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Three Phase Theory – Professor J R Lucas Level 2 – September 2001

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VYVB

Three Phase Theory - Professor J R Lucas 

To transmit power with single phase alternating current, we need two wires. However

you would have seen that distribution lines usually have only 4 wires. This is because

distribution is done using three phase and the 4th wire is the neutral. How does this help

? Since the three phases are usually 120o

out of phase, their phasor addition will be zero

if the supply is balanced

Three Phase Power

For a balanced star connected load with line voltage Vline and line current Iline:

Vstar = Vline / √3 

Istar = Iline 

Zstar = Vstar / Istar = Vline / √3Iline 

Sstar = 3VstarIstar = √3VlineIline = Vline2

/ Zstar = 3Iline2Zstar 

For a balanced delta connected load with

line voltage Vline and line current Iline:

Vdelta = Vline 

Idelta = Iline / √3 

Zdelta = Vdelta / Idelta = √3Vline / Iline 

Sdelta = 3VdeltaIdelta = √3VlineIline 

= 3Vline2

/ Zdelta = Iline2Zdelta 

The apparent power S, active power P and reactive power Q are related by:

S2 = P2 + Q2 

P = Scosφ 

Q = Ssinφ where cosφ is the power factor and sinφ is the reactive factor

Note that for equivalence between balanced star and delta connected loads:

Zdelta = 3Zstar 

Symmetrical Components

≡ 

Positive

Sequence

Negative

Sequence

Zero

Sequence

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Three Phase Theory – Professor J R Lucas Level 2 – September 2001

In any three phase system, the line currents Ia, Ib and Ic may be expressed as the phasor

sum of:

- a set of balanced positive phase sequence currents Ia1, Ib1 and Ic1 

(phase sequence a-b-c),

- a set of balanced negative phase sequence currents Ia2, Ib2 and Ic2 

(phase sequence a-c-b),

- a set of identical zero phase sequence currents Ia0, Ib0 and Ic0 (cophasal, no phase sequence).

The positive, negative and zero sequence currents are calculated from the line currents

using:

Ia1 = (Ia + αIb + α2Ic) / 3 

Ia2 = (Ia + a2Ib + αIc) / 3 

Ia0 = (Ia + Ib + Ic) / 3 

The positive, negative and zero sequence currents are combined to give the line currents

using:

Ia = Ia1 + Ia2 + Ia0 Ib = Ib1 + Ib2 + Ib0 = α2Ia1 + αIa2 + Ia0 

Ic = Ic1 + Ic2 + Ic0 = αIa1 + α2Ia2 + Ia0 

The residual current Ir is equal to the total zero sequence current:

Ir = Ia0 + Ib0 + Ic0 = 3Ia0 = Ia + Ib + Ic = Ie 

which is measured using three current transformers with parallel connected secondaries.

Ie is the earth fault current of the system.

Similarly, for phase-to-earth voltages Vae, Vbe and Vce, the residual voltage Vr is equal to

the total zero sequence voltage:

Vr = Va0 + Vb0 + Vc0 = 3Va0 = Vae + Vbe + Vce = 3Vne which is measured using an earthed-star / open-delta connected voltage transformer.

Vne is the neutral displacement voltage of the system.

The α - operator 

The α - operator (1∠120°) is the complex cube root of unity:

α = - 1 / 2 + j√3 / 2 = 1∠120° = 1∠-240° 

α2= - 1 / 2 - j√3 / 2 = 1∠240° = 1∠-120° 

Some useful properties of α are:

1 + α + α2 = 0 

α + α2 = - 1 = 1∠180° 

α - α2= j√3 = √3∠90° 

α2- α = - j√3 = √3∠-90°