Voltage Dips at Chemical Plant Captured
A brief voltage sag on January 29, 2009, less than a quarter of a second in duration, causes an entire chemical plant in South America to shut down. PSL’s inexpensive PQube® monitor captures the entire event on the plant’s 34.5 kV service, and converts it directly into picture files (below) and Excel files.
1 Normal voltage and current waveforms, before the sag. This is a 34.5 kV delta service. The upper graph shows the voltage, and the lower graph shows the current. Note that there is no current transformer on L2 – a normal configuration for delta systems.
2 A 3-phase fault occurs on the grid, upstream from the measuring point, starting a voltage sag that drops to 22.5% of nominal for just under ¼ of a second.
3 The reduced voltage, and possibly the voltage phase shift, cause the big 3-phase motors in the chemical plant to draw 400% of their normal current.
4 The motor circuit breakers trip, or their undervoltage relays remove the motors from the circuit.
5 A fuse or circuit breaker clears the fault, thus restoring the voltage. But only minor loads, drawing small amounts of current, are still turned on in the plant. The big loads have all shut down. A full plant re-start is required.