During the transmission of electrical energy from power plant generators to the consumer, about 12-18% of all generated electricity is lost in the conductors of overhead and cable lines, as well as in the windings and steel cores of power transformers.
When designing, it is necessary to strive to reduce electricity losses in all parts of the power system, since electricity losses lead to an increase in the capacity of power plants, which in turn affects the cost of electricity.
In networks up to 10 kV, power losses are mainly due to the heating of the wires from the action of the current.
Line power loss.
The active power losses (kW) and reactive power losses (kvar) can be found using the following formulas:
where Isettlement- rated current of a given section of the line, A;
Rl- active resistance of the line, Ohm.
Power losses in transformers.
Power losses in power transformers consist of losses that are independent and dependent on the load. The active power loss (kW) in the transformer can be determined using the following formula:
Loss of active power in the transformer
where ? Rst- losses of active power in the steel of the transformer at rated voltage. They depend only on the power of the transformer and the voltage applied to the primary winding of the transformer. ? Rst equate ? Px;
? Px- no-load losses of the transformer;
? Rob- losses in the windings at the rated load of the transformer, kW; ? Rob equate ? Rk.
? Rk- short circuit losses;
? = S / Snom- the load factor of the transformer is equal to the ratio of the actual load of the transformer to its rated power;
The reactive power loss of a transformer (kvar) can be determined using the following formula:
where ? Qst- losses of reactive power for magnetization, kvar. ? Qst equate ? Qx.
? Qx- magnetizing power of no-load of the transformer;
? Qras- losses of reactive power dissipation in the transformer at rated load.
The values ? Rst ( ? Px) and ? Rob ( ? Rk) listing in the catalogs of manufacturers of power transformers. The values ? Qst ( ? Qx) and ? Qras determined from the catalog data from the following expressions:
where IX- no-load current of the transformer,%;
Uк- short circuit voltage,%;
Inom- rated current of the transformer, A;
Xtr- the reactance of the transformer;
Snom- rated power of the transformer, kVA.
Loss of electricity.
Based on the power losses, the power losses can be calculated. You should be careful here. It is impossible to calculate the power losses by multiplying the power losses at any given load by the number of hours of operation of the line. This is not worth doing, because during the day or season, the consumed load changes and thus we get an unreasonably overestimated value.
Time of maximum losses ? - the conditional number of hours during which the maximum current flowing in the line creates energy losses equal to the actual energy losses per year.
By the time of using the maximum load or the time of using the maximum Tmah is the conditional number of hours during which the line, operating at maximum load, could transfer as much energy to the consumer per year as when operating on a real variable schedule. Let be W(kW * h) - energy transmitted through the line for a certain period of time, Rmax(kW) is the maximum load, then the time of using the maximum load:
Tmax = W / Pmax
Based on statistical data for individual groups of electrical consumers, the following values were obtained Tmah:
- For indoor lighting - 1500-2000 h;
- Outdoor lighting - 2000-3000 h;
- One-shift industrial enterprise - 2000-2500 h;
- Two-shift - 3000-4500 hours;
- Three-shift - 3000-7000 hours;
Loss time ? can be found on the schedule, knowing Tmah and power factor.
Energy losses in the transformer:
Energy losses in the transformer
where ? Watr–Total loss of active energy (kW * h) in the transformer;
? Wртр- total loss of reactive energy (kvar * h) in the transformer.