1、3535789 0024485 534 1s-3 1-01 CEEC-EE Engineer Technical Letter 1110-3-403 DEPARTMENT OF THE ARMY ETL 1110-3-403 U.S. Army Corps of Engineers Washington D.C. 20314-1000 30 June 1989 Engineering and Design ELECTRICAL POWER SYSTEMS FOR NONLINEAR LOADS 1. mD ose. This letter provides interim guidance a
2、nd design criteria to design and calculate 3-phase, 4-wire electrical distribution systems below 600 volts for buildings or parts of buildings where significant numbers of electrical equipment items will be furnished with nonlinear, solid state component loads. Most administrative and office buildin
3、gs fall into this category. . This letter applies to all HQUSACE/OCE 2 . Bgglicabilitv elements and field operating activities (FOA) having Army military design and construction responsibility. 1. 3. Discu ssion. Rapid development in the application of solid state switching mode power supply compone
4、nts in equipment such as computers, printers, uninterruptible power supply (UPS), motors with variable speed drives, ballasts, dimmers etc., has led to unexpected serious electrical distribution problem. are creating a different voltage and current phase relation in the alternating current (AC) dist
5、ribution systems. The above equipment items represent nonlinear electrical loads, which distort the shape of the sine wave and generate high harmonics, eddy currents, increase hysteresis losses and skin effects. Consequently, wiring systems, transformers, motors and generators are overheating and de
6、teriorating equipment performance. Such equipment is typically supplied from a single phase circuit stemming from a 3-phase, 4-wire distribution network. One of the most frequent results of nonlinear loads is overloading of the neutral conductor. These loads 4. Action to be Take n. The following act
7、ions should be taken on projects on a selective basis. applicable across the board. Projected electrical loads on new projects will be analyzed to determine whether or not they are considered potential nonlinear loads. The following are recommended design practices for circuits supplying nonlinear l
8、oads: These are not necessarily a. Use one 3-phase transformer with common core, delta connected primary and wye secondary, in lieu of using 3 single- phase transformers connected for 3-phase service. Transformers are available for high harmonic-content power distribution systems without derating. 3
9、496 Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-3535789 0024486 470 ETL 1110-3-403 30 Jun 89 b. Derate transfomer, motor and generator outputs, to prevent overheating or burnout. Where standby generators represent the only power source when the n
10、ormal power fails, sizing the generators should take into consideration the type of loads on the system. If a large amount of loads are the above listed ones, use a multiplying factor of 1.3 - 1.5 to upgrade the generator size in order to compensate for the predicted heat losses. A special voltage r
11、egulator should be specified to achieve proper voltage regulation in high harmonic content and distorted sine wave load conditions. c. The neutral conductor(s) between the transformer and the panelboard shall be a minimum of 1.73 times the ampacity of a phase conductor. d. The neutral bar in the pan
12、elboard shall be a minimum of 1.73 times the ampacity of a phase conductor. e. Provide isolation transformers close to high harmonic- producing equipment. f. Provide line filters to minimize the effects of the harmonics, in the close vicinity of the loads. g. A separate neutral conductor for each ph
13、ase to serve 120 volt outlet receptacles is preferred. This includes prewired workstations or system furniture. Where a shared neutral conductor for a 208Y/120 volt system must be used for multiple phases, the neutral conductor shall be a minimum of 1.73 times the ampacity of a phase conductor. h. P
14、rovide true root means square (RMS) sensing meters, relays and circuit breaker trip elements. 5. ImDlementation. This letter will have special implementation as defined in paragraph 6c, ER 1110-345-100. FOR THE DIRECTOR OF ENGINEERING AND CONSTRUCTION: HERBERT H. KENNON Chief, Engineering Division Directorate of Engineering and Construction 2 Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-
