Loadflow Study Guide

Load flow is one of the most commonly use power system analyses. It is also the foundation for other types of analysis and simulation. InterPSS loadflow algorithm can be summarized as follows:

For a set of specified bus voltage (magnitude and angle) and/or bus generation (Pg, Qg) at generator buses and specified

load (Pl,Ql) at load buses, find a set of bus voltage by iterative computation steps, such that the power

mismatch between specified bus quantities and calculated bus quantities is within a user specified convergence

tolerance.

The mismatch is computed based on bus Y-matrix and bus voltage. Currently, InterPSS loadflow implementation includes Newton Raphson (NR) method, Fast-Decoupled (PQ) method and Guess Siedel (GS) method. It also features Functional Load, PV Bus Limit control, PQ Bus Limit control, Remote Q adjustment, Transformer tap adjustment, and Phase-shifting angle adjustment.

Create Loadflow Project

If you are new to the InterPSS graphic One-line editor, please read the InterPSS Graphic Editor User Guide to get familar with the basic steps to create an InterPSS project.

Project/Network Data

To define a Loadflow project, select Transmission application type and ACFL network type.

• Description：Description of the project.
• BaseKVA：Base KVA for the Per Unit system for the project.
• BaseFreq：Base Frequency in Hz for the project.
• Include adjustments/Controls：Include advanced adjustment and control in the analysis.

Bus Data

InterPSS loadflow bus model is shown in the above diagram. In addition to the Bus object common attributes, three pieces of information, 1) Generation; 2) Load and 3) Shunt G+jB, need to be defined to describe an AclfBus object. The bus data entry dialog is shown in the following diagram.

• Swing - Swing/Slack generation bus
  • Vspec(pu)：Bus voltage magnitude
  • Angle(deg)：Bus voltage angle
• PV - PV generation bus
  • Pgen(pu)：Generator active power
  • Vspec(pu)：Bus voltage magnitude
• PQ - PQ generation bus
  • Pgen(pu)：Generator active power
  • Qgen(pu)：Generator reactive power
• Capacitor - Capacitor bus
  • Qcap(pu)：Rated capacitor reactive power. The reactive power is treated as an impedance in the Loadflow calculation.
• Non-Gen : None generator
• GenScripting : Bus custom model scripting. See How to Script InterPSS Bus/Branch Object for details.
• Const-P：Constant power load
• Const-I：Constant current load
• Const-Z：Constant impedance load
• NonLoad：none load bus
• LoadScripting : Bus custom model scripting. See How to Script InterPSS Bus/Branch Object for details.
  • Pload(pu)：Bus load active power
  • Qload(pu)：Bus load reactive power
• Shunt G+jB(pu)：Bus shunt admittance；

The other disabled fields on the Bus data dialog box are for advanced Loadflow features. They will be enabled when the Include adjustments/Controls choice in the project data panel is selected.

Branch Data

Line Branch Model

Transformer Branch Model

Phase Shifting Transformer Branch Model

InterPSS loadflow branch models are shown in the above diagrams. Please note: InterPSS simulation model also supports a phase-shift angle at the to-bus side also.

To define a Line branch, select Branch Type - Line

• R(pu)：Line resistance
• X(pu)：Line reactance
• 1/2B(pu)：Half line charging admittance
• MvaRating1：Branch Mva rating for violation check
• MvaRating2：Branch Mva rating for violation check
• MvaRating3：Branch Mva rating for violation check

To define a Transformer branch, select Branch Type - XFormer. The following additional fields need to be defined:

• From TurnRatio(pu)：From bus side transformer turn ratio
• To TurnRatio(pu)：To bus side transformer turn ratio

To define a Phase-shifting Transformer branch, select Branch Type - PhaseShift-Xfr. The following additional fields need to be defined:

• From Shift Angle(deg)：From bus side phase-shifting angle. From-side bus voltage is shifted by the angle value.
• To Shift Angle(deg)：To bus side phase-shifting angle. To-side bus voltage is shifted by the angle value.

BranchScripting - See How to Script InterPSS Bus/Branch Object for details.

Run Loadflow Analysis

When your project has loadflow information, no matter it is Transmission system or Distribution system, ACLF, ACSC or TranStability network type, the Loadflow Analysis menu item will allow you to perform a loadflow analysis run.

• Casename：Run case name.
• Description：Description for the case
• NR: Newton-Raphson Method
• PQ: Fast Decoupled Method
• GS: Gauss-Seidel Method
• Error tolerance：Error tolerance for loadflow convergence check
• Max iteration：Maximum iterations for the run.
• Adjust Change Step for non-Divergence: InterPSS will adjust loadflow run process such that loadflow will never diverge.
• Initialize Bus Voltage：Initialize bus voltage before running the case. InterPSS holds loadflow results in memory. Therefore, a loadflow run could continue from a previous run or start from flesh by initializing the bus voltage.
• Show Loadflow Summary：If checked, the loadflow summary information will be displayed after a Loadflow run.
• InterPSS Grid Computing : Enable grid computing to distribute the loadflow run to a remote computer. See more details at InterPSS Grid Computing Solution.

Loadflow Summary Display

If Show Loadflow Summary option is selected when running a loadflow analysis, the summary will be display, as shown in the following figure.

By clicking the Bus Style option, the summary can be displayed in the IEEE Bus Style Format, as follows:

You can click the SaveAs button to save the result to a text file.

Loadflow Result Annotation

Loadflow results will be annotated on the One-line diagram after a loadflow run.

Loadflow Report

Loadflow report can be generated after a loadflow run.

InterPSS report is template based. You can create your own report template and plug into InterPSS to create your own custom report. Please see How to Customize InterPSS Report for details.

Loadflow Lab

In most cases, you can get a converged loadflow solution by applying the NR, PQ or GS method. However, there may be situations where the available methods cannot produce a converged loadflow solution, because of data entering errors or ill-conditioned operation conditions. In such situation, you may want to find out the reason by "looking" into the loadflow calculation process to diagnosis the problem.

Loadflow is a non-linear problem. There is no math equation/method to exactly tell you what is the problem when a loadflow run diverges, at least we have not found such equation/method yet. What InterPSS provides is a new concept - Loadflow Lab, which allows you to run loadflow step-by-step, using different method at your choice, and applying loadflow adjustment/control at any time you want. The design goal of InterPSS Loadflow Lab is to provide an environment for you to tune your network to get a converge loadflow solution, as if you were sitting in a power system laboratory and adjusting control buttons to turn a power system to a desired operating condition.

Adjustment, such as the Remote Q Adjustment, applies certain adjustment of, for example, bus generator Q, to maintain voltage at a specified bus. The adjustment amount is calculated by InterPSS using sensitivity analysis. Sensitivity analysis is a linearization prediction, trying to predict the non-linear system behaviour using sensitivity factor(s). InterPSS allows you to control the adjustment amount by specifying a change multiplying factor.

Step-by-step Run

By clicking the "NR>" button, a NR step will be applied. You can watch the Power Mismatch change after the click. Normally, the mismatch will decrease. In case, the mismatch increases, click Details to find out the bus(s) with large mismatch to diagnosis the problem.

• NR> : Run a Newton-Raphson method step
• PQ-P> : Run a Fast-Decoupled method P calculation step
• PQ-Q> : Run a Fast-Decoupled method Q calculation step
• GS> : Run a Gauss-Seidel method step
• Reset : Reset the network to its initial condition (voltage = 1.0)

Applying Adjustment/Control

When you have active adjustment/control, you will see enabled buttons, which allows you to apply all adjustment/control(s) in a category or an individual adjustment/control. During the calculation process, an adjustment/control could be turned off. For example, a PV Bus Limit control will be turned off when a violation found and the bus is turned to a PQ Bus.

The status window will display internal messages indicating actions taken by InterPSS simulation engine. For example, in the following case, PV Bus has been turned to PQ Bus because of limit violation.

Advanced Loadflow Adjustment/Control

When the "Include adjustment/Controls" option in the project data dialog is selected, InterPSS will allow you to define some advanced adjustment/control in your loadflow study.

The following is a summary of this advanced feature. More detailed information can be found at AC Loadflow Analysis Reference Manual.

Functional Load

Functional load allows you to model a bus voltage dependent non-linear load

Pload = Pload(0) * [ Const-P-factor + Const-I-factor * Voltage + Const-Z-factor * Voltage^2 ]

Qload = Qload(0) * [ Const-P-factor + Const-I-factor * Voltage + Const-Z-factor * Voltage^2 ]

PV Bus Limit Adjustment

PV Bus Limit Adjustment allows you to define Qmax and Qmin for a PV Bus. If during the loadflow calculation process, the Q required to support the PV Bus voltage (Vspec) exceeds the limit, the bus will be changed to a PQ Bus with bus Q = Qmax or Qmin.

PQ Bus Limit Control

PQ Bus Limit Adjustment allows you to define Vmax and Vmin for a PQ Bus. If during the loadflow calculation process, the actual bus voltage exceeds the limit, the bus will be changed to a PV Bus with bus Vspec = Vmax or Vmin.

Remote Q Adjustment

Remote Q Adjustment allows you to use the bus Q to control a remote bus voltage or a remote branch Mva flow. In the above example, the bus Q is used to control Bus-4 voltage at ReVspec = 1.0. If the required Q to support the remote bus voltage exceeds the limit [Qmax, Qmin], the bus will be changed to a PQ bus with Q = Qmax or Qmin.

You can also use the bus Q to control reactive Mar flow on a remote branch. For example, in the above diagram, the bus Q is used to control reactive power flow on branch Bus2->Bus4. The Mar flow direction is From->To and Mar is measured at the to-bus side.

Transformer Tap Adjustment

You can use transformer tap to control a bus voltage. The bus could be the terminal bus of the transformer or any remote bus in the network. You need to specify the bus to be voltage-controlled is on the transformer from-bus side or to-bus side, and the control tap is on the from-side or to-side. Also, limit [max/min turn ratio] and change step length need to be specified. If the Step = 0.0, it is assumed that the transformer tap could be adjusted continuously.

You can also use transformer tap to control reactive Mar flow on the transformer branch.

Phase-Shift Transformer Adjustment

You can use phase-shift angle to control power flow on the phase-shift transformer. You need to specify power control side (From Side or To Side) and power flow direction (From->To or To-From). Angle control is within the limit [AngleMax, AngleMin].

Area Interchange Power Control

(Todo ...)