One of the main uses of SPS Software is to simulate electrical transients. Whether you are studying a line energization with mechanical circuit breakers or high-frequency switching with power electronic devices, SPS has specific tools and best practices to capture these phenomena accurately.
Here is a guide on how to approach transient simulations, choose the right line models, and set up your solver.
1. Simulating Transients with Circuit Breakers
Transients are usually triggered by switching events. In SPS, you can use the Breaker block to simulate line energizations, faults, or load switching.
When setting up a transient simulation (e.g., closing a breaker to energize a line), the accuracy of your results heavily depends on the physical models you choose for your components—especially transmission lines.
2. Choosing the Right Line Model for Transients
When studying fast transients, a simple equivalent impedance is not enough. Let’s compare three line models during an energization transient:
- Single PI Section Line: A simple PI model acts as a low-pass filter. It does not respond well to high frequencies and will miss fast transient peaks.
- Multiple PI Sections (e.g., 10 sections): This gives much better accuracy for transients. However, the discretization of the line into chunks can introduce artificial high-frequency oscillations.
- Distributed Parameters Line: This is the most accurate model for transient studies. It accurately represents the wave propagation time delay along the line without introducing artificial ringing.
3. Continuous vs. Discrete Simulation
SPS can simulate using either continuous (variable-step) algorithms or discrete (fixed-step) solvers.
- Continuous (Variable-step like
ode23tb): Usually faster for small systems because the solver takes large steps when the system is stable and tiny steps only during the transient event. - Discrete (Fixed-step): Highly recommended for large systems containing many states or nonlinear blocks (like power electronic switches).
The Discretization Trade-off:
When you discretize your system via the powergui block, the precision of the transient simulation is entirely controlled by your Sample Time (Ts). If you use a time step that is too large, you will lose accuracy and miss the transient peaks.
4. Rules of Thumb for Choosing a Time Step (Ts)
How do you know if your time step is small enough? The best method is to run the simulation with a small step, then increase it until the results start to deviate unacceptably.
However, here are the standard industry guidelines for SPS:
- 20 µs to 50 µs: Good for switching transients on standard 50-Hz or 60-Hz power systems, or systems using line-commutated devices (diodes and thyristors).
- \le 1 µs: Mandatory for systems using forced-commutated power electronic switches operating at high frequencies. For example, simulating a PWM inverter using IGBTs, MOSFETs, or GTOs at 8 kHz requires a maximum time step of 1 µs to accurately capture the dead times and switching transients.
