Why poor power factor causes blackouts before alarms
Introduction – Power factor is invisible until it hurts you
Power factor is one of the most misunderstood concepts on ships because nothing visibly “breaks” when it is bad. Lights stay on. Motors run. Generators appear healthy.
Meanwhile:
- stator currents climb
- AVRs work harder
- busbars heat
- load margins shrink
By the time alarms appear, you have already lost redundancy.
What power factor actually means on a ship
Power factor is the ratio between:
- real power (kW) — useful work
- apparent power (kVA) — what the generator must supply
Low PF means generators are working harder without producing useful output.
On ships, this directly affects:
- generator loading
- number of generators online
- blackout resilience
Regulatory expectations (not optional)
While IEC does not mandate a fixed PF, Class and PMS logic effectively enforce it.
Typical design expectations:
- minimum operational PF ≈ 0.8
- normal operation target ≈ 0.85–0.9
Operating below this:
- forces extra generators online
- increases fuel consumption
- reduces fault tolerance
PSC and Class will challenge chronic low PF conditions.
Why ships suffer PF problems more than shore plants
Shipboard loads include:
- large induction motors
- thrusters
- VFDs
- cranes and winches
- hotel loads switching rapidly
These cause:
- reactive power swings
- unstable voltage
- AVR stress
Without correction, PF degrades fastest during manoeuvring — when you least want it to.
Correction methods used at sea
- Fixed capacitor banks
- Automatic PF correction panels
- Synchronous machine excitation control
- PMS-managed reactive sharing
Each has risks. Poorly tuned capacitors can worsen harmonics or cause overvoltage.
This is ETO judgement territory.
Real-world failure pattern
Multiple blackouts investigated by Class show:
- no single fault
- generators “healthy”
- PF low for extended periods
- system collapsed during transient load
Low PF didn’t cause the blackout — it removed the margin that would have saved it.
Knowledge to Carry Forward
Power factor is not an efficiency metric.
It is a survivability margin.
An ETO who watches kW but ignores kVAR is only seeing half the system.
Tags
ETO, Power Factor, Marine Generators, Reactive Power, PMS, Blackout Prevention, Ship Electrical Systems