Protection Calculation

This section outlines how to perform a protection calculation using the grid model, including how to interpret the results. Grid protection calculation ensures that electrical faults - such as short circuits - are reliably detected and cleared by Fuses, Distance protection devices, or Definite Time Overcurrent Protection devices to maintain safety and system stability.

Grid

The calculation uses the fuses and protection devices defined in the grid model. Key results such as switch-off times, short-circuit currents, and limit indicators are displayed directly on the grid. Buses not deenergized by any device are highlighted in red with a lightning bolt. Select a bus and then click to trace whole path to the slack bus.

Where

• Orange = paths of the selected element to all network feeders incl. loops.
• Blue = electrically connected area.

Calculation

Branch Currents

Switch-off times are calculated by:

1. Determining minimum single-phase and two-phase short-circuit for each bus.
2. Calculating branch currents for a short-circuit at a given bus.
3. Using branch currents and characteristic curve of a fuse or stages of a protection device, switch-off times are calculated for considered device.

Fuse curves are interpolated using natural cubic spline interpolation as described in:

R.L. Burden, J.D. Faires, Numerical Analysis, 4th Ed., 1989, PWS-Kent, ISBN 0-53491-585-X, pp. 126–131.

Assumptions

The following assumptions are made for short-circuit calculation:

• 10% voltage tolerance
• 80°C branch temperature

For more details, refer to the short-circuit computation page and options dialog.

Limits

Switch-off time limits can be customized by clicking on :

• Grid Switch-off Time: Applies to fuses of type "Grid". Protects larger network areas (e.g. streets or neighborhoods). Has a higher threshold to avoid unnecessary disconnections affecting many customers.
• Connection Point Switch-off Time: Applies to fuses of type "Connection" and protection devices. Protects individual customer connections. Has a lower threshold to ensure quick fault isolation and equipment protection.

Limits determine visual indicators (green, yellow, red) for device dimensioning. Defaults can be restored anytime.

Radial vs. Meshed Grids

• Radial: One fuse or protection device is sufficient to disconnect the bus.
• Meshed: Multiple fuses or protection devices may be needed. After each disconnection, currents are recalculated and switch-off times adjusted proportionally.

Example

A short-circuit happens at Bus 1, which is connected to the slack bus via two paths. One branch has Fuse 1 and the other Fuse 2.

During a short-circuit at Bus 1, Fuse 1 sees the current 1kA and Fuse 2 sees the current 2kA. Because of the different characteristic curves, let us assume that at these currents, the switch-off times of Fuse 1 and Fuse 2 are 2s and 3s respectively. Hence, Fuse 1 switches first after 2s and the branch is disconnected, changing the grid topology.

The short-circuit currents are recalculated at this point, leading to a new lower current at Fuse 2 of 1.2kA. For this current, Fuse 2 has a switch-off time of 7.5s. However, Fuse 2 had already seen the current 2kA for 2s out of the actual switch-off time of 3s, meaning that it was already 2/3 strained.

This means that the switch-off time at the second current of 1.2kA is (1 - 2s/3s) * 7.5s = 2.5s, and not 7.5s. The total switch-off time of Fuse 2 is therefore 2s + 2.5s = 4.5s, which is also the time required to completely deenergize Bus 1.

Please note that this proportionality only affects fuses and definite time overcurrent protection devices, but not distance protection devices.

Results

The protection calculation is performed for all buses, providing fast and comprehensive insights into grid protection without needing to select individual buses.

Buses

Only shown if 'Switch-off times' was evaluated.

If you choose to view the results by buses, then you will get an overview of all buses of the grid with an indicator of their state in the table. Hover over the state indicator to show what it represents.

Click on show in the details column in the table to see the selectivity diagram for a certain bus. The characteristic curves and stages of all fuses and protection devices that are on the path of the short-circuit are displayed in the chart. Devices that are not directly involved in the switch-off of the bus, are gray, whereas devices that are involved are colored. The switch-off time and short-circuit current for all devices that are involved in switching off the short-circuit are displayed in the chart using X. The marker is colored according to whether or not the switch-off time is within the defined time limits.

Clicking on show will also show the respective devices, their state, switch-off time, short-circuit current and other information directly in the table, sorted by the order in which the devices would physically switch.

This result is designed to give you insight on whether further fuses or protection devices might be required to ensure all short-circuits are properly deenergized.

Fuses and Protection Devices

Only shown if 'Switch-off times' was evaluated.

You can also choose to view the results by fuses and protection devices, which will give you an overview over all installed devices in the grid with an indicator of their state in the table. The meaning of each state is described in the legend below the table. For each fuse or protection device the worst-case result for state, switch-off time and short-circuit current or short-circuit impedance is shown. The location of the short-circuit responsible for this worst-case result is also displayed and the bus can be viewed on the grid viewer by clicking on . In the case that a device switches off a short-circuit at several buses, you can click on to show the results for all buses that this fuse or protection device switches off.

This result is designed to give you insight on whether the existing fuses and protection devices are properly dimensioned, ensuring that short-circuits are not only deenergized, but deenergized within the defined time limits.

Lines

Only shown if 'Stability' was evaluated.

The table shows a list of all lines, as well as the maximum possible three-phase thermal short-circuit current that can occur in case of a fault. This current is the maximum of the fault currents of its adjacent buses. If the line has a limit defined (parameter 'Short circuit stability'), the limit is compared with the largest three phase thermal fault current. If the current exceeds the limit, the line is marked red in the table and colored red in the grid viewer.

The "Short circuit stability" parameter in the line model $I_{thr}$ is defined for an assumed short circuit duration of 1s. If you selected a time $T_k$ other than 1s in the short circuit options, the limit is adjusted to the selected time using the following formula:

$I_{thz}=I_{thr}\cdot \sqrt{\frac{T_{kr}}{T_k}}$

Legend:

Stage Plan

The stage plan visualizes the distance protection devices (stages, direction and non-direction final times) and self times of buses in an impedance-time-diagram. It is therefore necessary to define the path in the grid, for which to create the stage plan. This is an ordered selection of buses and can either be a feeder or a ring.

Create

The following steps will help you create a stage plan:

• In the panel 'Stage plan', click on + to start creating a stage plan.
• Hold Ctrl/Cmd and click on a bus in the grid to add it to the stage plan.
• You can only add buses that are adjacent to the previously selected bus.
• To remove falsely added buses, hold Ctrl/Cmd and click on the last added bus to remove it from the stage plan.
• To create a ring, select the same bus for both the start and end.
• The color denotes the selected path.
• Clicking on a self time in the stage plan will show the corresponding bus in the grid viewer.
• Clicking on a distance protection curve in the stage plan will show the corresponding protection device in the grid viewer.
• Click on 'Preview' to preview the stage plan while creating it, and click 'Save' to finalize it.
• Enable 'Inverted' to view the backward direction stage plan inverted, which aligns the buses in the forward and backward direction plots.
• Save the stage plan to be able to view and edit it later, assuming it is not invalidated by an incompatible grid change.

Auto-tune

If the parameters of the distance protection devices in your grid are not properly configured, you can either manually edit them in the grid viewer or use the Automatically parametrize distance protection devices option. When activated, this feature optimizes the stages, directional, and non-directional final times of the devices along the selected stage plan path using the input parameters below.

Note: Generated parameter values are rounded to three decimal places (mΩ and ms).

Input Parameters

• Close fault delay [ms]
  Time required for the device to switch off a close fault (first stage).

• Staging Time [ms]
  Time difference between the device stage and the highest time obstacle at that impedance (another device or bus self-time).

• Staging Factor [%]
  Relative distance to the next obstacle where the device should escalate to a higher stage. Stages may be delayed to ensure minimum grid impedance.

• Path Overshoot [%]
  Extension beyond the path impedance to ensure all faults along the path are cleared.

• Directional End Time Margin [ms]
  Additional time for the directional final stage compared to the last stage's switch-off time.

• Non-Directional End Time Margin [ms]
  Additional time for non-directional final stages across all devices on the path, relative to the highest directional final time.

If you're unsure about the effect of a parameter, adjust its value and click Preview to update the stage plan curves.

Transformers

This section provides insights into transformer protection by visualizing selectivity diagrams for transformers in the grid and any linked subgrids.

Overview

Under the Transformers tab, you can view selectivity diagrams for all transformers in the grid. If subgrids are linked, their transformers can also be loaded and analyzed.

The table includes:

• Transformer name and ID
• Associated grid
• Bus considered as the transformer station bus bar

For transformers in the same grid, the primary-side bus is pre-selected. For subgrid transformers, the linking bus in the main grid is pre-selected. You can change the bus bar to any bus between the transformer and the slack bus on the same voltage level as the primary side.

Selectivity Diagram

Click Show to view the transformer selectivity diagram, which includes:

• Distance Protection: Displays devices involved in clearing short-circuits at the selected bus bar. Curves show overcurrent thresholds and trigger times.
• Definite Time Overcurrent: Shows devices inside the transformer station, especially those in the low-voltage grid. Currents are transformed using the winding ratio.
• Fuses: Displays fuses on the lower voltage side, with currents transformed to the higher voltage side. Fuses behind other fuses or transformers are excluded.
• I’’k3max, MV: Maximum possible three-phase short-circuit current on the primary side.
• I’’k2min, LV, transformed: Minimum possible two-phase short-circuit current on the secondary side, transformed to the primary side using: IS / IP = NP / NS

Where:

• I = current
• N = number of turns
• P = primary side
• S = secondary side

Limits and Options

Click on to change the options of the protection calculation. There you can choose whether the minimum or maximum characteristic curve of fuses is used to determine the switch-off time of the fuse. By default, the maximum characteristic curve is used.

Click on to change the limits of the switch-off times. There you can set the limits for connection point and grid devices separately. You can also set different limits for warnings and violations, based on which the switch-off device will be colored in the results and the grid viewer.