Section II: Sequence Circuits for Loads

In this section we shall construct sequence circuits for both Y and Δ -connected loads separately.

• Sequence Circuit for a Y-Connected Load
• Sequence Circuit for a Δ-Connected Load

Sequence Circuit for a Y-Connected Load

Consider the balanced Y-connected load that is shown in Fig. 7.2. The neutral point (n) of the windings are grounded through an impedance Z_n . The load in each phase is denoted by Z_Y . Let us consider phase-a of the load. The voltage between line and ground is denoted by V_a , the line-to-neutral voltage is denoted by V_an and voltage between the neutral and ground is denoted by V_n . The neutral current is then

Therefore there will not be any positive or negative sequence current flowing out of the neutral point.

Fig. 7.2 Schematic diagram of a balanced Y-connected load.

The voltage drop between the neutral and ground is

Now

We can write similar expression for the other two phases. We can therefore write

Pre-multiplying both sides of the above equation by the matrix C and using (7.8) we get

Now since

We get from (7.26)

We then find that the zero, positive and negative sequence voltages only depend on their respective sequence component currents. The sequence component equivalent circuits are shown in Fig. 7.3. While the positive and negative sequence impedances are both equal to Z_Y , the zero sequence impedance is equal to

If the neutral is grounded directly (i.e., Z_n = 0), then Z₀ = Z_Y . On the other hand, if the neutral is kept floating   (i.e., Z_n = ∞ ), then there will not be any zero sequence current flowing in the circuit at all.

Fig. 7.3 Sequence circuits of Y-connected load: (a) positive, (b) negative and (c) zero sequence.