Design Guidance for Electrical Power Systems Serving Nonlinear Harmonic Loads

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DEPARTMENT OF THE NAVY

NAVAL

FACILITIES

ENGINEERING

COMMAND

200 STOVALL STREET

ALEXANDRIA.

22332.2300

IN REPLY REFER TO

04Al/CMM

27 Jan 1992

Design Policy Letter DPL-92-0001

FROM:

Commander, Naval Facilities Engineering Command

Subj:

DESIGN GUIDANCE FOR ELECTRICAL POWER SYSTEMS SERVING

NONLINEAR HARMONIC LOADS

Ref:

(a) Federal Construction Council Report No. 112, dtd 1 Nov 91

1. Purpose : To provide design guidance for electrical power systems serving

large concentrations of personal computers and industrial loads. This

guidance applies to all facilities where the estimated harmonic load exceeds

15 to 20 percent of the total building electrical load.

2 . Background: The majority of new government facilities and retrofit design

project8 will Include upgraded Electrical Utility Services for personal

computers, automatic data processing systems, electronic office equipment,

electronic ballasts, and solid state motor control equipment which may

contain switching mode power supplies.

This equipment, when connected to conventional building wiring systems,

can create overheating of transformers, generators, motors, and the

interconnected building wiring system. The excessive neutral currents are the

result of harmonic currents within the office equipment switching mode power

supplies. The resulting overtemperature conditions may damage the insulation

within transformers, motors, generator8 and cable systems. Reference (a),

recently issued by the Federal Construction Council, provide8 additional

information and references.

3 . Policy Statement: It is Naval Facilities Engineering Command

(NAVFACENGCOM) policy to use the following design practice8 when designing

electrical distribution systems for facilities which must accommodate

nonlinear and harmonic loads. Typical facilities Include communications

center8 laboratories, data processing and administrative building8 with

15 to 20 per cent of connected harmonic load.

(a) Electrical Design Analysis Shall include a calculation of estimated

nonlinear loads.

(b) Transformers shall he selected using procedure8 outlined in ANSI

C57.lll-1986. This standard provide8 a simple calculation method for

determining derating factors for low voltage, dry type distribution

transformers. Electrical Designers should consider using "K" rated

transformers which are rated for 220 degree centigrade maximum hot spot

temperature operation. This design procedure will ensure that the

transformer life expectancy will not be reduced when connected to

nonlinear loads.

APPENDIX X