrical voltage with a frequency of 50 Hz.
Rice. 2. Apparatus for testing the insulation of power cables and solid dielectrics AID-70/50
Table 3. Technical characteristics of AID-70/50
|
Parameter |
Magnitude |
|
Single-phase supply voltage alternating current, IN |
220+11 |
|
Parameters of the device on rectified voltage in continuous mode at the rated voltage in the network |
|
|
– highest operating voltage, kV, |
|
|
– maximum operating current, mA, |
|
|
Parameters of the device on alternating voltage in continuous mode at the rated voltage in the network |
|
|
– highest operating voltage (rms value), kV |
|
|
– maximum operating current (rms value), mA |
|
|
Power consumption, kVA, no more |
5.3 Procedure for testing insulation with increased voltage
Measure the insulation resistance of the object being tested.
Assemble the test circuit in the following sequence:
· prepare the test installation for operation in accordance with the manufacturer’s instructions;
· apply portable grounding to the high-voltage terminal of the test installation;
· make the necessary shutdowns (disconnections) of the electrical equipment under test;
· apply portable ground connections to the electrical equipment being tested or turn on the grounding blades;
· set the voltage regulator of the test installation to the position corresponding to the zero voltage value at the output;
· connect the high-voltage terminal to the object under test (bus, cable, wire, motor winding terminal, transformer, etc.);
· remove the portable grounding from the high-voltage terminal of the testing installation (from this moment on, making changes to the test circuit is strictly prohibited). Make all changes in the test circuit only with the high-voltage terminal disconnected and grounded;
· connect the test setup to the network.
Before removing the portable ground from the high-voltage terminal and connecting the test installation to the network, the work operator is obliged to loudly and clearly warn the team about the supply of voltage to the test object and make sure that his warning is heard by all members of the team.
After turning on the test setup, it is necessary to increase the output voltage from zero to the test value. The rate of voltage rise to 1/3 of the test value can be arbitrary. Thereafter, the rate of rise of the test voltage must be capable of being visually read by the measuring instruments, and once the specified voltage value is reached, it must be maintained constant for the required test time.
After the test time has expired, the voltage gradually decreases to zero, after which the test setup can be turned off. After this, it is necessary to re-measure the resistance of the tested insulation.
Testing insulation with increased voltage makes it possible to verify the presence of the necessary insulation strength margin and the absence of local defects that cannot be detected by other methods. High voltage insulation testing must be preceded by a thorough inspection and assessment of the insulation condition by other methods (measuring insulation resistance, determining insulation moisture, etc.).
The value of the test voltage for each type of equipment is determined by the established standards of the “Rules for the operation of consumer electrical installations”.
The insulation is considered to have passed the electrical test with increased voltage if there was no breakdown, overlap on the surface, surface discharges, an increase in the leakage current above the normalized value, or the presence of local heating from dielectric losses. If one of these factors is not met, the insulation fails the electrical test.
A typical scheme for testing the insulation of electrical equipment with increased alternating voltage is presented in Figure 3.
Rice. 3. Scheme for testing the insulation of electrical equipment with increased alternating voltage
The test installation consists of a regulating device TV1 (autotransformer), a step-up transformer TV2, a protection device QF (automatic circuit breaker), means for measuring current and voltage pV1, pV2, pA and additional resistance R, which is necessary to protect the installation in the event of a breakdown of the insulation of the test object.
Voltage measurement can be carried out either by an indirect method using special measuring transformers TV3, while the measuring transformer TV3 and voltmeter pV2 are connected to the secondary circuit of the step-up transformer (in Figure 5.5, the voltmeter V calibrated in kV is thus included), or by direct measurement of the test voltage directly on the test object using kilovoltmeters (the use of a TV3 measuring transformer is not required in this case).
The QF circuit breaker is designed to quickly disconnect the test setup when a large current occurs through the control transformer at the time of insulation breakdown. So this one circuit breaker limits the time the test voltage is exposed to the object during an insulation breakdown and protects the test installation from damage.
To test insulation with direct (rectified) voltage, test installations are used, which are schematically similar to installations for testing insulation with increased power frequency voltage, only a rectifier device is introduced into the circuit. An example test setup for DC testing is shown in Figure 4.
Rice. 4. Scheme for testing the insulation of electrical equipment with increased direct voltage
5.4. The procedure for testing using the AID-70 installation
5.4.1. Test preparation
Install the test voltage source (hereinafter referred to as the source) near the test object. Connect the object to the high voltage terminal of the source.
Ground the source with the flexible copper wire supplied with the device, the cross-section of which is 4 mm 2.
Connect the source cables to the corresponding connectors on the control panel.
Remove the device control panel from the source at a distance of at least 3 m.
Connect the control panel to the power supply and ground it using the network cable supplied with the device.
WORK WITHOUT GROUNDING IS PROHIBITED!
5.4.2. Testing
Persons present during testing must be at least 3 m away from the source and the tested object.
Insert the special key for the device into the control panel switch and turn on the required type of test voltage, and the green signal should light up.
When working on rectified voltage, in order to avoid failure of the source, as well as to correctly measure the value of the test voltage, strictly monitor the position of the “kV” toggle switch.
Rotate the test voltage regulator knob counterclockwise to set it to initial position all the way.
Turn on the test voltage with the button, and the red signal should light up.
By rotating the test voltage regulator knob clockwise and observing the kilovoltmeter readings, set the required test voltage value.
When testing capacitive objects, it must be remembered that after the voltage regulator knob stops rotating, the test voltage on the object continues to increase (the kilovoltmeter needle continues to deviate) as the capacity is charged.
In such cases, the voltage increase must be carried out slowly and smoothly, without exceeding the normalized value of the test voltage at the facility, and also without exceeding the highest operating voltage of the device, equal to 70 kV.
When working on a rectified test voltage, the load current of up to 1 mA should be measured with a microammeter, and the button that shunts this device should be pressed.
After completing the test, it is necessary to set the test voltage regulator knob, rotating it counterclockwise, to its original position until it stops.
Use the button to turn off the test voltage and only then disconnect the device from the network using a special key, setting it to position 0.
Control over the removal of residual capacitive charge from the test object must be carried out by observing the reading of the device’s kilovoltmeter - the kilovoltmeter needle should be at the numerical scale mark 0.
In the case of testing with a rectified voltage equal to 70 kV, a capacitive object with a capacitance value of more than 4 μF, after the end of the test and the voltage regulator knob is set to its original position all the way, the residual charge from the object must be removed using a special discharge rod with a limiting resistance, then turn off the test with a button voltage and only then disconnect the device from the network using a special key.
The use of a special discharge rod prevents failure of the secondary winding of the high-voltage transformer.
When testing capacitive objects with a rectified voltage below 70 kV, the value of the maximum permissible capacitance of the tested object, without the use of a special discharge rod, should be determined by the formula:
|
C = 19600 / U 2, |
(5.1) |
Where WITH– maximum permissible capacity of the test object without the use of a special discharge rod, μF;
U– test voltage, kV.
Page 31 of 56
§ 36. Insulation test with increased voltage
Applying increased voltage to the equipment under test can reveal insulation defects that cannot be detected by any other type of testing. If the insulation of the equipment under test can withstand increased voltage, significantly exceeding the rated voltage, you can be sure that the insulation will withstand not only the rated voltage, but also possible overvoltages during operation.
High voltage testing is the main and mandatory type of test for all types of insulation. However, due to the complexity of testing, it is permissible during the installation process not to test high-voltage equipment with increased voltage if this requires a voltage of 100 kV or more. High voltage testing is carried out primarily on alternating current, but it is advisable to test some types of equipment on direct current. This is due to the fact that testing high-capacity equipment requires a very powerful test facility weighing tens of tons and consuming power equal to hundreds and even thousands of kilovolt-amperes. In addition, direct current testing makes it possible to better identify local defects and use an additional criterion for assessing the quality of insulation in the form of through conduction current (leakage current), and in electrical machines the test voltage is evenly distributed along the winding.
When testing AC insulation, power frequency sources (50 Hz) are usually used. The high voltage test is carried out last, after all other types of measurements and tests required for this type of equipment have been completed.
The high voltage test cannot be carried out if there are visible insulation defects, the insulation does not meet the requirements of the standards for other types of tests, the condition of the oil of oil-filled devices does not meet the standards, as well as if the outer surface of the insulation of the equipment being tested is moistened (organic insulation) and contaminated.
The high voltage test should be carried out strictly observing safety requirements and, in particular, ensuring the permissible insulation distances from parts under test voltage.
Rice. 142. Scheme for testing insulation with increased voltage:
1 - automatic, 2 - voltage regulator, 3 - test transformer, 4 - button, 5 - voltage transformer, 6 - limiting resistance, 7 - arrester, 8 - output to the equipment under test
Insulation test with increased AC voltage.
These tests are performed according to the scheme shown in Fig. 142. First, check the operation of the circuit before connecting the equipment under test, smoothly raising the voltage slightly above the test voltage. Make sure that the test circuit is assembled correctly, the voltage regulator, measuring instruments and other equipment are working properly. Then reduce the voltage to zero, turn off the test installation and ground it on the high voltage side, connect the equipment under test to it, remove the grounding and, making sure that voltage regulator 2 is in initial position, at which the output voltage has a minimum value, turn on the machine 1 and smoothly increase the voltage supplied from the network to the test transformer 3, and therefore to the equipment under test.
In this case, the speed of voltage rise to 30-40% of the test voltage is not standardized, and in the future the voltage rise should be carried out at a speed not exceeding 2-3% of the test voltage every second. When it will be set value test voltage on the equipment under test, it is maintained for a time sufficient to inspect all insulation exposed to the test voltage. This time should be 5 minutes for hygroscopic insulation, such as bakelite, for which dielectric losses have not been measured and are not determined.
degree of humidification so that power losses can be assessed by the degree of heating after the test, and 1 min - for all other types of insulation and for hygroscopic insulation, for which dielectric losses were measured and the degree of humidification was determined.
The voltage in this circuit is measured by voltmeter VI, connected on the low voltage side of test transformer 3 and calibrated by voltage on the high voltage side. It is better to calibrate the voltmeter using a spark voltmeter connected to the high voltage winding of the test transformer.
It should be borne in mind that when testing equipment with parameters different from those at which voltmeter VI was calibrated, errors in assessing the supplied voltage are possible. Therefore, in the test circuit it is necessary to have a constantly switched on spark voltmeter, the distance between the balls of which should be such that a breakdown between them occurs at a voltage slightly greater (about 5%) than the normalized test voltage for this type of equipment. Thus, the spark voltmeter, being an indicator of the maximum voltage, in this case indirectly serves to protect the equipment under test from breakdown, preventing the supply of voltage exceeding the permissible according to the standards.
When testing equipment with increased AC voltage, it is advisable to measure the test voltage directly from the side of the test object, i.e. on the side of the high voltage winding of test transformer 3 and with a voltmeter V2 with voltage transformer 5.
Resistance 6 serves to limit the current in the test transformer and in the spark voltmeter during breakdown.
During testing, the test object must be carefully observed from a safe distance. In rare cases when it is difficult to judge the behavior of the insulation in light, it is recommended to observe in the dark.
After holding for the required time, the voltage is gradually reduced to 30-40% of the test voltage, after which the rate of voltage reduction is not standardized and it can be removed by turning off the machine.
The insulation is considered suitable for use if there has been no breakdown or overlap, no violation of the insulation has been noted according to instrument readings (sudden surges of current or voltage drop) or according to observations (emission of smoke and gas, strong sliding discharges along the surface, local heating after removal from test object under test voltage). Corona phenomena on live parts and insulation elements or small partial discharges on the surface of insulators are allowed.
The test voltage depends on the type of test, equipment and its rated voltage (Table 12).
Table 12 Power frequency test voltages 
* Test duration is 1 min, and the main insulation of instrument transformers, made of organic materials, is 5 min.
** The denominator shows the values of test voltages for dry and light-insulated transformers.
Continuation of the table. 12 
*** The numerator shows the values of the test voltages applied between the plates of the capacitors, and the denominator shows the values relative to the housing.
The power S of the test transformer (kV-A) is selected based on the value of the test voltage 11 (kV) and the capacitance C of the test object (pF) 
where 1 is the frequency of the test voltage, Hz. Expected test current
Approximate single-phase capacitance values for some test items are given below.
To test equipment with increased voltage, special NOM test transformers are used for voltages of 100-500 kV and rated powers of 25-500 kV-A, intended for testing substation equipment, as well as OM transformers for voltages of 15-35 kV and rated powers of 5-50 kV-A. A, intended for testing rotating machines. Test transformer rated current 
In addition to special test transformers, measuring voltage transformers, transformers from oil punches and kenotron devices, and power transformers are used to test insulation with increased AC voltage.
When connecting test transformers to the network, it is necessary to take measures to prevent the appearance of higher harmonics, for which it is necessary to supply linear rather than phase voltage to them.
Regulating devices must ensure smooth regulation of the voltage of the test transformer from 30% to the full test voltage and prevent the circuit from breaking during the regulation process. The most widely used are autotransformer control devices that provide smooth voltage regulation over a wide range, are economical and quite compact, allowing an output voltage higher than the network voltage. These include laboratory autotransformers LATP-1 and LATP-2, variators RNO (single-phase) and RNT (three-phase) and various theatrical voltage regulators. 
Rice. 143. Testing insulators in parts:
a - simultaneous, 6 and c - sequential
Induction regulators that do not contain sliding contacts with a movable short-circuited coil (AOSC, AOMK, ATSC and ATMC), with a magnetic shunt (TPR) and electric machine regulators (potential regulators) are reliable in operation and also provide wide ranges of voltage regulation.
In the absence of a transformer that provides the required test voltage, insulators can be tested in parts. As electrodes to which voltage is applied when testing insulators in parts, it is necessary to use metal elements of the composite insulator (flanges individual elements cascade voltage transformers, reinforcement of insulator columns, reinforcement of suspended insulators, etc.). Solid insulators are tested in parts using patch electrodes. When mass testing insulation in parts, it is useful to use special, easily installed (manually or insulating rods) and removable devices that allow you to quickly prepare the insulator for testing. When testing an insulator in parts, the test voltage should be increased by 10-20%. The test voltage applied to each part will be equal to 
where C/sp is the test voltage for the entire insulator, and u is the number of parts into which the insulator was divided during testing.
In Fig. 143, and a diagram of testing the insulator in parts is shown. All parts of the insulator are tested simultaneously. It is also possible to test individual parts of the insulator sequentially, for example, first the lower part (Fig. 143.6), then the one located above (Fig. 143, c), etc.
Measurements when testing equipment with increased voltage.
These measurements present a number of difficulties. There are two ways to measure voltage: on the low voltage side and on the high voltage side of the test transformer. The first method is much simpler, but it does not provide sufficient measurement accuracy, since the voltmeter is connected to the winding low voltage test transformer, and are calibrated according to the high voltage winding, based on the transformation ratio of the transformer at no load, or at rated load. The greater the load on the transformer during testing, the greater the error in measurement will be. of this object differs from the load that was present when calibrating the voltmeter. It should be noted that the measurement error can be both in the direction of overestimation and in the direction of underestimation of the voltmeter readings compared to the actual test voltage. Considering that the accuracy of voltage measurement when testing with increased voltage is allowed to be relatively low (error 5-10%), and also taking into account the simplicity and safety of voltage measurement using the first method, this method has become most widespread, especially when testing individual insulators, switchgear cells, small electrical machines power, as well as rectified voltage tests.
When testing particularly important objects, for example powerful generators, motors, transformers with significant electrical capacitance, the voltage must be measured from the side of the tested object. In this case, it is possible to directly switch on the voltmeter to the full test voltage (Fig. 146, a), through an additional resistance or voltage divider on active resistances (Fig. 146, b), through capacitive dividers (Fig. 146, c), through voltage transformers ( Fig. 146, d) and on part of the high-voltage winding of the test transformer (Fig. 146, e).
The simplest, most reliable and fairly accurate device (error 2-3%) is a spark voltmeter, which is a ball gap. There are tables from which, knowing the diameters of the balls, the distance between them, the type of test voltage current and the connection circuit (symmetrical or asymmetrical with one grounded ball), it is possible to determine the breakdown voltage under normal conditions (air pressure 760 mm Hg and temperature 20° WITH). During commissioning, spark voltmeters are used to calibrate voltmeters connected from the low-voltage winding of the test transformer, and to protect against accidental overvoltages during testing of particularly critical and expensive equipment, such as generators.
For adjustment work, a spark voltmeter with two polished brass balls with a diameter of 6.5 cm mounted on two bakelite stands, one of which is rigidly attached to the base, and the other can be moved along the guides, is convenient. The distance between the balls corresponding to a given voltage (to protect the equipment, this voltage should be 5-10% greater than the test voltage) is set with a micrometer screw on a scale graduated in kilovolts or millimeters.
A resistance (active from several kilo-ohms to several tens of kilo-ohms) is included in series with the balls of the spark gap, which serves to limit the current during breakdown of the ball gap (voltmeter) and protect the test transformer from overload and the surface of the balls from the action of the arc.
For testing, electrostatic voltmeters S-95 are also used for voltages up to 3 kV and S-96 for voltages up to 30 kV. They provide high accuracy of test voltage measurement and can be used when testing critical equipment and for calibrating voltmeters connected from the low-voltage winding of the test transformer. If the test voltage does not exceed the measurement limits for which electrostatic voltmeters are designed, the full test voltage may be applied to them. When measuring higher voltages, electrostatic voltmeters are conveniently used in conjunction with capacitive voltage dividers.
If commercially produced capacitive voltage dividers are not available, they can be assembled on site, for example from hanging insulators. To do this, assemble a garland with the number of insulators corresponding to the test voltage (2-3 at 35 kV, 6-7 at 110 kV, 14-15 at 220 kV and 28-30 in series, and the secondary ones are suspended in parallel on a grounded structure (for example, a portal outdoor switchgear) and calibrate a voltmeter connected in parallel to the last suspended insulator adjacent to the grounded structure on which the garland is suspended. at 500 kV), 
Rice. 147. Schemes for connecting voltage transformers when testing equipment with increased AC voltage: a - primary and secondary windings are connected in series, b - only the primary windings are connected in series, d - the primary windings are connected. It is better to calibrate the voltmeter using a spark voltmeter connected in parallel to the entire garland, leading to it is the voltage from the test transformer. Calibration can be performed at reduced voltage. 
Rice. 148. Installation for testing secondary switching AC voltage with increased voltage
When connecting conventional voltmeters through voltage transformers (Fig. 147), if the test voltage significantly exceeds the rated voltage of the measuring transformers, the use of identical voltage transformers with primary windings connected in series is allowed. Voltmeters can be connected to secondary windings connected in series (Fig. 147, a), to each secondary winding (Fig. 147, b), to only one secondary winding (Fig. 147, c) or to two secondary windings connected in parallel (Fig. 147, d). Voltages 11 x are determined: for the circuit (see Fig. 147, a) - Ux = Uvti*, for the circuit (see Fig. 147, b) - Ux=> = UvinB+Uv2nH, for circuits (see Fig. 147 , c and d) - Ux=2UvnB (mon - voltage transformer transformation ratio).
It should be noted that not all of these schemes are equivalent. The best one should be considered the scheme shown in Fig. 147, g, and the worst is shown in Fig. 147, v. The disadvantage of the circuits (see Fig. 147, a, b, c) is that with different open-circuit resistance of the voltage transformers, each of them will have a different voltage, which can be detected by the readings of voltmeters VI and V2 (see Fig. 147, b). This can lead to the fact that one and I transformers will be under high voltage, and the other under low voltage, and therefore, a measurement error and overload of one transformer is possible.
Control questions
What elements are included in the insulation equivalent circuit and what dielectric property characterizes each of these elements?
What test methods and instruments are used to determine the degree of moisture insulation?
Why is high voltage testing considered the main type of dielectric testing?
How is insulation tested under increased AC voltage?
In what cases is it advisable to test insulation with increased DC voltage?
What is the design of the AII-70 installation and how do they work on it when testing insulation with increased voltage of alternating and direct current?
Give brief description the main methods of measuring test voltage.
Why are ball arresters used when testing equipment with high voltage?
The test is carried out according to the diagram in Fig. 1, but instead of a megohmmeter, a testing installation is connected. The test voltage rises gradually. After setting the test voltage, the test time begins. After the test, the voltage is removed and the tested windings are grounded.
Testing the insulation of the windings of oil-filled transformers during their commissioning and major repairs without changing the windings and insulation is not necessary.
Insulation testing of dry-type transformers is mandatory.
During a major overhaul with a complete change of windings and insulation, testing with increased voltage is mandatory for all types of transformers. The test voltage value is 0.9 factory voltage.
Table 8
Industrial frequency test voltages in operation for electrical equipment of voltage classes up to 35 kV with normal and lightweight insulation
|
tension, tension |
Test voltage, kV |
|||
|
Power transformers, shunt and arc suppression reactors |
Devices, current and voltage transformers, current-limiting reactors, insulators, bushings, coupling capacitors, shielded conductors, busbars, switchgear and package transformer substations, electrode boilers. |
|||
|
Normal insulation |
Lightweight insulation* |
Porcelain insulation** |
Other types of insulation** |
|
Notes:
* Test voltages of sealed transformers are taken in accordance with the instructions of the manufacturers.
** Values in brackets apply to the gap between the contacts of switching devices.
Table 9
Test voltage values
|
Voltage class, kV |
At the factory |
During commissioning |
In operation |
The test voltage, indicated as a fraction, applies to electrical equipment:
Numerator - with normal insulation
Denominator - with lightweight insulation
Dry transformers are tested according to standards for lightweight insulation.
The duration of test voltage application is 1 minute. The need to test insulation with increased voltage must be agreed with the requirements of the manufacturer.
High voltage testing of accessible tie rods, bandages, yoke half-bandages, pressure yokes and electrostatic screens
The measurement diagram is not provided. The test voltage value is 1 kV, duration is 1 minute.
And today we will talk about testing cables with impregnated paper, plastic and rubber insulation with increased rectified current voltage.
Insulation monitoring of power cables with voltages above 1000 (V) is carried out using the applied voltage method, which makes it possible to detect defects that may, during further operation of the cable, reduce the electrical strength of its insulation.
Preparation for high voltage cable testing
Let me remind you right away that testing with increased voltage (high-voltage tests) is permitted to an employee over 18 years of age who has undergone special training and knowledge testing (reflected in the table for carrying out special work on his or her certificate). It looks something like this.
By the way, I specially created an online for you — you can test your knowledge.
Before testing the power cable with increased voltage of rectified current, it is necessary to inspect it and wipe the funnels from dust and dirt. If during inspection any insulation defects are visible or the outer surface of the cable is heavily contaminated, then it is prohibited to proceed with testing.
It is also worth paying attention to the ambient temperature.
The ambient air temperature should only be positive, because at a negative air temperature and if there are water particles inside the cable, they will be in a frozen state (ice is a dielectric), and such a defect will not appear during a high-voltage test.
Immediately before testing the cable with increased voltage, it is necessary to measure its insulation resistance. You can read more about this in the article .
As I said above, power cable lines are tested with increased rectified current voltage.
The increased rectified voltage is applied to each core of the power cable in turn. During testing, other cable cores and metal sheaths (armor, screens) must be grounded. In this case, we immediately check the insulation strength between the conductor and the ground, as well as in relation to other phases.
If the power cable is made without a metal sheath (armor, screen), then an increased rectified current voltage is applied between the core and other cores, which we first connect to each other and to the ground.
It is allowed to test all the cores of the power cable with increased voltage at once, but in this case it is necessary to measure the leakage currents for each phase.
We completely disconnect the power cable from the busbar, and separate the wires at a distance of more than 15 (cm) from each other.
We have figured out the circuit for testing rectified voltage power cables. Now we need to decide on the size and duration of the tests. To do this, open the reference books: PTEEP and PUE.
You can also use the electronic version of these books. I suggest you download right now and completely free electronic version.
I made the task a little easier for you and compiled a general table taking into account the requirements of the PUE (chapter 1.8, clause 1.8.40) and PTEEP (Appendix 3.1., table 10).
The duration of testing cable lines with voltage up to 10 (kV) with paper and plastic insulation after installation is 10 minutes, and during operation - 5 minutes.
The duration of testing cable lines with voltage up to 10 (kV) with rubber insulation is 5 minutes.
Now we will consider the standardized values of leakage currents and asymmetry coefficients when testing cable lines with increased rectified current voltage.
There are slight disagreements here between the PUE and PTEEP (values from PTEEP are indicated in parentheses).
If the power cable has cross-linked polyethylene insulation, for example, PvVng-LS(B)-10, then it is not recommended to test it with direct (rectified) voltage; moreover, the value of the test voltage differs significantly. I talked about this in more detail in a separate article about.
Power cable testing apparatus
Well, we have smoothly moved on to what is used to test cables with increased rectified voltage. In ours, we use either the AII-70, AID-70, or IVK-5 testing apparatus. The last two devices are used most often on the road.
We will talk about these devices in more detail in the following articles, and if you do not want to miss the release of new articles on the site, then subscribe to receive notifications by email.
Method of testing cables with increased voltage
Let's say we need to conduct operational tests of a 10 (kV) power cable of the AAShv brand (3x95).
Using an AII-70 or IVK-5 apparatus, we raise the test voltage to a value of 60 (kV) at a speed of 1-2 (kV) per second. From this moment the time countdown begins. During the entire 5 minutes, we closely monitor the magnitude of the leakage current. After the time has elapsed, we record the resulting leakage current and compare it with the values in the table above. Next, we calculate the asymmetry coefficient of leakage currents by phase - it should be no more than 2, but sometimes it can be more (see table).
The asymmetry coefficient is determined by dividing the maximum leakage current by the minimum leakage current.
After high-voltage testing of the cable, it is necessary to test it again.
The cable is considered to have passed the test when:
- During the test, no breakdown, surface flashover or surface discharges occurred
- There was no increase in leakage current during the test
- the cable insulation resistance has not decreased
It happens in practice that leakage currents exceed the values indicated in the tables. In this case, the cable is put into operation, but the period of its next test is reduced.
If during testing the leakage current begins to increase, but breakdown does not occur, then the test must be carried out for more than 5 minutes. If after this the breakdown does not occur, then the cable is put into operation, but the period of its next test is reduced.
Results and protocol for high voltage cable testing
After testing the cable with increased rectified voltage, it is necessary to draw up a protocol. Below I will give you the protocol form (example) used by our electrical laboratory (click on the picture to enlarge).
P.S. This concludes the article on testing cables with increased voltage. If you have questions about the material, ask them in the comments.
SUMMARY
Measurement is one of the main methods for monitoring the insulation of high voltage electrical equipment. During measurements, the absolute value of tg δ, changes in tg δ compared to previous measurements are monitored, and in some cases the dependence of tg δ on voltage is removed.
For measurements, a high-voltage measuring bridge is used according to the Schering scheme.
Monitoring partial discharges allows us to judge the rate of electrical aging of insulation. In the electrical PD monitoring method, the voltage surge across the insulation and the magnitude of the apparent charge are recorded.
Control questions
1. What properties of insulation are characterized by the dielectric loss angle?
2. How is insulation checked by measuring the dielectric loss angle?
3. What does the name mean?<четырехплечий уравновешенный мост переменного тока по схеме Шеринга>?
5. Explain the principle of operation of the Schering bridge and the possibility of measuring the dielectric loss angle. Write down the equilibrium equations for the bridge.
6. Why and how are partial discharges in insulation controlled?
INSULATION OF SEPARATE TYPES OF EQUIPMENT CONTROL BY INCREASED VOLTAGE. TESTS
High voltage insulation tests reveal local defects not detected by other methods ; In addition, this test method is a direct way to control the ability of the insulation to withstand the effects of overvoltages and provides some confidence in the quality of the insulation. A test voltage greater than the operating voltage is applied to the insulation, and normal insulation passes the test, while defective insulation breaks through.
During preventive or post-repair tests, the ability of the insulation to work without failure until the next regular tests is checked. Insulation testing with increased voltage provides only an indirect assessment of the long-term electrical strength of the insulation, and its main task is to check the absence of gross concentrated defects.
Test voltages for new equipment at manufacturing plants, GOST 1516.2-97 is determined, and when preventive trials The test voltage values are taken to be 10-15% lower than factory standards. This reduction takes into account the aging of the insulation and reduces the risk of accumulation of defects that arise during testing.
High voltage insulation monitoring under operating conditions is carried out for some types of equipment (rotating machines, power cables) with a rated voltage not higher than 35 kV , since at higher voltages the test setups are too bulky.
When testing with increased voltage, three main types of test voltages are used: increased power frequency voltage, rectified direct voltage and impulse test voltage (standard lightning impulses).
The main type of test voltage is power frequency voltage . Application time such voltage - 1 min, and insulation is considered to have passed the test , if during this time no breakdown or partial damage to the insulation was observed. In some cases, tests are carried out at higher frequency voltages (usually 100 or 250 Hz).
With a large capacity of the insulation being tested (when testing cables, capacitors), the use of high-power testing equipment is required, therefore such objects are most often tested increased DC voltage . As a rule, at a constant voltage, dielectric losses in the insulation, leading to its heating, are several orders of magnitude lower than at an alternating voltage of the same effective value; in addition, the intensity of partial discharges is much lower. In such tests, the load on the insulation is significantly less than in AC tests, so a higher DC voltage than the AC test voltage is required to break down the defective insulation.
When testing with constant voltage, the leakage current through the insulation is additionally monitored. The time for applying a constant test voltage is from 5 to 15 minutes. The insulation is considered to have passed the test if it has not broken through and the leakage current value has not changed or decreased by the end of the test.
The disadvantage of a constant test voltage is that this voltage is distributed throughout the thickness of the insulation in accordance with the resistances of the layers, and not in accordance with the capacitances of the layers, as with operating voltage or overvoltage. For this reason, the ratios of test voltages to operating voltages of individual insulation layers are significantly different.
The third type of test voltage is standard lightning impulses voltage with a rise of 1.2 μs and a duration to half-fall of 50 μs. Impulse voltage tests are carried out because the insulation during operation is exposed to lightning overvoltages with similar characteristics.
The effect of lightning impulses on insulation differs from that of 50 Hz voltage due to the much greater rate of voltage change, resulting in a different voltage distribution across complex insulation such as transformer insulation; in addition, the breakdown process itself at short times differs from the breakdown process at a frequency of 50 Hz, which is described by volt-second characteristics.
For these reasons, testing with power frequency voltage is in some cases not enough.
The impact of lightning overvoltages on insulation is often accompanied by the operation of protective arresters that cut off the overvoltage wave a few microseconds after its onset, and therefore, during testing, pulses that are cut off 2-3 μs after the start of the pulse are also used ( chopped standard lightning impulses ).
Pulse amplitude is selected based on the capabilities of the equipment that protects insulation from overvoltages, with some reserves, and based on the possibility of accumulation of hidden defects during repeated exposure to pulse voltages. Specific values of test pulses are determined according to GOST 1516.1-76.
Tests internal insulation carried out using the three-shock method. Three pulses of positive and negative polarity are applied to the object, first full and then cut. The time interval between pulses is at least 1 minute. The insulation is considered to have passed the test if no breakdowns occurred during the test and no damage was detected. The damage detection technique is quite complex and is usually carried out using oscillographic methods.
External insulation equipment is tested using the 15-shock method, when to the object at an interval of at least 1 min. 15 pulses of both polarities are applied, both full and cut. The insulation is considered to have passed the test if in each series of 15 pulses there were no more than two full discharges (overlaps).
7.2. Insulation testing of cables, transformers and high-voltage bushings
All types of tests can be divided into three main groups, differing in purpose and, accordingly, in volume and standards:
Testing of new products at the manufacturing plant;
Tests after laying or installing new equipment, tests after major repairs;
Periodic preventative testing.
Requirements for testing the insulation of cables, transformers and high-voltage bushings are set out separately for these three groups of tests.
1. Cables
Test voltages for cables are set in accordance with the expected level of internal and lightning overvoltages.
At manufacturing plants oil-filled cables and cables with low-viscosity impregnation are tested at increased industrial frequency voltage (about 2.5 U nom). To prevent damage to the insulation, cables with viscous impregnation and gas cables are tested with a rectified voltage of the order of (3.5..4) U nom, and U nom is linear at operating voltages of 35 kV or less and phase voltage at operating voltages of 110 kV or more.
After cable installation, after major repairs and during maintenance tests Cable insulation is tested with increased rectified voltage. The test time for cables with a voltage of 3..35 kV is 10 minutes for the cable after installation and 5 minutes after major repairs and during preventive tests.
For cables with a voltage of 110 kV, the time of application of the test voltage is 15 minutes per phase. The frequency of preventive tests ranges from twice a year to once every three years for different cables.
During testing leakage current is controlled , the values of which range from 150 to 800 µA/km for normal insulation. Measured before and after testing insulation resistance .
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