High-voltage testing is the transmission of high voltage through electrical equipment, namely power supply for various rooms: apartments, shops, schools, hospitals, traffic lights. Also used for street lighting.
This is a very important process, without which the work of many enterprises is impossible. This is due primarily to safety and labor protection.
This article will discuss the question of what electrical equipment, for what, in what order and how often it is checked.
High voltage test tasks:
- verification of insulation for compliance with regulations;
- identification of deficiencies that reduce the reliability of equipment;
- determination of damage sites;
- identification of failures in the operation of electrical equipment of substations.
Types of tests:
- Typical (verification of technical conditions).
- Control (directly upon release from the factory).
- Acceptance (completion of installation work, when the equipment is put back into operation).
- Operational (preventive tests and overhauls).
- Special (as part of special research programs).
- There are high voltage tests of various types of electrical equipment.
Power transformers
This type of electrical equipment is used in many areas of production, has two or more windings (this is a conductor, covered with a layer of insulation, keeps the wires in a certain position and cools). The winding can be made of copper or aluminum tapes and wires with cast epoxy insulation. It consists of several groups of coils connected in series with each other and filled with epoxy resin (protects from dust, environmental influences, provides mechanical strength).
The winding is made for operation under normal operating conditions at temperatures from -25 to +40. There are neutral and linear branches of structures.
The transformer is designed to convert the energy of one value into the electrical energy of another.
High-voltage transformer tests should be carried out in accordance with the rules adopted at the legislative level. At installation it is necessary to consider climatic conditions.
Power transformer includes:
- Windings stretched over a magnetic core (core). They come in low, medium and high voltage, made of burdened steel.
- A magnetic core placed in a special tank, on the roof of which windings are displayed.
- An exhaust pipe located on the cover (serves as protection against tearing, if any).
- Voltage regulation device.
- Expander (ensures constant filling of the tank with oil. If there is a change in air temperature or the load changes, it reduces the area of connection of oil and air.
- Oil line (connects the expander to the tank).
- Thermosiphon filter (filled with silica gel. Protects oil from oxidation and moisture).
Power tool
This is a tool with an electric energy source: drill, screwdriver, grinder, jackhammer, cutter and much more.
According to the regulations, for safety reasons, it is necessary to test these instruments after receiving them from the manufacturer. It is also desirable to carry out its tests after repair, replacement of components, within the framework of the schedule of preventive tests.
During a scheduled check, the data must be compared with the results of previous tests, including factory ones. Frequent use of the power tool is best checked once every 6-8 months.
The air temperature must be strictly positive, since if there are water particles in the cable, it will freeze at a negative air temperature. Ice is a dielectric, such an effect will not manifest itself in a high voltage test.
To avoid sad consequences, before starting work, exclude:
- Damage to the plug.
- Cable defects.
- Ground circuit integrity.
- The presence of a protective tube. It is located at the junction of the housing and the cable of the power tool).
Frequent inspection of the power tool will ensure safety, prevent breakdowns and extend the life of the equipment.
Electric motors
High-voltage motor testing is the most significant and at the same time vulnerable test element. It determines the reliability of equipment operation.
The most common cause of damage to electric motors is a complex of mechanical and thermal factors.
Progress in testing insulation of a high voltage motor:
- Determination of the resistance of the windings between the phases (using the same megohmmeter).
- Testing under conditions of increased voltage (frequency 50 Hz) is carried out using systems after assembling the engines (for 1 minute). For a successful check, there should be no slipping discharges and overlaps, no large increase in leakage current.
- Measurement of ohmic resistance (limit value of active resistance) in a cold state (at a direct current). The temperature should be no more than 3 degrees. Such a manipulation helps to determine the presence of coil faults, defective soldering sections.
- Measurements and external inspection of the gaps between the stator steel (stationary part of the generator or AC motor) and the rotor (rotating part of the machine inside the stator).
- Testing electrical equipment at idle.
- Checking the operation of engines under load.
- Assessment of engine performance under engine rotation conditions.
- Coil insulation test.
Test phases for AC motors:
- full cycle of measurements before operation;
- overhaul phase (once every several years, depending on the standards and instructions of the technical production manager);
- overhaul.
High voltage circuit breakers and their drives
These are important switching devices that are designed to turn the electrical circuit on and off. They are:
- gas-insulated;
- oil;
- air;
- vacuum;
- electromagnetic.
Testing of high-voltage circuit breakers is a prerequisite for installation, overhaul (approximately every 8 years) and periodic inspections (every 4 years).
Check Points:
- inspection;
- insulation test, resistance in direct current conditions;
- resistance of windings and contacts;
- comparison of data with the declared;
- control in conditions of high voltage (1 minute);
- control of mobility of contacts of the switch;
- measurement of the minimum operating time of the circuit breaker;
- information on the smallest voltage required to operate with an electromagnet;
- assessment of heating of working contacts (thermal imaging control).
Some types of high-voltage tests are carried out by numerous tests at the rated (that is, normal for which they were originally designed) voltage.
The tests are carried out using a special electric laboratory, which has the right to issue legal documentation.
High voltage cables
High voltage tests take place in stages:
- The core (insulated conductor) of the cable is connected to a rectified voltage.
- During the test of one core, the rest must be grounded.
Grounding is the connection of a network point (electrical installation, equipment) with a grounding device. It consists of a grounding conductor (also called a circuit) and a grounding conductor. Used for electrical safety. Protects equipment, people from high voltage and such phenomena as:
- breakdowns;
- improper operation;
- low temperatures;
- lightning strikes.
After checking one conductor, you must repeat the action with all the others.
This method of high voltage testing allows us to assess the insulation strength of each core.
The cable may be in the ground or on the drum throughout the process. This is a special wooden cable handling device.
There are various high voltage test methods. The choice of a specific option depends on the type of cable. For instance:
- Power cable with a metal screen. Cores that are not currently used are collapsed together and connected to the ground and the screen.
- Cable stitched into polyethylene. During high-voltage tests, cross-linked polyethylene cables cause voltage between the core and the sheath (protective layers around it).
- Cable without screen. The cores are tested separately from the others, which at this moment are grounded.
- Cable with metal shields on conductors. Each core with a sheath is tested, the rest in the process are grounded.
To increase the efficiency of the procedure (reduce time, damage to couplings), several cable lines connected to one section of the busbars of the central processor (CPU) can be tested.
Periodic inspection of electrical equipment is better to combine with the repair of electrical devices at the supply and end of the lines.
In order for the tests to be considered successfully passed, and the insulation to comply with the standards, there should be no increase in current above the norm or with heating from dielectric losses. If there was a surface overlap (breakdown), then the insulation test did not pass.
Before starting work, the insulation status must be monitored. Namely:
- resistance measurement;
- moisture determination.
Testing of a 10 kV high-voltage cable is carried out with voltage depending on the insulation material. She may be:
- rubber (2);
- paper, with viscous impregnation (5-6).
The test duration of a 10 kV high-voltage cable is no more than 5 minutes for each phase.
When testing other cables with voltages up to 1 kV, only the insulation resistance is measured for one minute. It must be at least 0.5 megohms.
The following will provide information on what specific problems can be identified during high voltage tests. It can be:
- installation errors of connectors and terminations;
- lead breakage;
- oil leak;
- a short circuit between the cores (for example, with corrosion of the metal sheath).
Cable power lines of foreign manufacture are checked according to the instructions in accordance with the instructions of the manufacturer.
If the cable is laid in the ground, then it is more advisable to conduct high-voltage tests in the summer. Thus, in case of breakdown of lines it will be easier to carry out repair work.
Insulation is checked using a special installation for high voltage tests.
Rectifier
These systems can be of several types:
- mobile;
- portable;
- stationary.
Any of them has:
- Test transformer.
- Remote Control.
- High voltage rectifier.
The rectification is carried out according to a half-wave circuit (this is a circuit conducting during one half of the alternating current cycle), and the transformer winding is powered by a control autotransformer.
The leakage current in high-voltage installations for cable testing is checked using a microammeter (has two poles: one is grounded, the second is connected to the secondary winding of the transformer). In this case, the R register is included in the circuit itself. It limits the current in case of cable breakdown.
Examples of installations for high voltage testing:
- HVTS-HP;
- RETOM-6000;
- VIST-120;
- AIST 50/70.
There are many others, their prices start at 100 thousand rubles.
Resistance measurement
For high-voltage tests and measurements, a megaohmmeter is used (“mega” is the measurement size, “ohm” is the unit, and “meter” is the measurement). This is a special device, an electronic device that is designed to establish resistances of large values. When testing, type M4100 / 1-5 (voltage from 100 to 2500 V) is used.
Megaohmmeters have a direct current generator (that is, a personal power source) and calculate the readings in megaohms.
Now let's look at how to use this item.
To do this, clamp Z (that is, earth) is connected to the installation case, and clamp L (line) is connected directly to the conductor.
This rule is valid for measuring insulation resistance to earth. And for other electrical circuits, clamps can be used in any position.
In addition to these two clamps, there is also an E (screen). It effectively refines measurements (especially at high resistances). This happens by eliminating the influence of leakage current (a physical phenomenon associated with poor insulation of electrical equipment).
Before starting work, set the insulation resistance. It must comply with megaohmmeter standards. This can be estimated by the handle of the generator. The correct data will be when the handle rotates 90-150 rpm with a rated voltage of 120 and an open external circuit. The value is fixed 60 seconds after the average speed of the generator handle is established. Thus, this value will be the insulation resistance.
For safety and accuracy, make sure:
- The cleanliness of wires, cable funnels, the most tested equipment.
- In the absence of voltage on the tested electrical equipment.
- That all parts with reduced insulation and test voltage are disconnected and shortened.
It is possible that the device data may be distorted in bad weather conditions (the surface of the insulating parts of the electrical installation may be wet). This question is also important in high-voltage tests; accuracy and safety depend on it.
Determine the level of moisture will help the absorption method. Its principle is that a megohmmeter reads 15 and then 60 seconds after applying voltage.
This method allows you to determine the moisture content of the insulation of transformers and electrical machines.
Mobile laboratory
Testing helps to achieve high-quality, long and stable service of an electrical object. It takes place with the help of the High Voltage Testing Laboratory (LVI). They are:
- LVI-1 (test of switchgears of overhead and cable lines, electrical equipment of substations).
- LVI-2 (search for places of insulation damage in cable lines).
- LVI-3 (conducting a full range of tests and determining the location of damage in power cables).
In this case, two systems for measuring high voltage are divided:
- SVN-20.
- SVN-100.
They have a state certificate of type approval of measuring instruments.
The operation of the laboratory of high-voltage tests will help not only to conduct tests, but also to make a forecast of possible energy losses, to correctly distribute the load.
Only highly qualified specialists can carry out work on it. They must have extensive experience in conducting high voltage tests and measurements.
Currently, you can use the services of a mobile high-voltage laboratory, which has all the necessary equipment. Its advantages:
- efficiency of work;
- tests in hard to reach places.
The main types of work on LVI:
- testing of grounding devices;
- repair of a damaged power cable;
- electrical testing;
- search for breakage, damage to cable lines;
- insulation resistance measurement;
- tests of arresters, power oil transformers and oil circuit breakers.
There are certain standards for high voltage testing. More information on this topic is contained in such regulatory documents as:
- "Electrical Installation Rules" () is the main technical document related to electrical equipment rules. It is used by engineers designing electrical installations of all types and modifications. This document applies to all created and repaired electrical appliances.
- “Rules for the Technical Operation of Consumer Electrical Installations” (PTEEP). These requirements are intended for engineering and technical personnel involved in the establishment, operation and repair of electrical appliances of power plants and networks.
Thermal imaging control is also required. It is produced for all switchgears, if the instruction does not contradict this.
Assessment of the state of backup electrical equipment is regulated by the "Rules on labor protection during the operation of electrical installations" (POT R M-016). The frequency depends on storage conditions.
With regard to electrical automation devices and relay protection, it should be said that it is necessary to test the insulation of equipment every time after a complete oil change.
There are many different rules described in the above documents. A complete list of cable line tests is regulated in the rules of the PTEEP (Appendix 3, clause 6), as well as in the PUE (Chapter 1.8, clause 1.8.40).
In addition, the high-voltage test standards provide recommendations for technical managers of energy enterprises. They should ensure the introduction of inspection of electrical appliances under operating voltage, which allows to identify shortcomings in the early stages of development. It is permissible to involve organizations accredited for the right to conduct relevant tests.
Voltage 10 kV is the most common class, which is used in most enterprises and industries. It is used to reduce damage to cable lines under operating voltage.
Almost all tests are carried out several times. This allows you to verify their accuracy.
Paperwork
As evidence of the verification, the protocols are high-voltage tests of electrical equipment with high voltage. This is a mandatory part of the audit, which is monitored by the relevant authorities.
Documents record the fact of timely inspection of electrical equipment, and are issued by specialists performing high-voltage tests.
It includes all electrical devices that are operated at the enterprise. Each of them must have an individual test certificate. It includes:
- the exact name of the model of equipment and its type;
- serial number that is stamped on the device itself;
- release date and all previous checks.
The test report is needed to confirm the verification and allow further operation of the equipment.
If there is no such document, control authorities will not allow continued use.
During the testing of new equipment, the compliance of real indicators with those stated by the manufacturer is established (temperature, power, permissible load).
A separate electrical safety test is carried out, an appropriate act is drawn up.
Documents should be executed immediately after verification. In addition, the period of control of the inspection service is also limited, so before starting the test, make sure that the company has the right to them.
The audit can be carried out by enterprises that are registered in Rostekhnadzor and have permission to provide services for high-voltage testing of electrical equipment.
Procedure
High voltage testing is limited to 10 minutes. This is associated with a risk of aging of the insulating layer. Cables with paper and polyethylene insulation can be tested for no more than 5 minutes. During the action, there should be no heating of electrical elements.
The magnitude of the voltage depends on the type of equipment. The norms are spelled out in the "Rules for the operation of electrical installations of consumers".
Verification is carried out by a team of at least two people. If the test involves working with voltages greater than 1000 V, one of the workers must have a sixth safety group, and the other a third.
The results of the check are made out in the register of norms and rules of work in electrical equipment.
If a voltage of less than 1000 V is applied, a third group is sufficient for both test participants.
Work can only be carried out by persons who have reached the age of 18 and have undergone professional training in the field of knowledge of test circuits and test rules in existing electrical installations. This is checked by a special mark in the certificate, which is called "Certificate for the right to carry out special work" and PUE (electrical installation rules).
Validation value
High-voltage tests are of great importance in the operation of electrical equipment both in the enterprise and in everyday life. They ensure the safety of work, extend the time the use of devices, identify violations.
In case of non-compliance with the verification standards or in its absence, enormous damage to the enterprise and working personnel can be caused.