Assignment Questions

Question 1

What are two reasons for why we step up the voltage from the generator?

Answer 1

By stepping up voltage for transmission, the current is reduced which lowers the losses and reduces the size of the
cross-sectional area of the conductor

Question 2

If the transmission line voltage is increased, what will happen to the transmission losses assuming the line remains the
same? What is the relationship of the changes?

Answer 2

If the voltage increases, the current decreases and the transmission line losses decrease exponentially if the
transmission line stayed the same

Question 3

If a 250kV transmission line has a resistance of 0.005Ω, calculate the losses on the line if the power transferred is
115MW (assume single phase)

Answer 3

P = V * I
I = P / V = 115(10 6) / 250(10 3) = 460A
PLOSS = I 2 * R = 460 2 * 0.005 = 1058W

Question 4

If the transmission losses are 1500W at 13800V, what would the losses be if the power flow was the same but the
voltage was increased to 34.5kV, assuming the transmission lines remained the same?

Answer 4

POLD = V OLD * I OLD
PNEW = V NEW * I NEW
POLD = PNEW
V OLD * I OLD = V NEW * I NEW
I NEW = V OLD * I OLD / V NEW
I NEW = 13800 * I OLD / 34500
I NEW = 0.4 * I OLD
PLOSSOLD = I OLD 2 * R
PLOSSNEW = I NEW 2 * R
PLOSSNEW / PLOSSOLD = (I NEW 2 * R) / ( I OLD 2 * R )
PLOSSNEW = PLOSSOLD * (I NEW 2 / I OLD 2 )
PLOSSNEW = 1500 * ( (0.4 * I OLD ) 2 / I OLD 2 )
PLOSSNEW = 240W

Question 5

What are the assumptions for an Ideal Transformer?

Answer 5

Has no losses, no internal voltage drops
o Winding resistance is zero
o Winding reactance is zero
o Power losses in core are zero
o Reactive losses in core are zero

Question 6

A transformer has 400 HV turns and 11 LV turns. What is the LV output if the HV has 13800V applied to it?

Answer 6

VHV / NHV = VLV / NLV
VLV = NLV * (VHV / NHV ) = 11 * (13800 / 400) = 379.5V

Question 7

A transformer has 120 LV turns and 720 HV turns. If the HV current is 0.833A, calculate the LV current

Answer 7

IHV / ILV = NLV / NHV
ILV = IHV * (NHV / NLV ) = 0.833 * (720 / 120) = 5.0A

Question 8

A transformer has a turns ratio of 10 and a high voltage of 230V, what is the low voltage?

Answer 8

a = NHV / NLV = VHV / VLV
VLV = VHV / a = 230 / 10 = 23V

Question 9

If a transformer has an output voltage of 120V at 8.33A and an input voltage of 240V, what is the input current?

Answer 9

VHV * IHV = VLV * ILV
IHV = (VLV * ILV ) / VHV = (120 * 8.33) / 240 = 4.165A

Question 10

What is the power rating of a 27600V – 120V transformer if the HV is rated current is 1.09A

Answer 10

S = V HV * I HV = 27600 * 1.09 = 30084VA = 30kVA

Question 11

What is the power rating of a transformer if it has 200 HV turns, 160 LV turns, the HV is rated at 600V and the LV
rated current is 20.8A?

Answer 11

I HV = I LV * (N LV / N HV ) = 20.8 * (160 / 200) = 16.64A
S = V HV * I HV = 600 * 16.64 = 9984VA
Or
V LV = N LV * (V HV / N HV ) = 160 * (600 / 200) = 480V
S = V LV * I LV = 480 * 20.8 = 9984VA

Question 12

A 500kVA transformer has a voltage ratio of 27600 to 4160. The 4160 volt winding has a tap at 2000V. If 500V is
applied to the HV, what voltage will appear across the tap

Answer 12

a = NHV / NTAP = VHV / VTAP = 27600 / 2000 = 13.8
VTAP actual = VHV actual / a = 500 / 13.8 = 36.23V

Question 13

What is the acceptable voltage range for a 13800V - 600V step-up transformer according to CSA?

Answer 13

Tolerance = ±0.5%
Output = 13800V
Upper limit = 13800 * 1.005 = 13869V
Lower limit = 13800 * 0.995 = 13731V

Question 14

What is the acceptable voltage range for a 13800V - 600V step-down transformer according to CSA?

Answer 14

Tolerance = ±0.5%
Output = 600V
Upper limit = 600 * 1.005 = 603V
Lower limit = 600 * 0.995 = 597V

Question 15

If the step-down transformer is rated 24940V – 480V and the actual output voltage is 478.5V, what is the % error?

Answer 15

%error = {(V Actual – V Rated ) / VRated } * 100 = {(478.5 – 480) / 480} * 100 = -0.3125%