Читать книгу Practical Power Plant Engineering - Zark Bedalov - Страница 71

The plant will include a number of large motors, for which cable sizing must be verified with respect to the voltage drop during its start. During the motor start, the voltage at the motor terminals must be >85% of the motor nominal voltage. Here is a quick check without going through a computer study.

For selecting the power cable for the motor we use National Electrical Code (NEC) for ampacities of Cu and Al cables [1]. Most of the engineers would have this booklet (Code) on their desks and use it for the various engineering activities ranging from the switchgear and cable selection to fire protection regulations. A similar Code is also available in Canada.

Let us calculate voltage drop for a motor rated as follows:

 Motor: 100 kW, 0.85 pf, 480 V

 Motor sub‐transient Impedance: Zm = 17%,

 Power cable length: 100 m.

 Select the cable. Calculate motor nominal current:

 Select the cable from NEC Ampacity table.

We look for a cable size for: 1.25 × 125 A = 155 A (∼25% margin was added).

 Cable selected from the code: 3c # 1/0, Cu, 90 °C, capable of carrying 170 A.

We calculate the voltage drop on the motor kW base (kWb): 100 kW = 1 pu.

Motor impedance on the motor base if not known can be assumed as: Z_m = 0.17 pu as per ANSI.

Cable impedance Z_Ω for cable 3c #1/0 AWG = 0.035 Ω/100 m. Value was taken from relevant tables.

 Calculate impedance Zc pu on per unit (pu) value for 100 m cable:

 Cable impedance in pu: .

 Calculate voltage drop ratio corresponding to the motor/cable impedance ratio:

 Therefore, the voltage % on motor terminals during the motor start is equals to 91.8% OK!For motor cable 100 m long: ΔV = 8.2% < 15% allowed.

This calculation assumed the 480 V bus is operating at 100% voltage at the time the motor is initiated to start.