UNSYMMETRICAL FAULT CALCULATIONS – PART – 35 – SEQUENCE NETWORK OF TRANSFORMER

The presence of a fault at any point in a system means generally that the system is put into an unbalanced state of operation.

In unsymmetrical fault calculations, each piece of equipment will have three value of impedance offered by each sequence current.

The zero sequence network is a single-phase component and its existence is dependent upon a closed path that must be completed through reference ground.

1. For positive or negative sequence networks, the neutral of the generator is taken as the reference bus. The reason is the positive and negative sequence components represent balanced sets and hence all the neutral point must be at the same potential for positive or negative sequence currents.

2. If there is no ground return, zero sequence currents cannot flow and in the corresponding zero-sequence networks are considered an open circuit.

3. In star connected transformer if the neutral is grounded zero-sequence current can flow in the winding.

4. If the star point is ungrounded zero sequence currents cannot flow in the winding.

5. In a delta connected, there is no neutral, hence zero-sequence currents cannot flow in the line because there is no return path. However, zero-sequence currents can circulate in the legs of the delta connected winding.

6. Due to the presence of zero sequence voltages the zero-sequence currents flowing around a delta winding.

7. When the magnetizing current in a transformer is neglected, the primary ampere-turns balance the secondary ampere-turns and current can flow in the primary only if the there is a current in secondary.

8. If the neutral point of the star connection is grounded through an impedance Zn and impedance 3Zn appears in series with Z0 in the zero-sequence network.

9. The zero sequence impedance depends upon earth connection. If there is a through the circuit for earth current, zero-sequence impedance is equal to positive sequence impedance otherwise it will be infinite.

10. Positive sequence impedance = Negative sequence impedance 

zero sequence impedance = Positive sequence impedance if there is a circuit for earth current (infinite, if there is no circuit for earth current)

UNSYMMETRICAL FAULT CALCULATIONS – PART – 34 – SEQUENCE NETWORK OF LOADED THREE-PHASE GENERATOR SEQUENCE NETWORKS

A single-phase equivalent circuit of power system consisting of impedances to the current of any one sequence only is called sequence network. [OR] An equivalent network for the balanced power system under an imagined operating condition such that only one sequence component of voltages and currents is present in the system.

Sequence network is composed of impedances offered to that sequence current in the system. Three sequence networks for a given power system namely

(i) Positive sequence network (ZO) – The positive sequence network for a given power system shows all the paths for the flow of positive sequence currents in the system.

(ii) Negative sequence network (Z1) – The negative sequence network for a given power system shows all the paths for the flow of negative sequence currents in the system

(iii) Zero sequence network (Z2) - The zero sequence network for a given power system shows all the paths for the flow of zero sequence currents in the system.

Each sequence network is replaced by a Thevenin’s equivalent circuit between two points i.e. each sequence network can be reduced to a single voltage and single impedance.

One point is the fault point ‘F’ and the other point is the zero potential of reference bus N.

Va1 = Vf - Z1 IR1, Va2 = -Z2 Ia2 and Va0 = -Z0 Ia0

Ia is the current flowing from the system into the fault, its components Ia1, Ia2 and Ia0.

Z1, Z2 and Z0 are the total impedance of the positive, negative and zero sequence networks up to the fault point.

For a fault on the unloaded generator with excitation voltage Eg the following will be the sequence voltage drops.

Va1 = Eg - Z1 IR1, Va2 = -Z2 Ia2 and Va0 = -Z0 Ia0

ELECTRICAL DIAGRAMS - PART - 05 DIFFERENCE BETWEEN IMPEDANCE AND REACTANCE DIAGRAM

ELECTRICAL DIAGRAMS – PART – 04 - SINGLE LINE DIAGRAM (SLD) IMPORTANCE AND APPLICATIONS

SINGLE LINE DIAGRAM

The single-line diagram is the blueprint for electrical power system analysis. It is the first step in preparing a critical response plan and to understand thoroughly familiar with the electrical distribution system layout and design.

IMPORTANCE OF SLD

1. The SLD is the roadmap for all future testing, service and maintenance activities.

2. It provides a picture of the facility at a moment in time.

3. It needs to change as your facility changes to ensure that your systems are adequately protected.

4. The SLD is making the electrical system easily understandable for any technical person inside/outside of the factory.

APPLICATIONS OF SINGLE LINE DIAGRAM

1. Short circuit calculations             2. Coordination studies

3. Load flow studies                        4. Safety evaluation studies

5. Electrical safety procedures         6. Efficient maintenance