The essence of seamless switching is the reconstruction of the grid voltage → off-grid voltage, or in the reverse direction the voltage phase/frequency/amplitude, within <20 ms. At this instant, if the "opposite side" at the PCS output terminal is a different load type, the physical behavior is completely different.
1. Different Loads Have Different "Reactions" to Switching
| Load Type | Physical Behavior at the Instant of Switching | Impact on PCS |
|---|---|---|
| Resistive (R) | Voltage and current in phase, no energy storage elements | Almost no impact |
| Inductive (L) | Residual magnetism of motor/transformer + back-EMF still maintained, residual voltage phase not fixed | Reverse current, phase misalignment → tripping |
| Capacitive (C) | Large phase difference between DC bus capacitor / compensation cabinet residual voltage and grid | 6–10× inrush current |
| Non-linear (rectifier/switching power supply) | Injects harmonic current | THD suddenly changes after switching |
| Sensitive load (IT/medical) | Extremely poor disturbance immunity | Even a 20 ms sag causes shutdown |
2. Three Core Matching Requirements at the Instant of Switching
- Phase synchronization — the phase difference between the residual voltage of an inductive load (motor) and the newly built PCS voltage directly generates inrush current
- Frequency matching — if the frequency corresponding to the motor speed differs from the PCS frequency, slip current will appear
- Amplitude matching — transformer magnetizing inrush current and capacitive load charging inrush current are both proportional to the voltage difference
This is why the loading rate of direct-on-line motor-type loads is limited to 15–20%, and that of variable-frequency type to 60% — not because the PCS cannot drive them, but because the surge at the instant of switching will trigger the protection.
3. Special Load Cases
- DyN11 isolation transformer (R4): tungsten-lamp loads / charging stations must go through DyN11, otherwise the PCS DC component triggers the insulation alarm
- Multi-module loading ≤ 40% (R5): simultaneous switching triggers inter-module circulating current protection
- Variable-frequency air conditioner: the DC bus capacitor is pulled down at the instant of switching, the under-voltage protection trips → switching fails
4. Engineering Countermeasures
- Make a load classification table before switching — classify all outgoing circuits by type
- Soft-start non-linear / impact loads — switch them on in batches 5–10 s after switching
- Retrofit motor-type loads with variable-frequency drives — change from direct-on-line to VFD (loading rate raised from 15% to 60%)
- Reserve STS capacity — energy-storage charging power + load consumption power
- Dedicated circuits for sensitive loads — avoid sharing the same bus with impact loads
One-sentence summary: the difficulty of seamless switching is not whether the PCS itself can build voltage, but that at the instant of switching the load will exert a reverse electrical stress on the PCS output terminal, and the nature of the reverse stress differs completely with load type — therefore switching strategies and loading rates must be designed separately for each load type.