Distribution System Guide

InterPSS divides power system simulation into two main categories: 1) Transmission system analysis and 2) Distribution system analysis. InterPSS distribution system analysis module is design for medium and low voltage power distribution system analysis. In the distribution system analysis, power system equipments, such as generator and motor, are described using nameplate values, such as Ohms, or HousePower. InterPSS distribution system analysis shares the same underlying power system simulation framework and core simulation engine with InterPSS transmission system analysis.

Introduction

In InterPSS distribution system analysis, power system equipments, such as generator and motor, are described using nameplate values, such as Ohms, or HousePower. It is design for medium and low voltage power distribution system analysis.

Distribution System Representation

Currently InterPSS distribution system analysis can by used to model the following bus equipments:

• Utility Bus - For representing interface to utility company. Normally, there is only one such interface in a distribution system. InterPSS, however, allows you to have multiple such interfaces defined in your project. In such a case, you have to make sure that the Utility bus voltage angles are specified correctly.
• Generator Bus - For representing generators for distribution system loadflow and short circuit analysis.
• Synchronous Motor Bus - For representing synchronous motor for distribution system loadflow and short circuit analysis
• Induction Motor Bus - For representing induction motor for distribution system loadflow and short circuit analysis
• Mixed Load Bus - For representing mixed static KVA load and motor load for distribution system loadflow and short circuit analysis
• NonContribute Bus - For representing a non-contributing bus

Currently InterPSS distribution system analysis could be used to model the following branch devices:

• Feeder Branch - For representing feeder, including cable for distribution system loadflow and short circuit analysis
• Transformer Branch - For representing 2-winding transformer for distribution system loadflow and short circuit analysis
• Breaker Branch - For representing circuit breaker for distribution system loadflow and short circuit analysis

Loadflow Analysis

After you build an InterPSS distribution system project, you have defined enough information for loadflow analysis. By running loadflow analysis, you can find bus voltage, voltage dip, branch KW/Kvar flow and branch current flow.

Short Circuit (SC) Calculation

After you build an InterPSS distribution system project, you have defined enough information for short circuit calculation. By running Short Circuit calculation, you can calculate short circuit current, when there is a bus fault or a branch fault. You can select any fault location and apply 3-Phase, Line-Ground, Line-Line or Line-Line-Ground fault.

SC Standard and Multi-Point SC Calculation

One of the main purposes of short circuit calculation is for equipment selection and protective device coordination. Normally one would calculate short circuit current value at the following three points:

• Momentary Peak Value at half-cycle for validating equipment withstand
• Interrupting Value at 3-5 cycle to make sure the breaker will operate correctly
• Steady-state value, 20-cycle and beyond to make that concerned equipment will not be overheated or for fuse selection.

Different countries and different manufactures have different standards. However, you always need to perform multiple short circuit calculations. You can then use the calculation results to find out X/R ratio and to determine the DC component for protective device coordination. InterPSS allows you to perform up to 5 point short circuit calculation. Future version will allow advanced user to plugin their own standard.

Voltage Profile Analysis

Load and generation, in distribution systems, change with time. You may want to define load profile, a set of load points, and/or generation profile, to study how bus voltage varies with load change. You can also define service profiles for transformers to switch on/off to study the effect of such switching on system loss to optimize reactive power flow.

Create Distribution System 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 Data

To define a Distribution system project, select Distribution application type. The Network Type is always ACSC, which means you can perform Loadflow and Short Circuit analysis.

• Description：Description for the project.
• BaseKVA：Base Kva for the Per Unit system for the project.
• BaseFreq：Base Frequency in Hz for the project.
• Load Schedule Analysis : Allows to define a set of load schedule points for voltage profile analysis.
  • Load Points : number of load schedule points
  • Period Length : load period length
• SC Standard : Currently InterPSS support Generic and ANSI

ANSI Standard

When select ANSI standard, you can define up to five short circuit calculation points.

Bus Data

InterPSS currently has six distribution bus types: Utility 、Generator 、SynMotor 、IndMotor 、MixedLoad and NonContribute.

Utility Bus

Utility Bus is treated as a Swing bus in loadflow calculation and a contributing bus in short circuit analysis.

• BusName : Bus name, optional
• Base Volt : Bus nominal voltage
• Area : Area number, optional
• Zone : Zone number, optional
• In Service : If unchecked, the bus will not not participate in the calculation.
• Voltage : Bus actual voltage
• Angle : Bus voltage angle. In most cases, there is only one Utility bus. The angle should be set to zero.
• SC MVA(3P): Three phase short circuit MVA, which will be used to calculate the internal impedance for the bus.
• SC MVA (1P) : Single phase short circuit MVA, which will be used to calculate the internal impedance for the bus.
• X/R (3P) : X over R for three phase SC MVA
• X/R (1P) : X over R for sinle phase SC MVA
• Grounding : For short circuit analysis
  • SolidGrounded：Solid grounded bus
  • Z-Grounded：Grounded through an impedance Z (GroundR + jGroundX)
  • UnGrounded：Not grounded

The following equations are used to calculate the internal impedance for the Utility Bus:

Generator Bus

• Rated KW : Generator equipment rating
• Rated V : Generator equipment rated voltage, which may be different than the bus Base/Norminal voltage
• PFactor : Generator power factor
• Loading(%)：Generator loading factor, used to calculate the actual generation output = RateKW*Loading/100.
• Include Load Schedule : Define generator loading schedule, if the Load Schedule Analysis option has been selected in the project data panel. You can define a set of P, Q factors to describe a generator P/Q profile.

• Short Circuit Impudence : Generator short circuit impedance. In case of ANSI standard, you need to enter Z for multiple points, as shown in the following diagram.

• ZUnit : Generator impedance unit on generator equipment rating and rated voltage base
• Grounding : Data entry aame as the Utility Bus

SynMotor Bus

Synchronous Motor bus. Input data is the same as the Induction Motor bus.

IndMotor Bus

For modeling an induction motor

• Rated HP: Motor equipment rating
• Rated V：Motor equipment rated voltage, which could be different then the bus Base Voltage
• PFactor : Motor equipment power factor
• Efficiency(%)：Motor equipment efficiency
• Loading(%) : Motor equipment loading
• Include Load Schedule : Define load schedule, if the Load Schedule Analysis option has been selected in the project data panel. You can define a set of P, Q factors to describe a load P/Q profile. Data entry same as generator bus.
• Short Circuit Impudence : Define motor short circuit impedance. Data entry same as the generator bus
• Grounding : Data entry same as the Utility Bus

MixedLoad Bus

For representing mixed static load and motor bus

• Total KVA : Total bus load rating
• Rated V : Bus rated voltage, which could be different then the bus Base Voltage
• PFactor：Bus load power factor
• Motor(%)：Motor load percent. It should be always less than 100%. The other part (100%-Motor%) is treated a static load.
• Include Load Schedule Define load schedule, if the Load Schedule Analysis option has been selected in the project data panel. You can define a set of P, Q factors to describe a load P/Q profile. Data entry same as the generator bus.
• Short Circuit Impudence : Define motor short circuit impedance. Data entry same as the generator bus.
• Grounding : Data as same as the Utility Bus

NonContribute Bus

NonContribute Bus has no contribution to loadflow analysis and short circuit calculation.

Branch Data

Feeder Branch

For representing a feeder.

• Name : Feeder branch name, optional
• Area : Feeder branch area number, optional
• Zone : Feeder branch zone number, optional
• In Service : If uncheck, the branch is treated as an open branch in the calculation
• Feeder Name : Feeder name. It will be used in the future for default feeder data lookup.
• Length: Feeder length
• X/1000, R/1000：Feeder X and R impedance Ohms per 1000 unit length
• X0/X1, R0/R1：Feeder X0/X1, R0/R1 ratio
• B1/1000 : Feeder positive sequence shunt admittance, microMhos/1000. Also applies to feeder negative sequence shunt admittance
• B0/B1 : Ratio B0/B1, B0 being zero sequence feeder shunt admittance.

Breaker Branch

For representing a breaker.

• Breaker Name : Breaker name. It will be used in the future for default data lookup.
• R：Breaker resistance in ohms
• Include Service Schedule : You define a breaker on/off schedule for multiple points, if the Load Schedule Analysis option has been selected in the project data panel.

Transformer Branch

For modeling a transformer branch

• Xformer Name: Transformer name, It will be used in the future for default data lookup.
• Rating : Transformer KVA rating
• Primary/Second Rated V：Primary and secondary rated voltage.
• X, R, X/R : Transformer X, R, on the transformer rating and rated voltage base
• X0/X1, R0/R1 : Ratio for calculating zero sequence X and R
• Primary/Secondary TurnRatio : Transformer primary and secondary turn ratio
• Include Service Schedule : You can define a transformer on/off schedule for multiple points, if the Load Schedule Analysis option is selected in the project data panel.

• Grounding : Allow you to define grounding for the primary and secondary side
  • Delta Connection：△-connection
  • Wye Connection：Y-connection. For Y-connection, grounding type and optional grounding Z need to be defined.

Run Distribution System Analysis

After completing a distribution system project, you can run Loadflow Analysis and SC Analysis by select the menuitem or the toolbar button. Advanced user can write The Custom Run Scripts to define a custom run process. Please see How to Script Custom Analysis Run for details.

Run Loadflow Analysis

In distribution system Loadflow analysis, InterPSS treats the Utility bus as a Swing bus; the Generator bus a PQ generation bus; SynModor bus, IndMotor bus and MixedLoad bus as a constant power load bus. The nameplate values will be transfered to PU to perform the Loadflow analysis.

Select a Loadflow method and enter necessary info, then click Run to run Loadflow analysis. If the Show Loadflow Summary check box is selected, you will be presented with the Loadflow result summary, as shown in the following diagram.

Select the Bus Style to change the Loadflow output in the IEEE BusStyle format. By click the SaveAs button, you can save the Loadflow results into a file. Click Close button to close the result disply box. Then the Loadflow results are annotated onto the One-line diagram, as shown in the following diagram:

Load Profile Analysis

When the Load Schedule Analysis option is selected in the project data panel, you can specify multiple Load points at load buses and/or generation points at generator buses. When you run Loadflod analysis, InterPSS will run multiple Loadflow for all the points. The following is a sample output of a Load Profile analysis output.

After a Load Profile Analysis run, by clicking a load bus, you can plot its P, Q and voltage profile.

Run Short Circuit Analysis

In distribution system SC analysis, InterPSS treats the Utility bus, the Generator bus, the SynModor bus, and the IndMotor bus as contributing source. Motor load part of the MixedLoad bus is also treated as contributing source. The nameplate values will be transfered to PU to perform the SC analysis. When you run SC analysis, you will be presented with the following dialog box:

For bus fault, select a bus and Bus Fault type

• 3P：Three phase to ground fault
• L-G：Single phase to ground fault
• L-L：Line to line fault
• LL-G：Line to line to ground fault
• L-G（r+jx）：Line to ground fault impedance, applicable to 3P, L-G and LL-G
• L-L（r+jx）：Fault impedance between lines, applicable to L-L
• Loadflow Prefault Voltage：Use loadflow voltage as prefault voltage
• Fixed Prefault Voltage：Fixed prefault voltage (1.0 pu). You can use the MFactor(%) to increase/decrease fault current contribution
• MFactor（%）： A multiplying factor can be defined increase/decrease fault current contribution

For branch fault, select a branch and Branch Fault type. In addition to data defined in the bus fault section,

• Fault Distance ：Define fault distance in % from the From-Bus.

Select the Run button to run SC calculation. After the run, you will be presented with the following result summary screen:

SC results also will be annotated on to the One-line diagram after a SC calculation run.

Run ANSI Multi-Point SC

When the ANSI standard is selected in the project data panel, you can define up to five SC points. Different generator/motor SC impedance could be specified for different SC points to perform multiple-point SC calculation. The following is a sample output of a Multi-Point SC run.

Loadflow and SC Report

After a successful Loadflow or SC run, you can create Loadflow and SC reports, by selecting the menuitems under the Output menu or the "R" button.

Loadflow Report

Currently, you can create Loadflow analysis report in two formats: 1) Summary format and 2) the IEEE Bus Style format. Last reports are saved under the parent project, as shown in the following diagram.

A Sample Loadflow Summary Report

A Sample Loadflow IEEE Bus Style Report

SC Report

The following is a sample SC report.

Please note : InterPSS report is template driven. 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.

Sample Distribution System Project

In the InterPSS release distribution, there is a distribution system sample project folder : Distribution, in the Sample Workspace, where you can find distribution system samples. You can run these samples to explore InterPSS distribution system analysis capabilities.

• PDCBook_2-11.ipss - Sample from Book "Electric Power System Protection and Coordination" by Michael Anthony, Figure 2.11, page 73
• PDCBook_2-12.ipss - Sample from Book "Electric Power System Protection and Coordination" by Michael Anthony, Figure 2.12, page 75
• PDCBook_2-11_MPoints.ipss - Same as PDCBook_2-11, with three SC points
• PDCBook_2-11_LoadProfile.ipss - Same as PDCBook_2-11, with 24 load points.