Protection Functions

The DIgSILENT PowerFactory protection analysis tool is an extension of the basic functional model library, containing additional devices like CTs, VTs, relays, fuses and more complex protection schemes including user-defined modeling 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, which then updates the settings both in the database and simulation environment.

All protective devices are also fully functional under steady state as well as transient conditions, allowing device response assessment under all possible simulation modes, including load flow, fault analysis, RMS and EMT.

Protection model library and functionality

The DIgSILENT PowerFactory protection analysis tool contains a comprehensive protective device model library and includes:

Time overcurrent relays for 1 phase, 3 phase, ground and negative sequence time overcurrents. Additionally, the relay characteristics can incorporate the following standards and solution methods:
- IEC 255-3
- ANSI/IEEE
- ANSI/IEEE squared
- ABB/Westinghouse CO (Mdar)
- Linear approximation
- Hermite-spline approximation
- Analytical expressions vi build-in formula editor and analyser
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.
Distance relays for phase, ground, and zone distance protection. Provision is available for incorporating overcurrent and under-impedance starting.
Different characteristics for distance relays steps including:
- MHO
- Polygonial
- Tomatoes
- Lens
- Circle
- R/X Blinder
- Offset characteristic
Voltage relays for undervoltage, instantaneous voltage, voltage balance and unbalance.
Additional devices include Breaker fail, Motor protection, Generator, and Out-of-step relays.
Apart from the standard circuit breakers the model library contains Low voltage circuit breakers and Fuses

Additional to these protection functions and relays DIgSILENT PowerFactory provides further devices and characteristics for more detailed protection system modeling, such as:

Current and Voltage transformers that include saturation effects
Conductor, cable damage curves, cable overload curves and inrush peak current modeling
Transformer damage curves (ANSI/IEEE Standard C57.109 – 1985) and inrush peak current modeling
Motor starting curves, cold and hot stall, in-rush peak current modeling, and any user defined curves

All protection device models are implemented within the composite model frame environment. This allows the user to easily design and implement their own models, by utilizing the graphical user interface for constructing block diagrams (see also stability section).

Protection Co-ordination

The co-ordination of overcurrent-time protection is done graphically on basis of the current-time diagram. Relays settings are modified using Drag & Drop for moving the characteristics. Short-circuit currents, calculated with the short circuit command, are shown in the diagram as a vertical line. In addition the corresponding tripping times of the relays are displayed. Co-ordination between relays at different voltage level is also available. Therefore currents are automatically based on the leading voltage level, which can be selected by the user.

For distance protection co-ordination two powerful graphical features are integrated. The R-X diagram for displaying the tripping area of distance relays and the line impedances. Several relays can be visualized in the same R-X diagram. This might be useful for comparison of two relays that are located at both ends of the same line. The relay characteristics with the connecting line will be shown in the same R-X diagram. After 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 time of the relay. For dynamic simulation, measured impedances of the relays can be displayed online. Therefore it's possible to detect problems graphically like a power swing.

The time-distance diagram is used for checking the selectivity between relays along a co-ordination path. The relays of a co-ordination 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 co-ordination 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 by cutting the given positive sequence impedance of the lines with the impedance characteristic of the relays.

Short circuit method: this is the main method for checking the selectivity. Along the co-ordination path short-circuits (user-defined fault type) are calculated. The tripping times for the time-distance curve are determined using the calculated impedances. The starting signal of a relay is considered, too.

As special feature blocking signals or POTT (Permissive over-reach transfer tripping) PUTT (Permissive under-reach transfer tripping) are also taken into account. In addition to tripping curves of distance relays the curves of overcurrent relays can be displayed and co-ordinated 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. As an option the breaker opening time can be ignored.

Protection diagrams:

Relay settings are modified very easily be clicking twice on the characteristics/curves. The settings dialog for the clicked zone pops up and the parameters can be modified. After confirmation of the changes with the OK button the dialog is closed and the characteristics/curves are adapted to the new settings immediately. The most important settings and commands in protection diagrams can be accessed easily using the context-sensitivity menu.