Check machines stay in step

What it tells you. A stability simulation applies a disturbance, such as a fault, and follows your machines for a few seconds. It shows whether they stay in synchronism or whether one swings away from the rest. Use it to check that a fault cleared in a given time does not cost you a generator.

This study follows swings over seconds, using RMS values. To see waveforms in the first cycles, use the EMT study instead.

Before you start

You do not need to run a load flow first. The simulation solves its own starting point, so the load flow data must be complete. Check the data below. Select an element to see its data in the inspector.

Data	Where to enter it
Machine Rotor model, Inertia constant, Damping and Transient reactance xd′	Inspector, Stability
Machine Subtransient reactance xd″ and Stator resistance, for the Round rotor model	Inspector, Short circuit
Machine Exciter, Governor and Stabiliser, if you model them	Inspector, Stability
External grid Short-circuit power max, which sets how strong the grid is	Inspector, Short circuit
The events: faults, switching and load steps, with their times	Events dialog, described below
Simulation time and Step size	Study case, under Stability simulation

The Classical rotor model needs only the inertia, damping and transient reactance. The Round rotor model adds the circuits of a real rotor. An exciter holds a machine's voltage. A governor holds its speed or power. A stabiliser damps swings through the exciter.

Set up the study

Each event has a time, a type and an element.

Select the Calculate tab.
Under Dynamics, select Events. The Study case dialog opens at Simulation events.
Select Add event.
In Time (s), enter when the event happens.
In Event, choose Three-phase fault at, Clear fault at, Switch out, Switch in or Set load to (% of initial).
In Element, choose the busbar, branch, machine or load.
For a fault, enter the fault resistance in R Ω and reactance in X Ω. Leave them empty for a bolted fault. For a load step, enter the new level in Value.
Repeat steps 3 to 7 for each event.
Select Apply.

A bolted fault has no impedance in the fault itself. Give every fault a Clear fault at event, or it lasts to the end.

To add a fault quickly, right-click a busbar and select Add fault to simulation. This adds a fault at 0.1 s and its clearing at 0.2 s. Edit the times in Events.

To change the other settings, press Ctrl+, to open Study case, then scroll to Stability simulation.

Setting	What it does
Simulation time	Sets how long to simulate. The default is 3 s.
Step size	Sets the integration step. The default is 0.005 s. Shorten it to resolve fast controls.
Load P held as constant power and Load P held as constant current	Set how each load's active power follows the voltage. The remainder is constant impedance. Both default to 0 %.
Load Q held as constant power and Load Q held as constant current	Set the same for reactive power. Both default to 0 %.
Constant power and current below	Turns those shares into constant impedance below this voltage. The default is 0.7 p.u.

The method is described in the engine description.

Run it

On the Calculate tab, select Simulation, or press Alt+R.
Wait for the progress bar in the status bar to finish.

The Stability tab opens in the results panel. The Output tab logs the result in one line. It also lists any event that was skipped.

To stop a run that takes too long, press Ctrl+.

Read the results

The top of the Stability tab shows a pill and a summary.

The pill says All machines stay in synchronism, or when synchronism was lost, such as Loss of synchronism at 0.515 s.
Angles against says what the rotor angles are measured from. It is the external grid when the network has one. Otherwise it is the centre of inertia, the average angle of all machines weighted by their inertia.
Steps is the number of time steps. Time is how long the calculation took.

The plot shows one quantity for every machine. Choose the quantity from the list at the top right. The list holds Rotor angle, Speed, Active power, Reactive power, Field voltage, Mechanical power and Busbar voltage.

The panel beside the plot has a checkbox for each trace, so you can hide or show it. It also lists the events with their times.

Drag the time cursor along the plot. The diagram shows each busbar's voltage and each machine's rotor angle and power at that time. Busbars below 0.8 p.u. are coloured.

To save the plotted traces, select Export table as CSV.

The rotor angle is how far a machine's rotor is ahead of the reference. A stable machine swings and then settles. An unstable one keeps moving away. PowerStudio reports a loss of synchronism when a machine is more than 180° from another.

Check your results

Flat before the event. Every trace should stay still until the first event. If not, the starting point is not in balance.
Starting power. At time 0, each machine's Active power equals its P in the load flow. On the IEEE 14-bus sample, Generator 1 starts at 232.39 MW.
Fault voltage. A bolted fault takes the voltage at its busbar to 0.000 p.u. The voltage recovers after the fault clears.
Speed. Speeds swing around 50 Hz, or your system frequency. A speed that keeps rising points to a machine that is running away.
Time simulated. The pill judges only the time you simulated. If machines are still swinging at the end, raise Simulation time to look at later swings.
Shorter fault. Halve the fault duration, and move any Switch out event with it. The swings in rotor angle should get smaller. On the IEEE 14-bus sample, Generator 2 then swings by 4.2° instead of 8.0°.

If something goes wrong

What you see	What it means	What to do
A red pill, Loss of synchronism at a time	A machine moved more than 180° from another	This is a result, not an error. Clear the fault sooner, or check the inertia and the exciter.
Event at a time skipped in the Output tab	The event did nothing, for example a clearing with no fault	Check the event's element and time in Events.
Stability simulation cannot run, and a message about constant power and constant current shares	The shares for a load add up to more than 100 %	Lower them under Stability simulation in Study case.
Stability simulation cannot run, and a message about round-rotor data	A machine's Round rotor data is not consistent	Correct its data, or set its Rotor model to Classical.
All traces are flat	The study case has no events	Add events in Events.
Traces look jagged	The Step size is too long for the fast controls	Shorten the Step size.
The pill Calculated before the last edit	You changed the network after the run	Run Simulation again.

Try it on the IEEE 14-bus sample

Select the Help tab.
Under Samples, select IEEE 14-bus system.
On the Calculate tab, select Events. The sample has 3 events.
Read them. Three-phase fault at Bus 4 is at 0.1 s. Clear fault at Bus 4 and Switch out Line 4-5 are both at 0.18 s.
Select Cancel, then select Simulation.
In the Stability tab, read the pill. It says All machines stay in synchronism. The summary says the angles are against the centre of inertia, with 600 steps.
Choose Rotor angle. Generator 1 stays between 2.6° and 4.2°. Condenser 8 swings furthest, to −23.5°.
Choose Busbar voltage. Bus 4 falls to 0.000 p.u. at 0.1 s. It is 1.006 p.u. at 3 s.
Choose Speed. The speeds stay between 49.84 Hz and 50.20 Hz.

The speeds are still swinging at 3 s. The pill judges only the 3 s that you simulated. Raise Simulation time under Stability simulation to look further.

Find the margin on the Riverside sample

The Riverside sample has a single generator, the CHP unit, against a strong grid. It shows how a longer fault ends in a loss of synchronism.

Select the Help tab, then select Riverside distribution under Samples.
Select Events. The sample has a Three-phase fault at Hilltop at 0.1 s and Clear fault at Hilltop at 0.25 s.
Select Cancel, then select Simulation.
The pill says All machines stay in synchronism. In Rotor angle, Hilltop CHP swings between −23.0° and 48.3°, measured against the grid.
Select Events, set the time of Clear fault at Hilltop to 0.45 s, then select Apply.
Select Simulation. The pill now says Loss of synchronism at 0.515 s.

A clearing time of 0.35 s still holds. The margin lies between 0.35 s and 0.45 s. Use that range to judge how fast the protection must be.