Load Flow Study & Analysis

What is Load Flow Study?

The definition of a load flow study, or power flow studies, is a numerical analysis of the flow of electric power in any electrical system. A load flow study is also an assessment of the steady-state conditions of the electrical system. Its goal is to determine the flow of power, current, voltage, real power and reactive power in a system under any load conditions. During the design phase of a new project or when evaluating changes and additions to existing electrical system, a load flow study is necessary to ensure system voltages and current remain within safe limits and whether additional equipment or services will be required. The flow of electric power in any electrical system is referred to as “Load Flow” – Omazaki Engineering is a consultant serving load flow study and analysis. If you are looking for an electric load or power flow analysis and study consultants for your project in Indonesia and South East Asia, please contact us by sending an email to cs@omazaki.co.id or by filling in the form in contact.

The load flow study is one of the most difficult studies in the power systems studies. This study evaluates the ability of the system to adequately supply loads while remaining within required voltage and current ranges. The power flow study report will determine voltage and power factor across all buses, as well as current or power flow across all feeders.

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Importance of Load Flow Studies

Power flow studies are very helpful when making future plans by considering and analyzing various hypothetical situations related to electricity. For example, if the transmission line is to be removed from the system for maintenance, is the remaining line capable of serving the load without exceeding its rated value? A load flow study will answer this.

Through load flow studies we can get information about the voltage level (V) and the voltage phase angle (δ) on each bus under steady-state conditions. This is important because the magnitude of the bus voltage must be maintained within a defined limit. After the bus angle and voltage level are calculated using the power flow, the magnitude and deviation of the reactive (Q) and real (P) power through each lines can be calculated. Also based on the difference between the power flow at the sending and receiving ends, the losses in a given line can also be calculated. In addition, we can also find out more and less load status. Power flow solutions are essential for continuous evaluation of power system performance so that appropriate control measures can be taken if required.

In load flow studies, engineers and load flow studies consultant should be able to answer these questions:

• What are the voltage levels at all system nodes during operation?
• Are the power system elements (transformer, generator, cables, etc.) adequate or overloaded?
• Where is the weakest point of the system?

Load or power flow analysis is essential for the operation of the power system under current operating conditions, up-grading and future capacity expansion.

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Objectives and Purposes of Load Flow Study

The objective of load flow calculations is to determine the steady-state operating characteristics of the power system for a given load and generator real power and voltage conditions. Once we have this information, we can calculate easily real and reactive power flow in all branches together with power losses.

The other objectives of load flow studies is so that you are able to plan ahead and account for different hypothetical situations. For instance, let’s say a transmission line must be taken offline for maintenance reasons, are the lines that are remaining going to be able to handle the required loads without their rated values being exceeded?

So that load flow studies are commonly used to investigate:

• Component or circuit loading
• Bus voltage profiles (magnitude, phase angle, etc)
• Real and reactive power flow
• Power system losses
• Proper transformer tap settings

Conducting a load flow study using multiple scenarios helps to ensure that the power system is adequately designed to satisfy desired performance criteria for the most economical expenditure of initial capital investment and future operating costs.

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How to Perform Load Flow Study?

Steps in Load Flow Studies

The load flow study involves the following three general steps:

• Modelling of power system components and network.
• Development of load flow equations.
• Solving the load flow equations using numerical techniques.

Methods in Load Flow Analysis

There are three methods for calculating the power systems data:

• Gauss-Seidel System
  Gauss-Seidel System is one of the most common types of analysis. The advantages of this system are its simplicity in operation, limited computational power required, and less time to complete. However, its slow rate of convergence results in many iterations. A greater number of buses increases these iterations.
• Newton–Raphson Method
  Newton–Raphson method is a more sophisticated method, using the quadratic convergence, and can be used for more complex situations. This method takes fewer iterations to reach convergence, and therefore also takes less computer time. It also is more accurate since it is less sensitive to complicating factors such as slack bus selection or regulation transformers. One disadvantage is that programming can be complicated and requires a large computer memory.

• Fast Decoupled Load Flow System
  Main advantage of this method is that it uses less computer memory. The speed of calculation is 5x faster than the Newton–Raphson method, making it a popular choice for real-time management of power grids. However, it can be less accurate since assumptions are used to obtain fast calculations. Since it is more difficult to change this computer program to look for other problems such as power system security or flow, its scope is limited.

Ways to Perform Load Flow Studies

There are two ways to perform load flow studies

• Mathematical Analysis
• Software Analysis

Mathematical Analysis

There are number of steps to be done while mathematically analyze the load flow. They are:

1. Step 1 – Represent the system by its single-line diagram.
2. Step 2 – Convert all quantities to per unit
3. Step 3 – Draw the impedance diagram.
4. Step 4 – Obtain the Ybus matrix.
5. Step 5 – Classify the buses (swing bus, or generator bus, or load buses)
6. Step 6 – Start answering the missing variables, by assumptions (unless it is specified otherwise).
7. Step 7 – Find approximations for the real and reactive power that we are given, using the assumed and given values for voltage/angles/admittance.
8. Step 8 – Write the Jacobian Matrix for the first iteration of the Newton Raphson Method.
9. Step 9 – Solve for the unknown differences, use Cramers Rule.
10. Step 10 – Repeat step 7 – 9 iteratively until we obtain an accurate value.

Software Analysis

Power system analysis software is an excellent tool for studying power systems, but should not be used as a substitute for knowledge and experience. There many software are available to help to analyze the load flow, such as ETAP, SKM, EasyPower, PSS, Neplan, and the others. Using software simplifies the carrying out of a load flow study.  However, the selection of input data required, level of detail to model, verification and interpretation of the output and utilising this to achieve the required design still requires the input of a skilled electrical engineer.

Software is utilized in most of the realistic or real-time conditions since they are easier.  In doing this, the electrical engineer builds a network of nodes interconnected by admittances (impedances).

Each system node has four key parameters:

• Active power (P)
• Reactive power (Q )
• Voltage magnitude (V)
• Voltage phase angle (δ)

In defining nodes in a software model, the engineer typically considers three types:

• Load Bus [P-Q bus], a bus where the real and reactive power are specified.
• Generator Bus [P-V bus], a bus in which the voltage and real power generation is known.
• Slack Bus (Swing bus), where the voltage magnitude and phase are assumed known.

In a study most, nodes are of the load bus type.  The generator bus type is all the nodes that have a generator connected.  While more than one slack bus could be defined, it is usual to have only one, and this is chosen as the connection point to the main grid supply.

The engineers and load flow study consultant will construct the network mock-up by setting busbar, grid connections, interconnections, generators and major items of equipment. The software will then carry out the necessary calculations.

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Contact Omazaki Engineering if you need load flow study and analysis consultants for your electrical facilities or new projects in Indonesia and South East Asia.

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Related Articles

• Power System Study & Analysis – General Overview
• Short Circuit Study & Analysis
• Protection Coordination Study
• Motor Starting Study and Analysis
• Power System Transient Stability Study
• Harmonic Study and Analysis
• Arc Flash Study & Assessment
• Power Quality Study & Assessment
• Voltage Drop Study & Analysis
• Voltage Imbalance (Unbalance) Study

References

• ETAP – Load Flow Software
• Load Flow Analysis – https://www.bice-eeconsulting.com/services/load-flow-analysis/
• Load Flow Study & Analysis – https://3phtechservices.com/load-flow-study-analysis/
• Power Flow Study – https://en.wikipedia.org/wiki/Power-flow_study
• Load Flow Study and Analysys – https://carelabz.com/load-flow-analysis/