RU2800650C1 - Method for protecting power oil filled transformer from internal electric arc - Google Patents

Abstract

FIELD: electrical engineering.

SUBSTANCE: invention relates to methods for protecting power oil-filled transformers from an internal electric arc. The objective of the invention is to increase the speed of protection of power transformers in the event of the occurrence and development of an electric arc inside the transformer tank. The task is achieved by the fact that after the formation of an electrically conductive channel of high conductivity from mechanical charged microparticles in the oil of the transformer, as a result of which the ohmic resistance of the oil sharply decreases, a protective shutdown device and a current transformer are switched on in the ground bus circuit of the transformer tank, the signal from which is supplied to turn off the automatic high voltage circuit breaker, which break the circuit of the external electrically conductive channel and the higher voltage circuit with the appearance of a leakage current in the ground bus of the transformer tank of a given value at the initial stage of the formation of the internal electrically conductive channel before the development of an electric arc in the transformer oil.

EFFECT: increase the speed of protection of power transformers in the event of the occurrence and development of an electric arc inside the transformer tank.

1 cl, 7 dwg, 2 tbl

Description

The invention relates to the field of electrical engineering, namely, to methods for protecting power oil-filled transformers from an internal electric arc.

Known methods and devices for built-in and external protection of power transformers [1, 2, 3, 4].

Power oil-filled transformers have built-in protection against local overheating and partial discharges inside the tank and external protection against short-circuit currents inside the tank.

The built-in protection devices include gas relays, which should turn off the transformer during the development of local overheating and the formation of partial discharges inside the transformers.

External devices include current cutoff, overcurrent protection and current differential protection. During the operation of transformers, various defects and damages can occur [5, 6].

There are two types of external protection against short circuit currents inside the tank. In the old relay protection circuits, a short circuiter and a separator were installed at the higher voltage terminals of the transformer [1]. Currently, a microprocessor-based current cutoff is used [2] with its own response time of the current cutoff of 50–80 ms. According to the authors, current cutoff in combination with maximum current protection and gas protection provide reliable protection for transformers with a power of up to 4000 MB A [2]. However, under the conditions of selectivity, the overcurrent protection must have a time delay and, therefore, cannot be fast. For this reason, overcurrent protection is a back-up protection for the transformer.

Differential longitudinal protection against short circuit currents inside the transformer tank.

This protection is based on the principle of comparing currents at the beginning and at the end of the windings of a power transformer [1, 2]. The section between the current transformers installed on the higher and lower sides of the power transformer is considered to be a protected area.

In the event of a short circuit in the protected area, the currents in the secondary windings of the current transformers will be different and current will flow through the current relay winding. If the value of the current is equal to the tripping current, or more than it, then the relay will work and the damaged area will be switched off through the intermediate relay. Therefore, differential protection should operate in case of phase-to-phase short circuits, in case of inter-turn circuits and in case of damage to high-voltage bushings 110-330 kV. The intrinsic tripping time of the differential protection is 58 ms. The disadvantage of current cutoff and differential protection is that they do not have time to prevent and stop the development of an electric arc in transformer oil.

The objective of the invention is to increase the speed of protection of power transformers in the event of the occurrence and development of an electric arc inside the transformer tank.

The task is achieved by the fact that a residual current device and a current transformer are included in the ground bus of the transformer tank, the signal from which is sent to turn off the circuit breaker in the higher voltage circuit, and the residual current device breaks the circuit of the external electrically conductive channel with the appearance of a leakage current in the ground bus of the transformer tank a given value at the initial stage of formation of an internal conductive channel of high conductivity from mechanical charged microparticles in the oil of the transformer before the development of an electric arc inside the transformer. Brief description of the damage:

1. P.S. Miradino, Bobruisk Electric Networks, 2012 Damaged bushing of 330 kV autotransformer with RlP insulation brand new. Damaged tank and magnetic system. The day before, the modernization of relay protection at the Miradino-330 substation was completed. The transformer protection has tripped. (Fig. 1).

2. JSC "BMZ" 2017 Damaged input 330 kV transformer TKTOR-L-125000-330. The input was pulled out of the transformer tank and thrown a few meters. According to the conclusion of the factory commission, the gas relay, current cutoff and maximum current protection worked (Fig. 2).

3. Minsk CHPP-3, 2018. The input of the 220 kV block transformer was damaged and the transformer was damaged. The transformer protection has tripped (Fig. 3, Fig. 4).

In all cases, the external current protection did not protect the transformers from damage to the internal electric arc. According to American experts, explosions of bushings in the US power systems account for 3.2%, 16% of transformer damage occurs for unknown reasons [6]. As a rule, the ignition of oil-filled transformers is attributed to unidentified causes. An analysis of damage to transformers in the Republic of Belarus shows that these fires are caused by an internal electric arc in the transformer.

To simulate an electric arc in transformer oil, an experimental setup was created [7]. The composition of the experimental setup (figure 5, 6):

1.1. High-voltage installation AID-70M for creating alternating voltage up to 70 kV and direct voltage up to 60 kV.

1.2. Vessel with transformer oil.

1.3. Partial discharge probe M4202 for registration in a vessel.

1.4. Thermal imager "Testo787".

1.5. Video cameras transmitting a signal to a personal computer.

1.6. Impurities: copper filings, particles of electric cardboard, colored hydrogel.

One electrode is grounded. The second electrode is connected to AID-70 with AC voltage. The distance between the electrodes could be changed from a minimum of 1 cm to a maximum of 18 cm.

Mechanical impurities were added to the vessel with oil: balls of colored hydrogel, small copper filings and small pieces of electrical cardboard. After applying a minimum (2-3 kV) voltage to the electrode, the impurity balls began Brownian motion, repelled from the walls of the vessel, from the electrodes, and repelled from neighboring balls. After increasing the potential between the electrodes, some of the mechanical impurities/balls began to line up in a circuit between the electrodes over the entire gap.

As the voltage increased, impurities were collected in a line of any configuration between the electrodes. With a further increase in voltage, a luminous channel is formed - a streamer. Charged microparticles, merging one with one, form a channel of high conductivity. Streamer - partial discharges move from the instrument electrode to the grounded electrode.

After the occurrence of partial discharges, a leakage current appears, which is recorded by the AID-70M device. Each time before the arc discharge, the leakage current of the AID-70M device increased, and the protection of the device was triggered.

Partial discharges were registered by the German partial discharge probe M 4202 (Lemke-5). Due to the large contamination of the oil, partial discharges of more than 1000 pC were recorded already at a voltage of 4 kV. After short-term exposure (one minute) to partial and electrical discharges, a sample of oil No. 2 was taken. After a long exposure to electrical discharges for 3-5 minutes, a sample of oil No. 3 was taken. Chromatographic analysis of gases dissolved in oil was carried out in the laboratory of the Gomel Electric Networks using a Kristall-2000M instrument. The results of the analysis are presented in table.2.

As follows from Table 2, the percentage of gases increased significantly in sample No. 3. It is checked how the ratio of gases has changed, following the accepted methods of IEC and Dornenburg.

Sample No. 2: C ₂ H ₂ /C ₂ H ₄ \u003d 0.247 / 0.229 \u003d 1.08; CH ₄ /H ₂ =0.46; C ₂ H ₄ /C ₂ H ₆ =3.55, indicating arc discharges.

Sample No. 3: C ₂ H ₂ /C ₂ H ₄ \u003d 1.75; CH ₄ /H ₂ =0.454; C ₂ H ₄ /C ₂ H ₆ \u003d 3.2. In this case, arc discharges are also observed.

High-energy discharges turning into an electric arc were visually observed. The device recorded partial discharges of more than 1000 pC. Chromatographic control of combustible gases was an additional method for evaluating the effect of partial discharges on transformer oil. In FIG. Figure 7 shows the average curve of leakage current versus the amount of impurities and the applied voltage. Air bubbles, moisture microparticles were modeled with hydrogel balls 1 mm in diameter. Under the influence of partial discharges, the balls were destroyed and the leakage current, and partial discharges decreased.

The way to protect power oil-filled transformers from an internal electric arc is as follows.

To prevent the development of an internal electric arc, a residual current device and a current transformer are switched on in the ground bus of the transformer tank, the signal from which is sent to turn off the circuit breaker in the higher voltage circuit, and the residual current device breaks the circuit of the external electrically conductive channel with the appearance of a leakage current in the ground bus of the transformer tank a given value at the initial stage of formation of an internal conductive channel of high conductivity from mechanical charged microparticles in the oil of the transformer before the development of an electric arc inside the transformer.

At JSC "BMZ" partial discharges in the tank of 13 transformers and leakage currents in their grounding buses were measured. These measurements confirmed the relationship between partial discharges in the transformer oil and the leakage current in the earth bus [Appendix 1].

To prevent the formation of an electrically conductive channel of high conductivity inside the tank from mechanically charged microparticles in the mass of the transformer, it is necessary to regenerate the oil based on the results of a comprehensive technical diagnostics of the transformer.

For transformers that have worked out the owl normative period, it is necessary to carry out the following measurements every 6 months under the load of the transformer:

- chromatographic control of combustible gases in transformer oil with analysis by two, but better by three methods to increase reliability;

- physical and chemical tests of transformer oil;

- after these measurements, partial discharges in the transformer tank and in the bushings are controlled;

- vibroacoustic inspection of the transformer tank is performed in the frequency range of 10-5000 Hz. The established practice in Belarus and Russia to evaluate the dynamic resistance of windings and magnetic core by the overall vibration level in the range of 10-1000 Hz is formal and unreliable;

- thermographic inspection of the transformer tank and high-voltage bushings must be carried out from the ground and from the car cradle from 3 sides through 120 degrees. The established practice of thermographic inspection of transformers only from the ground is unreliable.

The residual current device has shown its effectiveness not only in protecting a person from electric shock when accidentally touching bare conductors, but also in protecting electric motors from insulation breakdowns when they are flooded with water.

Information sources:

1. Lipkin B.Yu. Power supply of industrial enterprises and installations: A textbook for students of technical schools. - 3rd ed. Revised - M. Higher School, 1981, 376 pages.

2. Evminov L.I. Relay protection and automation of power supply systems.: textbook method, manual / L.I. Evminov, G.I. Seliverstov: Ministry of Education, Republic of Belarus. Gomel, Mrs. those. un. them. P.O. Sukhoi - Gomel: GSTU im. P. O. Sukhoi, 2016. 531 pages.

3. METHOD FOR PROTECTING POWER TRANSFORMER OF AC TRAFFIC SUBSTATIONS FROM INTERNAL DAMAGE. Grigoriev N.P., Dynkin B.E., Parfianovich A.P., Trofimovich P.N. Patent for invention RU 2710023 C2, 05/22/2018, Application No. 2018118911 dated 12/24/2019

4. DEVICE FOR DIFFERENTIAL PROTECTION OF THE POWER TRANSFORMER. Suvorov A.A., Glazyrin V.E., Erushin V.P., Timofeev I.P. Patent for invention RU 2497256 C1, 07.06.2012, Application No. 2012123731/07 dated 06/07/2012.

5. N.V. Gruntovich, O.V. Fedorov, D.R. Frost, B.B. Tretyakov, P.M. Kolesnikov. Analysis of problematic issues of operation of oil-filled transformers. Energy and Management, 2017 - No. 3 (96), p. 2-6.

6. W. Bartley. Overview of transformer damage. "Energy and Management", 2011, No. 1, pp. 40-43.

7. N.V. Gruntovich, E.V. Vorobieva, P.V. Lychev. Influence of partial discharges on physical and chemical properties of transformer oil Vestn. Gom. state tech. in-ta im. BY. Sukhoi. - 2019. - No. 4. - S. 37-45.

Claims (1)

A method for protecting oil-filled power transformers from an internal electric arc, characterized in that a residual current device and a current transformer are connected to the ground bus of the transformer tank, the signal from which is sent to turn off the circuit breaker in the higher voltage circuit, and the residual current device breaks the circuit of the external electrically conductive channel from the appearance of a leakage current in the ground bus of the transformer tank of a given value at the initial stage of the formation of an internal conductive channel of high conductivity from mechanically charged microparticles in the transformer oil before the development of an electric arc inside the transformer.