重温根基：开关柜、电力变压器和UPS

图1：用来发电机并机的大型开关柜。供应：约翰斯顿有限责任企业
Figure 1: A large switchgear lineup is used for paralleling generators. Courtesy: Johnston, LLC
开关柜的额定电压范畴从1000伏下列到200千伏以上。额定电压高过1000伏的中压开关柜有多种配置，采取壁挂式装载、吊顶式装载或装载在专属的独立式金属框架中，并应用空气、气体、真空或油成为绝缘介质。本文将重点研讨户内式高压开关柜。Switchgear is available in a wide range of voltage ratings, from below 1,000 volts to more than 200 kilovolts. Medium-voltage switchgear, rated above 1,000 volts, is manufactured in a variety of configurations. Assemblies are available for exterior padmount installation, vault installation or installed in dedicated freestanding metal buildings, with air, gas, vacuum or oil as insulating media. This discussion will focus on interior low-voltage switchgear.
开关柜的代替品是配电柜。配电柜往往只要更少的空间，也更廉价。这二者往往由多个垂直部份产生，此中每个部份都用金属隔板封闭并为过电流爱护装置、监管设施和操控装置设计了开孔，其他部份还含盖1个主过电流爱护装置、计量装置、智能操控和监管体系、馈线过电流爱护装置及其组合，亦或选用其余特定设施的组合。现在过流爱护装置已然很少选用带熔丝的刀闸开关，往往选用带过流爱护性能的断路器。The alternative to switchgear is switchboard construction. Switchboards generally require less space and are less expensive. Both are typically constructed of a number of vertical sections. Each section is enclosed in sheet metal, with openings in front for overcurrent protection devices, monitoring equipment and control devices. A section may contain a main overcurrent protection device, metering devices, automatic control and monitoring systems, overcurrent protection devices for distribution feeders or a combination of these or other equipment specific to the installation. Overcurrent protection is typically accomplished with circuit breakers, with fused switches are less frequently.
高压开关柜根据UL 1558《金属封闭高压电力断路器开关设施规范》生产。配电柜根据UL 891《配电柜规范》生产。与UL 891相比，UL 1558含盖了不少提升牢靠性、耐久性和可养护性的需要。
LV switchgear is constructed to UL 1558: Standard for Metal-Enclosed Low-Voltage Power Circuit Breaker Switchgear. Switchboards are constructed under UL 891: Switchboards. UL 1558 incorporates a number of requirements that enhance the reliability, durability and maintainability over UL 891.
开关柜断路器往往采取四位垂直层叠式单独装载方法，且每个断路器与其余断路器都完全隔开。在典型的开关柜设计中，程度和垂直母线封闭在断路器隔间后的母线隔间中，该母线隔间应用绝缘隔断与柜内其他部份完全隔开。
Switchgear breakers are typically installed four high in a vertical section, individually mounted. Each circuit breaker is separated by solid barriers from other breakers and from the rest of the assembly. In a typical switchgear assembly, the horizontal and vertical buses are enclosed in a bus compartment to the rear of the breaker compartments and this bus compartment is isolated from the rest of the assembly using insulating barriers.
电缆端接排位于配电柜的后部隔间，通过绝缘隔板和母线隔间完全分开开。依据UL 891规范，配电柜不须要在组件之间供应相近级别的隔离。但UL 1558划定，为了以免修理职员在装载或养护期时碰触到相邻断路器的通电部份，提升开关柜的牢靠性和可养护性，须要设置这类隔间和隔板。当短路故障构成电弧时，这也将相邻部件的毁坏降到最低。
Finally, the cable connections are in the rear compartment, which is isolated from the bus compartment with an insulating barrier. These separations and barriers, prescribed by UL 1558, are intended to increase the reliability and maintainability of switchgear by limiting the possibility of contact between conductors attached to adjacent breakers during installations or maintenance and to minimize any damage to adjacent components in the event that an arcing fault should develop.Switchboards, under UL 891, are not required to provide the same level of isolation between components.

图2：已装载断路器的开关柜简化剖面图，图中展现了断路器室、垂直母线隔间和电缆连通隔间。供应：约翰斯顿有限责任企业
Figure 2: This simplified section view of an installed switchgear breaker shows the breaker compartment, vertical bus and cable connections. Courtesy: Johnston, LLC
高压开关柜中装载的断路器必需适合UL 1066《配电柜箱体中的高压交流和直流断路器规范》的需要。本规范需要断路用具有30周波的耐压值，和承受0.5秒而不毁坏的故障电流品级，且具有瞬时或延时跳闸性能。通过设置适合的过电流爱护设定值，促使下游断路器动作，断开故障点，而不会导致上游断路器越级跳闸。
Circuit breakers installed in LV switchgear are required to meet UL 1066: Standard for Low-Voltage AC and DC Power Circuit Breakers Used in Enclosures. This standard requires that circuit breakers have a 30-cycle withstand rating, describing the level of fault current that they can tolerate for 0.5 seconds without damage. The instantaneous trip function can thus be delayed, to allow downstream breakers to clear a fault without tripping the switchgear breaker, facilitating selective coordination.
配电柜生产须要遵循UL 489《塑壳断路器、塑壳开关和断路器外壳》规范。根据本规范生产的断路器只要拥有3个周期的耐受时间，即0.05秒（3′1/60Hz）。针对这类断路器，只具有瞬时过流跳闸性能，不带延缓过流跳闸性能，也可应用带熔丝的刀闸。封闭式开关的应用规范为NEMA KS1《重载外壳防护封闭式开关》。
The switchboard standard allows breakers built to UL 489: Molded-Case Circuit Breakers, Molded-Case Switches and Circuit-Breaker Enclosures. Breakers built to this standard are required only to have a 3-cycle withstand, 0.05 seconds. For these breakers, the instantaneous trip function cannot be delayed to facilitate selective coordination. Use of fused switches is also allowed. The applicable standard for enclosed switches is NEMA KS1: Heavy Duty Enclosed and Dead-Front Switches.
开关柜额定铭牌属性含盖：
Switchgear ratings include:
绝缘程度
Insulation level
最大连续电流
Maximum continuous current
最大电压
Maximum voltage
电源频率
Power frequency
短路耐受电流
Short-circuit withstand current
短时耐受电流（热安稳电流）
Short time withstand current
在典型装载中，高压开关柜连通到厂用变压器或设备变压器的二次侧。采取中压供电方法时，电力变压器可与开关柜用螺栓固定在一块，生成1个单元。由此构成的组件称为“模块化变电站”。开关柜的馈线断路器往往为大型设备负载（如冷机、大型变压器或大型UPS）或其余配电设施供电，含盖高压配电柜、电机操控核心、配电箱或其余开关设施组件等。
In a typical installation, LV switchgear is connected to the secondary of a power transformer — either the utility’s service transformer or a facility transformer. Where service is at medium voltage, a power transformer may be close-coupled to the switchgear, with the two assemblies bolted together to form a single unit. The resulting assembly is called a “unit substation.” The distribution breakers of the switchgear will typically serve feeders to large facility loads, such as chillers, large transformers or large UPSs — or other distribution equipment, such as switchboards, motor control centers, panelboards or, rarely, other switchgear assemblies.
电力变压器
Transformers
在牢靠性和可养护性方面，开关柜比配电柜构造拥有显著的优势。在1个特定项目上应用哪品种型取决于多种原因。配电柜只要相当小的占地面积，便可供应相近的配电和爱护性能，因而可用空间将决议能否选取配电柜。而开关柜的价钱要高得多，出资本钱比配电柜高约60%至100%。因而，假设项目预算紧绷，就倾向选用配电柜。况且，在须要采用过流爱护选取性协作的项目中，特别是在NEC第700.28条需要严密协作的应急体系上，就只可选开关柜成为处理计划。
Switchgear has definite advantages over switchboard construction in terms of reliability and maintainability. The decision of which system to use on a particular project will depend on a variety of factors. Switchboard construction requires a considerably smaller footprint to provide the same distribution and protection functions, so available space will have an impact on the selection. Switchgear is considerably more expensive, with a cost penalty on the order of 60% to 100%, so a tight project budget will bias the decision toward switchboard constructions. And, in projects where selective coordination is challenging, particularly on an emergency system where strict coordination is required by NEC Article 700.28, switchgear may be the necessary solution.
变压器是一类交流电磁装置，通过磁场将电能从1个或多个一次绕组回路传输到1个或多个二次绕组回路。一次绕组回路和二次绕组回路往往运作不同的电压和电流，它们之间的比例由变压器的特征决议。NEC《国度电气标准》第450条描绘了变压器的有关需要。
A transformer is an alternating-current electromagnetic device that magnetically moves power from one or more primary circuits to one or more secondary circuits. The primary and secondary circuits secondary circuits typically operate different voltages and currents, with the ratio between them determined by the transformer’s characteristics. Requirements for transformers are described in NEC Article 450.
变压器在当代生活中无处不在，它拥有各类不同的特征、额定值和用处。在高功率运用场所，电力企业应用大型电力变压器连通不同电压品级的输电体系。在小型终端，微型信号变压器被用来将通信设施连通到以太网体系，微型变压器乃至被印刷在集成电路中。而设备配电体系中应用的变压器介于这2个极度运用场所之间。Transformers are ubiquitous in modern life, with a variety of characteristics, ratings and uses. On the high-power end of the scale, electric utilities use large power transformers to connect transmission systems operating at different voltages. On the small end, tiny signal transformers are used to connect communication equipment to Ethernet systems and microscopic transformers have even been printed in integrated circuits. Transformers used in facility distribution systems fall between those extremes.
变压器的工作原理是磁感应。这是一类电磁原理，即导体在周边磁场改变的状况下，导体两头会构成电压。磁感应是19时代由许多科学家发掘并量化的，他们的奉献这样重要，以致于电学丈量单位和物理学定律都用他们的名称来命名。因为篇幅限定，下文描绘变压器运作原理时，咱们再简洁地讲讲磁感应的原理。
A transformer operates on the principle of magnetic induction, an electromagnetic principle that states that a voltage will develop across a conductor in the presence of a changing magnetic field. Magnetic induction was discovered and quantified in the 19th century by scientists whose contributions were so significant that their names have been attached to electrical units of measure and laws of physics. A thorough treatment of magnetic induction would require many times the space available here, so it will be treated qualitatively in this discussion of transformer operation.
1个简洁的变压器也许由1个称为“铁芯”的铁环构成，1个一次线圈和1个二次线圈分别围绕该铁环生成的回路，称为“绕组”，如图3所示。当交流电流过一次绕组时，其构成的磁场大小和方向随填写功率而改变。
In an elementary implementations, a simple transformer might consist of an iron ring, called the “core,” with one primary and one secondary each making multiple loops around the ring, called “coils,” as shown in Figure 3. When the primary is energized with alternating current, the primary coil generates a magnetic field that varies in magnitude and direction with the input power.

图3：单相变压器线圈回路表示图。供应：约翰斯顿有限责任企业
Figure 3: A simplified diagram shows a single-phase transformer coil. Courtesy: Johnston, LLC
从理论上讲，全部变压器空间都有磁场，但铁芯的磁特征将几乎一切磁场都集中在环体内，在环体内，磁场同时通过一次绕组和二次绕组。流经二次绕组的时变磁场通过磁感应构成出感应电压。一次绕组数和二次绕组数的商称为“匝数比”，此中匝数指的是绕铁芯的导线匝数。最终得悉，二次电压等于一次电压除以匝数比。
In theory, that magnetic field exists throughout all space, but the magnetic characteristics of the iron core concentrate nearly all of the magnetic field within the body of the ring, where it passes through both the primary and secondary coils. The time-varying magnetic field running through the secondary coil induces a voltage across those coils by magnetic induction. The quotient of the number of primary loops and the number of secondary loops is called the “turns ratio,” where turns refers to turns of wire around the core. In the end, the secondary voltage is equal to the primary voltage divided by the turns ratio.
真正的变压器比这里描绘的原理模型要高难得多。比如，设备中装载的大多数变压器全是三相装置，其铁芯几何构造必需容纳3个一次线圈和3个二次线圈。变压器往往在二次线圈上配备分接头，在体系负载、市电电压或其余原因造成电压持久低过或高过常态值的运用场所，能够通过度接头将流出电压调节至略高过或低过铭牌额定电压。变压器铁芯往往由特殊钢板制成，用绝缘粘合剂粘合在一块，而不是用实心铁或钢，以减小铁芯在运作时的涡流。典型的设备变压器装载在金属外壳内，往往带有透风孔。
Real-world transformers are much more complex than the na?ve implementation described here. For example, most transformers installed in facilities are three-phase units, whose core geometry must accommodate three primary and three secondary coils. Transformers are often provided with taps on the secondary coil — additional connection points whose output voltage is slightly higher or lower than the nominal voltage, for use in applications where voltages lower or higher than normal chronically occur due to system loading, utility voltage levels or for other reasons. Transformer cores are typically made from sheets of specialty steel, bonded together with an insulating adhesive, rather than solid iron or steel, to reduced magnetically induced currents that circulate in the core during operation. A typical facility transformer is mounted inside a metal enclosure, usually with openings for ventilation.
变压器的一次绕组和二次绕组之间没有电的连系。一二次绕组之间的磁场促使二次绕组流出电压到达特定值。但理论上，绕组导体与其周边场景之间的电压尚未定论。针对大多数体系，此中一根二次绕组导线必需有意接地，以保证二次导线上的电压不会偏离地电位太远。但这个规则并非应用于可以容忍单点接地故障的体系，比如医疗设备中的隔离电源体系。
No conductive connection exists between the primary and secondary coils of a transformer. The magnetic interaction between the coils forces the voltage between the secondary conductors to a specific value, but the voltage between either conductor and its surroundings is, in theory, undefined. For most systems, one of the secondary conductors must be intentionally connected to ground, to ensure that the voltage on the secondary doesn’t stray too far from earth potential. Exceptions to that rule are systems that must be tolerant of a single ground fault, such as isolated power systems in medical facilities.

图4：装载在操控间里的1个隔离的配电柜，柜内正下方为隔离变压器。
Figure 4: An isolated power panel is installed in an operating room.
The transformer is visible at the bottom of the enclosure. Courtesy: Johnston, LLC
变压器额定铭牌属性含盖：
Transformer ratings include:
容量，往往以千伏安表示，即变压器可向其负载供应的最大视在功率。
Capacity, typically expressed in kilovolt amperes, the maximum apparent power that the transformer can supply to its loads.
一次电压或路线电压——一次绕组的工作电压。
Primary voltage or line voltage — the operating voltage of the primary coil.
二次电压或负载电压——二次绕组的工作电压。
Secondary voltage or load voltage — the operating voltage of the secondary coil.
温升（单位：摄氏度）——变压器满载运作时，变压器线圈热度与场景热度之间的差值。
Temperature rise, typically expressed in degrees Celsius — the difference between the temperature of the transformer coils and ambient temperature when the transformer operates at full load.
标准中往往显现的变压器的其余特点含盖相数、变压器抽头的数目和电压调整范畴、外壳防护品级、绝缘介质、阻抗和效益。Other features of transformers that normally appear in specifications are number of phases, number and spacing of transformer taps, enclosure characteristics, insulation medium, impedance and efficiency.
变压器的效益不是100%。固然大部份电能都能从一次绕组输送到二次绕组，但有类功率会因热量而损失，这类消耗能够被描绘为负载消耗（铜损--首要是因为线圈导体的电阻造成的消耗）并且空载消耗（铁损--首要是因为铁芯内外的磁效应造成的消耗）。这两类型号的损失是互相依赖的，由于设计减小一类型号的损失就会提升另一类型号的损失。
Transformers are not 100% efficient. While most of the input power is delivered to the secondary terminals, some is lost as heat. These losses can be characterized as load losses, primarily due to resistance of the coil conductors and no-load losses, primarily due to magnetic effects inside and outside of the core. These two types of losses are interdependent, in that designing to reduce one type of losses can raise the other.
比如，能够应用大尺寸导线来生产线圈，减低其串联电阻进而减低负载消耗（铜损）。但是，远离铁芯的外层的大尺寸的导领会减低线圈和铁芯之间的磁耦合效益，并提升空载消耗（铁损）。针对大多数变压器，美国能源部的划定描绘了所需的效益程度，并划定变压器效益将在35%或靠近35%的负载程度下进行优化。这就标志着须要掂量考质变压器的负载消耗和空载消耗。
For example, load losses can be reduced by constructing the coils from larger wire, reducing their series resistance. However, larger conductors will place outside layers further from the core, reducing the effectiveness of magnetic coupling between the coil and core and raising no-load losses. For most transformers, Department of Energy rules describe required efficiency levels and specify that transformer efficiency will be optimized at a load level at or near 35%. Those regulations generally dictate what trade-offs are between load losses and no-load losses are permissible.
不间断电源
Uninterruptible power supplies
UPS是一类电气设施，它能够持续供应近乎完美的交流电源，电气牢靠性靠近100%。UPS往往用来追随要害负载。UPS既可为数百伏安的桌面用电设施供电，也能够为额定功率为数千千瓦的大型公司体系供电。
A UPS is an electrical assembly that is designed to provide nearly perfect alternating current power continuously, with nearly 100% reliability. A UPS is typically deployed to support electrical loads that are critical to the business conducted in a facility. UPSs are available in very small desktop units to power loads in hundreds of volt-amperes, to very large enterprise systems rated in thousands of kilowatts.
UPS的性能是在主进线电源（往往是电力设备）显现断电故障或供电品质超越容许范畴时，为其负载供应高端量的电源。UPS在停电、低电压、电压突降和突升、单相断电和其余体系烦扰时期保持其负载的供电，防范断电和毁坏。
The function of a UPS is to provide high-quality power to its load when the primary power source, usually an electric utility, fails or becomes unacceptable. A UPS maintains power to its load during blackouts, brownouts, voltage sags and swells, loss of a single phase and other system disturbances, protecting from both loss of power and from damage.
一切UPS都含盖1个储能体系，往往配备化学电池（铅酸、镍镉、锂离子）储能。当填写电源产生故障时，UPS从电池中提炼能量，将其转换为交流电并将其输送至负载。现在已有多种UPS拓扑型号获得全面地运用。
All UPSs contain an energy storage system, most often in the form of chemical batteries (lead-acid, nickel-cadmium, lithium-ion). When the input power fails, a UPS draws energy from its batteries, converts it to AC and delivers it to the load. A number of schemes for providing replacement power, called “topologies,” are in common use.
“双变换”UPS，也称为在线式UPS，内部整流器将填写的交留连续转换为直流电。构成的直流电通过内部逆变器为负载构成交流电源，并保持体系电池的充电。假设交流填写中止，电池将向直流母线供电，并持续转换为交流电源，连续不停地向负载供电。
A “double-conversion” UPS, also called an online UPS, continuously converts incoming AC to direct current using an internal rectifier. The resulting DC power is used to generate AC power for the load, using an internal inverter and to maintain the charge on the system batteries. Should the AC input be disrupted, the batteries provide power to the DC bus and the conversion to AC and delivery to the load continue without interruption.
术语“双变换”指的是UPS持续地将交流转换为直流，接着将该直流转换回交流。在这类拓扑型号中，流出交流电的品质不取决于填写电源的品质，由于流出的交流电是由独立的直流总线逆变构成出去的。因为电能转换是持续的，填写侧电源扰动不会牵连负载持续用电。这类拓扑型号被认定是高度牢靠的。它往往也比其余拓扑型号价钱更低廉、效益更低。
The term “double-conversion” refers to the fact that the UPS continuously converts AC to DC and then converts that DC back to AC. With this scheme, the quality of the output AC does not depend on the quality of the input power, since the output is independently generated from the DC bus. Because conversion is continuous, there is no requirement for detection of input power disturbances in order to protect the load. This topology is seen as highly reliable. It is also generally more expensive and less efficient, than the alternatives.
因为双变换UPS连续构成流出交流电，UPS内部故障也许会使要害负载的电源持续性面对危害。为了解决这个破绽，UPS内置了1个静态开关——1个连通在填写和流出之间的高速电子开关——将填写电源直接连通到负载。UPS监管其本身的流出，假设流出超越可接受的限定，UPS将关闭静态开关并和负载断开。
Because a double-conversion UPS continuously generates the output AC, a failure inside the UPS can put the continuity of power to the critical load at risk. To address this vulnerability, these units typically include a static switch — a high-speed electronic switch connected between input and output — that will connect the input power directly to the load. The UPS monitors its own output and, should the output fall out of acceptable limits, the UPS closes the static switch and disconnects itself from the load.

图5：双转换不间断电源原理图。供应：约翰斯顿有限责任企业
Figure 5: A block diagram of a double-conversion uninterruptible power supply is shown. Courtesy: Johnston, LLC
“单变换”或“备用”拓扑型号的UPS持续将其填写功率直接传播给负载，而填写是可接受的。UPS监管填写电源能否存在烦扰，假设显现任意烦扰，UPS会断开填写电源，并开启通过逆变器从电池中为负载供电。该流程须要在填写烦扰和改换电源之间进行延缓，以便探测、从新校准体系并开启逆变器。因而，备用UPS应用于体系烦扰容限很高的负载。固然这类拓扑型号不如“双变换”牢靠。但是，它的效益更高，由于在常态运作时，它的整流器或逆变器不会显现消耗。
A “single-conversion” or “standby” UPS continuously passes its input power directly to the load, while the input is acceptable. The UPS monitors the input power for disturbances and, should any appear, it disconnects input power and begins to serve the load from its batteries through its inverter. This process necessitates a delay between the input disturbance and the commencement of replacement power, for detection, realignment of the system and starting the inverter. A standby UPS is therefore applicable to loads with a higher tolerance for system disturbances. This topology is seen as less reliable than double conversion. It is, however, more efficient, since it doesn’t incur losses in its rectifier or its inverter in normal operation.
不间断电源体系额定铭牌属性含盖：
Ratings for uninterruptible power systems include:
满载运作时间——用来设计电池容量
Full load runtime — a function of the battery capacity
填写电压
Input voltage
最大流出视在功率，以伏安表示
Maximum output apparent power, expressed in volt-amperes
最大流出瓦数，以瓦为单位
Maximum output wattage, expressed in watts
流出电压
Output voltages
UPS功率往往根据全部生命周期中预料最大负载的125%进行选取。信息核心须要依据营业成长估算负载的急暴增长，而这类增长偶尔不能实行，进而耗费了UPS过剩的容量。为了解决这个问题，许多体系供应了模块化的、可热插拔的电源和电池模块，以便紧随负载的加大进行扩容和加大连续供电时间。
A UPS is typically sized at about 125% of its expected maximum load, estimated for its entire life cycle. Data center applications call for estimates of aggressive load growth that sometimes don’t materialize, stranding excess capacity. To address this issue, some systems are available with modular, hot-swappable power supply and battery modules to allow incremental capacity and runtime upgrades as load increases.
不间断电源须要平常养护，与其余一切设施同样，在应用流程中，它们偶尔也会显现故障。针对某类体系，将负载直接连通到市电的外部养护旁路是养护和修理活动的十足前提。更敏感的体系须要绝对水平的冗余。不间断电源模块能够并联或串联，以供应冗余容量，并在冗余单元之间进行通信和监管。
UPSs require routine maintenance and, like everything else, they sometimes fail in service. For some systems, a wrap-around maintenance bypass, connecting the load directly to utility power, is an adequate provision for maintenance and repair activities. More sensitive systems will require a level of redundancy. Units can be connected in parallel or in series to provide redundant capacity, with communication and monitoring among redundant units.
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