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

ELECTRICAL DIAGRAMS - PART – 03 - DIFFERENCE BETWEEN BLOCK AND LAYOUT DIAGRAM

BLOCK DIAGRAM - It is a functional drawing which shows and describes the main operating principles of the equipment or devices. It consists of principle functions or parts represented by blocks and is connected through lines that show the relationship between the blocks.

LAYOUT DIAGRAM - A drawing meant to show the physical arrangement of the wires and the components they connect is called layout design. 

ELECTRICAL DIAGRAMS PART – 02 - DIFFERENCE BETWEEN PICTORIAL AND SCHEMATIC DIAGRAM

ELECTRIC CIRCUIT – An electrical circuit is a path through which an electrical current flows. A closed circuit makes electrical current flow possible. An open circuit does not allow electrical current to flow.

An electric circuit made up of the basic elements resistance, inductance, and capacitance in a simple arrangement such that its performance would duplicate that of a more complicated circuit or network.

PICTORIAL CIRCUIT DIAGRAM - A pictorial circuit diagram uses simple images of components.

SCHEMATIC CIRCUIT DIAGRAM - A schematic circuit diagram is used to give a visual representation of an electrical circuit to an electrician.

ELECTRICAL DIAGRAMS PART – 01 - DIFFERENCE BETWEEN VECTOR AND PHASOR DIAGRAM

VECTOR - A quantity having direction as well as magnitude, especially as determining the position of one point in space relative to another.

VECTOR DIAGRAM - Vector diagram shows the direction and relative magnitude of a vector quantity by a vector arrow. Vector diagrams can be used to describe the velocity of a moving object during its motion. Vector diagrams can be used to represent any vector quantity. For example, acceleration, force, and momentum.

PHASOR - A phasor is a line whose direction represents the phase angle in electrical degrees and whose length represents, the magnitude and of the electrical quantity.

PHASOR DIAGRAM - The graphic representation of the phasors of sinusoidal quantities taken all at the same frequency and with proper phase relationships with respect to each other is called a phasor diagrams. The relative position of the phasors or phase difference which is important in a.c. calculations. 

UNSYMMETRICAL FAULT CALCULATIONS – PART – 32 – DOUBLE LINE TO GROUND AND LINE TO LINE FAULT - PROBLEM

A 33 kV bus bar has a three phase fault level of 1000 MVA. The negative and zero sequence source reactances are 2/3 and 1/3 of positive sequence reactance. The zero sequence resistance of the source is 50 ohms. A 30 MVA, 33 / 132 kV solidly grounded delta/star transformer having a reactance of 0.2 per unit. Is fed from 33 kV bus. Find fault current and fault MVA at 132 kV bus for the following faults. (a) Double line-to-ground fault and (b) Line to Line fault.

UNSYMMETRICAL FAULT CALCULATIONS – PART – 31 – THREE PHASE FAULT AND SINGLE LINE TO GROUND FAULT - PROBLEM

A 33 kV bus bar has a three-phase fault level of 1000 MVA. The negative and zero sequence source reactances are 2/3 and 1/3 of positive sequence reactance. The zero sequence resistance of the source is 50 ohms. A 30 MVA, 33 / 132 kV solidly grounded delta/star transformer having a reactance of 0.2 per unit. Is fed from 33 kV bus. Find fault current and fault MVA at 132 kV bus for the following faults. (a) Three phase fault and (b) Single line to ground fault.

UNSYMMETRICAL FAULT CALCULATIONS – PART – 30 – LINE-TO-LINE FAULT - PROBLEM

A 25 MVA, 11 kV, three-phase, 50Hz generator has Z1 = j0.25 p,u., Z2 = j0.35 p,u., Z0 = j0.1 p.u. and Zn = 0.3 ohms. If a line to line fault occurs on Y and B phases. Calculate the fault current and per phase voltages.