Reactive power must be generated immediately near its consumers, i.e. it is necessary to install compensating devices step-up a power factor of loads. An electric system elements transfer of reactive power is accompanied by watt losses, cause inadmissible supply voltage derating or supply voltage rise, account for reduce of electric system capacity.
In order that make sure that a setting of compensating devices is necessary negative consequences induced by electric system transfer of reactive power must be estimated.
Electric system element watt losses are found by the expression:
, (1)
Where U - the voltage of transmission network; R - the resistive impedance of the transmission network element; 
and 
- respectively losses specified by the active P and the reactive Q power.
Watt losses are directly proportional to the resistive impedance of the transmission network element. That means that watt losses depend on the transfer length. In addition, these losses are in inverse proportion to square of mains voltage. Therefore the long transmission of electric power at a high voltage.
Relative watt losses can be judged by the expression:
. (2)
For example, the natural weight-average power factor of industrial consumers is 
(
) [1], therefore relative watt losses are equal to 
. Whereas the economic value of power factor is equal to 
(
) and corresponding relative losses of it is equal to 
.
Reactive power flows are the complementary load of electric system elements. Therefore when reactive power is transmitted quantity of electrical energy transmitted to consumers must be restricted. That opposes to the connection of new consumers and may result in serious power failures.
The complementary load of electric system elements 
induced by reactive power transfer is can be judged by the expression:
(3)
Where 
and 
are natural and economic values of the power factor.
I.e. the complementary load of electric system elements vary depending on the relation of natural and economic power factor values. Analysis shows that power factor of industrial consumers much less then the economic value of it [1].
The relative complementary load of electric system is:
. (4)
Electric system load was increased by 24-35% when the transmitted reactive power is more then its economic value.
One of electric system reliability criterions is the voltage of network nodes. Inadmissible supply voltage derating is produced by considerable consumption of reactive power during hours of the peak electrical demand. And vice versa reactive power generation during hours of the small load results in a considerable supply voltage rise. That decreases the work reliability of electric system and the quality of electrical energy.
Thus reactive power compensation is necessary in electric systems. That permitted to reduce relative watt losses 
by 33-37%, to increase the network throughput by 20-25%.
References:
1. Electrical handbook. Book 3. Power generation and electrical energy distribution. Moscow, MEI, 2004, 964 p.
The work was submitted to III international scientific conference «Actual problems of science and education», Cuba, March, 19-29, 2008, came to the editorial office 21.04.2008.