Friday, November 18, 2011

Power in the Data Center and Balancing 3-phase power - some questions and answers

First, clearing up some misconceptions I've heard and giving out some basic data:

  • This conversation is for power typically used in the US.
  • 120V power is one phase wire to neutral wire.
  • 208V power, across 2 phase wires, is still considered single-phase.
  • There is no "two phase" power in common use these days.  Trust me.
  • It is really important to balance use of the phases of 3-phase power.

Most data centers use large UPS systems for the room (before distribution of power to power panels and racks).  Smaller server rooms sometimes use rack-based UPS systems (after distribution to racks).

The step-down transformer sometimes used taking 208V down to 110V wastes energy.  I ballpark it at 10%.  In most server rooms 208V power is used for equipment like servers and storage.  Here are a few examples of how that power is often distributed and how to balance power across the phases in each case:
  • A 3-phase power panel with 208V circuit-breakers wired to outlets near each rack.  This method is the usual method in older data centers, and requires an electrician for every change, which can be expensive over time.
    • Balance power by placing gear in racks using under-utilized phases.  For example - Phase A is under-utilized by B and C phases are over-used.  Use racks with AB and/or CA to boost use of A more than B and C.
  • 3-Phase from Starline Busway or equivalent to 208V plugin modules which provide outlets.  This type of distribution is quite flexible and a bit more expensive up front.  No electrician is needed for minor power distribution changes over time.
    • Balance power with a short outage and switch modules to change phases in use for the outlet.
  • 3-Phase to the PDU/power strip in the rack.  The equipment to do this is typically more expensive than other power distribution equipment.
    • Balance power by changing which outlets you use in the rack.  Power must be balanced in each rack.

The real question in balancing 3-phase power is this: how much out-of-balance is too much?  Our standard is that power must not be more than 10% out of balance.  It is "ok" if a very temporary migration takes power use 15% out-of-balance for less than a few hours.

I have seen power panels 25% or more out of balance.  This typically causes some problems:
  • Heat - this condition is detectable as heat
  • Circuit Breaker outages - circuit breakers can flip
  • Waste of power - Power will be induced in the neutral wire and be wasted
  • Loss of capacity - this wasted power is lost and cannot be used, lowering the maximum capacity available.

1 comment:

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