Reliability Calculation

This section explains how to run a reliability calculation to assess the impact of grid element failures on supply continuity.

General

The reliability calculation performs an automated stochastic analysis to identify critical supply interruptions. It is based on connection analysis and does not require a power flow calculation.

Input

Failures can be modeled for the following grid elements:

• Branches (lines, connections, transformers): Failure leads to immediate disconnection at the point of failure, regardless of breakers or protection devices.
• Busbars: Failure is equivalent to the failure of all connected branches.
• Switches: Failure is treated as the failure of the branch where the breaker is installed.
• Generators: Failure disconnects the generator but does not affect the rest of the grid.

Reliability models can be edited in the component library and assigned to grid elements. For elements without a defined model, a default can be set in the calculation options.

Results

The following reliability indicators are calculated for the entire grid model:

• SAIDI [h]
  System Average Interruption Duration Index – average outage duration per customer.

• SAIFI [-]
  System Average Interruption Frequency Index – average number of outages per customer.

• CAIDI [h]
  Consumer Average Interruption Duration Index – average outage duration for affected customers.
  $CAIDI=\frac{SAIDI}{SAIFI}$

• CAIFI [-]
  Consumer Average Interruption Frequency Index – average number of outages per affected customer.

The following customer results are shown for each connected load and generator individually:

• H [/a] – Interruption frequency (failures per year)
• T [h] – Average interruption time
• Q [h/a] – Unavailability (duration × occurrences)
• W [kWh/a] – Not-delivered energy
• P [kW/a] – Not-delivered power

Calculation Example

Schematic Grid Model

Load A

All grid elements between load A and the network feeder are ideal, therefore all values are 0.

Load B

Load B is affected by the unideal transformer, which has two 1h outages and one 3h outage.

• $H=2/a+1/a=3/a$
• $T=\frac{2\cdot 1h+1\cdot 3h}{2+1}=1.666h$
• $Q=2\cdot 1h/a+1\cdot 3h/a=5h/a$
• $W=2kW\cdot 5h=10kWh$
• $P=2kW\cdot 3/a=6kWh/a$

Load C

Load C is affected by the unideal transformer, and in addition the unideal line with one 4h outage.

• $H=2/a+1/a+1/a=4/a$
• $T=\frac{2\cdot 1h+1\cdot 3h+1\cdot 4h}{4}=2.25h$
• $Q=2\cdot 1h/a+1\cdot 3h/a+1\cdot 4h/a=9h/a$
• $W=2kW\cdot 9h=18kWh$
• $P=2kW\cdot 4/a=8kWh/a$

Global Reliability Indicators

• $SAIDI=\frac{1.666h+2.25h}{3}=1.306h$
• $SAIFI=\frac{5h/a+9h/a}{3}=4.667/a$
• $CAIDI=\frac{1.306h}{4.667/a}=0.28h$
• $CAIFI=\frac{5h/a+9h/a}{2}=7h/a$

Where:

• N = total number of customers
• M = number of affected customers

Troubleshoot

• Empty result table?
  Your grid might not contain any (connected) loads and generators.

• All values are 0?
  All elements might be ideal. This can be fixed by selecting reliability models for your grid elements or setting non-ideal fallback types in the options. Other causes could be that the model names don't match a reliability model in the component library, or that all outage types are disabled in the options.

• Loads or generators are missing?
  There might be open switches in your grid model such that some loads and generators are not connected to the slack bus. Only connected loads and generators are evaluated.