The busbar reactance is not normally sufficiently large to affect the total reactance of a power system and hence is not included in the calculations when establishing the short-circuit currents and reactive volt drops within a power system. The exception to this is when considering certain heavy current industrial applications such as furnaces, welding sets, or roll heating equipment for steel mills. In these cases the reactance may be required to be known for control purposes, or to obtain busbar arrangements to give minimum or balanced reactance. This may be important because of its effect on both volt drop and power factor, and hence on the generating plant kVA requirement per kW of load, or on the tariffs payable where the power is purchased from outside.
The busbar impedance is made up of three components: resistance, inductance and capacitance. The values of these components are given an ohmic value which in the case of inductance and capacitance is dependent on the frequency of the system. They are defined as follows:
Resistance:
where Rf = resistance at frequency f (Hz), W
Ro = d.c. resistance
S = skin effect ratio
K = proximity ratio
Inductance:
where XL = inductive reactance, W
f = frequency, Hz
L = inductance, H
Capacitance:
where Xc = capacitive reactance, W
f = frequency, Hz
C = capacitance, F
Impedance:
where X = XL - XC
The value of XC is usually very much smaller than XL, and XL is usually much larger than Rf. The value of X is taken to be positive with the sign of XL - XC to indicate whether the system has a positive or negative power factor.
The volt drop in a busbar system is estimated as follows from the usual formula:
VB = I ZB
where VB = volt drop, V
I = current flowing in the conductor, A
ZB = busbar impedance, W
However, to find the magnitude of the load voltage VL available, the busbar volt drop VB must be subtracted vectorially from the supply voltage VS:
VS = supply voltage, V qL = angle of load, °
VB = busbar volt drop, V fB = angle of busbar, °
VL = load voltage, V RB = busbar resistance, W
I = current, A XB = busbar reactance, W
The apparent volt drop in the busbar trunking, phase to neutral, is given by:
Multiply by Ö3 for phase to phase volt drop.
The above formula gives a very close approximation as long as the busbar system volt drop remains small in comparison to the system voltage.
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