Load Flow Studies of a Power System | Electrical Engineering

In this article we will discuss about:- 1. Introduction to Load Flow Studies of a Power System 2. Importance and Objectives of Power Flow Studies.

Introduction to Load Flow Studies of a Power System:

In a 3-phase ac power system, active and reactive power flows from the generating stations to the load through different network buses and branches (transmission lines). Active power P and reactive power Q is supplied by generators at generator buses. Active power is drawn by loads from load buses. Reactive power Q is supplied or drawn from the load buses by shunt compensation elements (shunt capacitors, reactor elements, static VAR system). The flow of active and reactive power is called the power flow or load flow. The voltages of buses and their phase angles are affected by the power flow and vice-versa.

Power flow studies provide a systematic mathematical approach for determination of various bus voltages, their phase angles, active and reactive power flow through different branches, generators and loads under steady state conditions. The power flow study in a power system constitutes a study of paramount importance.

Load flow studies are carried out to study short circuit conditions for any interconnected power system. These are also required for planning the operation of power systems under existing conditions, its improvement and future expansion. Such studies facilitate us in determination of best size as well as the most favourable locations for the power capacitors both for power factor improvement and also for raising the network voltages.

The load flow studies also help us in determination of the best location as well as optimal capacity of the proposed generating stations, substations and new lines. Thus load flow studies are very important for planning existing system as well as its future expansion.

For optimized operation of an interconnected system some information’s, such as bus-bar voltage levels, machine excitation, tap-change and reactive compensation requirements are required which are provided by load flow studies.

The main information obtained from load flow studies comprises the magnitudes and phase angles of load bus voltages, reactive power at generator buses, active and reactive power flow in transmission lines, other variables being specified. Such information is essential for the continuous monitoring of the current state of the system and for analysis of the effectiveness of alternative plans for future system expansion to meet the increased load demand.

For many years, load flow studies were carried out by means of special purpose analog computer, called the ac network analyzer, but the advent of high-speed digital computers has tended to replace their use for large system studies.

This change from the ac network analyzer to the digital computer has resulted in greater flexibility, economy, accuracy and quicker operation. However, for system studies of a more local character, the network analyzer is still used, particularly in the initial planning stages.

Analysis of power system networks is possible by using either mesh current or nodal voltage techniques. The latter is more amenable to digital computer analysis, so that the nodal iterative approach to the solution of load or power flow problems has now become firmly established.

The main advantages attributed to the use of nodal voltage methods are:

(i) The number of equations is smaller,

(ii) The system may be described in terms of its node numbers and the interconnecting impedances,

(iii) Parallel branches can be dealt with separately without adding to the number of equations and

(iv) Cross­-over branches (if present) do not introduce any difficulties in respect of the formation of the admittance matrix.

The steps in the collection of system data are given below:

1. A single line diagram of the system is drawn.

2. Assuming a balanced 3-phase system, the transmission system is represented by its positive-phase- sequence network of linear-lumped series and shunt branches. The line impedances and shunt admittances are then determined in per unit values, including transformer impedances, shunt capacitor ratings and transformer tappings.

3. Node self and mutual admittances are determined, using nodal analysis.

4. The shunt capacitance and per unit resistance and reactance between the terminating buses of the lines are found.

5. The operating conditions are selected. The static operating state of the system is then specified by the constraints on power and/or voltage at the network buses.

After having the above type of data, a suitable mathematical model of the system, adequately describing the relationships between voltages and powers in the interconnected system, is formed. Power and voltage constraints at various buses in the network are then specified and the load flow equations are solved numerically. On determination of different bus voltages, actual load flow in all transmission lines is computed.

The mathematical formulation of the load flow problem results in a system of nonlinear equations. These equations can be written in terms of either the bus admittance matrix or bus impedance matrix.

The former is more amenable to digital computer analysis, because of the ease with which the bus admittance matrix could be formed and modified for network changes in subsequent cases. Further this approach (bus admittance matrix) is the most economical from the point of view of computer time and memory requirements.

Importance and Objectives of Power Flow Studies:

The power flow (or load flow) studies are the essential and vital part of power system studies. Data about active and reactive power flows through the branches and the bus voltage under steady state is required by power system engineers. Power flow study provides such data.

Power flow studies are extremely important and essential for power system planning, designing, expansion design and for providing guidelines to control room operating engineers (power controllers) in the following activities:

1. Providing operating instructions to generating station and substation control rooms for loading, reactive power compensation relay settings, tap-setting and switching sequence. Selecting the optimum settings of over-current relays.

2. Analysing the effect of rearranging the circuits on the power flows, bus voltages.

3. Preparing software for online operation, control and monitoring of the power system.

4. Analysing the effect of temporary loss of generating station or transmission path on the power flow.

5. Knowing the effect of reactive power compensation on bus voltages.

6. Calculation of line losses for different power flow conditions.

7. Evaluation of the operating performance of a power system under normal steady state.

8. Planning expansion of system. Introducing HVDC line, inter-connection, EHV ac lines etc.

9. Obtaining initial input data for various other power system studies such as economic load dispatch, reactive power and voltage, control, state estimation, fault calculations, generation planning, transmission planning etc.