LOW FREQUENCY MAGNETIC INTERFERENCE IN HIGH-RISE BUILDINGS
LOW FREQUENCY MAGNETIC INTERFERENCE IN HIGH-RISE BUILDINGS
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高层建筑中的低频磁干扰
DOI：
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发表时间：
2001
期刊：
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通讯作者：
Ya
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文献类型：
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作者：
J. Burnett;Ya
随着大型商业建筑中对电力需求的增加，对敏感设备的磁干扰问题也增加了。然而，低频（ELF）电磁干扰通常不会在建筑物配电系统的设计和安装中得到解决。在办公楼的背景磁场水平的上限的发展进行了讨论的物理布局载流导体。介绍了一种数值计算方法，并讨论了磁场缓解的技术途径。为了实现一个兼容的磁环境良好的做法，在电气装置的设计概述。引言在香港，正如在许多亚洲城市一样，高层空调商业楼宇是经济活动的重要资产。这些类型的建筑物容纳越来越复杂和敏感的电子设备，并消耗非常大的功率。为了满足不断增长的租户需求，这种建筑物中的配电系统需要大的载流导体。因此，建筑物中极低频（ELF）磁干扰的发生率增加。随着建筑物中磁场强度的增加，已经报告了许多ELF干扰问题[1-5]。最常见的问题是计算机显示器不稳定。已知[5-10]，当磁场低至1μT时，显示器屏幕上会出现抖动等不稳定性。由于电气安装设计人员和承包商缺乏认识，导致磁场的背景水平超过敏感设备的敏感度水平的情况并不少见。香港的一座高层商业建筑将为几个1500 kVA：11000/380 V变压器提供11 kV馈电。通常采用三极中性（TPN）母线槽（空气绝缘铜条或绝缘母线槽）或大型多芯电缆进行低压配电。这些电缆承载接近2400 A的电流，而到租户楼层的水平电缆通常承载高达200 A的电流。租户的负载，产生显着的谐波电流，由于照明和计算机，是从一个共同的上升。由于累积谐波电压降[11]，高层的谐波电压失真可能非常高，并且由负载不平衡和负载谐波引起的中性导体电流可能超过带电导体电流。对于满载电路，中性电流可能超过导体载流能力，导致过热和故障。与配电网相关的工频和谐波频率电流在此类建筑物中产生显著的极低频磁场环境。在Transformer室、主配电盘室附近、沿着走廊和垂直管道中，可发现电流高达2400 A的导线。由于变压器和电动机等电气设备的磁路磁阻较低（图1），因此大电流电缆和母线是香港建筑物中最重要的ELF磁干扰源。由于高层建筑中的电力电缆或母线相对较长，并且平行运行，因此可以从安培定律导出导体系统产生的磁场及其周围磁场的简单表达式。图1显示了电缆或母线导体的几种典型配置，这些配置通常用于香港建筑物。在平衡电流条件下，磁场B由下式给出：
With increasing demands for electric power in large commercial buildings the problem of magnetic interference to sensitive equipment has increased. However, low-frequency (ELF) electromagnetic interference is not generally addressed in the design and installation of building electrical distribution systems. The development of the upper limit of the background magnetic field levels in office buildings is discussed in relation to the physical layout of current carrying conductors. A numerical computation method is described, and technical approaches or for magnetic field mitigation are discussed. To achieve a compatible magnetic environment good practice in the design for electrical installations are outlined. INTRODUCTION In Hong Kong, as in many Asian cities, high-rise, airconditioned commercial buildings are important assets to economic activity. These kinds of buildings house increasingly sophisticated and sensitive electronic equipments and draw very significant power. To meet the ever-increasing tenant demands electrical distribution systems in such buildings require large current carrying conductors. Consequently, the incidence of extremely low frequency (ELF) magnetic interference in buildings has increased. A number of ELF interference problems have been reported as the magnetic field strengths in buildings increase [1-5]. The most common problem reported is the instability on computer monitors. It is known [5-10] that instabilities such as jitter appears on monitor screens with the magnetic field is as low as 1μT. The lack of awareness by electrical installation designers and contractors has led to a not uncommon situation whereby the background level of magnetic fields exceeds the susceptibility levels of sensitive equipment. A high-rise commercial building in Hong Kong will be provided with an 11 kV feed to several 1500 kVA: 11000/380 V transformers. Low voltage distribution by triple-pole and neutral (TPN) busbar risers (either air insulated copper bar or insulated busduct) or large multi-core cables is common practice. These risers carry currents approaching 2400A, and horizontal cabling to tenant floors typically carry up to 200A. Tenants loads, which generate significant harmonic currents due to lighting and computers, are fed from a common riser. Harmonic voltage distortion at high floors can be very high due to cumulative harmonic voltage drop [11], and neutral conductor currents caused by load unbalance and the load harmonics can exceed live conductor currents. For fully loaded circuits the neutral current can exceed the conductor current-carrying capacity resulting in overheating and failure. CHARACTERISTICS OF ELF SOURCES The power-frequency and harmonic frequency currents associated with the electrical distribution network create significant ELF magnetic environments in such buildings. Conductors carrying up to 2400A will be found near transformer rooms, main switchboard rooms, along corridors, and in vertical ducts. Since electrical equipment such as transformers and motors have low reluctance magnetic circuits (Figure 1) it is the heavy-current power cables and busbars which are the most significant sources of ELF magnetic interference in Hong Kong buildings. Since the power cables or busbars in a high-rise building are relatively long and are run in parallel, simple expressions of the magnetic field produced by and surrounding the conductor system can be derived from the Ampere’s law. Figure 1 shows several typical configurations of cable or busbar conductors, which are normally used in Hong Kong buildings. Under the balanced current conditions, the magnetic field B is given by: