What is the maximum load allowed for oil transformers?

Oil-filled transformers are a special type of device that are used primarily when operating in areas with unusual temperatures and high humidity.

They are demanding in terms of voltage, current and load, so the question of what load is allowed for oil transformers is quite common among specialists. It is not possible to determine it in absentia; it is necessary to pay attention to the technical characteristics and conditions, features of its use, and individual nuances.

What are oil transformers and where are they used?

Oil transformers are used to operate in unusual environmental conditions. They will be the best choice when you have to work at low or high temperatures, in environments with high humidity characteristics. In particular, the oil transformer operates at temperatures up to 40 degrees with a plus sign and 60 degrees with a minus sign.

The device with oil inclusions is placed in the open air, since it will not be spoiled by environmental precipitation. The mechanism parts are reliably protected from the aggressive effects of snow, rain, and lightning by a special thick case. The temperature regime for the operation of the transformer is extended, so devices of this type are used for combination with mast and pole type substations.

The quality of transformer oil determines the uninterrupted operation and efficiency of equipment. The addition of non-original fluid, the ingress of impurity particles or dust, dirt leads to a decrease in the characteristics of electrolytes. As a result, holes appear between the equipment housing and the winding. Due to the minimum oil level, it begins to exceed the maximum temperature. Whether or not overloading standard oil transformers in emergency modes is allowed is a controversial issue. This is possible, but it will have a detrimental effect on the operation of the device.

Important characteristics during the operation of the equipment are the presence of correct insulation of the oil and the windings themselves, the establishment of the correct temperature and humidity conditions. Special installed monitoring systems cope with this. Online technology monitors the set temperature, analyzes the content of impurities in oil and gas, and monitors humidity. Emergency mode is activated if the parameters do not meet the standards. You cannot turn on the load - this will lead to failure.

Permissible loads and overloads in transformers

Technical characteristics are determined depending on the nominal values ​​at which the wear of structural parts will occur within 20 years. As a result of using the device within wide limits, it will not only not show efficiency and will work intermittently, but will also fail. Natural wear is observed in the windings, which will no longer show their nominal values ​​when an electrical impulse passes through.

An oil transformer can be used when the highest temperature of its winding does not exceed 98 degrees. If the air temperature increases by 8 degrees, then the service life is reduced by half (Montzinger’s rule). The point is located on the winding layer of the coil, which is at the top.

It is clear that achieving ideal values ​​is not possible due to a number of factors. Transformers operate under variable load, and operating parameters are changed using a special cooling medium. Increased wear of the insulation is observed if an overload is applied. It is not present when the characteristics are set smaller, but then there is no point in using insulation in oil transformers.

Experts clearly answer the question of which modern transformers can be switched on at rated load - almost all oil transformers. But a number of mandatory conditions must be met:

• the rated temperature of the highest point of the winding should not exceed the set value;
• overload is allowed to exceed the nominal maximum by 5 percent;
• for windings with branches, the coefficient is 1.05 of the established one.

Often, staff do not have the ability to track and build an individual schedule. In this case, use the developed tables. They carry out a calculation according to which, if there are systematic overloads, the characteristics are calculated depending on the temperature of the upper layers of oil and the temperature of the natural environment.

For oil engines, in the winter season it is possible to exceed by 1 percent the percentage of underload in the warm season. Indicators are determined in summer - plus 15 degrees, in winter - 5 degrees. Negative temperatures are calculated according to the same schedule.

Due to the fact that the equipment is used in natural conditions, that is, in nature, in the city, specialists are interested in the answer to the question of which transformers can be turned on at the rated load at any negative air temperature. Although the operating sheet states that the procedure can be done up to 60 degrees with a minus sign, in practice this turns out not to be the case.

The overload indicators that occurred and air humidity are taken into account. If the equipment has already been systematically switched on and off in emergency mode, then failures and breakdowns are already observed in the structure of the windings. Using the windings at full power leads to negative consequences.

To avoid equipment breakdown and possible emergency shutdown, it is recommended to calculate the oil TM indicators in advance, taking into account the degree of its wear.

The duration of the overload in minutes is allowed if an overload is observed in terms of current strength as a percentage of the rated possible:

• 30 — 120;
• 45 — 80;
• 60 — 45;
• 75 — 20;
• 100 — 10.

The voltage of any part of the winding of oil equipment should not exceed the highest operating voltage on the parts. During an overload, specialists take a set of measures aimed at replacing a worn-out device with a backup one. At the same time, they are unloaded to their nominal characteristics - the most necessary ones, which are not essential, are turned off.

5.3. RULES FOR TECHNICAL OPERATION OF POWER PLANTS AND NETWORKS OF THE RUSSIAN FEDERATION

5.3. Power transformers and oil shunt reactors

5.3.1. When operating transformers (autotransformers) and shunt oil reactors, the conditions for their reliable operation must be met. Loads, voltage levels, temperatures of individual elements of transformers (reactors), oil characteristics and insulation parameters must be within the established standards; cooling devices, voltage regulation, and other elements must be kept in good condition.

5.3.2. It is necessary to monitor the correct installation of transformers (reactors) equipped with gas protection devices. The cover must have a rise towards the gas relay of at least 1%, and the oil line to the expander must have a rise of at least 2%. The exhaust pipe cavity must be connected to the expander cavity. If necessary, the membrane (diaphragm) on the exhaust pipe must be replaced with a similar one supplied by the manufacturer.

5.3.3. Stationary fire extinguishing equipment, oil receivers, oil drains and oil collectors must be in good condition.

5.3.4. The station (substation) numbers must be indicated on the tanks of outdoor transformers and reactors. The same numbers should be on the doors and inside transformer points and chambers.

The phase colors must be applied to the tanks of single-phase transformers and reactors. Transformers and reactors for outdoor installations should be painted in light colors with paint that is resistant to weathering and oil.

5.3.5. The electric motors of cooling devices for transformers (reactors) must be powered, as a rule, from two sources, and for transformers (reactors) with forced oil circulation - using an automatic transfer switch.

5.3.6. Load voltage control devices (OLTC) of transformers must be in automatic mode. By decision of the technical manager of the power system, it is allowed to install a non-automatic voltage regulation mode by remotely switching the on-load tap changer from the control panel, if voltage fluctuations in the network are within the limits that satisfy the requirements of electricity consumers.

Switching the on-load tap-changer device of a transformer under voltage manually (with a handle) is not allowed.

5.3.7. Ventilation of transformer substations and chambers must ensure the operation of transformers in all rated modes.

5.3.8. On transformers and reactors with forced circulation of air and oil (DC type cooling) and on transformers with forced circulation of water and oil (C type cooling), cooling devices must be automatically turned on (off) simultaneously with the transformer or reactor on (off). Forced oil circulation must be continuous regardless of the load. The procedure for turning on (off) cooling systems must be determined by the factory instructions.

It is not allowed to operate transformers and reactors with artificial cooling without switched on alarm devices for stopping the circulation of oil, cooling water or stopping the fans.

5.3.9. On transformers with forced air circulation and natural oil circulation (cooling system D), the fan motors should automatically turn on when the oil temperature reaches 55 degrees. C or rated load, regardless of oil temperature and switches off when the oil temperature drops to 50 degrees. C, if the load current is less than the rated one.

The operating conditions of transformers with the blower switched off must be determined by the factory instructions.

5.3.10. When oil-water cooling of transformers, the oil pressure in the oil coolers must exceed the pressure of the water circulating in them by at least 0.1 kgf/cm2 (10 kPa) at a minimum oil level in the transformer expander.

The water circulation system must be turned on after turning on the working oil pumps at a temperature of the upper layers of oil not lower than 15 degrees. C and turns off when the oil temperature drops to 10 degrees. C, unless otherwise specified in the factory technical documentation.

Provisions must be made to prevent oil coolers, pumps and water lines from freezing.

5.3.11. The oil in the conservator of an idle transformer (reactor) should be at the level of the mark corresponding to the temperature of the oil in the transformer (reactor).

5.3.12. At rated load, the temperature of the upper layers of oil should be (unless other temperature values ​​are specified by the manufacturers) for the transformer and reactor with DC cooling - no higher than 75 degrees. C, with natural oil cooling M and cooling D - no higher than 95 degrees. WITH; For transformers with C cooling, the oil temperature at the inlet to the oil cooler should be no higher than 70 degrees. WITH.

5.3.13. Continuous operation of transformers is allowed (with a power not exceeding the rated one) at a voltage on any branch of the winding that is 10% higher than the rated voltage for a given branch. In this case, the voltage on any winding should not be higher than the maximum operating voltage.

For autotransformers with neutral taps for voltage regulation or intended to operate with series regulation transformers, the permissible voltage increase must be specified by the manufacturer.

5.3.14. For oil transformers, long-term overcurrent of any winding is allowed by 5% of the rated branch current, if the voltage on the branch does not exceed the rated one.

In addition, for transformers, depending on the operating mode, systematic overloads are allowed, the value and duration of which are regulated by the standard operating instructions for transformers and the manufacturers' instructions.

In those autotransformers, to the low voltage windings of which a generator, synchronous compensator or load is connected, the current control of the common part of the high voltage winding must be organized.

5.3.15. In emergency modes, short-term overload of transformers above the rated current is allowed for all cooling systems, regardless of the duration and value of the previous load and the temperature of the cooling medium within the following limits:

Oil transformers Current overload, %………………. 30 45 60 75 100 Overload duration, min. ……….. 10 Dry transformers Current overload, % ………………. 20 30 40 50 60 Overload duration, min. ……….. 60 45 32 18 5

Permissible long-term overloads of dry-type transformers are established by the factory instructions.

5.3.16. In case of emergency shutdown of cooling devices, the operating mode of transformers is determined by the provisions of the factory documentation.

5.3.17. Turning on transformers at rated load is allowed:

with cooling systems M and D at any negative air temperature;

with DC and C cooling systems at ambient temperatures not lower than minus 25 degrees. C. At lower temperatures, the transformer must be preheated by switching on a load of about 0.5 rated without starting the oil circulation system until the temperature of the upper oil layers reaches minus 25 degrees. C, after which the oil circulation system must be turned on. In emergency conditions, it is allowed to turn on the transformer at full load, regardless of the ambient temperature;

with a cooling system with directed oil flow in the windings of transformers NDC, NTs in accordance with factory instructions.

5.3.18. The on-load tap-changer switching devices of transformers are allowed to be put into operation at a temperature of the upper oil layers of minus 20 degrees. C and above (for submersible resistor on-load tap-changers) and minus 45 degrees. C and higher (for on-load tap-changer devices with current-limiting reactors, as well as for switching devices with a contactor located on the support insulator outside the transformer tank and equipped with an artificial heating device).

The operation of on-load tap-changer devices must be organized in accordance with the provisions of the manufacturers' instructions.

5.3.19. For each electrical installation, depending on the load schedule, taking into account the reliability of power supply to consumers and the minimum of energy losses, the number of simultaneously operating transformers must be determined.

In distribution power networks with voltages up to 15 kV inclusive, measurements of loads and voltages of transformers must be organized during periods of maximum and minimum loads. The timing and frequency of measurements are established by the technical manager of the power facility.

5.3.20. The neutrals of the windings of 110 kV and higher autotransformers and reactors, as well as transformers of 330 kV and higher, must operate in solid grounding mode.

It is allowed to ground the neutral of transformers and autotransformers through special reactors.

Transformers of 110 and 220 kV with a neutral test voltage of 100 and 200 kV, respectively, can operate with an ungrounded neutral provided it is protected by a surge arrester. When justified by calculations, it is allowed to work with an ungrounded neutral of 110 kV transformers with a neutral test voltage of 85 kV, protected by a surge arrester.

5.3.21. When a gas relay is triggered by a signal, an external inspection of the transformer (reactor) must be carried out, gas must be selected from the relay for analysis and testing for flammability. To ensure the safety of personnel when sampling gas from the gas relay and identifying the reason for its operation, the transformer (reactor) must be unloaded and turned off. The time it takes to unload and disconnect the transformer should be minimal.

If the gas in the relay is non-flammable, there are no signs of damage to the transformer (reactor), and its shutdown caused a lack of electricity supply, the transformer (reactor) can be immediately put into operation until the reason for the gas relay triggering the signal is determined. The duration of operation of the transformer (reactor) in this case is established by the technical manager of the power facility.

Based on the results of gas analysis from the gas relay, chromatographic analysis of the oil, and other measurements (tests), it is necessary to establish the reason for the gas relay to respond to a signal, determine the technical condition of the transformer (reactor) and the possibility of its normal operation.

5.3.22. In the event of automatic shutdown of the transformer (reactor) due to the action of protection against internal damage, it can be put into operation only after inspection, testing, gas and oil analysis and elimination of identified violations.

If a transformer (reactor) is switched off by protections whose operation is not related to its damage, it can be switched on again without checks.

5.3.23. Transformers with a capacity of 1 MVA or more and reactors must be operated with a system of continuous oil regeneration in thermosyphon or adsorption filters.

The oil in the transformer conservator (reactor), as well as in the tank or conservator of the on-load tap-changer, must be protected from direct contact with ambient air.

For transformers and reactors equipped with special devices that prevent oil moisture, these devices must be constantly turned on, regardless of the operating mode of the transformer (reactor). The operation of these devices must be organized in accordance with the manufacturer's instructions.

The oil of oil-filled bushings must be protected from oxidation and moisture.

5.3.24. The transformer (reactor) must be connected to the network by pushing to full voltage.

Transformers operating in a block with a generator can be turned on together with the generator by raising the voltage from zero.

5.3.25. Inspections of transformers (reactors) without shutdown are carried out within the time limits established by the technical manager of the power facility, depending on their purpose, installation location and technical condition.

5.3.26. Repair of transformers and reactors (overhaul, current) and their components (on-load tap-changer, cooling system, etc.) is carried out as necessary, depending on their technical condition, determined by measurements, tests and external inspection.

Repair periods are set by the technical manager of the energy system (energy facility).

5.3.27. Preventive testing of transformers (reactors) must be carried out in accordance with the scope and standards of electrical equipment testing and factory instructions.

Permissible loads for general purpose equipment

According to GOST 14209 - 85, permissible loads for oil transformers are established by the equipment manufacturer. Previously there was GOST 14209 - 69, which was replaced by the specified one. It saves the load calculation model using the insulation wear indicator, the heated point of the winding layers, and the type of diagram. What is saved is that:

• the base winding temperature (maximum) does not exceed 98 degrees;
• emergency overloads are possible up to 115 degrees;
• the permissible maximum for systematic overloads is 95 degrees (a graph for determining operation is constructed);
• There is a six-degree rule for insulation wear.

Other provisions regarding permissible heating rates for points have changed. For overloads, the indicator is 140 (standard model for transformers from 110 kV and below), 160 (equipment with emergency overloads of 110 kV and below). For equipment 110 kW and above, a threshold of 140 degrees is maintained for systematic and emergency conditions. Values ​​subject to conditions (the temperature of the heating point corresponds to the nominal technical characteristics) for systematic - 1.5, for emergency - 2.

Environmental parameters are taken into account. The duration of the load curve is considered if the change in temperature indicators does not exceed 12 degrees and the characteristics are positive. An adjustment schedule is entered if changes exceed the set threshold of 12 or the temperature has become negative.

Graphical methodology is used to determine the oil temperature rise (depending on the characteristics of the hottest point of the winding and the environment). The graph clearly demonstrates a departure from the rated power and load, which allows you to correct the results by introducing additional cooling.

Insulation wear is determined by calculation tables - the experimental method does not work.

The use of electronic computers monitors the performance of the windings and the performance characteristics of the transformer oil.

Online electrical magazine

When operating power transformers, it is necessary to overload them at certain hours of the day so that, due to underload at other hours, the daily wear of the winding insulation from overheating is not higher than the wear that corresponds to the nominal operating mode of the transformer, since a change in the insulation temperature by 6 °C causes a change its service life is doubled.

The duration t times per day of the permissible periodic overload of the transformer, estimated by the excess load coefficient K2, depends on the initial load coefficient K1 of the transformer, its rated power Snom, the cooling system, the constant heating time and the equivalent temperature of the cooling air corresponding to a given period of the year.

Coefficients K1 and K2 are determined by the ratios of the equivalent initial and maximum currents to the rated current of the transformer, while equivalent values ​​are understood as their root mean square values ​​before the arrival of a larger load and during the period of its maximum.

Graphs of the load capacity of transformers K2 (K1), corresponding to different durations t of periodic overload (Fig. 1), allow for a given initial state of the transformer, characterized by the coefficient K1 determined from the daily load schedule I(t) 10 hours before the arrival of its maximum, and this duration t of periodic overload, find the permissible overload coefficient K2 for the period of the highest load of the transformer.

Rice. 1. Graphs of the load capacity of three-phase transformers with a rated power of up to 1000 kVA with natural circulation of air and oil and a constant heating time of 2.5 hours at an equivalent cooling air temperature of 20 °C.

The equivalent temperature of the cooling air is its constant temperature, at which the same wear of the insulation of the windings of a transformer carrying a constant load occurs, as with the existing variable air temperature. With a virtually constant load and the absence of periodic daily and seasonal fluctuations, the equivalent cooling air temperature is assumed to be 20 °C.

If the maximum of the average load curve I(t) in the summer is less than the rated power of the transformer, then in the winter months an additional 1% overload of the transformer is allowed for each percentage of underload in the summer, but less than 15%, while the total load should be less than 150 % nominal.

In emergency cases, short-term overload of transformers above the rated value is allowed, which is accompanied by increased wear of the winding insulation and a decrease in the service life of the transformers (see table).

Permissible short-term overloads of transformers during emergency conditions

Such overloads are permissible for all cooling systems, regardless of the previous mode, the temperature of the cooling air and the location of the transformers, provided that the oil temperature in the upper layers is not higher than 115 ° C. In addition, for oil-filled transformers operating with an initial load coefficient K1 < 0.93, an overload of 40% above the rated current is allowed for less than 5 days during peak loads with a total duration of less than 6 hours per day, while taking all measures to enhance the cooling of the transformer.

When there is a variable load on a substation with several transformers, it is necessary to draw up a schedule for turning on and off parallel operating transformers in order to achieve economical modes of their operation.

In real conditions, it is necessary to deviate somewhat from the design mode so that the number of operational switchings of each transformer does not exceed 10 during the day, i.e., it would not be necessary to turn off the transformers for at least 2 - 3 hours.

When operating transformers in parallel, the total load on the transformer substation must provide sufficient load to each of them, as judged by the evidence of the corresponding ammeters, the installation of which is mandatory for transformers with a rated power of 1000 kVA and above.

Modern transformers operating at high magnetic induction should not be in operation with a significant increase in the primary voltage, because this is accompanied by an increase in the loss of electronic energy due to heating of the magnetic circuits. A long increase in the primary voltage when the transformer load is not higher than the rated one is allowed up to 5% of the voltage of this branch, and when it is loaded at 25% of the rated power - up to 10%, which can be allowed at a load not higher than the rated duration for up to 6 hours per day.

The degree of load unevenness across the transformer phases should not exceed 20%. It is defined like this:

Kn = (Imax - Iav / Iav) x 100,

where, Imax is the current of the overloaded phase at the moment of greater load on the transformer, Iav is the average current of the 3 phases of the transformer at the same moment.

Electrician school

The influence of loads on the operation of an oil transformer

If the maximum temperature of a winding turn and environmental parameters are taken into account, then systematic loads on an oil-type transformer do not lead to its wear or failure. Taking into account the graphs created by specialists, you can correlate the necessary values ​​and select the load level that corresponds to stable and uninterrupted operation.

The normal service life will not decrease, since the insulation does not wear out. An oil-type transformer will last at least twenty years.

The main problem is that it is not so easy for a novice specialist to determine on his own using graphs or flowcharts the required degree of load. Therefore, to get started, choose models from modern manufacturers with clear operating sheets. They prescribe design indicators, indicate not only the load parameters under different modes, but also provide independent calculation schemes.

Emergency overloads lead to wear and tear of the insulation. As a result, the transformer windings become unusable. Breakdowns occur within them or at the boundaries of the transformer structure and windings. Electric current cannot flow uninterruptedly and short circuits are possible. The service life specified by the manufacturer is reduced many times over.

Methods are used to compensate for insulation wear (when operating at reduced load parameters), but this does not always lead to positive expected results.

Loading Guide for Power Oil Transformers

When choosing transformers, in addition to their rating data, possible short-term and long-term overloads in operation must be taken into account. When checking the admissibility of loads and overloads of transformers, the following definitions are used [11].

Distribution transformer - a three-phase transformer with a rated power of no more than 2500 kVA×A or a single-phase rated power of no more than 833 kVA×A of voltage classes up to 35 kV inclusive, that is, a step-down transformer with ON cooling and without switching winding taps under load.

Medium power transformer - a three-phase transformer with a rated power of no more than 100 MV×A or a single-phase transformer with a rated power of no more than 33.3 MV×A

The load on transformers continuously changes throughout the day. In this case, part of the day the transformer load may be less than the rated one, the temperature of the hottest point of the winding will be less than the long-term permissible one and the transformer will be underused for heating. The optimal mode for a transformer should be such a mode in which the wear of its insulation would be close to the design one.

The design or natural wear life of a transformer operating in nominal mode is considered to be approximately 20 years. A longer period is considered inappropriate from the point of view of its obsolescence. This period is determined by the aging of the insulation. For normal daily wear of transformer insulation with cooling type M, the temperature of the hottest point of the winding (the hottest inner layer of the winding) in a long-term mode should not exceed 98°C. If the temperature is increased by 6°C, the service life of the insulation will be reduced by almost half (6 degree rule).

In order for the actual service life to be closer to the natural one, the transformer must be loaded in accordance with the so-called load capacity. The load capacity of a transformer is understood as such a set of loads and overloads under which the wear of the winding insulation during the cycle does not exceed the wear corresponding to the nominal operating mode.

Let's consider possible operating modes according to GOST 14209.

Cyclic load mode

Load mode with cyclic changes (usually the cycle is equal to a day), which is determined taking into account the average wear value over the duration of the cycle. The cyclic load mode can be a systematic load mode or a continuous emergency overload mode.

Systematic load mode is a mode during part of the cycle of which the temperature of the cooling medium may be higher and the load current exceeds the rated current, but from the point of view of thermal wear, such a load is equivalent to the rated load at the rated temperature of the cooling medium. This is achieved by lowering the coolant temperature or load current during the rest of the cycle. The load (overload) of the transformer, which is allowed by its load capacity and which, over the duration of the cycle of the load schedule, does not cause a reduction in the normal service life of the transformer (due to reduced wear during hours of reduced load) is called systematic. The mode, during part of the cycle of which the temperature of the cooling medium may be higher and the load current exceeds the rated one, but from the point of view of thermal wear, such a load during the cycle (day) is equivalent to the rated load, is called a systematic load mode.

When planning loads, this principle can be extended to long periods, during which cycles with a relative insulation wear rate of more than one are compensated by cycles with a wear rate of less than one.

Long-term emergency overload mode is a load mode that occurs as a result of prolonged failure of some network elements, which can be restored only after reaching a constant value of the transformer temperature rise. The permissible duration of such a load is greater than the thermal time constant of the transformer. This condition is expected to occur rarely, but can last for weeks or even months and cause significant thermal wear. However, such a load should not cause an accident due to thermal damage or reduced dielectric strength of the transformer.

Short-term emergency overload mode is a mode of extremely high load caused by unexpected impacts that lead to significant disruptions to the normal operation of the network. In this case, the temperature of the hottest point of the conductors reaches dangerous values ​​and in some cases a temporary decrease in the electrical strength of the insulation occurs. The permissible duration of such a load is less than the thermal time constant of the transformer and depends on the temperature reached before the overload; Typically, the duration of the overload is less than half an hour. They must be reduced as quickly as possible or the transformer must be switched off for a short time to avoid damage.

The PUE [1] introduces definitions of two long-term consumer modes: normal and post-emergency. The normal mode of an electrical energy consumer is a mode in which the specified values ​​of its operating parameters are ensured. Post-emergency mode is the mode in which the consumer of electrical energy is located as a result of a disruption in its power supply system until the establishment of a normal mode after the elimination of the failure. If the post-emergency mode of the consumer is caused by the failure of one of the transformers, then its duration will be determined by the time of repair or replacement of the damaged transformer.

Let's draw a parallel between the consumer modes and the transformer load modes.

The regime of systematic loads of the transformer occurs in the normal mode of the consumer's power supply circuit, provided that at some time intervals of the day the load current of the transformer exceeds the rated one.

The mode of prolonged emergency transformer overloads occurs in the post-emergency mode of the consumer's power supply system, when one of the transformers is under repair and the second has taken over its load, provided that the temperature of the most heated part of the winding and the oil temperature have exceeded the normally permissible values, but remain less than the maximum acceptable values. In this case, there is no danger of thermal damage or a decrease in the electrical strength of the transformer insulation, but there is increased thermal wear of the transformer. Therefore, the magnitude and duration of long-term emergency transformer overloads in the post-emergency consumer mode should be limited, for example, by unloading the transformer by turning off part of the electrical receivers for the entire repair period or only during the hours of the transformer’s daily maximum load. Thus, in the general case, the mode of prolonged emergency transformer overloads occurs in the post-emergency mode of the consumer's power supply system after unloading the transformer by turning off part of the electrical receivers.

The mode of short-term emergency overloads occurs due to an increase in the load power of the transformer due to the operation of an ATS during an emergency shutdown of one of the transformers in the time interval from the operation of the ATS to the unloading of the transformer, provided that the temperature of the most heated part of the winding or the oil temperature exceeded the maximum permissible values. In real time, the mode of short-term emergency overloads is an intermediate mode between the normal and post-emergency modes of the consumer.

The choice of the number of transformers is discussed in subsection 7.2 of this manual, the choice of the type and design of transformers is discussed in subsection 7.3, the choice of transformer power and checking for permissible overloads is discussed in subsection 7.4.

The load capacity of a transformer is the property of a transformer to carry a load in excess of the rated load under certain operating conditions (preceding the load of the transformer, the temperature of the cooling medium) is called load capacity [12].

Mode of permissible systematic loads. Systematic overload mode is acceptable for an unlimited time if:

— insulation wear during the cycle does not exceed the nominal value (NRACH < 24 hours);

— temperature of the upper layers of oil QM <950С;

— temperature of the upper most heated point of the winding QННТ <1400С;

— the highest load current is not more than 1.5 INOM (SC.M<1.5).

Mode of permissible long-term emergency overloads. If the wear of the insulation per day is > 24 hours. then the overload mode is classified as emergency. The emergency overload mode is acceptable. If

– if its duration during the cycle does not exceed the calculated permissible (TPER<TDOP);

— temperature of the upper layers of oil QM <1150С;

— temperature of the upper hottest point of the winding QННТ <1600С;

— the highest load current is not more than 2.0 (INOM (SC.M<2.0).

The mode of permissible short-term emergency overloads is standardized by PTEEP.

overcurrent, % 30 45 60 75 100 overload duration, min. 10

overcurrent, % 20 30 40 50 60 overload duration, min. 60 45 32 18 5.

Current and temperature limits for loads exceeding the rated load according to GOST

When the load exceeds the rated load, GOST 14209 [11] recommends not to exceed the current and temperature limits given in the table.

Temperature and current limits for load conditions exceeding rated load

The maximum values ​​of load current, temperature of the hottest point of the windings and oil temperature in the upper layers given in Table 6.3 should not be exceeded. For distribution transformers with a power of no more than 2500 kVA for short-term emergency overload modes, the maximum values ​​of the oil temperature in the upper layers and the hottest point are not established, since in practice it is impossible to control the duration of emergency overload of distribution transformers. It should be borne in mind that at a temperature of the hottest point exceeding 140-160 °C, gas bubbles may be released, reducing the electrical strength of the transformer insulation.

13. Sequence and preliminary determination of the power of power transformers.

The selection of the optimal power of transformers should be made in accordance with the size and nature of the electrical loads. In this case, both economic requirements (in normal mode) and possible short-term and long-term overloads in operation must be taken into account.

There is a distinction between the choice of power of distribution transformers and GPP transformers.

The choice of power of distribution transformers should be made on the basis of technical and economic calculations. Preliminary power selection is made either by specific load density or by recommended load factors.

The power of transformers is determined by the specific load density for workshops with a known load density in kVA/(square meter). When the load density is less than 0.2 kVA/m2, it is advisable to use transformers with a power of 1000 kVA or less. For a load density of 0.2 kVA/m2 or more, it is more appropriate to use transformers with a capacity of 1600-2500 kVA.

The power of transformers is determined by load factor in the absence of data on the specific load density. Including, both for workshops and for field facilities. In this case, for transformers of distribution substations, the following load factors should, as a rule, be accepted:

- for objects with a predominant load of category I with two-transformer substations - 0.65-0.7;

- for workshops with a predominant load of category II with single-transformer substations with mutual redundancy of transformers - 0.7-0.8;

- for workshops with a predominant load of category II, with the possibility of using a centralized reserve of transformers, and for workshops with loads of category III - 0.9-0.95.

The preliminary selection of the power of GPP and PGV transformers should be made in accordance with the technological design standards for step-down substations with a higher voltage of 35-750 kV. In this case, when one transformer goes out of operation, the remaining transformer must ensure the operation of the enterprise while the retired transformer is being replaced, taking into account possible load limitations without compromising the main activity of the enterprise and using the permissible overload of the transformer.

The most advantageous transformer power corresponds to the minimum given costs, which take into account the capital costs of construction and installation of transformer substations (including the cost of transformers) and current costs associated with operation, including the cost of electricity losses in transformers.

On the other hand, it is known that minimal power losses in a transformer occur at a load factor

,

where RO

and
РК
- certified values ​​of power losses in steel (no-load losses) and in windings (load losses).
For step-down power transformers used in electrical networks of industrial enterprises, bE
​​are in the range of 0.36 - 0.59.
However, with such a low load on transformers, the installed capacity of the transformer increases, and therefore the share of capital costs for transformers increases. Therefore, the optimal transformer load factor b
, which takes into account not only power losses in the transformer, but also power losses in the supply network, capital costs for the construction and installation of transformer substations, is usually higher than
bE
.
At the design stage, it is recommended for GPP transformers b
= 0.65-7 [13].

In this case, the rated power of distribution substations transformers and GPP transformers is determined by the expression

, (6.1)

where ST

— the total calculated load power transmitted through
N
transformers in the fifth year of operation.

For GPP as ST

It is recommended to take
the calculated (maximum) power SP
(
SM
) of the half-hour maximum, and for all other transformers, including distribution substations, the average power
SСМ
for the busiest shift.

When the temperature of the cooling medium differs from the standard one (20°C), when choosing the rated power of the transformer, the temperature of the cooling medium must be taken into account. The temperature of the cooling medium affects the thermal conditions of the transformer, and, consequently, the permissible load factor. If the temperature of the cooling medium differs from the standard one, and the transformer load does not change significantly for some time, then when calculating the permissible load of the transformer in GOST 14209 [11], it is recommended to recalculate the permissible current (power) of the load. In this case, the value of the acceptable load factor b

in formula (6.1) can be multiplied by the temperature factor. The values ​​of this coefficient for continuous operation are given in Table 6.1 for different temperatures of the cooling medium [11].

Table 6.1

Permissible load factors for continuous operation at different coolant temperatures

(cooling type M. D. ONAN, ON, OF

and
O.D.
)

During operation, the transformer load may change and at certain times may exceed the rated load. To take into account possible short-term and long-term overloads in operation, it is advisable to select the rated power of transformers in the following sequence. First, the appropriate power of the transformer is preliminarily calculated according to (6.1) based on the recommended load factors in normal mode and two options with rated powers (from the standard range) closest to the calculated value. In general, it is advisable to take the nearest higher and the nearest lower rated power. However, if the calculated rated power value turns out to be close to one of the standard values, then it may be advisable to accept the standard rated power values ​​in both options, either up or down.

Selected transformers are checked:

- on the permissible systematic load in the normal mode of the power supply circuit;

- for permissible short-term emergency overload (when the automatic transfer switch is triggered) without taking into account the unloading of the transformer;

- for permissible long-term emergency overload in post-emergency mode of the power supply circuit, taking into account the unloading of the transformer by operating personnel by disconnecting some of the non-critical electrical receivers (consumers), if such a disconnection is possible and acceptable for a given consumer.

Permissible systematic and emergency overloads for oil transformers with a power of up to 100 MVA inclusive are established in GOST 14209-97 [11], and for dry transformers and transformers with non-flammable liquid dielectric - in standards or technical conditions for specific groups or types of transformers.

Mode of permissible systematic loads. Systematic overload mode is acceptable for an unlimited time if:

— insulation wear during the cycle does not exceed the nominal value (NRACH < 24 hours);

— temperature of the upper layers of oil QM <950С;

— temperature of the upper most heated point of the winding QННТ <1400С;

— the highest load current is not more than 1.5 INOM (SC.M<1.5).

Mode of permissible long-term emergency overloads. If the wear of the insulation per day is > 24 hours. then the overload mode is classified as emergency. The emergency overload mode is acceptable. If

– if its duration during the cycle does not exceed the calculated permissible (TPER<TDOP);

— temperature of the upper layers of oil QM <1150С;

— temperature of the upper hottest point of the winding QННТ <1600С;

— the highest load current is not more than 2.0 (INOM (SC.M<2.0).

The mode of permissible short-term emergency overloads is standardized by PTEEP.

overcurrent, % 30 45 60 75 100 overload duration, min. 10

overcurrent, % 20 30 40 50 60 overload duration, min. 60 45 32 18 5.

If the transformers in both options pass all tests, then technical and economic calculations are carried out and the final decision is made based on a comparison of the options at the given costs.

14. Conversion of real daily load graphs into equivalent two-stage rectangular graphs

To determine the admissibility of a particular operating mode of a transformer, it is necessary to construct heating curves and evaluate its thermal regime during the cycle. In engineering practice and design, thermal heating curves are not plotted. The thermal conditions and wear of the transformer insulation are determined indirectly by the coefficients of the equivalent load curve.

At the same time, to check transformers for overload, it is necessary to convert the real daily load schedule into an equivalent two-stage one. The form of a two-stage load graph is shown in Figure 6.1. The real daily load schedule and the two-stage one should be equivalent in terms of power losses in the transformer. In this case, they will be equivalent in terms of the heating temperature of the transformer, and therefore in terms of the wear rate of the insulation. In Figure 6.1 K

1 and
K
2 are two load stages, with
K
1 being the initial or preliminary load
, K
2
being
the maximum load.
The maximum load duration is t
(or h) hours.

Figure 6.1 - Equivalent two-stage load diagram

Both exact and approximate transformation methods are used.

Refined method. The transformation of the original daily transformer load graph into a loss-equivalent two-stage rectangular graph representing the load as a fraction of the rated current (or rated power) is performed in the following sequence.

First, the load graph is presented in relative units: in fractions of the rated current (or rated power)

On the resulting graph of the transformer load, draw a line of rated current l

n, (or rated power).
When plotted in relative units, this line corresponds to K
= 1 (Figure 6.2).

At points A

and
B
intersection of the nominal line with the curve of the original load graph, an overload section of duration h
'
.

The remainder of the original schedule with a smaller load is divided into t

intervals of duration D
t
(usually D
t
= 30 min.) and determine the initial load
K1
of the equivalent schedule

(7.2)

where S1, S2, Sm —

average load values ​​at the corresponding intervals Dt.

Overload section h'

on the original load graph, divide it into
p
intervals Dh and determine the preliminary excess overload of the equivalent load graph

(7.3)

Where , , —

average load values ​​at the corresponding intervals Dh.

Compare the value with the maximum load factor Kmax

original graph: if , you should take
K
2 = ;
if, however, one should take K
2
=
0.9
Kmax
, and the duration
h
of the overload of the equivalent load schedule should be calculated using the formula

(7.4)

If the initial daily transformer load curve contains two similar maximums of different durations, the values ​​of h

and
K
2 are determined by the maximum of the longer duration, and the value of
K
1
is
determined as the root mean square value of the remaining load.

If the original daily transformer load schedule contains several successive close maxima, the values ​​of K

2 and
h
are determined from the coverage of all maxima, and the value of K1 is determined as the rms value of the remaining load.

1 —

original load schedule,

2 —

equivalent rectangular load diagram

Figure 7.2 - Conversion of the original transformer load curve into an equivalent two-stage rectangular

Simplified methods for converting a real load schedule into an equivalent two-stage one

In Simplified methods for converting a real load curve into an equivalent two-stage coefficient K2 of the maximum load is taken equal to the maximum load factor of the real schedule, and the duration of the overload and the initial load are determined by a graphical-analytical method.

Load diagram with one maximum. In this case, the value of t

should be determined as shown in Figure 6.2. A horizontal straight line K2 is drawn, corresponding to the maximum load. The straight line K1 is drawn (selected) and the time interval t is selected so that between the real schedule and the equivalent two-stage one, the conditions for equality of areas are met: 1=2+3+4 and a+b=c+d.

Fulfillment of the first of these conditions means that the value of K1

is defined as the average load value for the section of the load graph without a maximum. The fulfillment of the second of these conditions means that the area of ​​the rectangle with sides K1, t is equal to the area under the maximum of the real load graph.

Figure 6.2 - Load graph with one maximum