Power System Analysis

1. SEQUENCE IMPEDANCES OF
TRANSFORMERS
Name: Muhammad Moin
MUET Jamshoro

2. SEQUENCE IMPEDANCE
 The sequence impedance of the network describes
the behavior of the system under asymmetrical fault
conditions. The performance of the system
determines by calculating the impedance offered by
the different element of the power system to the
flow of the different phase sequence component of
the current. Every power system component (static
or rotating) has three values of impedance one for
each symmetrical value of current. The sequence
impedance of power system is of three types
namely positive sequence impedance, negative
sequence impedance and zero sequence
impedance.

3. POSITIVE SEQUENCE IMPEDANCE –
 The impedance offered by the network to the flow of positive
sequence current is called the positive sequence impedance.
The positive sequence means all the electrical quantities are
numerically equal and displaces each other by 120º.

4. NEGATIVE SEQUENCE IMPEDANCE –
 The impedance offered by the flow of negative sequence
current in the circuit is called the negative sequence
impedance. Here the negative sequence indicates all the
electrical quantities are having equal magnitude and
displaced 120º each other in opposite direction.

5. ZERO SEQUENCE IMPEDANCE –
 The impedance offered to zero sequence current is
called the zero sequence impedance.

6.  The impedance of the positive, negative and zero
sequence component is given by the ratio of the
phase sequence voltage to the phase sequence
current of the system.
 There is no mutual impedance between
various symmetrical components. The each
sequence impedance considered separately, which
simplifies the calculation of asymmetrical fault
calculations.

7. SEQUENCE IMPEDANCES OF TRANSFORMERS
 In power transformer, the core losses and the
magnetization current are on the order of 1 percent of
the rated value; therefore, the magnetizing branch is
neglected. The transformer is modeled with the
equivalent series leakage impedance. If the phase
sequence changed. Therefore, the positive and
negative sequence impedance are same. Also if the
transformer permits zero sequence current flow at all,
the phase impedance to zero sequence is equal to the
leakage impedance.
 We have
Z0=Z1=Z2=Zl

8. ZERO SEQUENCE OF TRANSFORMER
 Y-Y connections with neutrals ground- We know
that the zero sequence current equals the sum of
phase currents. Since both neutral are grounded,
there is a path for the zero sequence current to flow
in the primary and secondary, and the transformer
exhibits the equivalent leakage impedance per
phase as shown in below fig:

9.  Y-Y connection with the primary neutral
grounded - The primary neutral is grounded, but
since the secondary neutral is isolated, the
secondary phase current must sum up to zero. This
means that the zero-sequence current in the
secondary is zero. Consequently, the zero
sequence current in the primary is zero, reflecting
infinite impedance or an open circuit as shown in
Figure

10.  Y-∆ with grounded neutral - In this configuration
the primary currents can flow because there is
zero-sequence circulating current in the ∆-
connected secondary and a ground return path for
the Y-connected primary. Note that no zero-
sequence current can leave the ∆ terminals, thus
there is an isolation between the primary and
secondary sides as shown in Figure

11.  Y-∆ connection with isolated neutral - In
this configuration, because the neutral is
isolated, zero sequence current cannot flow
and the equivalent circuit reflects an infinite
impedance or an open as shown in Figure

12.  ∆ -∆ connection - In this configuration zero-
sequence currents circulate in the ∆ -
connected windings, but no currents can
leave the ∆ terminals, and the equivalent
circuit is as shown in Figure

13. THANKS