Variations in Input Power: COTS Power Supply Holdup

July 21, 2014
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This is the fourth post in the series providing an introduction to COTS power supplies, which explains the requirements for holdup in the case of variations in the input power.

Mil-STD 704 and 1399 are designed to ensure operational performance between aircraft or shipboard electrical systems, external power and airborne or shipborne end equipment. The key requirement for COTS power supplies is that the power supply will continue to operate under defined variations in the power supplied, and if it ceases to operate under abnormal conditions, it will recover properly. Mil-STD 704 deals with airborne and 1399 deals with shipborne systems, and as there are many similarities between the two, we will focus primarily on Mil-STD 704.

The power source present might be:

  • 115/200 V (single or three phase) 60 or 400 Hz (fixed frequency)
  • 115/200 V (single or three phase) 360-800 Hz (variable frequency)
  • 28 VDC and 270 VDC

The standard defines three different Operation Categories:

  1. Normal operation, which occurs when the system is operating as intended in the absence of any fault or malfunction that degrades performance beyond established requirements. COTS supplies must meet these needs, in particular the transient envelopes for input power discussed below.
  2. Abnormal operation is when a malfunction or failure in the electric system has taken place and the protective devices of the system are operating to remove this malfunction or failure from the remainder of the system before the limits for abnormal operation are exceeded. COTS supplies are only expected to recover when normal operation is restored.
  3. Emergency operation follows the loss of the main generating equipment when a limited electric source, independent of the main system, is used to power a reduced complement of distribution and utilization equipment selected to maintain flight and personnel safety. COTS supplies are not generally used in these applications.

Electric starting operation, which is a specialized case of normal electric system operating conditions, must also be considered as the normal voltage limits may be exceeded due to the high electric demand. This, however, is generally not applicable to applications where COTS is used.

Generally with fixed frequency sources of 60 and 400 Hz, modern PFC supplies have no difficulty with normal operation. The variable frequency requirement of 360 to 800 Hz poses more of a problem, due to adjustments that may be needed in the PFC loop response for different frequency ranges.

The main challenge to COTS supplies in normal operation lies in the hold-up time that is required because of the low voltage transients. Recovery to the point where the PFC boost can begin to effectively operate may be up to 70 milliseconds, significantly higher than the standard 20 milliseconds demanded in commercial applications.

Supply Holdup - COTS

Supply Holdup Requirements (click to enlarge)

 

In the final part of this series, Jeff describes some general considerations that should be taken into account when developing COTS power systems.

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