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# Lesson 36 AC Three Phase Power - PowerPoint PPT Presentation

Lesson 36 AC Three Phase Power. Learning Objectives. Compute the real, reactive and apparent power in three phase systems Calculate currents and voltages in more challenging three phase circuit arrangements. Apply the principles of Power Factor Correction to a three phase load. Review.

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### Lesson 36 AC Three Phase Power

• Compute the real, reactive and apparent power in three phase systems

• Calculate currents and voltages in more challenging three phase circuit arrangements.

• Apply the principles of Power Factor Correction to a three phase load.

AC Power Summary

Power Triangle

• The power triangle graphically shows the relationship between real (P), reactive (Q) and apparent power (S).

Active Power (P) to Wye (Y) Load

• Because we are considering a balanced system, the power per phase (P) is identical and the total active power (PT) is simply PT = 3 P.

• Using line voltage () and line current (IL=I):

EAN = 277-30 V . Compute PΦ, PT.

S

Q = V I sin 



P

Reactive Power (Q) to Wye (Y) Load

• The reactivepower per phase (Q) is given

Reactive Power (Q) to Wye (Y) Load

• Because we are considering a balanced system, the power per phase (Q) is identical and the total reactive power (QT) is simply QT = 3 Q.

• Using line voltage (VL ) and line current (IL):

EAN = 277-30 V . Compute QΦ, QT.

Apparent Power (S) to Wye (Y) Load

• The apparentpower per phase (S) is given

S = VI

Q



P

• The power factor (FP) is given

S

Q



P

EAN = 277-30 V . Compute SΦ, ST, and FP.

Power to a Delta () Load

Active Power (P) to Delta () Load

• Total active power (PT) is simply PT = 3 P.

• Using line voltage (VL=V) and line current ( ):

• Which was the EXACT same equation as for Y loads

• The equations for calculating total reactive and apparent power are also identical to the Wye load versions:

EAN=120-30 V.

Determine per phase and total power (active, reactive, and apparent). Determine total powers (active, reactive, and apparent) by multiplying the per-phase powers by 3.

EAN=120-30 V.

Determine total powers (active, reactive, and apparent) by using these formulas:

You must pay attention to the problem statement!

Does it ask for total or per-phase power?

What kind of power? S, P, or Q?

Where is the power?

Generator

Line Impedances

Pline=?

Qline =?

Sgen=?

Pgen=?

Qgen=?

Power factor (FP) tells us what portion of the apparent power (S) is actually realpower (P).

FP = P / S = cos 

Power factor angle

 = cos-1(P / S)=cos-1(FP)

For a pure resistance,  = 0º

For a pure inductance,  = 90º

For a pure capacitance,  = -90º

Review

NOTE:  is the phase angle of ZT, not the current or voltage.

In order to cancel the reactive component of power, we must add reactance of the opposite type. This is called power factor correction.

Review

Capacitors will be connected in parallel with each load phase

Calculate the reactive power (Q) of ONE PHASE of the load

Insert a component in parallel of the load that will cancel out that reactive power e.g. If the load has QΦ=512 VAR, insert a capacitor with QΦ=-512 VAR.

Calculate the reactance (X) that will give this value of Q Normally the Q=V2/X formula will work

Calculate the component value (F or H) required to provide that reactance.

EAB=4800 V. Frequency 60 Hz.

Determine value of capacitor which must be placed across each phase of the motor to correct to a unity power factor.