What is Load Flow Analysis / Power Flow Analysis and Why is it Done

Load Flow Analysis / Power Flow Analysis

Power systems across the world subjected to great demands owing to expansions in the networks. Rapid development of a nation in every sphere is interlinked with its power transmission capacity. This can be done by adding new lines and by upgrading existing ones by adding new devices like FACTS. A stable power transmission network ensures prosperity.

Voltage instability in any network may lead to system collapse, when the bus voltage drops to such a level from which it cannot recover. In such a situation, complete system blackouts may take place. Hence voltage stability analysis is very important for successful process and planning of power system and for decreasing system losses. In this context, Load Flow or Power Flow Study and Analysis has been found useful by researchers in Voltage Stability Studies and Contingency Analysis.

Voltage Stability

Voltage stability is capacity of a power system to manage acceptable voltages at all buses in the system under normal conditions and after subjected to Voltage instability results in voltage collapse. Voltage collapse is the process by which the voltage falls to a low, unacceptable value as a result of an avalanche of events accompanying voltage instability. Voltage failure usually appears in power systems which usually heavy loaded failure and has reactive power shortages. In recent years voltage instability has attracted attention of power system planning and operating engineers as well as researchers. This is due to the frequent voltage collapses occurring in different parts of the world. Therefore Voltage Stability Analysis is important for researchers and power system planners to prevent such incidents from occurring.

Voltage Stability Analysis

The different methods used are:

  • P-V curve method.
  • V-Q curve method and reactive power reserve.
  • Methods based on peculiarity of power flow Jacobean matrix at the point of voltage collapse.
  • Continuation power flow method.
  • Optimization Method

Load Flow Analysis

To begin the Voltage Stability Analysis of a power system, computation of the complex voltages at all the buses is essential. After this, power flows from a bus and the power flowing in all the transmission lines are to calculate. A computational tool for this purpose is Load Flow Analysis. This analysis helps compute the steady state voltage magnitudes at all the buses, for a particular load condition. Load flow is mainly used in planning studies, for designing a new network or expansion of an existing one. The next step would be to compare the calculated values of power flows and voltage with the steady state device limits, to estimate the health of the network

Load Flow Study

  • Power flow analysis is very important in planning and designing the future expansion of power systems or addition to existing ones like adding new generator sites, meeting increase load demand and locating new transmission sites.
  • The load flow solution yields the nodal voltages and the phase angles, the power injection, power flows and the line losses in a network.
  • The best place, as well as the optimal capacity of a generating station, substation and new lines can regulate by load flow study.
  • Minimization of System transmission losses and prevention of line overloads. The operating voltages of the buses being determined, it aids in voltage stability analysis and voltage levels at certain buses can keep within the closed tolerances. The power flow problem formulated assuming the power system network to linear, bilateral and balanced. However, the power and voltage constraints impose non-linearity in the power flow formulation and iterative techniques are essential for the solution. The different conventional techniques for solving the power flow problem are:
  • Gauss-Seidel (GS) Method
  • Newton Raphson (NR) Method
  • Fast Decoupled Load Flow (FDLF)

Voltage Stability and Line Compensation

One of the prime causes leading to voltage instability is reactive power imbalance in the power system network. This occurs when there is a sudden and unpredicted increase or decrease in reactive power demand in the system. Occurrence of voltage collapse can only be prevented by either reducing the reactive power load or by providing further supply of reactive power before the system reaches the point of voltage collapse. During situations of outage in some critical lines, the generators are capable of supplying limited reactive power. But in the process, the real powers of the generators compromised while supplying this reactive power. In long transmission lines, the line length and the degree of shunt compensation are the most important factors affecting the power frequency voltages under normal and fault conditions. An open-end or unloaded line experiences a rise in the receiving end voltage related to sinusoidal input voltage, known as Ferranti effect. On the other hand, an overloaded line experiences a sequential reduction in voltage leading to voltage collapse at the weakest bus. To stabilize the line voltage, reactive power (VAR) compensation required, which is control of reactive power to enhance power system network performance. The two important features of reactive power compensation are:

  1. Load Compensation and
  2. Voltage Support.

The aim of voltage support is to decrease the voltage variations at a given terminal of a transmission line.

Line inductance compensation done by sequence capacitors and the line capacitance to earth by shunt reactors. Optimal placement of sequence capacitors are at different places along the line, when that of the shunt reactors is in the stations at the end of the line. In this way, the voltage drop/rise between the ends of the line can decrease both in amplitude and phase angle.