CN214250510U - System for heating transformer or reactor - Google Patents
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- CN214250510U CN214250510U CN202022838248.9U CN202022838248U CN214250510U CN 214250510 U CN214250510 U CN 214250510U CN 202022838248 U CN202022838248 U CN 202022838248U CN 214250510 U CN214250510 U CN 214250510U
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Abstract
Embodiments of the present disclosure provide systems for heating a transformer or reactor. The system comprises: a variable frequency power supply coupled to a winding of the transformer or reactor, the variable frequency power supply configured to provide ac power to the winding at a frequency below a power frequency; a detection apparatus comprising a voltage sensing device and a current sensing device and coupled to a line between the variable frequency power supply and the winding; a power analyzer coupled to the detection device, the power analyzer configured to obtain a current active power on the winding based on the measurement result; a resistance calculation device coupled to the power analyzer and configured to calculate a present resistance of the winding; and a temperature estimation device coupled to the resistance calculation device and configured to estimate a present temperature of the winding based on the calculated present resistance. The scheme of the disclosure can perform efficient heating and drying on the transformer or the reactor by using a low-capacity power supply, and accurately estimate the temperature of the winding without a temperature sensor so as to avoid insulation damage.
Description
Technical Field
The present disclosure relates to the field of power distribution technology, and more particularly, to a system for heating a transformer or reactor.
Background
Power transformers and reactors are common and important electrical devices in power or distribution systems. For various reasons, power transformers and reactors may have insulation materials that are wet or have too high a water content during production and use. This situation may cause the insulation level of the transformer and the reactor to be lowered, thereby seriously affecting the safe operation of the transformer and the reactor.
In order to ensure normal use of the transformer and the reactor, moisture in the insulating materials of the transformer and the reactor is generally removed before the transformer and the reactor are put into use or in the case where the transformer and the reactor are out of service for a long time. Currently, there are various ways to remove moisture in the insulation material of transformers and reactors. For example, a transformer or a reactor may be placed in a drying chamber and subjected to heating and ventilation treatment, thereby effectively removing moisture in the transformer or the reactor; a coil may be wound around the outside of the oil tank of the oil-filled transformer, so that when current flows through the coil, drying is performed using eddy current heating of the tank case.
However, the existing drying methods have many problems. For example, as the voltage class of the power system is continuously increased, the size of the transformer and the reactor is increased, and the conventional drying method may not be suitable for the large transformer and the reactor, and it is difficult to achieve the desired drying effect. In addition, during the heating and drying process, if the temperature is too high, the insulation material of the transformer or the reactor may be damaged, so that the service life of the insulation material is affected, and even the insulation of the winding fails.
SUMMERY OF THE UTILITY MODEL
The present disclosure provides a system for heating a transformer or a reactor, capable of controlling the temperature of a winding at a level that avoids damage to insulation while ensuring a heat drying effect.
According to an aspect of the present disclosure, there is provided a system for heating a transformer or a reactor, the system comprising: a variable frequency power supply coupled to a winding of the transformer or reactor, the variable frequency power supply configured to provide ac power to the winding at a frequency below a power frequency; a detection apparatus comprising a voltage sensing device and a current sensing device and coupled to a line between the variable frequency power supply and the winding; a power analyzer coupled to the detection device, the power analyzer configured to receive measurements from the detection device associated with the voltage and current output by the variable frequency power supply to the winding, and to obtain a present active power on the winding based on the measurements; a resistance calculation device coupled to the power analyzer and configured to calculate a current resistance of the winding based on the obtained current active power; and a temperature estimation device coupled to the resistance calculation device and configured to estimate a present temperature of the winding based on the calculated present resistance.
In some embodiments of the present disclosure, the temperature estimation apparatus includes a display device and is configured to present the estimated current temperature on the display device.
In certain embodiments of the present disclosure, the variable frequency power supply includes a ac-dc-ac frequency converter.
In certain embodiments of the present disclosure, the variable frequency power supply is coupled on one side to the mains power supply network and on the other side to a primary side winding of a transformer or a winding of a reactor, the secondary side winding of the transformer being short-circuited.
In certain embodiments of the present disclosure, the temperature estimation device is further coupled to the variable frequency power supply, the temperature estimation device being configured to vary the ac power output by the variable frequency power supply to the winding based on the estimated current temperature.
In certain embodiments of the present disclosure, the temperature estimation device is configured to send a control signal to the variable frequency power supply to reduce the ac power output by the variable frequency power supply to the winding when the estimated current temperature exceeds the threshold temperature.
In certain embodiments of the present disclosure, the temperature estimation device is configured to estimate the present temperature of the winding based on the calculated present resistance, the measured resistance of the winding at the ambient temperature, and the resistance temperature constant of the winding.
In certain embodiments of the present disclosure, the power analyzer is configured to determine an apparent power and a power factor of the ac power provided to the winding based on the measurement results to obtain the current active power.
In certain embodiments of the present disclosure, the variable frequency power supply provides ac power to the winding at a frequency less than or equal to one hundredth of the frequency of the power frequency.
In certain embodiments of the present disclosure, the variable frequency power supply provides ac power to the winding at a frequency less than or equal to one thousandth of the frequency of the power frequency.
The summary is provided to introduce a selection of concepts in a simplified form that are further described below in the detailed description. The summary is not intended to identify key features or essential features of the disclosure, nor is it intended to limit the scope of the disclosure.
Drawings
The foregoing and other objects, features and advantages of the disclosure will be apparent from the following more particular descriptions of exemplary embodiments of the disclosure as illustrated in the accompanying drawings wherein like reference numbers generally represent like parts throughout the exemplary embodiments of the disclosure.
Fig. 1 shows a system 100 for heating a transformer according to an embodiment of the present disclosure.
Fig. 2 shows a system 200 for heating a reactor according to an embodiment of the disclosure.
Detailed Description
Embodiments of the present disclosure will be described in more detail below with reference to the accompanying drawings. While embodiments of the present disclosure are shown in the drawings, it should be understood that the present disclosure may be embodied in various forms and should not be limited by the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the disclosure to those skilled in the art. Alternative embodiments will become apparent to those skilled in the art from the following description without departing from the spirit and scope of the disclosure.
The term "include" and variations thereof as used herein is meant to be inclusive in an open-ended manner, i.e., "including but not limited to". Unless specifically stated otherwise, the term "or" means "and/or". The term "based on" means "based at least in part on". The terms "one example embodiment" and "one embodiment" mean "at least one example embodiment". Other explicit and implicit definitions are also possible below.
According to an embodiment of the present disclosure, a solution for heating a transformer and a reactor is provided. In this scheme, a variable frequency power supply is used to supply low frequency ac power to a transformer and a reactor, so that the insulation materials of the transformer and the reactor are dried by generating heat in the winding by the low frequency current. Meanwhile, based on the corresponding relation between the resistance and the temperature, the scheme monitors the resistance change of the winding in real time, and obtains the real-time temperature of the winding based on the monitored resistance, so that the variable frequency power supply can be controlled to adjust the temperature of the winding in time. Due to the adoption of the low-frequency heating mode, the capacity of the variable-frequency power supply for heating is effectively reduced, more importantly, the temperature change of the heated winding can be monitored in real time under the condition that a temperature sensor is not needed, and therefore the heating and drying effect can be ensured, and meanwhile, the temperature of the winding is controlled at the level of avoiding damage to the insulation.
Fig. 1 and 2 show systems 100, 200 for heating transformers and reactors according to embodiments of the disclosure. The systems 100, 200 are shown as three-phase systems, but it should be understood that the systems 100, 200 in fig. 1 and 2 are merely exemplary and may be any number of phases depending on the number of phases of the transformer and reactor being heated. For example, the systems 100, 200 may also be single phase systems.
The transformer 160 as a heating target may include a primary side winding 161 and a secondary side winding 162. As an example, the primary side winding 161 and the secondary side winding 162 are represented in a simplified equivalent circuit form. The primary side winding 161 may include an a-phase winding equivalent resistance Rta and an a-phase winding equivalent inductance Lta connected in series, a B-phase winding equivalent resistance Rtb and a B-phase winding equivalent inductance Ltb connected in series, and a C-phase winding equivalent resistance Rtc and a C-phase winding equivalent inductance Ltc connected in series. Similarly, the secondary side winding 161 may include an a-phase winding equivalent resistance Rta 'and an a-phase winding equivalent inductance Lta' connected in series, a B-phase winding equivalent resistance Rtb 'and a B-phase winding equivalent inductance Ltb' connected in series, and a C-phase winding equivalent resistance Rtc 'and a C-phase winding equivalent inductance Ltc' connected in series. It is understood that the transformer 160 may be any type of power transformer, such as a single-phase transformer, a three-phase transformer, a dry-type transformer, an oil-filled transformer, an autotransformer, and the like.
The reactor 270 as a heating target may include a winding 271. As an example, the winding 271 is represented in the form of a simplified equivalent circuit. The winding 271 may include an a-phase winding equivalent resistance Rra and an a-phase winding equivalent inductance Lra connected in series, a B-phase winding equivalent resistance Rrb and a B-phase winding equivalent inductance Lrb connected in series, and a C-phase winding equivalent resistance Rrc and a C-phase winding equivalent inductance Lrc connected in series. It is understood that the reactor 270 may be any type of reactor, such as a single-phase reactor, a three-phase reactor, a dry reactor, an oil-immersed reactor, a filter reactor, a current-limiting reactor, and so forth.
According to embodiments of the present disclosure, the system 100, 200 may include a variable frequency power supply 110, the variable frequency power supply 110 coupled to windings 161 and 162 of a transformer 160 or a winding 271 of a reactor 270, the variable frequency power supply 110 configured to provide ac power to the windings at a frequency below a power frequency.
Specifically, there are inductances such as Lta, Lta ', Ltb ', Ltc, and Ltc ' and Lra, Lrb, and Lrc in the windings of the transformer 160 and the reactor 270. Assuming that the total equivalent inductance of the primary side winding and the secondary side winding of each phase is L, the total equivalent resistance is R, and the power supply frequency is f, the transformer winding inductance is X2 pi fL, and the total impedance is
From this equation it can be seen that when using a 50Hz frequency power supply, the inductive reactance X can reach several kilo ohms for a power transformer of a power distribution system, whereby the impedance Z at the input port on the primary side of the transformer will be high. At the same time, current is applied to the transformer windingWhen heating, the heating power is P ═ I2R, where the heating current I needs to reach the rated current of the winding, which may be several hundred amperes in size. Thus, in the case of using a 50Hz frequency power supply, the voltage (U ═ I × Z) applied to the primary side of the transformer may reach the order of tens of kilovolts to hundreds of kilovolts. For a heat drying transformer or reactor, such voltage levels seriously affect personnel safety, and would also require a power supply with a very large output capacity and high cost.
To overcome the above-mentioned drawbacks, the variable frequency power supply 110 of the present disclosure can supply low frequency current lower than the power frequency from a heating transformer and a reactor. The reduction in frequency makes it possible to reduce the inductive reactance X (═ 2 pi fL) of the winding of the transformer or reactor, whereby the voltage and capacity of the power supply will be correspondingly reduced, which increases the safety of the heating process and reduces the cost of the heating system.
In certain embodiments of the present disclosure, the frequency of the ac power provided by the variable frequency power supply 110 to the windings 161 and 162 of the transformer 160 and the winding 271 of the reactor 270 may be less than or equal to one hundredth of the frequency of the power frequency. Further, in certain embodiments of the present disclosure, the frequency of the ac power provided by the variable frequency power supply 110 to the windings 161 and 162 of the transformer 160 and the winding 271 of the reactor 270 may be less than or equal to one-thousandth of the frequency of the power frequency. As the power frequency decreases to less than one percent, even less than one thousandth of the power frequency, the inductive reactance of the winding will also decrease to less than one percent, even less than one thousandth of the power frequency, as can be seen from the inductive reactance X ═ 2 pi fL. Thereby, the voltage and capacity of the power supply will be significantly reduced, thus improving safety and reducing costs. In addition, the volume and weight of the power supply can be greatly reduced, thereby possibly meeting the requirements of field implementation.
In certain embodiments of the present disclosure, the variable frequency power supply 110 may include a ac-dc-ac frequency converter. The AC-DC-AC frequency converter has the advantage of high power factor, and can effectively inhibit harmonic distortion at the power grid side. However, it is understood that the variable frequency power supply 110 may include other types of frequency converters.
In certain embodiments of the present disclosure, the variable frequency power supply 110 is coupled on one side to the mains power supply network and on the other side to the primary side winding 161 of the transformer 160 or the winding 271 of the reactor 270, the secondary side winding 162 of the transformer 160 being short-circuited.
Specifically, when the primary side winding 161 of the transformer 160 is coupled to the variable frequency power source 110, both ends of each phase winding of the secondary side winding 162 of the transformer 160 may be short-circuited or both grounded. With this connection, when a current flows through the primary side winding 161 of the transformer 160, an induced current will also be generated in the secondary side winding 162, thereby enabling simultaneous heating of the primary and secondary side windings of the transformer. The variable frequency power supply 110 provides power from the power grid to the transformer 160 or the reactor 270, and as current flows through the winding resistances Rta, Rta ', Rtb ', Rtc, and Rtc ' of the transformer 160 and the winding resistances Rra, Rrb, and Rrc of the reactor 270, electrical energy is converted to heat energy and heats the windings to remove moisture from the winding insulation.
According to embodiments of the present disclosure, the system 100, 200 may include a detection arrangement including a voltage sensing device 121 and a current sensing device 122, and coupled to lines between the variable frequency power supply 110 and the windings 161 and 162 or the winding 271.
As an example, the detection means may comprise a voltage sensing device 121 and a current sensing device 122 on each phase line. The voltage sensing device 121 and the current sensing device 122 can detect the voltage and current applied to the transformer winding or the reactor winding in real time. For example, the voltage sensing device 121 may be a voltage divider and the current sensing device 122 may be a current sensor. It should be understood that the detection means may also be any type of sensing device that detects voltage and current in real time.
During the process of the power supply 110 supplying power and heating the transformer 160 and the reactor 270, the temperature of the transformer winding or the reactor winding will rise with the accumulation of heat, so that the moisture in the insulating material can be removed. However, if the temperature of the winding is too high, the insulation properties of the insulation material may be adversely affected, and in severe cases, the insulation of the winding may even fail. In order to prevent the temperature from being too high, the scheme of the disclosure can realize the monitoring of the temperature of the winding by acquiring and analyzing electrical quantities such as voltage and current, so that the insulation can be prevented from being damaged due to the temperature from being too high.
According to an embodiment of the present disclosure, the system 100, 200 may comprise a power analyzer 130, the power analyzer 130 being coupled to the detection device, the power analyzer 130 being configured to receive from the detection device measurements associated with the voltage and current output by the variable frequency power supply 110 to the transformer winding or the reactor winding, and to obtain the present active power on the winding based on the measurements.
As an example, the power analyzer 130 may obtain voltage measurements from the voltage sensing device 121 and current measurements from the current sensing device 122 in real time. The power analyzer 130 may analyze and calculate the measured voltage and current in real time to obtain the current active power on the winding.
In certain embodiments of the present disclosure, the power analyzer 130 is configured to determine the apparent power and power factor of the ac power provided to the transformer winding or the reactor winding based on the measurement results to obtain the current active power. By way of example, the power analyzer 130 may determine the power factor through analysis and calculation of the voltage and current. Furthermore, the apparent power can be directly obtained by the present magnitude of the voltage and current. Thus, with the aid of the voltage and current measurements, the power analyzer 130 can determine the real active power of the windings of the transformer or reactor.
According to an embodiment of the present disclosure, the system 100, 200 may comprise a resistance calculation device 140, the resistance calculation device 140 being coupled to the power analyzer 130 and configured to calculate a current resistance of the transformer windings 161 and 162 or the reactor winding 271 based on the obtained current active power.
As an example, the resistance calculation device 140 may calculate the magnitude of the resistance R according to the relationship between the winding resistance R and the active power P. The relationship between the winding resistance R and the active power P can be expressed as the equation R ═ P/I2Where I is the current on the winding, the magnitude of which may be derived fromA measurement of the current at the detection device. Therefore, the current resistance of the windings 161 and 162 or the winding 271 can be obtained by a simple calculation. It is noted that the present resistance of the present disclosure is a resistance that is determined in real time at some point in the heating process or for some short period of time. From the properties of the conductive material, the winding resistance changes with temperature and has a substantially unique correspondence with temperature, so that the change in winding resistance may actually reflect a change in temperature across windings 161 and 162 or winding 271.
According to an embodiment of the present disclosure, the system 100, 200 may further comprise a temperature estimation device 150, the temperature estimation device 150 being coupled to the resistance calculation device 140 and configured to estimate a current temperature of the transformer winding or the reactor winding based on the calculated current resistance.
In particular, the winding resistance has a substantially unique correspondence with temperature, and therefore the temperature of the winding can be estimated from the current resistance using this characteristic. After obtaining the current resistance, the temperature of the winding may be estimated by calculation, or the correspondence between the winding resistance and the temperature may be stored in a memory in advance and the temperature of the winding may be estimated in a table look-up manner. For estimation and control of winding temperature in a process of heating a transformer and a reactor, it is very advantageous to judge a change in temperature based on a change in resistance, which can avoid installing temperature sensing devices such as temperature sensors in the transformer or the reactor, which have disadvantages of complicated installation, high cost, and low reliability.
In certain embodiments of the present disclosure, the temperature estimation device 150 is configured to estimate the current temperature of the winding based on the calculated current resistance, the measured resistance of the transformer windings 161 and 162 or the reactor winding 271 at the ambient temperature, and the resistance temperature constant of the winding.
In particular, the winding resistance can be measured directly by a resistance measuring instrument in the ambient temperature T1 before the winding heating is carried out, whereby the winding resistance R1 in the ambient temperature T1 can be obtained. Further, assuming that the current resistance calculated by the resistance calculation device 140 is R2 and the winding material is copper (the resistance temperature constant of copper is about 235), the current temperature T2 can be obtained by the equation T2 ═ R2 ═ 235+ T1)/R1-235, where the resistance temperature constant in the equation may be different according to the winding material. It can be seen that the present disclosure requires only conventional voltage and current sensing devices to assess the temperature of the windings in real time and accurately, without any need for any temperature sensors, which is very advantageous for large transformer and reactor heating processes.
In some embodiments of the present disclosure, the temperature estimation apparatus 150 may include a display device and be configured to present the estimated current temperature on the display device. In particular, the display device may present the estimated real-time temperature to the operator so that the operator may operate appropriately to reduce the temperature of the windings if the winding temperature is too high.
In certain embodiments of the present disclosure, the temperature estimation device 150 may be further coupled to the variable frequency power supply 110, the temperature estimation device 150 being configured to vary the ac power output by the variable frequency power supply 110 to the transformer windings 161 and 162 or the reactor winding 271 based on the estimated current temperature. Specifically, the temperature estimation device 150 may send a control signal to the variable frequency power supply 110 to increase or decrease the ac power output by the variable frequency power supply 110 according to the current temperature. Compared with manual operation, the temperature control of the heating process can be made more efficient by the temperature estimation device 150 judging and automatically controlling the variable frequency power supply 110, thereby improving the heating and drying effect and avoiding the damage of the heating to the winding insulation.
In certain embodiments of the present disclosure, the temperature estimation device 150 may be configured to send a control signal to the variable frequency power supply 110 to reduce the ac power output by the variable frequency power supply 110 to the transformer windings 161 and 162 or the reactor winding 271 when the estimated current temperature exceeds the threshold temperature. Specifically, when the estimated current temperature exceeds the threshold temperature, the temperature estimation device 150 may automatically control the variable frequency power supply to reduce the output power in time to reduce the temperature of the heated winding, thereby more effectively avoiding damage to the winding insulation.
Further, it is understood that instead of converting the present resistance to the present temperature and comparing it to the threshold temperature, the present resistance may be directly compared to the threshold resistance. As previously described, the present resistance has a substantially unique correspondence with temperature, and thus this characteristic can be used to compare the present resistance to a preset threshold resistance (which corresponds to a threshold temperature) to determine whether the temperature of the winding is too high. In the direct resistance comparison method, the threshold resistance may be calculated in advance based on the threshold temperature (the threshold resistance Rth may be obtained by the equation Rth ═ R1 × (235+ Tth)/(235+ T1), where Tth is the threshold temperature, T1 is the ambient temperature T1, and R1 is the resistance value of the winding resistance measured at the ambient temperature), and the calculation of the threshold resistance may be completed once before the heating process starts, without performing the switching calculation between the resistance and the temperature before each comparison, thereby reducing the calculation amount and the delay, and obtaining better heating and temperature control effects.
Through the embodiment of the disclosure, the transformer and the reactor can be sufficiently heated and dried by using a power supply with lower capacity, and the winding temperature of the transformer or the reactor is controlled within a safe range through efficient and accurate temperature evaluation and control under the condition of no need of a temperature sensor, so that the damage of overhigh temperature to the winding insulation is effectively avoided.
Many modifications and other embodiments of the disclosure set forth herein will come to mind to one skilled in the art to which this disclosure pertains having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. Therefore, it is to be understood that the embodiments of the disclosure are not to be limited to the specific embodiments disclosed and that modifications and other embodiments are intended to be included within the scope of the disclosure. Moreover, while the above description and the related figures describe example embodiments in the context of certain example combinations of components and/or functions, it should be appreciated that different combinations of components and/or functions may be provided by alternative embodiments without departing from the scope of the present disclosure. In this regard, for example, other combinations of components and/or functions than those explicitly described above are also contemplated as within the scope of the present disclosure. Although specific terms are employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation.
Claims (10)
1. A system for heating a transformer or a reactor, characterized in that the system comprises:
a variable frequency power supply (110) coupled to a winding (161, 162; 271) of the transformer (160) or the reactor (270), the variable frequency power supply (110) configured to provide alternating current power at a frequency lower than a power frequency to the winding (161, 162; 271);
a detection arrangement (121, 122) comprising a voltage sensing device (121) and a current sensing device (122) and coupled to a line between the variable frequency power supply (110) and the winding (161, 162; 271);
a power analyzer (130) coupled to the detection device (121, 122), the power analyzer (130) being configured to receive measurements from the detection device (121, 122) associated with the voltage and current output by the variable frequency power supply (110) to the winding (161, 162; 271) and to obtain a present active power on the winding (161, 162; 271) based on the measurements;
a resistance calculation device (140) coupled to the power analyzer (130) and configured to calculate a present resistance of the winding (161, 162; 271) based on the obtained present active power; and
a temperature estimation device (150) coupled to the resistance calculation device (140) and configured to estimate a present temperature of the winding (161, 162; 271) based on the calculated present resistance.
2. The system of claim 1, wherein the temperature estimation apparatus (150) comprises a display device and is configured to present the estimated current temperature on the display device.
3. The system of claim 1, wherein the variable frequency power supply (110) comprises a ac-dc-ac frequency converter.
4. The system according to claim 1, characterized in that the variable frequency power supply (110) is coupled on one side to a mains supply network and on the other side to a primary side winding (161) of the transformer (160) or a winding of the reactor (270), the secondary side winding (162) of the transformer (160) being short-circuited.
5. The system of claim 1, wherein the temperature estimation device (150) is further coupled to the variable frequency power supply (110), the temperature estimation device (150) being configured to vary the alternating current power output by the variable frequency power supply (110) to the windings (161, 162; 271) based on the estimated current temperature.
6. The system of claim 5, wherein the temperature estimation device (150) is configured to send a control signal to the variable frequency power supply (110) to reduce the AC power output by the variable frequency power supply (110) to the windings (161, 162; 271) when the estimated current temperature exceeds a threshold temperature.
7. The system according to claim 1, characterized in that the temperature estimation device (150) is configured to estimate the present temperature of the winding (161, 162; 271) based on the calculated present resistance, the measured resistance of the winding (161, 162; 271) at ambient temperature, and a resistance temperature constant of the winding (161, 162; 271).
8. The system according to claim 1, characterized in that the power analyzer (130) is configured to determine an apparent power and a power factor of the alternating current power provided to the winding (161, 162; 271) based on the measurement results to obtain the current active power.
9. The system of claim 1, wherein the variable frequency power supply (110) provides AC power to the windings (161, 162; 271) at a frequency less than or equal to one percent of the power frequency.
10. The system according to claim 7, characterized in that the frequency of the alternating current power supplied by the variable frequency power supply (110) to the windings (161, 162; 271) is lower than or equal to one thousandth of the mains frequency.
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| CN202022838248.9U CN214250510U (en) | 2020-11-30 | 2020-11-30 | System for heating transformer or reactor |
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| CN202022838248.9U CN214250510U (en) | 2020-11-30 | 2020-11-30 | System for heating transformer or reactor |
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