Summary of Calculation Formulas for Heating of Electrical Equipment
Mar 17, 2026| I. Heating Capacity of High and Low Voltage Switchgear / Panels
The heat dissipation of high-voltage switchgear panels can be calculated using the following formula:
Q=(Ig/Ie)2qe(kW)
Ig: Operating current of the high-voltage switch (A)
Ie: Rated current of the high-voltage switch (A)
qe: Heat dissipation of the high-voltage switch at rated current
High-voltage switchgear is divided into incoming switchgear and feeder switchgear. Generally, the heat generation of incoming switchgear is greater than that of feeder switchgear
The heat dissipation of low-voltage switchgear panels can be calculated using the following formula:
Q=e×∑P(kW)
e: Utilization factor of the panel
x: Actual loss factor of the panel
∑P: Sum of power losses of all electrical components in the low-voltage panel (kW)
● Due to different purposes and operating currents of various panels in the power station, generally, the larger the operating current, the greater the heat generation of electrical components in the panel. For centrally arranged distribution panels, it is recommended to obtain more accurate heat generation data from the equipment manufacturer.
● Specifically, for important distribution panels, electric heaters are installed inside to protect electrical components, prevent excessive humidity and reduce insulation performance. The power of each panel is generally about 0.3 ~ 0.5 kW, which should be considered in centrally arranged relay protection rooms.
II. Transformer Heat Generation
The heat dissipation of transformers mainly refers to the energy loss inside the transformer, which consists of two parts: Copper Loss (resistive loss) and Iron Loss (magnetic loss). Copper loss varies with the load, while iron loss is independent of the load and can be regarded as a constant. Typically, the copper loss at rated load is defined as the short-circuit loss, and the iron loss at rated voltage is defined as the no-load loss.
The losses of self-cooled, air-cooled, and dry-type transformers are all dissipated into the surrounding air. In contrast, for water-cooled transformers, most of the losses are carried away by the water cooling system, while a small portion is dissipated into the air due to the oil temperature being higher than the ambient air temperature.
Generally, in enclosed factories, underground power stations, and pumped storage power stations, the main transformers arranged in the factory buildings or underground mostly adopt water cooling. However, other transformers in the power station, such as station service transformers, lighting transformers, emergency transformers, and excitation transformers, mostly adopt air-cooled or dry-type transformers.
The heat dissipation of air-cooled transformers can be simply calculated by the following formula:
Q=Pk+Pd(kW)
Pk - No-load loss of the transformer (kW)
The heat dissipation of water-cooled transformers can be calculated by the following formula:
Q=5.5×(ty−tn)1.25A×10−3(kW)
Where:ty - Average oil temperature of the transformer tank (generally between 65-70°C)
tn - Ambient indoor temperature (°C)
III. Heat Generation of Busbars and Cables
In power stations, the connection between generators and transformers mostly uses self-cooled enclosed busbars. The heat generation of busbars includes two parts: heat generation from busbar power loss and induced heat dissipation from the enclosure.
Since both ends of the main busbar are connected to generator and transformer equipment respectively, the air between the busbar and the enclosure is actually enclosed. The enclosure acts as protection and electromagnetic shielding to reduce the impact of the busbar's electromagnetic field on surrounding electrical equipment and the environment, without reducing the heat dissipation of the busbar. The heat from busbar power loss is transferred to the air between the busbar and the enclosure, then to the environment through the enclosure shell. The induced heat dissipation from the enclosure is directly transferred to the environment.
The heat dissipation caused by busbar power loss can be calculated by the following formula:
qs=3×I2RΣφsL×10−3(kW)
I: Operating current of the busbar (A)
RΣ: Equivalent resistance per unit length of the busbar (Ω/m)
φs: Proportional coefficient of power loss dissipated to the environment
L: Length of the busbar (m)
The induced heat dissipation of the busbar enclosure can be calculated using the following formula:
qk=3×I2RkφkL×10−3(kW)
I: Phase current of the busbar (A)
RZ: DC resistance of the busbar at operating temperature (Ω/m)
Rk: DC resistance of the busbar enclosure at operating temperature (Ω/m)
φs: Skin effect coefficient of the busbar
φk: Skin effect coefficient of the busbar enclosure
L: Length of the busbar (m)
IV. Heat Generation of Reactors
Reactors are used in large-capacity power distribution devices to limit short-circuit currents, and can also be used as filter reactors in rectification devices.
The heat dissipation of a reactor can be calculated using the following formula:
Q=η1η2P(kW)>Where:
η1: Utilization factor of the reactor, generally taken as 0.95
η2: Load factor of the reactor, generally taken as 0.75
P: Power loss of the reactor at rated power (kW), determined by rated current, rated reactance and model
Reactors are composed of windings, with large heat capacity and heat generation, and it takes a period of time to reach stable heat generation. For reactors operating continuously, the heat generation is stable; for intermittently operating reactors, the heat generation should be determined according to the operating time and the heat generation characteristic curve of the reactor.
V. Heat Generation of Generator Sets
The heat dissipation of generator sets mainly comes from two aspects: one is the heat transfer through the cover plate and casing enclosure structure, and the other is the heat brought by the leakage of the cooling circulating air of the generator set.
Large and medium-sized generator sets usually adopt closed air self-circulation cooling mode: the loss of the generator winding is transferred to the cooling air, and then the heat of the air is taken away by the cooling water through the set's water cooler. According to measured data, the temperature of the air discharged from the stator generally does not exceed 65℃, while the temperature of the air entering the rotor is generally not lower than 5℃.
The heat dissipation of the generator casing can be calculated by the following formula:
qk=KA(tg−tn)(W)
K: Heat transfer coefficient of the generator casing (W/(m²·℃))
A: Surface area of the generator casing (m²)
tg: Average temperature of the generator's cooling circulating air (℃)
tn: Indoor ambient temperature (℃)
Heat Dissipation from Generator Air Leakage
The heat dissipation caused by generator air leakage can be calculated by the following formula:
qf=βvcγ(tf−tn)
β: Leakage coefficient (0.3% for steel cover plates)
v: Cooling air circulation volume (m³/h)
c: Specific heat capacity of air (W/(kg·℃))
γ: Air density (1.2 kg/m³)
tf: Leakage air temperature (℃)
tn: Indoor ambient temperature (℃)
Key Note: The calculation of air leakage heat loss largely depends on the cooling air volume (v). Due to differences in design standards between domestic and international manufacturers, the specified air volume can vary significantly (e.g., 200 m³/h vs. 120 m³/h for a 300MW unit). For accurate results, it is recommended to obtain the official cooling air volume parameters from the generator manufacturer rather than relying solely on manual calculations.
VI. Heat Generation of SFC Static Frequency Converter Starting Device
SFC (Static Frequency Converter) is a static frequency conversion starting device, mainly used for starting pumped-storage power station units under pumping conditions. It consists of input reactors, output reactors, filters, power cabinets, and DC reactors.
For a pumped-storage power station with a single unit capacity of 300 MW, the capacities of each component in the SFC device provided by a foreign manufacturer are as follows:
SFC Device Capacity
| No. | Equipment Name | Running (kW) | Standby (kW) |
|---|---|---|---|
| 1 | Input Reactor | 27 | 3 |
| 2 | Output Reactor | 63 | 0 |
| 3 | Filter | 83 | 28 |
| 4 | Power Cabinet | 15 | 6 |
| 5 | DC Reactor | 200 | 0 |
| 6 | Total | 388 | 37 |
As we can see, the heat generation of the SFC device reaches 388 kW when calculated at full load. According to the actual operation analysis and statistics of some operating pumped-storage power stations, the startup of one unit (from static dragging to grid connection) takes only 240 seconds, and the startup time for six units is about 25 minutes.
Based on the SFC device operating characteristic curve provided by the foreign manufacturer:
Input reactors, output reactors, and DC reactors reach 20% of their rated heat generation after 25 minutes of operation.
Filters and power cabinets reach approximately 70% of their rated heat generation.
According to this calculation, the heat generation of the SFC device is about 126.6 kW, which is 32.6% of the rated heat generation.
The heat generation of the SFC device is closely related to its capacity and operating time. To determine the equipment heat generation more accurately, it is necessary to request the equipment operating characteristic curve from the relevant manufacturer, and then calculate it based on the equipment capacity and operating time.
VII. Heat Generation of Lighting Equipment
For large and medium-sized power stations, the lighting power tends to increase due to the demand for lighting in architectural decoration and landscape design. With the development of lighting equipment, the lighting application in power stations has shifted from incandescent lamps and fluorescent lamps to high-brightness light sources such as iodine-tungsten lamps and metal halide lamps. However, the heat dissipation of lighting equipment is stable: as long as the voltage and power are stable, the heat dissipation remains unchanged.
Part of the electric energy consumed by lighting is directly converted into heat, which is dissipated to the surroundings through convection and conduction. Light energy radiates outward in the form of infrared radiation, which cannot be directly absorbed by air but passes through the air to be absorbed by surrounding objects, and then transferred to the air. The part converted into light is also first projected to surrounding objects, absorbed by the objects and then converted into heat, which is then transferred to the air and other objects through convection, conduction or radiation.
The heat generation of lighting equipment is calculated as:
Q=n1N(kW)
n1: Power consumption coefficient of the ballast, generally taken as 1.2
N: Total installed power of the lighting equipment (kW)