How are power transformers repaired?

FORMAT-ENERGO LLC

Based on the scope of work, repairs of transformers are divided into current and overhaul. Current (or revision) repairs of transformers are carried out at the site of their installation with mandatory disconnection from power sources.

Current repairs mean maintenance of the transformer without removing the active part. An inspection of transformers with their disconnection, but without removing the active part, is carried out as necessary, but at least once every 3 years, and for 35/6-10 kV transformers at least once a year.

Major repairs are done at electrical repair plants. Major repairs are carried out with the removal of the active part and are of two types - 1st category of complexity (without disassembling the active part) and 2nd category of complexity (with disassembly).

Overhaul of the 1st category of complexity includes opening the transformer with removal of the active part, repairing or replacing (if necessary) components and assemblies of the transformer, drying the active part, restoring or replacing transformer oil and sorbents.

Overhaul of the 2nd category of complexity, in addition to the work included in the overhaul of the 1st category of complexity, includes repair of the active part with its disassembly to restore or replace the windings, main insulation, and in rare cases, repair of the magnetic system with re-insulation of the plates.

Overhaul of transformers with removal of the active part is done for the first time 6 years after commissioning, and subsequently as necessary, depending on the measurement results and the condition of the transformer. However, depending on the type of transformer, conditions of transportation, installation and operation, the timing of current and major repairs may be changed.

There are the following overhaul intervals for transformers of the first and second dimensions:

• for new transformers – 5-6 years
• for transformers with partial sealing and oil protection after major repairs (including cases of installation of expanders and thermosiphon filters during repairs) – 4-5 years
• for transformers that have undergone a major overhaul (without sealing and oil protection) – 2-3 years

Depending on the purpose, repairs are divided into planned preventive or post-emergency. The first is carried out according to a predetermined schedule. Repair of an emergency transformer is carried out based on the fact that a malfunction has occurred with it.

By nature of execution there are restoration repairs, reconstruction and modernization.

Refurbishment repair is the elimination of all faults, the replacement of all unusable components and parts with new ones, the same as those being replaced, without making any design changes to them. Such repairs are carried out on new transformers produced at the modern technical level that have failed for various reasons. This includes current and major repairs of the 1st and 2nd complexity categories.

Transformers of an outdated design that do not have sufficient fittings, as well as transformers that require changes in their design and changes in the characteristics of the nominal parameters, are subject to reconstruction or modernization, which are classified as overhaul of the 3rd category of complexity.

Reconstruction is understood as changing the design of any parts of the transformer without changing its nominal parameters.

By modernization we mean precisely a change in the nominal data of a transformer - power, voltage, current, winding connection diagram, etc., and during modernization, as a rule, the design of individual parts of the transformer changes: windings, taps, inputs, etc.

Repair standards and repair instructions in this section are given for power transformers for general industrial use with voltages up to 35 kV and power up to 16,000 kVA, transformers for powering converters and electric furnaces, dry transformers; autotransformers with a power of up to 250 kVA, voltage stabilizers for 220 - 380 V with a power of up to 100 kVA; complete transformer substations with voltage up to 10 kV and power up to 1000 kVA.

Operation and repair of the listed equipment must meet the requirements of PTE and PPB.

To ensure the safety of work during maintenance and repair of power transformers, the personnel involved for these purposes must have a qualification group in accordance with the PTE and PPB.

1. Maintenance

1.1.

The scope of maintenance of power transformers includes regular/extraordinary inspections.

1.2.

Regular inspections of transformers (without shutting them down) are carried out at the following times:

• in electrical installations with permanent duty personnel - once a day;
• in installations without permanent personnel on duty - at least once; per month, at transformer points - at least once every 6 months.

1.3.

Depending on local conditions, the design and condition of power transformers, the specified periods for inspections of transformers without shutdown can be changed by the person responsible for electrical equipment.

1.4.

Extraordinary inspections of transformers are carried out:

• with a sharp change in air temperature;
• each time the transformer is disconnected by gas or differential protection.

2. Typical nomenclature of repair work during routine repairs

2.1.

During routine repairs, maintenance operations are carried out in full, as well as the following work: cleaning of insulators, oil glass, tank and transformer cover; tightening of all bolted connections and cleaning of contact connections; removing dirt from the expander; checking, disassembling and cleaning (if necessary) oil indicators; adding oil to the transformer, adjusting the oil pressure in the bushings; checking transformers for leaks (for gas-filled ones), inspection, cleaning and repair of cooling devices; checking the condition of parts of switching devices accessible for inspection; voltage position check; repair of the grounding network; checking thermosiphon filters (if necessary, replacing the sorbent); checking temperature and pressure control devices (for gas-filled transformers); measurement of winding insulation before and after repair.

2.2.

Simultaneously with the current repair of transformers, current repairs of bushings are carried out.

3. Typical nomenclature of repair work during major repairs

3.1.

During a major overhaul, all routine repair operations are performed, as well as the following work: draining (pumping) oil from the tank and taking a sample for chemical analysis; dismantling electrical devices, voltage switch and expansion tank; disconnecting leads from coils; removal from the tank and inspection of the core; dismantling radiators; cleaning the tank inside; loosening and loosening (if necessary) of the upper yoke of the magnetic core with depressing and removing the coils, replacing them or repairing the insulation of the low and high voltage windings, drying and impregnation of the windings, if necessary, changing the intersheet insulation and re-lamping the electrical steel of the magnetic core after assembly without windings, stopping the high voltage coils and low voltage on the magnetic cores, welding of leads onto the coils; installation of connecting devices and insulating strips, wedging of windings; checking tie rods with a megger and replacing defective insulation, repairing voltage switches and taps; repair of the expander cap, radiators, taps, thermosiphon filters (with replacement of silica gel); replacing gaskets; replacement of nitrogen in gas-filled transformers; repair (replacement) of insulators (inputs); repair of cooling and oil purification devices; repair (replacement) of oil pumps, fans; tank painting; changing the oil in the bushings; filling transformer oil (filling with quartz sand); checking instrumentation, signaling and protective devices.

3.2.

For explosion-proof transformers and transformer substations, the following work is additionally performed: checking the status of interlocks; checking explosion protection elements, shells; coating of explosion-proof surfaces with a thin layer of grease CIATIM-202, CIATIM-203.

4. Standards for frequency, duration and labor intensity of repairs

4.1.

In table 11.1 shows repair standards for indoor transformers under normal environmental conditions. For outdoor transformers, the repair frequency is taken with a coefficient of 0.75.

Table 1

Standards for the frequency, duration and labor intensity of repairs of transformers and complete substations
Name, type, brand and brief technical characteristics of the equipment

Frequency of repairs (numerator) and downtime for repairs (denominator), h		Labor intensity of one repair, person-hour
Maintenance	major renovation	Maintenance	major renovation
Three-phase, two-winding, oil transformers, (U up to 10 kV) power, kVA:
up to 25	25 920/2	103 680/54	13	65
40	25 920/4	103 680/72	17	86
63	25 920/4	103 680/96	21	103
100	25 920/4	103 680/108	25	124
160	25 920/6	103 680/128	30	150
250	25 920/6	103 680/144	36	179
400	25 920/6	103 680/156	43	216
630	25 920/8	103 680/172	51	258
1000	25 920/8	103 680/184	62	310
1600	25 920/8	103 680/216	75	375
2500	25 920/10	103 680/268	89	447
4000	25 920/12	103 680/360	108	540
6300	25 920/16	103 680/384	129	647
7500	25 920/18	103 680/432	139	693
10000	25 920/24	103 680/486	155	777
12500	25 920/36	103 680/508	170	850
16000	25 920/42	103 680/540	188	938
Three-phase oil transformers (U up to 10 kV) with 12 voltage levels complete with high voltage equipment for electric arc furnaces, power, kVA:
485	4320/10	34 560/252	95	456
630	4320/12	34 560/360	102	494
1000	4320/14	34 560/382	119	580
1600	4320/16	34 560/424	143	694
2000	4320/22	34 560/536	172	893
The same, single-phase, power, kVA:
250	4320/6	34 560/184	57	266
400	4320/8	34 560/206	67	309
630	4320/8	34 560/212	71	333
1000	4320/10	34 560/216	86	399
1600	4320/12	34 560/296	95	485
2500	4320/16	34 560/420	142	750
Three-phase transformers for resistance electric furnaces (U=380 V), power, kVA:
25	8640/2	69 120/72	15	80
40	8640/4	69 120/98	21	105
63	8640/4	69 120/108	24	138
100	8640/6	69 120/144	32	162
160	8640/6	69 120/144	38	190
250	8640/8	69 120/206	42	209
360	8640/8	69 120/216	49	247
The same, single-phase, power, kVA:
25	8640/2	69 120/54	11	57
40	8640/2	69 120/72	15	76
63	8640/2	69 120/88	19	95
100	8640/4	69 120/96	23	114
160	8640/4	69 120/108	27	133
250	8640/4	69 120/116	28	146
360	8640/4	69 120/144	34	171
630	8640/6	69 120/156	40	200
Single-phase furnace transformers (U = 35 kV) for powering RKZ-48F furnaces with regulation of operating voltage under load with a power of 16,700 kVA, type 30NTs-30000/35 and 30NTs-33000/35	2160/18	8640/108	114	754
Single-phase furnace transformers (U=110 kV) for powering furnaces RKZ-72F and RKZ-80F with regulation of operating voltage under load with a capacity of 26,700 kVA A, type 30NTs-54000/110	2160/18	8640/108	133	812
Three-phase transformers for submersible pumps, power, kVA:
up to 40	17 280/4	69 120/98	21	105
63	17 280/4	69 120/108	28	138
100	17 280/6	69 120/118	32	162
160	17 280/6	69 120/144	38	190
Dry transformers for powering mercury converters, power, kVA:
75	17 280/2	34 560/36	9	49
160	17 280/2	34 560/54	13	67
250	17 280/4	34 560/66	15	76
400	17 280/4	34 560/72	17	86
Transformers for powering selenium rectifiers with a primary voltage of 660 V, power, kVA:
0,1-0,16	17 280/1	34 560/1	—	—
0,25-0,4	17 280/1	34 560/2	1	3
0,63-1,0	17 280/1	34 560/4	2	5
1,6-2,5	17 280/1	34 560/6	2	8
6-8	17 280/2	34 560/9	3	14
11-14	17 280/2	34 560/12	5	24
19-25	17 280/2	34 560/36	9	47
Three-phase oil autotransformers for smooth regulation and stabilization with a primary voltage of 380 V, power, kVA:
up to 25	8640/4	69 120/98	19	101
40	8640/4	69 120/108	25	132
63	8640/6	69 120/144	34	159
100	8640/6	69 120/196	40	191
160	8640/8	69 120/216	44	230
250	8640/8	69 120/258	53	275
Three-phase transformers for powering power tools, kVA:
up to 0.63	8640/1	69 120/2	1	3
1-1,6	8640/1	69 120/6	1	8
2,5-4	8640/2	69 120/12	3	16
Low power transformers for local lighting of power supply of control circuit systems, power, kVA:
0,16-0,25	8640/1	—	1	—
0,4-0,63	8640/1	—	1	—
1,6-2,5	8640/2	69 120/6	2	10
4-6	8640/2	69 120/8	3	15
8-10	8640/4	69 120/16	5	17
Dry transformers for powering semiconductor converters with mains winding voltages up to 660 V, power, kVA:
up to 16	8640/4	69 120/24	7	34
18-23	8640/6	69 120/36	9	42
30-32	8640/6	69 120/48	10	51
35-51	8640/8	69 120/64	12	60
52-74	8640/8	69 120/76	15	73
75-104	8640/10	69 120/78	17	90
112-117	8640/12	69 120/82	19	96
142-147	8640/14	69 120/94	21	105
148-159	8640/16	69 120/96	23	114
202-220	8640/18	69 120/100	25	124
235-250	8640/20	69 120/102	26	129
275	8640/24	69 120/108	27	135
320	8640/24	69 120/108	30	151
Oil transformers with voltage regulation under load for powering semiconductor converters with a network winding of 6300-10,000 V, rated power, kVA:
345-681	8640/48	69 120/144	105	485
796-1580	8640/64	69 120/168	133	665
2040-2570	8640/83	69 120/216	162	808
4030-5090	8640/104	69 120/240	209	1045
The same, single-phase oil modulation for voltage regulation in an alternating current network with voltage up to 380 V rated power, kVA:
12	8640/8	69 120/24	5	27
20	8640/12	69 120/64	16	76
45	8640/16	69 120/88	21	105
115	8640/24	69 120/96	34	171
210	8640/32	69 120/96	38	190
375	8640/40	69 120/116	49	247
The same, three-phase rated power, kVA
25	8640/18	69 120/72	17	85
50	8640/24	69 120/96	27	133
63	8640/24	69 120/108	31	152
Single-transformer complete substations up to 10 kV for indoor installation, power, kVA:
160-250	8640/6	69 120/268	57	285
400-630	8640/8	69 120/360	76	380
1000	8640/10	69 120/360	95	475
The same, outdoor installation, power, kVA:
250-400	8640/8	69 120/330	68	342
630-1000	8640/16	69 120/432	114	570
Three-phase dry autotransformers for smooth regulation and stabilization at voltages up to 380 V, rated power, kVA:
25	8640/2	69 120/36	8	40
40	8640/2	69 120/36	10	51
63	8640/2	69 120/54	13	68
100	8640/4	69 120/72	16	81
160	8640/4	69 120/72	19	99
250	8640/4	69 120/72	27	133
High frequency autotransformers for powering electric furnaces with a frequency of 2400-10,000 Hz, voltage 800 V, rated power 500 kVA A	8640/28	69 120/114	36	180
High frequency transformers for powering electric furnaces with voltage 400 V, frequency 800-10000 Hz, power 200 kVA	8640/24	69 120/108	30	152
The same, voltage 800 V, frequency 2400 - 10000 Hz, power 800 kVA	8640/32	69 120/120	40	200
Switches type:
RNO-9, RNO-13, RNO-21	8640/24	—	28	—
RTN-9, RNT-13, RNT-18	8640/28	—	34	—
Three-phase stabilizers with voltage 220-380 V, dry, rated power, kVA:
10	8640/2	69 120/16	4	19
16	8640/2	69 120/20	5	24
25	8640/2	69 120/24	7	33
40	8640/2	69 120/36	9	43
63	8640/2	69 120/36	11	57
100	8640/2	69 120/36	13	67

Notes.

1.

The following correction factors are introduced to the given labor intensity standards: for power transformers 25-30 kV - 1.3; for power transformers with aluminum windings - 1.1; for dry transformers - 0.4; for transformers with voltage regulation under load, with the exception of transformers for arc furnaces - 1.25; for transformers with split windings - 1.1.

2.

The labor intensity of a major overhaul is given for the repair of transformers with a change of windings. During major repairs without changing the windings, the following coefficients should be applied: for general purpose transformers, resistance electric furnaces, submersible pumps, power supply to mercury converters, power supply to selenium rectifiers, power supply to electric tools, local lighting and power supply to control circuit systems, power supply to semiconductor converters, autotransformers and stabilizers - 0 .45; for transformers complete with high voltage equipment for electric arc furnaces - 0.6; for single-transformer complete indoor substations - 0.73; for single-transformer complete substations for outdoor installation - 0.70.

.4.2.

Downtime for major repairs of transformers is given for repairs with replacement of windings. When repairing without changing the windings, a coefficient of 0.67 is applied.

5. Standards for consumption of materials and spare parts for current and major repairs

5.1.

Material consumption rates for major repairs of general industrial and special transformers are given in Table. 2 - 4.

5.2.

Material consumption rates for major repairs of general industrial and furnace transformers are given separately for repair conditions with and without changing windings.

5.3.

Transformer oil, rubber and silica gel are excluded from the material consumption standards for the repair of dry transformers.

5.4.

For transformers with a voltage of 35 kV or more, the material consumption standards should be taken with a factor of 1.3.

table 2

Material consumption standards for major repairs without changing the windings of three-phase general industrial and furnace transformers, per 100 man-hours of repair

Material	Power, kVA
	up to 40	41 — 100	101 — 250	251 — 1000	1001 — 4000	4001 — 7500	7501 and more
Section steel, kg	2,37	4,68	8,1	13,57	29,06	62,37	84,84
Electrodes, kg	0,03	0,05	0,16	0,34	0,83	1,84	2,55
Fasteners, kg	0,72	1,17	2,44	4,52	10,38	18,41	25,45
Solder, kg
tin-lead	0,01	0,01	0,03	0,05	0,08	—	—
copper-phosphorus	0,01	0,02	0,03	0,07	0,09	—	—
The wire
installation, m	0,72	1,17	1,63	2,13	2,58	3,07	4,24
copper (aluminum), kg	15,78	25,81	58,65	140,18	303,06	484,82	670,20
Electrical insulating cardboard, kg	0,72	1,17	1,95	3,17	6,64	10,43	14,42
Cable paper, kg	0,35	0,58	0,81	1,36	2,49	3,68	5,09
Varnished fabric 700 mm wide, m	0,09	0,15	0,24	0,45	0,87	1,84	2,55
Ribbon
keeper, m	—	—	40,73	90,44	170,21	257,7	356,31
taffeta, m	7,17	11,74	19,55	40,7	99,64	171,8	237,54
asbestos electrical insulation, kg	0,03	0,04	0,06	0,11	0,33	0,55	0,76
Electrical insulating varnishes, kg	0,44	0,82	1,3	3,39	6,64	11,05	15,27
Ground enamels, kg	0,93	1,52	3,26	5,65	12,87	19,64	27,15
Aviation gasoline, kg	0,35	0,58	0,98	1,58	3,74	6,14	8,48
Solvents, kg	—	—	1,13	1,81	4,15	7,36	10,18
Rubber, kg
oil resistant	0,07	0,12	0,49	0,90	2,08	3,68	5,09
profile	0,07	0,12	0,19	0,29	0,37	0,55	0,76
Twisted cord, kg	—	—	0,19	0,68	1,49	2,27	3,14
Wiping material, kg	0,14	0,23	0,49	0,90	2,08	4,30	5,94

Table 3

Material consumption standards for major repairs with replacement of windings of three-phase general industrial and furnace transformers, per 100 man-hours of repair

Material	Power, kVA
	up to 63	64 — 250	251 — 1000	1001 — 2500	2501 — 6300	6301 — 10000	10001 or more
Section steel, kg	16,15	33,90	56,53	113,7	165,5	254,3	312,5
Channels, kg	—	15,39	45,22	102,3	144,5	217,5	265,3
Wire, kg:
bandage	0,06	0,08	—	—	—	—	—
piano	0,02	—	—	—	—	—	—
Electrodes, kg	0,08	0,31	0,68	1,5	2,55	4,35	5,3
Fasteners, kg	2,91	8,31	14,70	30,3	45,3	72,6	88,4
Aluminum alloy castings, kg	1,21	2,00	—	—	—	—	—
Copper, kg:
tire	2,10	6,16	16,51	60,6	80,8	116,2	141,5
bar	1,86	6,16	16,28	36,0	52,0	80,0	97,3
Copper tape, kg	0,24	0,77	1,58	5,1	7,6	10,9	13,3
Solder, kg:
tin-lead	0,01	0,06	0,10	0,19	—	—	—
copper-phosphorus	0,03	0,07	0,14	—	—	—	—
The wire:
installation, m	1,62	3,08	4,26	4,8	5,0	7,4	8,8
copper (aluminum), kg	42,80	127,74	327,85	644,4	899,8	1344,5	1636,2
Insulating cardboard, kg	5,33	16,16	37,31	90,2	118,6	168,6	205,2
Paper, kg:
cable	0,48	1,15	2,26	4,2	5,8	8,7	10,7
telephone	1,62	3,08	8,14	19,0	28,0	43,6	53,1
Varnished fabric 700mm wide, m	0,53	1,54	4,75	8,0	14,5	28,1	31,8
Tape, m:
keeper	53,30	153,90	287,15	496,5	686,4	1017,4	1238,2
taffeta	26,65	76,95	205,75	405,6	546,9	799,4	972,8
Getinax, kg	—	—	31,65	72,2	99,1	159,9	194,6
Asbestos electrical insulating tape, kg	0,16	0,31	0,61	1,6	2,2	3,3	4,0
Electrical insulating varnishes, kg	3,23	8,46	22,61	45,5	66,0	101,7	123,8
Ground enamels, kg	1,05	3,08	5,65	11,7	15,9	24,0	28,3
Aviation gasoline, kg	0,81	1,85	3,39	6,8	9,6	14,5	17,7
Solvents, kg	—	2,31	3,62	7,6	11,5	18,1	22,1
Rubber, kg:
oil resistant	0,08	0,46	0,90	1,9	2,8	4,4	5,3
profile	0,24	0,54	0,93	1,06	1,4	2,0	2,5
Silica gel, kg	1,21	4,62	9,04	19,0	28,0	43,6	53,0
Twisted cord, kg	—	—	0,68	1,35	1,85	2,7	3,2
Wiping material, kg	0,40	0,92	1,81	3,4	5,8	10,1	12,3
Hardwood, m³	0,01	0,03	0,07	0,15	0,25	0,39	0,5

Table 4

Material consumption standards for major repairs of dry special transformers, autotransformers and stabilizers, per 100 man-hours of repair

Material	Three-phase dry stabilizers	Three-phase autotransformers	Dry transformers for powering mercury converters	Dry transformers for powering semiconductor converters	Dry transformers for powering power rectifiers, control circuits and local lighting
Section steel, kg	7,79/1,56	1,71/0,34	13,30/2,66	16,72/3,34	3,80/0,76
Wire, kg:
bandage	0,03	0,01	0,05	0,07	0,02
piano	0,01	0,03	0,02	0,03	0,006
Electrodes, kg	0,04/0,02	0,01/0,004	0,07/0,03	0,08/0,04	0,02/0,01
Fasteners, kg	0,40/0,39	0,31/0,09	2,39/0,67	3,01/0,84	0,68/0,19
Aluminum alloy castings, kg	0,58	0,13	1,00	1,25	0,29
Copper, kg:
tire	0,97	0,21	1,66	2,09	0,48
bar	0,90	0,20	1,53	2,92	0,44
Copper tape, kg	0,12	0,03	0,20	0,25	0,06
Solder, kg:
tin-lead	0,006/0,004	0,001/0,001	0,01/0,01	0,01/0,01	0,003/0,002
copper-phosphorus	0,01/0,01	0,003/0,002	0,02/0,01	0,03/0,02	0,007/0,003
Installation wire, m	0,78/0,39	0,17/0,09	1,33/0,67	1,67/0,84	0,38/0,19
Copper/aluminum cable, kg	20,64/8,04	4,53/1,88	35,25/14,63	44,3/18,39	10,07/4,18
Electrical insulating cardboard, kg	2,57/0,39	0,56/0,09	4,39/0,67	5,52/0,84	1,25/0,10
Paper, kg:
cable	0,23/0,19	0,05/0,04	0,40/0,33	0,50/0,42	0,11/0,10
telephone	0,79	0,17	1,33	1,67	0,38
for gluing electric steel	0,66	0,06/0,01	1,13	1,42	0,13/0,02
Varnished fabric 700mm wide, m	0,26/0,05	5,64	0,44/0,09	0,55/0,11	12,54
Ribbon:
keeper, m	27,70	2,82/0,86	43,89	55,18	6,27/1,9
taffeta, m	12,85/3,9	0,02/0,003	21,95/6,65	27,59/8,36	0,04/0,01
asbestos electrical insulation, kg	0,08/0,02	0,15	0,13/0,03	0,17/0,03	0,32
Paper for gluing electric steel, kg	0,26/0,04	0,34/0,06	0,42/0,06	0,64/0,011	0,76/0,13
Electrical insulating varnishes, kg	1,56/0,27	0,11/0,11	2,66/0,47	3,34/0,59	0,01/0,02
Ground enamels, kg	0,51/0,51	0,84/0,84	0,86/0,86	1,09/1,09	0,25/0,025
Aviation gasoline, kg	0,39/0,19	0,09/0,04	0,67/0,33	0,84/0,42	0,19/0,10
Rubber, kg:
oil resistant	0,04/0,04	0,01/0,01	0,07/0,078	0,08/0,08	0,02/0,02
profile	0,12/0,04	0,03/0,01	0,20/0,07	0,25/0,08	0,06/0,02
Silica gel, kg	0,58	0,13	1,00	1,25	0,29
Wiping material, kg	0,19/0,08	0,04/0,02	0,33/0,13	0,42/0,17	0,10/0,04
Hardwood, m³	0,004	0,001	0,01	0,01	0,002

Note. The numerator indicates the consumption of materials with changing windings, the denominator - without changing windings

5.5.

Material consumption rates for routine repairs of three-phase general industrial and furnace transformers are set at 20% of the corresponding consumption rates for major repairs without changing windings according to the following nomenclature: steel, electrodes, fasteners, solder, wire, cable, electrical insulating cardboard, cable paper, varnished cloth , kiper tape, taffeta tape, asbestos electrical insulating tape, electrical insulating varnishes, ground enamels, aviation gasoline, solvents, oil-resistant rubber, profile rubber, twisted cord, cleaning material.

5.6.

Material consumption rates for routine repairs of special transformers are determined by applying the coefficients given in Table 5 to the corresponding material consumption rates for major repairs without changing windings.

Table 5

Coefficients for determining material consumption rates for routine repairs of special transformers

Material	Three-phase dry stabilizers and autotransformers	Transformers for powering mercury converters and power tools	Transformers for local lighting and power supply of control circuits and selenium rectifiers
Section steel, kg	0,19	0,21	0,13
Electrodes, kg	0,2	0,29	0,10
Fasteners, kg	0,19	0,20	0,14
Solder, kg:
tin-lead	0,17	0,15	0,12
copper-phosphorus	0,15	0,17	0,21
The wire:
installation, m	0,19	0,20	0,14
copper (aluminum), kg	0,20	0,20	0,14
Electrical insulating cardboard, kg	0,19	0,20	0,14
Cable paper, kg	0,20	0,23	0,11
Varnished fabric 700 mm wide, m	0,22	0,26	0,17
Ribbon:
taffeta, m	0,19	0,26	0,13
asbestos electrical insulation, kg	0,91	0,33	0,1
Electrical insulating varnishes, kg	0,19	0,18	0,12
Ground enamels, kg	0,28	0,15	0,13
Aviation gasoline, kg	0,20	0,23	0,11
Rubber, kg:
oil resistant	0,21	0,17	0,12
profile	0,21	0,12	0,12
Wiping material, kg	0,20	0,21	0,19

5.7. For power transformers, the consumption standards for spare parts for current and major repairs are established as general for all types of power transformers and are given in Table. 6.

Table 6

Standards for the consumption of spare parts for the repair of power transformers

Spare parts	Consumption rate for 10 units of the same type of equipment, pcs.
	major renovation	Maintenance
Windings, set
high voltage	2	—
low voltage	2	—
Feedthrough insulators, set.	2	1
Bushings through passage, set.	2	1
Radiator tap, pcs.	2	—
Thermal alarm, pcs.	1	—

5.8.

A safety stock of transformers should be provided only in the absence of a hot reserve in the amount of 10% of the operated number of transformers.

Varieties

There are several types of power equipment repairs. If you need to service a large high-power device, for example, 1600 kVA, 2500 kVA, 6300 kVA, etc., you cannot do without special skills. Special equipment should only be inspected and repaired by a qualified professional.

The following types of repairs are distinguished:

1. Maintenance. Produced according to the schedule established by regulations. In this case, the operation of the equipment does not stop.
2. Routine repair of power transformer. Requires disconnecting the device from the network. Refers to preventive actions.
3. Overhaul of transformers. Measures are taken to eliminate malfunctions that arose during operation of the unit, as well as in case of aging and wear of the system. After 10–15 years of operation of the installation, it must be reconstructed.

In addition to the presented actions, between-repair and “post-repair” tests are carried out. For devices with a power of more than 110 kV, the first major maintenance is required 12 years after the start of operation. For other varieties, similar actions are performed based on test results and general condition.

Inspection of “power oil transformers” and units with an oil cooling system such as “TSZM”, “TSZN” and other varieties that contain adjusting elements (OLTC) is required once a year. For devices without an on-load tap-changer, this type of maintenance is performed every two years.

For other types, maintenance is required at least once every four years. There are also special instructions. They are used in places of increased pollution.

Maintenance with dry cooling

In this case, a cast insulation system is assumed. Current repairs are carried out in accordance with the regulations, however, the number of points may vary depending on the operating conditions or location of the transformer. Typically the list includes the following procedures:

1. Check the cooling system once every six months. The functionality of the temperature controller is checked, as well as the quality of the fans rotating on a constant basis.
2. Dirt is being cleaned. Must be repeated every six months or quarter. It is done more often if the environment is conducive to it.
3. Check the housing for cracks and damage. Eliminate them if necessary.
4. Checking the integrity of insulating and protective systems. Replacement in case of malfunctions. Held annually.
5. Inspection of the quality of winding fixation. A similar check is also carried out during technical inspection. In the absence of proper quality, the transformers are completely rewinded.

Overhaul of power transformers

During a major overhaul, the cover must be opened and all components thoroughly checked.
After which it is tested in accordance with regulatory documents. Repair of large power oil transformers (TMG) is carried out directly at the installation site using prefabricated structures, without sending it to a repair shop. If there are transformer towers built near switchgears or repair areas of machine rooms with access roads, then they will also be used. Repair of oil transformers (TMG) should include a complete replacement of old oil with new one. Low power transformers (welding, pulse, etc.) are repaired in specially equipped workshops or repair shops. These premises must reliably protect disassembled transformers from dust and various atmospheric precipitations getting on their parts. Types of particularly important work that should only be performed by highly specialized workers with the skills and knowledge:

• Delivery of TMG to the repair site. Its loading, unloading and transportation;
• Removing contact terminals;
• Repair of the active part of the transformer;
• Moving and installing individual components and assemblies.

Moreover, workers must be able to efficiently perform not only electrical work, but also rigging. After completing the appropriate training, passing exams, and also receiving a supporting document. The technological process of repairing a transformer must be carried out efficiently and strictly according to schedule; then this unpretentious equipment will last for decades. Testing a transformer after repair comes down to:

• determination of the transformation ratio. It is defined for all existing windings and branches;
• measuring the insulation resistance of windings;
• supplying increased voltage to the primary winding. Each winding is subjected to this test. The technology of this process is performed using a step-up autotransformer. It is this that makes it possible to increase and decrease the test voltage smoothly

Maintenance with oil cooling

The complexity generally depends on the design features and operating conditions. Among the general procedures when carrying out routine repairs of power transformers of this type, the following should be noted:

1. Inspect the housing for external defects.
2. Cleaning the device from dirt.
3. Measuring the resistance level on winding insulation.
4. Elimination of minor faults in the cooling system, fittings and attachments.
5. Tightening fasteners if they have been loosened.
6. Adding oil and eliminating leaks.

The entire process is carried out only at the installation site of the transformer without transporting it.