Protection Functions
The basic functional model library of DIgSILENT PowerFactory’s protection analysis tool has been extended to include additional devices such as CTs, VTs, relays, fuses and more complex protection schemes including user-defined modelling capabilities. Additionally, there are specially designed interactive VIs (Virtual Instruments) for displaying system quantities and, more importantly, for modifying protection settings in the graphical environment. This last feature is especially useful, as coordinated settings between different protection schemes can be modified via the cursor in the graphical environment, following which the settings in both the database and the simulation environment are also updated.
All protective devices are fully-functional under steady-state and transient conditions, allowing device response assessment under all possible simulation modes, including load flow calculation, fault analysis, RMS and Instantaneous Values (EMT) simulation.
PowerFactory’s main protection features are:
- Extensive relay database
- Accurate steady-state relay checking via short-circuit and load flow (balanced & unbalanced)
- Precise dynamic relay checking with RMS and EMT simulations
- Consideration of current transformer saturation
- Diagrams for overcurrent and distance coordination:
- Time-overcurrent diagrams
- R-X characteristic diagrams
- Time distance diagrams
- Automatic Protection Coordination Wizard for time-overcurrent protection schemes
Protection Model Library and Functionality
The DIgSILENT PowerFactory protection analysis tool contains a comprehensive protection device model library. All relays are modelled for steady-state calculations (short-circuit, load flow), RMS and EMT simulation modes. The definition of relay types is highly flexible via block diagrams. For RMS and EMT simulation purposes, relays may be extended and adopted to cope with user specific requirements via the PowerFactory DSL language The features of the protection model library are listed below.
Fuses are represented by their melting curves. It is possible to take minimum and maximum melting curves into account.
Time-Overcurrent Relays for 1-phase, 3-phase, ground and negative sequence time over-currents. Additionally, the relay characteristics can incorporate the following standards and solution methods:
- IEC 255-3, ANSI/IEEE and ANSI/IEEE squared
- ABB/Westinghouse CO (Mdar)
- Linear approximation, Hermite-spline approximation
- Analytical expressions via built-in formula editor and analyzer (DSL)
Instantaneous Overcurrent Relays for 1- phase, 3-phase, ground and negative sequence time over-currents.
Directional Relays for overcurrent, power, ground current, and any combination of time and instantaneous overcurrent relays. Additionally, voltage and current polarization is used for the detection of negative and zero sequence components considering also dual polarization. Optional: with voltage memory.
Distance Relays for phase, ground and zone distance protection. Provision is available for incorporating overcurrent and under-impedance starting units (U-I or Z) as well as angle under-impedance.
Different characteristics are available for distance relay zones including:
- MHO, offset MHO
- Polygonal, offset polygonal
- Tomatoes, lens and circle
- R/X Blinders and quadrilateral
Support of various polarizations such as:
- Self-polarized
- Cross polarized (90ø connection)
- Positive, negative sequence polarized
- Optional: voltage memory
Zero sequence and parallel line compensation
Voltage Relays for under-voltage, instantaneous voltage, voltage balance and unbalance.
Additional devices such as: Breaker Fail, Motor Protection, Generator Protection, Differential Protection, Reclosing Relays, Low Voltage Circuit Breakers, and Out-of-Step Relays.
In addition to these protection functions and relays, DIgSILENT PowerFactory provides further devices and characteristics for more detailed protection system modelling, such as:
- Current and voltage transformers that include saturation effects
- Conductor, cable damage curves, cable overload curves and inrush peak current modelling
- Transformer damage curves (ANSI/IEEE Standard C57.109-1985) and inrush peak current modelling
- Motor starting curves, cold and hot stall, in-rush peak current modelling, and any user-defined curves
All protection device models are implemented within the composite model frame environment. This allows users to easily design and implement their own models, by utilizing the graphical user interface for constructing block diagrams.
Output & Graphical Representation
Time-Overcurrent Diagrams
- Overcurrent curve adjustment using drag & drop
- Display of tripping curve tolerances during drag & drop
- User-defined labels
- Tripping times are automatically displayed for calculated currents in time-overcurrent diagrams
- Display of an unlimited number of overcurrent curves in diagrams
- Simple creation and addition of diagrams via single line graphics
- Display of motor starting curves, conductor/cable and transformer damage curves
- Balloon help showing name of relay, etc.
- Double-click on curves to change relay settings
- Additional axis for voltage levels
R-X Characteristic Diagrams
- Display branch impedances with several options
- Automatic display of calculated impedances
- Adding relays with offset
- Flexible display of zones (starting zones, etc.)
Time Distance Diagrams
- Different methods for calculating curves: kilometrical or short-circuit sweep method
- Forward and/or reverse diagram
- Selectivity check of distance and overcurrent relays/fuses in same diagram
- Separate overreach zone representation
- Additional axis showing relay locations and busbars/terminals
- Selectable x-axis scaling (length, impedance, reactance, 1/conductance)
Single Line Diagram
- Colouring of switches according to relay locations, relay tripping times
- Display of relay tripping times in result boxes
- Additional text boxes for relay settings
Relay Setting Report
Relay Tripping Report
Overcurrent-Time Protection
The coordination of overcurrent-time protection is performed graphically using the current-time diagram as the basis. Relay settings are modified using drag & drop to move characteristics. Short-circuit currents calculated by the short-circuit command, are shown in the diagram as a vertical line. In addition, the corresponding tripping times of the relays are displayed. Coordination between relays at different voltage levels is available. Therefore, currents are automatically based on the leading voltage level, which can be selected by the user.
Distance Protection
For distance protection coordination, two powerful graphical features are integrated. The first of these features is the R-X diagram for displaying the tripping zone of distance relays and the line impedances. Several relays can be visualized in the same R-X diagram. This can be useful for the comparison of two relays that are located at different ends of the same line. The relay characteristics and the impedance characteristic of the connecting line will be shown in the same R-X diagram. Following short-circuit calculations, the measured impedances are visualized with a marker in the shape of a small arrow or cross. From the location of the marker the user can see the tripped zone and its associated tripping time. For dynamic simulation, measured impedances of the relays can be displayed, thereby visualizing the functioning of power swing blocking or out-of-step tripping relays.
The second powerful graphical feature is the time-distance diagram, which is used for checking the selectivity between relays along a coordination path. The relays on a coordination path can be displayed in diagrams for forward, reverse or for both directions. Consequently, it is very easy to check the selectivity of the relays along a coordination path. Two different methods for calculation of the tripping curves are provided. These are the kilometric and the short-circuit method.
- Kilometric method: The reach of the zones is calculated from the intersection of the given positive sequence impedance of the lines, and the impedance characteristic of the relays.
- Short-circuit method: This is the main method for checking the selectivity. Short-circuits (user-defined fault type) are calculated along the coordination path. The tripping times for the time-distance curve are determined using the calculated impedances. The starting signal of a relay is also considered.
A special feature of the distance protection is the consideration of blocking signals or POTT (permissive over-reach transfer tripping), PUTT (permissive under-reach transfer tripping), which are also taken into account. In addition to tripping curves of distance relays, the curves of overcurrent relays can be displayed and coordinated in the same diagram using the short-circuit method.
Both the kilometric and the short-circuit method consider breaker opening times in the calculation of tripping times. The breaker opening time can be optionally ignored.
Protection Coordination Wizard
The Protection Coordination Wizard for automatic overcurrent protection relay settings calculation is used to:
- Verify that the settings (thresholds, time delays and curve shapes) of the overcurrent devices satisfy the requirements to achieve the protection of the circuit respecting the selectivity constraints and guaranteeing “normal operation” of the system.
- Calculate the settings (thresholds, time delays and curve shapes) to satisfy protection, selectivity and “normal operation”.
The Protection Coordination Wizard has the ability to verify/calculate the selectivity for each protective device using the settings of:
- one phase inverse element
- two phase definite time elements
- one phase ground inverse element
- two ground definite time elements
In order to protect the system, the rules implemented in the wizard calculate the relay settings of:
- one phase inverse element
- two phase definite time elements
- one ground definite time element
