Load Flow Analysis
Within the Load Flow analysis environment, the accurate representation of a variety of network configurations and power system components is possible.
- DIgSILENT PowerFactory offers a selection of calculation methods, including a full AC Newton-Raphson technique (balanced and unbalanced) and a linear DC method. The enhanced non-decoupled Newton-Raphson solution technique with current or power mismatch iterations, typically yields round-off errors below 1 kVA for all buses. The implemented algorithms exhibit excellent stability and convergence. Several iteration levels guarantee convergence under all conditions, with optional automatic relaxation and modification of constraints. The DC load flow, solving for active power flows and voltage angles, is extremely fast and robust (linear system; no iterations required).
- Any combination of meshed 1-, 2-, and 3-phase AC and/or DC systems can be represented and solved simultaneously, from HV transmission systems, down to residential and industrial loads at LV voltage levels. Neutral conductors can be modelled explicitly.
- The Load Flow tool accurately represents unbalanced loads, generation, grids with variable neutral potentials, HVDC systems, DC loads, adjustable speed drives, SVSs and FACTS devices, etc., for all AC and DC voltage levels.
- DIgSILENT PowerFactory offers a new, intuitive and easy-to-use modelling technique which avoids the definition of bus types such as SL, PV, PQ, PI, AS, etc. PowerFactory simply provides the control mechanisms and device characteristics which are found in reality.
More Load Flow Analysis Features
- Consideration of reactive power limits: detailed model for generator Mvar capability curves (including voltage-dependency).
- Practical station control features with various local and remote control modes for voltage regulation and reactive power generation. Reactive power is automatically adjusted to ensure that generator output remains within its capability limits.
- Various active power control modes, e.g. as dispatched, according to secondary or primary control, or inertial response.
- Supports device characteristics, such as voltage-dependent loads and asynchronous machines with saturation and slip dependency, etc.
- Comprehensive area/network power exchange control features using Secondary Controllers (SCO) with flexible participation factors.
- Transformer OLTC able to control local or remote bus voltages, reactive power flows and voltage-drop compensation (LDC) within distribution systems. Special transformer controller model for parallel transformers. Transformer tap adjustment supports discrete and continuous methods.
- Device controllers for shunts, doubly-fed asynchronous machines and other power electronics elements such as self-commutated converters (VSC), thyristor/diode converters or integrated FACTS devices.
- Local and remote control mechanisms for SVCs. Automatic and continuous control of TCR and TSC switching is performed within component ratings to hold the voltage at a given value.
- Correct representation of transformer vector groups and phase displacement.
- Shunts can be modelled to consist of a combination of series and/or parallel connected capacitors, reactors and resistors. Shunts can be connected to busbars and feeders or to the remote ends of cables and lines. Filters may consist of any number of shunt combinations, and automatic shunt switching can be included in the automatic voltage regulation.
- Support of the Virtual Power Plant model for generator dispatch based on merit order algorithm.
- Feeder load scaling to control power flows at feeder entry point – including nested and parallel feeders.
- Full support of any parameter characteristic and scale to allow parametric studies or easy definition of loading scenarios or load profiles.
- All operational data (generation and demand patterns, switch positions, etc) can be saved and maintained in distinct Operational Scenarios.
Further Special Functions
- Analysis of system control conditions
- Consideration of protection devices
- Determination of ‘Power at Risk’
- Calculation of Load Flow Sensitivities. Evaluation of expected active/reactive power flow and voltage changes in the network based on the effect of demand/generation or transformer tap change.
- Support of DPL scripts; e.g. to perform load balancing, determination of penalty factors or any other parameter required.
Load Flow Results
- Implicit calculation of a large number of individual result variables and summary figures
- Display of any variable within the single line graphic, station diagram, and a tabular Flexible Data Page
- Various colouring modes for the single line graphic to visualize quantities such as calculated loading and/or voltage levels
- Detailed analysis reporting, which can list overloaded system elements, unacceptable bus voltages, system islands, out-of-service components, voltage levels, area summaries, and more
- Detailed textual output with pre-defined or user-defined filters and levels
- DPL interactivity with all results
- Result export to other software applications such as MS-EXCEL
DIgSILENT PowerFactory features fault calculation functionality based on international standards as well as the most accurate DIgSILENT General Fault Analysis (GFA) method.
The following features and options are supported by all implemented fault analysis methods:
- Calculation of fault levels at all busbars.
- Calculation of short-circuit quantities at a selected busbar or along a defined section of line/cable, including all branch contributions and busbar voltages
- Calculation of all symmetrical components as well as phase quantities.
- User-definable fault impedance
- Provision of specially designed graphs and diagrams including all quantities typically required by the protection engineer
- Thermal overloads highlighted on the single line graphic for busbars and cables, with all equipment overloads available in a summary text report
- Calculation of Thevenin impedances as seen from the faulty node
- Calculation of apparent phase impedances (magnitude and angle) at any location along a transmission line/cable or busbar, for all branches, selected subsets thereof, or 1, 2 or 3 nodes from the faulted node
IEC 60909 and VDE 0102/0103
PowerFactory provides a strict and complete implementation of the most frequently used standard for component design world-wide; the IEC 60909 and VDE 0102/0103 fault calculation standard, according to the most recently published versions.
- Calculation of the initial symmetrical peak current Ik” and short-circuit power Sk”, peak short-circuit current ip, symmetrical short-circuit breaking current Ib, and thermal equivalent current Ith (IEC 60909-0 2001). Both minimum and maximum short-circuit currents can also be calculated based on network voltage c-factors
- Support of all fault types (three-phase, two-phase, two-phase to ground, single-phase to ground)
- Calculation of Ik with selectable “Decaying Aperiodic Component”
- Selectable method for calculating the peak short-circuit current in meshed networks
- User-definable fault impedance, conductor temperature and c-voltage factor.
- Fault calculation can optionally include or exclude motor contribution to the fault current
- Provision of specially designed graphs and diagrams required by the protection engineer for protection coordination and design
IEEE 141 / ANSI e 37.5
PowerFactory provides a thorough implementation of the IEEE 141/ANSI e37.5 fault calculation standard according to the latest published version. Special features are:
- Transformer tap positions can be included in the fault current calculation
- User-defined fault impedance and pre-fault voltage can be included in the fault current calculation
G 74 and IEC 61363
Complete Method/Multiple Faults
DIgSILENT PowerFactory’s Complete Method is especially designed for protection coordination purposes or for analysing observed system contingencies. It provides the required algorithms and precision for determining the “true” or “operational” short-circuit currents without considering the simplifications or assumptions typically made in standard fault analysis.
In addition to the high precision network model, multiple faults which occur simultaneously in the system or unusual fault conditions such as inter-circuit faults or single-phase interruptions can be analysed.
- The Multiple Fault Analysis executes a complete network analysis based on subtransient and transient representations of electrical machines taking into account all specified network devices with their full representation and pre-faulted load conditions.
- Combination with IEC60909 principles for the calculation of aperiodic components and peak short-circuit currents
- Calculation of peak-break and break-RMS currents
- Consideration of a complete multi-wire system representation. Applicable to single-phase or two-phase networks.
- Analysis of multiple fault conditions
- Calculation of any asymmetrical, single or multiple fault condition with or without fault impedance, including single- and double-phase line interruptions.
DC Short-Circuit Calculation
PowerFactory offers the following DC short-circuit calculation options:
- DC short-circuit according to IEC 61660
- DC short-circuit sccording to ANSI/IEEE 946
The maximum and minimum short-circuit currents can be analysed from various DC based models such as:
- AC/DC converters (rectifier/inverter) in bridge connection (ElmRec and ElmRecMono)
- Smoothing capacitors (only for IEC 61660) (ElmShnt)
- Batteries (ElmBattery)
- DC motor/generator (ElmDcm)
The DC short-circuit calculation can be initialised using the results of a load flow calculation (optional). With this option selected, instead of taking a constant pre-fault voltage factor into account, the load flow calculation is used to determine the pre-fault voltages in the DC system. Upon completion to the calculation, the user may access a complete set of result variables as definded in the standards, among them:
- Peak short-circuit current
- Quasi-steady-state short-circuit current
- Time to peak
- Rise and decay times, rate of rise
- Equivalent system resistence and inductance, network time constant, etc.
Fault Analysis Results (all Methods)
PowerFactory offers many reporting options, including detailed reporting on all short-circuit levels for all faults, or alternatively, a specific report for a particular fault type. Special protection reports can also be generated to include impedance, current and voltage information.
- Display of any variable within the single line graphic, station diagram and Flexible Data Page
- Fully flexible filter mechanisms to display objects in colour mode
- Detailed analysis reporting, which can list overloaded system elements, unacceptable bus voltages, system islands, out-of-service components, voltage levels, area summaries and more
- Detailed text output with pre-defined or user-defined filters and levels
- DPL interactivity with all results
- Result export to other software applications such as MS-EXCEL or MS-ACCESS
Load Flow Sensitivities
Supplementing PowerFactory’s voltage stability analysis suite is the Sensitivity Analysis tool. It is often required to not only know the critical point of a system, but also how this critical point is affected by changes in system conditions. PowerFactory’s Sensitivity Analysis tool performs a static voltage stability calculation according to the following options:
- Sensitivity to a single busbar (calculation of the voltage sensitivities of all busbars and branch flow sensitivities according to variations in power (ΔP and ΔQ) at the selected busbar).
- Option to calculate sensitivities with respect to all busbars simultaneously.
- Sensitivity to a transformer tap position change (calculation of the voltage sensitivities of all busbars and branch flow sensitivities according to changes of a transformer/quad booster tap).
- Modal analysis
- Identification of “weak” and “strong” parts of the network based on modal transformation of the δv/δQ sensitivity matrix.
- Eigenvalue calculation on the δv/δQ sensitivity matrix, with a user-defined number of eigenvalues to be calculated.
- Results of eigenvalues are displayed (in descending order according to magnitude), and branch/bus sensitivities can be displayed for each mode.
Asynchronous Machine Parameter Identification
Overhead Line and Cable Parameter Calculation
DIgSILENT PowerFactory incorporates the automatic calculation of the electrical parameters of any cable/overhead line configuration starting from layout and geometric characteristics which are typically available in manufacture’s datasheets. The calculation is applicable over a wide range of frequencies and supports the step-up process of highly accurate line and cable models for harmonic analysis, frequency sweep and EMT-simulation among others. The supported options are described below.
Overhead Line Parameter Calculation
- Any combination of line circuits (1-, 2- and 3-ph), neutral conductors and earth wires, with/without automatic reduction of earth wires
- A flexible definition of tower types and tower geometries, including conductor sags, allowing a multiple combination of tower geometries and conductor types that avoids entry of redundant data
- Circuit-wise, symmetrical and perfect transposition and user-defined phasing for the definition of any non-standard transposition scheme
- Solid and tubular conductor types, including sub-conductors for phase circuits and earth wires
- Skin effect
- Equivalent impedance and admittance matrices in natural, reduced and symmetrical components
Cable Parameter Calculation
- Multi-phase single core and pipe type cable systems
- Flexible definition of cable layouts, including conducting, semi-conducting and insulating layers
- Compact and hollow core shapes, filling factor for stranded conductors
- Consideration of skin effect
Calculation of layer impedances and admittances in natural, reduced and symmetrical components, including sheath and armour reduction, cross-bonding
Basic MV/LV Network Analysis
- Feeder Plots: Graphical display feature (Virtual Instrument, VI) to increase transparency in grid loading and voltage profile analysis along the feeder. Displayed result variables are freely configurable. Full interactivity is given via the VI to access all relevant data of the components belonging to the feeder.
- Schematic Visualisation of Feeder: Automatic generation of single line diagram to visualize components of the feeder with distance/index view.
- Feeder Load Scaling: A load flow calculation feature that allows the automatic adjustment of individual bus loads to match a specified total feeder load. The selection of loads which are to participate in the feeder scaling procedure is user-defined. This method allows for complex scaling scenarios with nested and parallel feeders.
- Backbone Calculation: A calculation that allows the determination and visualisation of the main connections between meshed feeders. Various methods are available to determine backbones, ranging from purely topological citeria, and cross-section analysis, to more sophisticated methods that score quality of power restoration.
Low-Voltage Network Analysis
PowerFactory integrates enhanced features designed especially for the analysis of LV networks. These functions enable the user to:
- Define loads in terms of numbers of customers connected to a line
- Consider load diversity
- Perform a load flow analysis that considers load diversity for calculating maximum voltage drops and maximum branch current
- Perform cable reinforcement optimization to either automatically reinforce selected cables, or to provide a report of recommendations
- Perform voltage drop and cable loading analysis
- Perform statistical calculations of neutral currents caused by unbalanced single-phase loading and load diversity, to represent a realistic network
Stochastic Load Modelling
On the basis of defined ‘customer units’ the user may specify a number of customers connected to a line. Load flow options are provided to define the load per unit customer according to:
- Power per customer unit
- Power factor
- Coincidence factor for an infinite number of loads (i.e. ‘simultaneity factor’)
In addition, the user may select one of two methods for considering the stochastic nature of loads:
- Stochastic evaluation (theoretical approach, also applicable to meshed networks)
- Maximum current estimation (application of stochastic rules for estimating maximum branch flow and maximum voltage drops)
The Load Flow with stochastic load modelling then provides maximum currents for each branch component, maximum voltage drops, and minimum voltages at every bus bar.
The usual variables for currents and voltages in this case represent average values of voltages and currents.
Losses are calculated based on average values; the maximum circuit loading is calculated using maximum currents.
The PowerFactory Feeder Tools comprise a set of tools for radial systems to change voltage levels, phase technology or to optimise phasing from a particular point downwards.
Voltage and Phase Technology Change Tool
- Automatic change of the voltage level and/or phase technology inside a pre-defined feeder
- Automatic replacement of type data (for transformers, lines, loads and motors) according to pre-configurable type mapping tables – including automatic creation of new compatible types if necessary