Design for calculation and selection of relay protection for 220kV networks

Contents

Contents

Introduction

1. Determination of three-phase short-circuit currents by the method of equivalent EMF at times and

1.1 Determination of initial super-transient current

1.1.1 Preparation of substitution scheme

1.1.2 Definition of substitution scheme parameters

1.1.2.1 Determination of reduced resistance of substitution scheme

1.1.3 Conversion of substitution scheme excluding loads

1.1.4 Determining the nature of loads

1.1.5 Conversion of substitution scheme taking into account loads

1.1.6 Determination of initial super-transient and shock currents

1.2 Determination of steady-state short-circuit current

1.2.1 Preliminary selection of generator operation modes and conversion of substitution circuit

1.2.2 Determination of short-circuit currents in generator branches

1.2.3 Check of correct selection of generator operation modes

2. Determination of three-phase short circuit currents by method

typical curves

2.1 Preparation of substitution scheme and its transformation

2.2 Determination of currents

2.3 Comparison of results of calculation of super-transient and steady-state short-circuit currents by method of equivalent EMF and by method of calculation curves

2.4 Plotting Dependency

3. Calculation of various types of asymmetric short circuit by method

calculation curves

3.1 Drawing up a direct sequence diagram

3.1.1 Determination of total resistance

3.2 Development of reverse sequence diagram

3.2.1 Determination of total resistance

3.3 Generation of zero sequence diagram

3.3.1 Determination of total resistance

3.4 Calculation of two-phase short circuit

3.4.1 Determination of resultant resistances

3.4.2 Comparison of non-symmetry coefficients

3.5 Calculation of two-phase ground fault

3.5.1 Determination of resultant resistances

3.5.2 Comparison of non-symmetry coefficients

3.6 Calculation of single-phase short circuit

3.6.1 Determination of reverse sequence current

3.6.2 Comparison of non-symmetry coefficients

Conclusion

List of sources used

Appendix A. Graphs of current dependencies of asymmetric short-circuit types

Appendix B. Current and voltage diagrams of asymmetric short-circuit types

Summary

In this course work, three-phase short-circuit currents are calculated using the method of equivalent EMF and typical curves. The results of the two methods are compared. Short-circuit currents are also determined under asymmetric modes by the method of calculation curves, and current and voltage diagrams are plotted at asymmetric types of short-circuit.

The note contains: 46 pages, including 40 figures, 5 tables, and 2 annexes.

Introduction

The course "Transient processes in power supply systems" is one of the profiling for electric power specialties and specializations.

Of the entire variety of electromagnetic transients in the electrical system, the most common are the processes caused by:

a) actuation and disconnection of motors and other electric power receivers;

b) short circuit in the system, as well as repeated switching on and off (simultaneous or cascade) short circuit;

c) the occurrence of local asymmetry in the system (for example, the disconnection of one phase of the transmission line);

d) the action of forcing the excitation of synchronous machines, as well as their excitation (that is, quenching their magnetic field);

e) non-synchronous activation of synchronous machines.

Any short circuit that is not provided for by normal operating conditions is called a short circuit between phases, and in systems with grounded neutrals (or four-wire ones) it is also called a short circuit of one or more phases to earth (or to a zero wire).

In three-phase systems with grounded neutral, the following main types of short circuits at one point are distinguished:

a) three-phase;

b) two-phase;

c) single-phase;

d) two-phase to earth, that is, a fault between two phases with simultaneous closure of the same point to earth.

The three-phase short circuit is symmetrical because all phases remain under the same conditions. On the contrary, all other types of short circuits are asymmetric, since at each of them the phases are already in different conditions; therefore, current and voltage systems at these types of short circuit are distorted to one degree or another.

The overwhelming number of short circuits is associated with a ground fault, while a three-phase short circuit is very rare.

Determination of three-phase short-circuit currents by the method of equivalent EMF at times and

1.1 Determination of initial super-transient current

1. Creation of substitution scheme

The calculation of the super-transient short-circuit mode must be started by clarifying the nature of the loads, for which a substitution scheme is drawn up without taking into account the loads.

From the obtained values ​ ​ of residual voltages, we determine that only the load H1 is generating, since

Thus, this load is included in the substitution scheme with EMF equal to 0.85, and loads H2 and H3 with EMF equal to zero.

We define the reduced values of load resistances.

Since the resistances of generalized loads in steady state are as follows: other than in super-transition, we recalculate them. EMF of loads in steady-state short-circuit mode are zero.

1.2.1 Preliminary selection of generator operation modes and conversion of substitution circuit

Preselection of generator operation modes and conversion of substitution scheme.

Based on the fact that the short circuit on the line, we assume that all generators operate in the LPV mode, for which

The initial substitution scheme for calculating the steady-state short-circuit mode is shown in Figure 17.

Determination of three-phase short-circuit currents using typical curves

2.1 Preparation of substitution scheme and its transformation

The substitution scheme is received partially converted.

Conclusion

In this course work, three-phase short circuit currents were calculated using two methods. As a result, it was found that the method of calculating short-circuit currents by the method of typical curves gives a more accurate result than the method of equivalent EMF. Also, with asymmetric short circuits, the largest short circuit current occurs with a two-phase ground short circuit.

List of sources used

1. Ulyanov S.A. Electromagnetic processes in electrical systems. Textbook for electrical and energy universities and faculties, - M.: Energy, 1970;

2. Rules for the design of electrical installations, - M.: Energoatomizdat, 1985;

3. Neklepaev B.N., Kryuchkov I.P. "Electric part of stations and substations," - M.: Energoatomizdat, 1989;

4. Pilipenko V.T. Methodological guidelines for coursework in the discipline "Transitional processes" (Part 1). - Orenburg: RIC GOU OGU, 2004. – 49 pages.