High-voltage tests: purpose, algorithm, test methods, standards, protocol and compliance with safety rules

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

High voltage testing is the transmission of high voltage through electrical equipment, namely the power supply for various premises: 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, first of all, to safety and labor protection.

This article will discuss what electrical equipment, for what, in what order and how often is tested.

High voltage test tasks:

• check insulation for compliance with regulations;
• identifying deficiencies that reduce equipment reliability;
• locating damage;
• detection of failures in the electrical equipment of substations.

Types of tests:

1. Typical (check specifications).
2. Control (immediately upon release from the factory).
3. Acceptance (completion of installation work when the equipmentrecommissioned).
4. Operational (preventive testing and overhaul).
5. Special (within special research programs).
6. There are high voltage tests for different 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 that holds 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 and filled with epoxy resin (protects from dust, environmental influences, provides mechanical strength).

Winding is made for operation in normal operating conditions at temperatures from -25 to +40. There are neutral and linear branches of designs.

The transformer is designed to convert the energy of one value into electrical energy of another.

High voltage testing of transformers must take place in accordance with the rules adopted at the legislative level. Installation must take into account climatic conditions.

Power transformer includes:

• Windings stretched over the magnetic circuit (core). They come in low, medium and high voltage and are made of laminated steel.
• A magnetic circuit placed in a special tank, on the roof of which the windings are brought out.
• Exhaust pipe,located on the lid (serves as protection against tearing, if any).
• Voltage adjustment device.
• Expander (ensures constant filling of the tank with oil. If there is a change in air temperature or a change in load, it reduces the area of \u200b\u200bconnection of oil and air.
• Oil line (connects expansion tank to tank).
• Thermosiphon filter (filled with silica gel. Protects oil from oxidation and moisture).

Power tool

This is a tool with an electric power source: drill, screwdriver, grinder, jackhammer, cutter and more.

Regulations require these tools to be tested for safety after they are received from the factory. It is also desirable to test it after repair, replacement of components, as part of the preventive test schedule.

During a routine inspection, the data must be compared with the results of previous tests, including factory ones. Power tools with frequent use should be checked every 6-8 months.

The air temperature must be strictly positive, because if there are water particles in the cable, it will freeze at negative air temperature. Ice is an insulator, this effect will not appear in a high voltage test.

To avoid sad consequences, exclude before starting work:

• Damage on power plug.
• Cable defects.
• Ground continuity.
• Availabilityprotective tube. It is located at the junction of the body and the cable of the power tool).

Checking your power tool frequently will ensure safety, prevent breakdowns and prolong the life of your equipment.

Electric motors

Testing a high-voltage motor is the most significant and at the same time vulnerable element of the test. It determines the reliability of equipment operation.

The most common cause of damage to electric motors is a combination of mechanical and thermal factors.

High voltage motor insulation test flow:

• Determining the resistance of the windings between phases (using the same megaohmmeter).
• Checking under high voltage conditions (frequency 50 Hz) is carried out using systems after assembling the motors (for 1 minute). For a successful test, there should be no sliding discharges and overlaps, a large increase in leakage current.
• Measurement of ohmic resistance (limit value of active resistance) in a cold state (at direct current). The temperature should not exceed 3 degrees. Such manipulation helps to determine the presence of turn short circuits, defective soldering areas.
• Measurement and external inspection of the gaps between the steel of the stator (the fixed part of the generator or AC motor) and the rotor (the rotating part of the machine inside the stator).
• Testing electrical equipment at idle.
• Checking the operation of engines under load.
• Assessing engine performance under engine rotation conditions.
• Turn insulation test.

AC motor test phases:

• full cycle of measurements before operation;
• overhaul phase (once every few years, depending on the regulations 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 an electrical circuit on and off. They are:

• SF6;
• oil;
• air;
• vacuum;
• electromagnetic.

Testing high-voltage circuit breakers is a prerequisite for installation, overhaul (about every 8 years) and periodic inspections (every 4 years).

Checkpoints:

• inspection;
• insulation test, DC resistance;
• resistance of windings and contacts;
• comparison of data with the declared ones;
• high voltage control (1 minute);
• monitoring the mobility of circuit breaker contacts;
• measuring the minimum tripping time of a circuit breaker;
• information about the lowest voltage required to operate an electromagnet;
• assessment of heating of working contacts (thermal imaging control).

Some types of high voltage tests are carried out by multiple samples at nominal (that is, normal, for which they were originally designed) voltage.

Testing is carried out with the help of a special electrical laboratory, which has the right to issue legal documentation.

High voltage cables

High-voltage testing is taking place in stages:

1. The core (insulated conductor) of the cable is connected to the rectified voltage.
2. When testing 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 purposes. Protects equipment, people from high voltage and phenomena such as:

• breakdowns;
• improper operation;
• low temperatures;
• lightning strikes.

After checking one conductor, you must repeat the action with all the others.

This high-voltage test technique allows you to evaluate the insulation strength of each core.

The cable can be in the ground or on the drum during the whole process. This is a special wooden device for cable transportation.

There are different ways of high voltage testing. The choice of a particular option depends on the type of cable. For example:

1. Power cable with metal screen. Wires that are not currently in use are rolled together and connected to ground and shield.
2. Cable sewn in polyethylene. During high-voltage testing of XLPE cable, stress is applied between the core and the sheath(protective layers around her).

• Cable without screen. The cores are tested separately from the rest, which are at this moment grounded.
• Cable with metal screens on the cores. Each core is tested with a sheath, the rest are grounded in the process.

To increase the efficiency of the procedure (reduce time, reduce damage to couplings), you can test several cable lines connected to one section of the central processing unit (CPU) buses.

Periodic inspection of electrical equipment is best combined with the repair of electrical devices at the supply and end ends of the lines.

In order for the tests to be considered successful, and the insulation to comply with the standards, there should not be an increase in current above the norm or with heating from dielectric losses. If a surface flashover (breakdown) occurs, the insulation fails the test.

Before starting work, the condition of the insulation must be checked. Namely:

• resistance measurement;
• moisture determination.

Testing a 10 kV high-voltage cable is carried out with voltage depending on the insulation material. She can be:

• rubber (2);
• paper, with viscous impregnation (5-6).

The duration of testing 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, measure only the insulation resistance for one minute. It must be at least 0.5 MΩ.

Next, information will be provided on what specific problems can be identified inhigh voltage test time. These could be:

• installation errors of couplings and terminations;
• core break;
• oil leak;
• short circuit between the cores (for example, due to corrosion of the metal sheath).

Cable power lines of foreign production 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 expedient to carry out high-voltage tests in the summer. Thus, in the event of a breakdown of the lines, it will be easier to carry out repairs.

Insulation is tested using a special high voltage tester.

Rectifying plant

These systems can be of several types:

• mobile;
• portable;
• stationary.

Each of them has:

1. Test transformer.
2. Control panel.
3. High voltage rectifier.

The rectification is done in a half-wave pattern (this is the circuit that conducts during one half of the AC cycle), and the transformer winding is powered by a regulating autotransformer.

Leakage current in high-voltage cable test facilities is checked using a microammeter (has two poles: one is grounded, the other 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 the event of a cable breakdown.

Examples of high voltage test setups:

• HVTS-HP;
• RETOM-6000;
• VIST-120;
• STORK 50/70.

There are many others, their prices start from 100 thousand rubles.

Resistance measurement

For high-voltage tests and measurements, a megaohmmeter is used ("mega" - the size of the measurement, "ohm" - the unit, "meter" - to measure). This is a special device, an electronic device that is designed to establish high resistance values. The test applies type M4100/1-5 (voltage from 100 to 2500 V).

Megohmmeters have a DC generator (that is, a personal power source) and calculate readings in megaohms.

Now let's see how to use this item.

To do this, the Z terminal (that is, ground) is connected to the installation case, and the L terminal (line) is connected directly to the conductor.

This rule is valid for measuring insulation resistance to earth. And for other electrical circuits, the clamps can be used in any position.

Besides these two clamps, there is also 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 the standards for megohmmeter. You can evaluate this by the handle of the generator. The correct data will be when the knob is rotated 90-150 rpm with a nominal voltage of 120 and an open external circuit. The value is fixed 60 seconds afterhow the average frequency of rotation of the generator handle was established. Thus, this value will be the insulation resistance.

For the safety and accuracy of the operation, make sure:

• In the cleanliness of wires, cable funnels, the most tested equipment.
• In the absence of voltage on the electrical equipment under test.
• That all parts with reduced insulation and test voltage are disconnected and shortened.

The device data may be distorted under bad weather conditions (the surface of the insulating parts of the electrical installation may be wet). This issue is also important in high-voltage testing, and accuracy and safety depend on it.

Determine the level of moisture will help the absorption method. Its principle is that readings are taken from the megaohmmeter 15 and then 60 seconds after the voltage is applied.

This method allows you to determine the moisture content of the insulation of transformers and electrical machines.

Mobile laboratory

Checking helps to achieve high-quality, long and stable service life of an electrical object. It takes place with the help of a high-voltage testing laboratory (LVI). They are:

• LVI-1 (testing switchgears of overhead and cable lines, electrical equipment of substations).
• LVI-2 (search for places of insulation damage in cable lines).
• LVI-3 (carrying out a full range of tests and locating faults in power cables).

At the same time, twohigh voltage measurement systems:

1. SVN-20.
2. SVN-100.

They have a state certificate of type approval of measuring instruments.

Operation of the high-voltage testing laboratory will help not only to conduct tests, but also to predict possible energy losses, to correctly distribute the load.

Only highly qualified specialists can work on it. They should have extensive experience in high voltage testing and measurement.

Currently, you can use the services of a mobile high-voltage laboratory, which has all the necessary equipment. Its benefits:

• efficiency of work;
• testing in hard-to-reach places.

Main types of work at LVI:

• checking grounding devices;
• repair damaged power cable;
• testing electrical equipment;
• search for a break, damage to cable lines;
• insulation resistance measurement;
• testing 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:

1. "Electrical Installation Rules" (PUE) is the main technical document related to the rules of electrical equipment. It is used by engineers designing electrical installations of all types and modifications. This document applies to all created and repairedelectrical appliances.
2. "Rules for the technical operation of electrical installations of consumers" (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 instructions do not contradict this.

Assessment of the state of standby electrical equipment is regulated by the "Rules on labor protection during the operation of electrical installations" (POT R M-016). Frequency depends on storage conditions.

With regard to electrical and relay protection devices, it should be said that equipment insulation tests are necessary every time after a complete oil change.

There are many different rules described in the above mentioned documents. A complete list of cable line tests is regulated in the PTEEP rules (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 makes it possible to identify deficiencies 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 in order to reduce the damage of cable lines under the workingvoltage.

Almost all tests are carried out several times. This allows you to verify their accuracy.

Documentation

As evidence confirming the check, there are reports of high-voltage tests of electrical equipment with increased voltage. This is a mandatory part of the verification, which is controlled by the relevant authorities.

Documents record the fact of timely inspection of electrical equipment, and are issued by specialists who carry out high-voltage tests.

It includes all electrical devices that are operated at the enterprise. Each of them must have an act of individual testing. It includes:

• exact equipment model name and type;
• serial number stamped on the device itself;
• date of issue and all previous checks.

A test report is needed to confirm that the test has been carried out and to allow further operation of the equipment.

If there is no such document, regulatory authorities will not allow continued use.

During the testing of new equipment, the correspondence of real indicators with those declared by the manufacturer (temperature conditions, power, allowable load) is established.

A separate test for electrical safety is carried out, an appropriate act is drawn up.

Documents must be completed immediately after verification. In addition, the period of control by the inspection service is also limited, therefore, before the start of thetests to make sure the company is eligible.

Verification can be carried out by enterprises that are registered with Rostekhnadzor and have permission to provide services for high-voltage testing of electrical equipment.

Procedure of actions

High voltage testing is limited to 10 minutes. This is due to the risk of aging of the insulating layer. Cables with paper and polyethylene insulation can be tested for no more than 5 minutes. During the operation, no heating of electrical elements should occur.

Voltage depends on the type of equipment. The norms are prescribed in the "Rules for the operation of electrical installations of consumers".

Checking is done by a team of at least two people. If the test involves working with a voltage of more than 1000 V, one of the workers must have the sixth safety group, and the other - the third.

The results of the check are recorded in the accounting journal of the rules and regulations for working in electrical equipment.

If less than 1000 V is applied, a third group is sufficient for both test subjects.

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 schemes and test rules in the conditions of existing electrical installations. This is checked by a special mark in the certificate, which is called the "Certificate for the right to carry out special work" and PUE (electrical installation rules).

Check value

High voltage testing is of great importance inoperation of electrical equipment both at the enterprise and at home. They ensure the safety of work, extend the time of use of devices, detect violations.

In case of non-compliance with the standards of verification or in the absence of it, great harm can be done to the enterprise and workers.