You require load flow analysis whenever you design a new project or plan to change the existing system. It is essential to conduct research beforehand to ensure that the current and voltage are within safe limits. Moreover, it checks for the additional services the equipment may require for maintenance. If you fail to conduct this step, you may bear the consequences of repairing it. Today, we are going to have a clear picture of load flow.
Table of Contents
What is the Load Flow Study?
We define load flow or power flow study as the numerical analysis of power flowing through the electrical systems. In addition, you can analyze the steady conditions using power flow calculation. The phenomena determine the flow of current, changes in voltages, real and reactive power flow of an electric circuit under any load circumstances.
The electricians see the power flow study as one of the most challenging yet essential power system analyses. Thus, it checks the ability of the power grid to supply the load adequately without overflowing the current and voltage magnitude. The resulting report determines the power changes across all the buses and the current load flow in generating feeders. Additionally, it also points out if there are any overloading problems in the system.
Why is the Load Flow Study critical?
A deterministic load flow study helps you tackle the hypothetical scenarios to prepare you for the upcoming problems. It is vital as if you want to remove any line for the maintenance of the system, analysis can tell you if the system is still capable of supporting the load.
Moreover, you can conduct the load flow study to check the unknown voltage magnitude V values and its phase angle (δ). Hence, complete the test when load buses are under steady-state conditions. Also, the voltage magnitude and angle determine the reactive and real power through each line. Thus, the difference between input and output power flow explains the reactive power losses occurring in the system.
Load Flow metrics are essential to calculate, as continuous evaluations of the electrical system will help you take control measures on time. Thus, you have an upgraded design that can work even if you plan to expand power systems in the future.
Objectives of Power Flow Load Study
The aim for testing the flow load is simple, to determine the power characteristics in a steady-state of the electric power lines. In addition, it makes you able to plan if you come across outrage of power grids at any moment.
In general, the load flow study investigates
- The flow of reactive and real power
- Voltage drops and power loss in an electrical system
- Load on flow items (it may be an individual component or a whole circuit)
- Voltage magnitude and voltage phase angle
- Adequate settings in power generating transformer
Ways to perform Load Flow Study
In practice, you can perform the energy flow analysis using two methods.
To perform the mathematical analysis of the flow load of the system, you have to follow these simple steps.
- Draw the plan in a signal line format.
- Convert the quantities into appropriate units
- Draw an impedance diagram of the circuit
- Create a Ybus matrix
- Classify the under observation buses as swing bus, generator bus, or load bus.
- Assume the missing variables of the equation if not specified.
- Find the approximate values for real power and reactive power with the help of the values you have assumed.
- Note down the first iteration in the Jacobian Matrix if you are using Newton Raphson Method.
- If there are unknown differences, solve them using Crammer’s Rule
- Repeat the 7 to 9 steps until you have an accurate value.
There are various solution methods available in the form of software for power system analysis. However, you cannot use them without any experience or knowledge. The applications such as EasyPower, ETAP, and SKM save the time you spend on calculations. But, to select the appropriate input data and level of detail and interpret the output, you must be a skilled electrical engineer.
Generally, you can work with software in scenarios where you require realistic, quick, and real-time readings. For this purpose, the electricians also need to create a network based on the impedance of the nodes.
Load Flow Analysis Method
To solve the mathematical problems, you can opt for different solution methods. Commonly used are:
For the Newton Raphson method, you will start with hypothetical values of all variables. It includes voltage values and phase angle on load bus and the value of voltage angle at generator bus. Then you will write it down in the form of the Taylor series and ignore the higher terms. The resulting values are
Where P and Q have mismatch equations.
Here, J is the Jacobian Matrix that you can describe as
You will solve the linear system of the equation to check for the following hypothetical value of voltage magnitude |V| and phase angle θ. Where
You will continue this algorithm until the values meet the condition to terminate. So, the mismatch conditions will be below the average tolerance level.
You may know the Gauss-Seidel method as the successive displacement method as well. In general, the GS method uses the iterative technique to solve the load flow problem until f(x) approaches zero.
Thus, the GS method solves the problem with the following steps;
- First, arrange the equation in the form of x=g(x) so that you can calculate the unknown variable.
- Then, calculate the value for g(x) using the hypothetical value of x.
- In the iterative step, use the improved vale and find the value for g(x) again.
- Continue repeating the steps until the rate of deviation from the actual value is minimal.
The successive displacement method may slowly converge to the value as you may experience in the other ways. So, you will not solve the matrix equation in this method. Additionally, it does not require high processing speed if you use software to calculate, thus saving resources.
Fast Decoupled Load Flow Method
The fast-decoupled method is based on the same principles as the Newton Raphson method. However, it uses the approximate values of solid coupling among active and reactive energy flows. Moreover, it improves the Jacobian matrix since it avoids multiple iterations.
In this method, the matrix is inverted only once, obtaining results much faster than the Newton method. Hence, you can use the fast-decoupled flow method for quick management of power stations.
Holomorphic Embedding Load Flow Method
The Holomorphic Embedding Load-Flow Method or HELM is an advanced solution to power system equations. Instead of using the iterative approach for analysis, it is the direct method. Yet, it ensures that you calculate the correct operating branch and don’t waste resources trying the other possible paths.
Back-Forward Sweep (BFS) Method
The BFS method takes advantage of modern distribution grids, i.e., in a radial form. In this method, you again choose the values of voltage. Next, you divide the whole system into two parts and solve one of them. Then using the results, you solve the other equations. You will repeat the process until the values no longer diverge.
Therefore, you will get the voltage values from the current (Backward sweep) or the current values from the voltage (forward sweep).
Steps to perform the Load Flow Study
To study the flow load of the system, you need to follow these simple steps;
- Creating a model of the power generation system that includes all the components and how they are attached
- Developing power flow equation
- Solving the flow load equations using numerical formulas
Modeling the Power System
You can model the power flow system for the analysis and management of the power grid. As a result, it will provide equations for the flow efficiency in each transmission line. Since the flow velocity and the load impedance are the functions of the voltage square, the above equations are non-linear.
Modeling the Components
To model the flow items, you will get
In a transmission line, you can represent the model in the form of nominal π
Where R+jX is the impedance of the line and Y/2 is the impedance of the half charging line.
As for the off-nominal transformer, the transformation ratio (a) will be
Transformation Ratio (a) = Top ratio/Nominal ratio
Converting it into a nominal π model makes the circuit like
Solving the circuit, you can get the values of admittance Y1, Y2, and Y3. Some observations that you will make are
Thus, the values of Y2 and Y3 will entirely depend upon the transformation ratio instead of positive or negative. Since, as you think about what does the positive and negative values mean,
- If Y is negative, the system is absorbing the reactive power and behaving like an inductor.
- But if Y is positive, the system is generating reactive power and acting as a capacitor.
Modeling the System
Now, consider an electrical system with two busses as follows
As you model the whole circuit, you will get to know that
Power generation at the bus “i” is
And the demand at the bus “i” is
Hence, you can define the net power bus “i” gets through the following equation.
Developing Load Flow Study Equations
As you are done modeling the system, you will create a basic equation with the help of its nodal analysis.
For example, for a matric as
Here, the Yij is the element of the bus impedance matrix, Vj is the matrix of bus voltage, and Ii is the current at each node. You can describe the equation for nodes at the bus I as
Formulation of power-flow study
For each bus in the system, the following variables are associated,
While solving the equation for any circuit, you will know the values of the two above variables. With the help of known values, you will then find out the values of the other two variables. These variables depend upon the type of bus in the power system among these three categories.
Load bus (P-Q bus) – the type of bus where there is no generator. In this type of bus, you know the value of real and reactive power, i.e., V and δ. Thus, it would help if you calculated the bus voltage.
Generator Bus (P-V bus) – In this type, you know voltage magnitude values and keep them constant by adjusting the current field. Also, you will assign the real power values to each generator and then find the values for other variables.
Slack Bus – the generator bus or the reference bus. Hence, you will assume that it has a fixed voltage phase angle and magnitude. You know the values of P and Q to find the V and δ.
Solving the Load Flow Equations
All the equations that you get in the load flow study are non-linear. So, you cannot solve them through an analytical process. Because of the following reason, you will solve it through the procedure of mathematical analysis.
To summarize, you will create an impedance matrix of the system and estimate the voltage values. Then using the above power equations, you will substitute the actual values you know and determine the other parameters. Next, update the voltage using any numerical algorithm and repeat the process till all deviations are minimal.
Load flow study uses mathematical analysis methods to find unknown power parameters. It checks that whether the system can handle maintenance or future expansions. Remember that the planning engineer needs to choose the appropriate form of analysis. Moreover, you can opt for excellent software to carry out the calculations as well.
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