how to figure current on a 115 kv high voltage line with 40 mw and 30 vars?

Answer 1

Supposing a three phase line
Power = sqrt3 * V *I *cos @ where tan@= 30/40

so cos@=0.8 and I = 40*10^6/(sqrt3*115*10^3 *0.8) =251 amp
In another way the apparent power is 50 MVolt Amp=sqrt3*V*I so 50*10^6/sqrt3*115*10^3 =251 amp

Answer 2

Assuming a balanced 3 Phase system and that you meant 30 (M)VARS,

You need the MVA and voltage To calculate current in the 3 Phase system

Since MW is active power and MVA is reactive power (the phasors are 90 degrees), take the square root of the sum of the squares of the MW and MVAR to get MVA or Mega Volt Amps.

MVA=sq rt(40MW^2+30MVAR^2)=50MVA

VA=Volts*Amps*sq rt(3), solving for Amps

Amps=VA/(volts*sq rt(3))

Translating into the units given,

Amps=50,000,000VA/(115,000 Volts*1.73) = 251 Amps(rms)/phase.

(If it is single phase eliminate the 1.73 in the previous equation).

Note: if the current phasor leads the voltage phasor the power factor is leading (over excited syncronous machine or capacitive load) and if the current phasor lags the voltage phasor the power factor is lagging (induction motors or inductive type loads). Syncronous condensers are used to absorb the capacitive VARS from long transmission lines.

Answer 3

Use the formula I = Kw X 1000 / 1.73 X E X PF

(PF = KW / KVA) where KVA sqd = KW sqd + KVAR sqd
are you sure you meant 30 vars not mvars?

This will work for any 3 phase circuit