Know Your Electrical Network Through Power System Study – Load Flow
Load-flow studies are performed to determine the steady-state operation of an electric power system. It calculates the voltage profile, voltage drop on each feeder & at each bus, real power (MW/KW) & reactive power (KVAR/MVAR) power flow in all branch and feeder circuits, power factor, transformer LTC setting, generator’s MVAR demand (Qmax & Qmin), total generation v/s power demand, MW & MVAR losses.
Load flow analysis has great importance:
a) To verify the operation of a network under various load and generation conditions
b) To plan the future growth of both loads and generation
c) To determine the best economical operation for existing systems
d) To establish initial conditions for stability studies
e) To help identify the need for additional capacitive or inductive VAR support, to maintain system voltages within acceptable limits
Required input data to carry out the load flow study
This clause presents input data organization in general terms along with some comments on data preparation. The data are divided into system data, bus data, load data, source data, branch data, and transformer data
2. System Data
Most load flow programs perform their calculations using a per unit representation of the system rather than working with actual volts, amperes, and ohms. Selecting the nominal voltage to be the base voltage simplifies the analyses and reduces the chance of errors in the interpretation of results
3. Bus data
Buses represent the nodes of the electrical system and can be classified based on their conditions of load and/ or generation. The bus data describe each bus and the load and shunts connected to that bus. Bus ID, name, and/or number, Bus classification (swing/voltage controlled/load bus), Bus service (in/out) status, Bus nominal voltage, Bus rating/continuous amps, Initial voltage, and angle
4. Load types & data
In a load flow simulation, the voltage magnitudes and phase angles of load terminals are calculated and they change according to overall loading and network conditions.
The load data are used to represent the load at various locations.
5. Source Data
- Generator Data – The data defines the generator power output and how voltage is controlled by the generator.
- Swing (slack) Source – Load flow models require at least one swing source in every isolated subsystem.
- Voltage control generator – The active power supplied is kept constant at its scheduled value, i.e., the generator is base loaded (the governor is in droop mode). This represents a generator where the voltage is controlled by the excitation system and the real power is controlled by the prime mover.
- Branch data – Data are also entered for each branch in the system. Herein the term branch refers to all elements that connect two buses including transmission lines, cables, series reactors, series capacitors, and transformers. In the real system, there may be multiple elements in series (e.g., an overhead transmission circuit that transitions into a cable circuit); when using modern simulation software that does not impose practical limits on the number of nodes in the model, it is preferable to treat each of these elements separately connected by a “node.”
- Transformer Data – Additional data are required for transformers. These can either be entered as part of the branch data or as a separate data category, depending on the particular load flow program being used. Depending on the design of the software. Transformers with rated primary or secondary voltages that do not match the system nominal (base kV) voltages on the terminal buses will require an off-nominal tap representation in the load flow model (and possibly require the corresponding adjustment of the transformer impedance).
- Example system input data – Note that the amount of input parameters required depends on the complexity and capability of the simulation tool used to perform a load flow analysis and the needs of the analysis. As always, the power system model needs to be as simple or as complex as is needed to solve the problem at hand.
Standard to be used
IEEE: 3002.2-2018 – IEEE Recommended Practice for Conducting Load-Flow Studies and Analysis of Industrial and Commercial Power Systems
In all the realistic cases the Load Flow analysis cannot be solved analytically, hence iterative solutions implemented in computers must be used for the analysis. The Consultant has used ETAP version 21.0.1 for carrying out the simulations.
Predominantly there are three main iterative numerical methods to solve non-linear algebraic equations which are.
It should be evident to designers and operators of industrial and commercial power systems, as well as to utility system engineers, that a tool that predicts the actual performance of their electrical systems (under various steady-state operating conditions) is very valuable and essential for the short- and long-term operation of the power systems. That is why load flow is called “the mother of all studies.” Load flow analysis is used to conduct conceptual design, and to determine equipment size, transformers tap settings, switching interlocks logic, operational limits, etc.
It is therefore essential to study the system to keep the operating voltages within the ideal operating range (e.g., 98% to 102%), within an acceptable range (e.g., 95% to 105%), and within the manufacturing range (e.g., 90% to 110%).