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外文原文和译文

外文原文

Selection of Power Transformer and Switchgear

The selection of the transformer can have a major impact on the cost of a substation, since the transformer represents the major cost item. Nameplate rating is only a guide to transformer application, and should only be used as a first step in the selection process.The transformer is available as a self-cooled unit, or it can be purchased with additional steps of forced cooling that use fan or fans and oil pumps. Transformer ratings can be in-creased from 25% to 66% by the addition of fans and pumps. The nameplate rating is based on a continuous load producing a 55'C to 65'C conductor temperature rise over ambi-ent. Since many transformers do not carry continuous loads, advantage can be gained from the thermal time lag to carry higher peak loads without exceeding the temperature limits.Transformer ratings are based on the assumption that only an extremely slow deterioration of insulation wi!l take place with normal operation. A substantial increase in rating can be achieved by accelerating the loss of insulation life. This increase in rating might approach 200% for an hour or two, and approximately 120% for 24 hours. For substations that are designed to carry full load under the outage of any one transformer, a high emergency rat-ing for a 24-hour period (e. g. until the failed unit can be replaced) could mean the selection of smaller transformers and a substantial saving in substation cost.

The selection of the transformer should involve a careful evaluation of a number of other factors

(1) Impedances should be selected considering their effect on short-circuit duties and lowside breaker ratings both for initial and future station developments. In addition, impedance is important to achieve a proper load division in the parallel operation of trans-formers.

(2) No load tap ranges should be selected to provide an adequate low-side bus voltage.

(3) If the high-side or low-side voltages vary over a wide range during the load cycle, it may be necessary to provide bus regulation. The actual regulation can be calculated using the system and load characteristics. If regulating equipment is needed, it may be desirable to provide it in the transformer by using load tap changer (LTC) equipment. If the need for bus regulation is not presently evident, but may be required in the future, it may be economical to leave space in the station for future regulators, and buy transformers without LTC equipment.

A great many transformers, including all the large ones and all the high voltage ones are immersed in mineral oil which serves the double purpose of cooling and insulating the windings. Small transformers have enough tank surface to radiate all the heat caused by their losses without exceeding the permissible temperature rise. As size increases, the loss-es increase faster than the tank surface which soon becomes inadequate. Various methods have been developed to get the heat out of the tank more effectively. Large power trans-former being built today are commonly of the forced-oil-cooled type. In this design the oil is pumped both through the external cooling devices (air or water heat-exchangers) and through internal channels that are located nearest the points where the heat is generated.Thus the transfer of heat to and from the oil is far more effective than in the plain self-cooled or fan-cooled unit where the oil is allowed to circulate by convection. When oil-to-air heat exchanger (a group fans on each radiator) is used, this cooiijl|Je is designated as type of FOA. While oil-to-water heat exchanger is used, it is called POW.

For applications where any liquid, even a nonflammable one, is objectionable, the dry-type transformer is used. The ventilated dry-type unit is cooled by a continuous natural draft of air and consequently is not suitable for locations where the air is wet or dirty. For these locations a completely enclosed unit, the sealed dry-type, is available, having a core-and-coil in a tank that is sealed and filled with an insulating gas. Dry-type transformers are completely nonflammable, using organic material such as epoxy resin.

Switchgear is a general term covering switching and interrupting devices, also associ-ated devices with control, metering, protective and regulatory equipment. Swichgear is the vehicle for performing two distinctly different functions.Under normal conditions, it is means of carrying out a multitude of routine switching operations, e.g. disconnecting and

isolating any piece of apparatus for maintenance or replacement ; disconnecting a generator from the system when it is no longer required to serve the load ; sectionalizing a line for in-spection, maintenance or construction purposes; transferring loads; isolating regulators; by-passing circuit breakers ; and performing the reverse of these various operations. Under abnormal conditions, switchgear provides the means for automatically disconnecting the part of the system in trouble to prevent excessive damage and to confine the trouble to the smallest possible part of the system. Under these conditions the switchgear equipment is performing a protective function.

Svitchgear mainly includes circuit breaker, disconnecting switch, load-break switch and fuse. The disconnect switch is the simplest switch on the basis of function, operating only in the absence of appreciable current. This switch cannot open normal load current and its function is to disconnect or connect transformers, circuit breakers, other pieces of equipment and short length of high voltage conductors only.after current through them has been interrupted by opening a circuit breaker or load-break switch. It may, however, open minute "charging" currents to these unloaded equipments being disconnected.When opening, the switch blade is swung upward roughly 900, creating a long, simple gap in air.

A load-break switch will switch normal load currents but will not interrupt short circuit currents. A wall switch in a home fits this classification. However, circuit breakers will per-form the switching functions of the above two classes, but will, if applied within rating, interrupt all short circuit currents that may occur on the system. Fuses consist essentially of a fusible element and an arc-extinguishing means. In some fuses, the fusible element is made of silver, but usually it is tin, copper, aluminum, or some alloy. This element is of proper shape and cross-section to carry rated current continuously and to melt in accordance with a specific time-current characteristic on heavy overload or short circuit.

Circuit breakers and disconnect switches should not be blown open or otherwise dam-aged by short circuit currents within their short time ratings. The circuit breakers and disconnecting switches should be designed or protected to withstand normal operating voltages across the device in the open position. It is desirable to get rid of a short circuit as promptly as possible, now breakers can interrupt short circuit currents within two cycles (on power frequency basis). This is the total time from energizing the trip coil until the

interruption of current on the primary arcing contacts. Shorter arc durations ultimately mean less contacts burning, resulting in less maintenance and a better breaker. Faster interruption of circuit breakers in the transmission voltage offers significant advantage to the transmission system. Faster switching time is advantageous not only in terms of system stability, but also from the standpoint of minimizing possible equipment damage during the fault period, and allowing faster reclosing time.

The term "immediate reclosure" means the immediate return to service of a circuit that has been tripped automatically. In this case, the breaker is reclosed without intention-al delay. To be of much help to stability, it is essential that the breaker be reclosed just as quickly as possible. This requirement has brought about the development of breaker operating mechanisms that are not only high-speed opening, but are also high-speed reclosing.

外文译文

电力变压器和开关装置的选择

变压器的选择对变电站的造价有主要的影响，因为变压器为变电站造价中的主要项目。铭牌额定值仅是变压器应用的一个指导，只是选择过程的第一步。可选用自冷式变压器，也可购买带有风扇或风扇加上油泵这种强迫冷却措施的变压器。增加风扇和油泵可使变压器额定值增加25％到66％。铭牌额定值是以产生55℃至65℃导线温升的持续负荷为依据的。由于许多变压器并不带持续负荷，这就可利用发热时间的滞后来带上更高的峰值负荷而不至于超出温升的限定值。变压器的额定值是以这样的假定为依据的，即正常运行时绝缘仅发生极缓慢的老化。过度超出额定容量将加速绝缘寿命的缩短。负荷增大到200％额定值时可运行1或2小时，增大到约120％额定值时可运行24小时。对于设计为任何一台变压器停运时仍能带满负荷的变电站，其24小时的高紧急情况额定值可能意味着可选择容量较小的变压器，这可大大节省变电站的造价。

1、变压器的选择原则

（1）选用变压器的技术规范和参数应符合国家标准和行业标准，一般应按GB/T6451(油浸式变压器)、GB/T10228(干式变压器)、JB/T2426(所、厂用变压器)选择。选用时应明确是升压变压器，降压变压器，配电变压器，厂用变压器，联络变压器，单相、三相变压器，有载调压、无励磁调压，自冷变压器，还是风冷变压器。应选用通过省部级或相应级别鉴定的设备，优先选用国家经贸委和国家电力公司推荐的产品。

（2)农网线路供电半径一般应满足下列要求：400V线路≤0. 5km；10kV线路≤15km；35kV线路≤40km ；110kV线路≤150km。农网主变压器的容量与配电变压器的容量之比宜采用1:2.5，配电变压器与用电容量之比宜采用1：1.5~1.8。

93）农村变电所的建设应坚持“密布点、短半径”的原则，向“户外式、小型化、低造价、安全可靠、技术先进”的方向发展，设计时考虑无人值班。设计标准可考虑10年负荷发展要求，一般可按两台主变压器考虑。

(4)新建和改造的配网台区，应按“小容量、密布点、短半径”的原列建设改造.应选用低损耗配电变压器，目前主要是采用S9型和新S9型和少量非晶合金变压器。

配电变压器的容量选择，要根据当地经济和生活用电水平，并考虑5年以上发展水平确定。64，73系列高能耗配电变压器要全部更换掉。预装式箱式变电站小区应以规划布点为主，先确定一个合理的供电范围，再根据供电范围的用电负载情况计算配电变压器容量，城市可采用大容量一步到位方案，一般以每户 4. 5—7. 5kW或每户60W/m2计算。

(5)配电变压器的高压侧应采用国家定型的新型熔断器和金属氧化物避雷器。低压侧出线导线截面积不得小于35mm2(铝线〕，总开关应采用空气断路器，并加装漏电保护器。

（6)供电电压的允许偏差.可按G812325-1990 (电压允许偏差)、SD325-1989《无功导则》、GB50052- 1992《供配电规范》规定，即：220V用户的电压允许偏差值为系统额定电压的±5% —10%；380V用户为±7%；10kV用户为±7%；35kV用户为10%。电力线路的电流小于经济电流密度。如果高压线路超过了压降5%，低压线路超过了4%，据《架空配电线路技术规程》规定，应选用有载调压变压器。配置有载调压后，调压幅度可达±10%—±15%，甚至更大。且比无励磁调压级数多，调压精度高，调压的运行情况还可在线监侧。在安装线路压降补偿装备后，可以实现逆调压，以降低电压波动幅度。

2、选择变压器时应仔细评估一些其它的因素：

（1）阻抗的选择要考虑它们对短路负荷和低压侧断路器额定值的影响作用，变电站初期情况和将来的发展都要考虑到。此外，要实现变压器并联运行时的恰当负荷分配，阻抗值是重要的。

（2）无载分接范围选择应能提供正确的低压侧母线电压。

（3）若在负荷变化周期中，高压侧或低压侧的电压有较大范围的变化，就有必要提供母线电压的调节。实际调节量可利用系统特性和负荷特性来计算。若需要调节设备，可要求在变压器中采用有载分接开关（LTC）设备。若目前对母线电压调节并不明确需要，但将来可能需要，较为经济的做法是在变电站中留有将来装电压调节器的地方，购买不带LTC设备的变压器。

大量的变压器，包括所有的大型变压器和所有高压变压器是浸于矿物油中的，矿物油起冷却和将绕组绝缘的双重作用。小型变压器具有足够的油箱表面积来散发所有由损耗引起的热而使之不超过允许的温升。随着容量的增大，损耗的增大要超过油箱表面积的增大，因而表面积就不够了。已经研究了各种方法使热量更有效地从油箱中

散发出来。大型变压器通常为强迫油冷型的。在此种类型的设计中，用油泵使油通过外部的冷却装置（空气或水热交换器）和通过最靠近热产生部位的内部通道，这样热被转移至油，并再从油转移走，从而就比普通自冷或风冷式变压器更为有效，因此时变压器油被循环而不只是对流。当采用油一空气热交换器（每个散热器上一组风扇）时，此种冷却方式称为FOA。而当采用油一水热交换器时，此种冷却方式称为FOW。

在液体甚至是不可燃液体不允许使用的场所，就采用干式变压器。风冷干式变压器通过持续的自然通风来冷却，因此不适合于潮湿或多尘的场所。对于这些场所，采用全封闭的千式变压器，在箱体中充以绝缘气体来隔绝铁芯和线圈。干式变压器是完全不燃的，采用如环氧树脂这样的有机材料。

开关装置是包括开关设备和切断设各的总名称，也包括附属的控制、测量、保护和调节设各。开关装置执行两种明显不同的功能。正常情况下，开关装置执行众多的例行开闭操作，例如，断开并隔离一台设备以进行维修和替换；当发电机不需要向负荷提供电力时，将其从系统上断开；将线路分段以进行检查、维修或施工；转移负荷；断开调节器；旁路断路器；以及执行这些不同操作的逆操作。在异常情况下，开关装置为将系统有问题部分自动断开以避免过度损坏和将问题尽可能限制到最小范围提供了措施。在此情况时，开关装置执行的是保护功能。

开关装置主要包括断路器、隔离开关、负荷开关和熔断器。按功能来讲，隔离开关是最简单的开关，仅在很小电流下操作。隔离开关不能开断正常负荷电流，其功能只能是在变压器、断路器或其它设备和高压短导线中的电流由断路器或负荷开关断开后再进行断开和接通。然而，隔离开关也可开断负荷已被断开设备的微小“充电”电流。隔离开关分间时，问刀片向上转过大约90°，形成一个简明的长空气间隙。负荷开关可开断正常负荷电流但不能开断短路电流。家用墙上的开关就属于此种类型。然而，断路器可执行上述两种开关的开断功能，但若在额定范围内使用，也可开断出现于系统中所有短路电流。熔断器基本由可熔断元件和熄弧装置组成。在某些熔断器中，可熔断元件用银制成，但通常为锡、铜、铝或某些合金。此可熔断元件恰当的形状和横截面使其可连续通过额定电流，而对于严重过负荷或短路，按规定的时间一电流特性而熔断。

断路器和隔离开关在短时额定范围内应不能被短路电流熔断或损坏。断路器和隔离开关的设计或保护应能在断开状态下耐受正常的工作电压。对于短路需尽快地摆脱，现在，断路器可在两个周波（对于工频）内开断短路电流。此时间为从跳闸线圈受电

到主触头电弧电流开断为止的总时间。归根结底，电弧持续时间越短，触头越不易烧熔，维修量越少，而断路器性能越好。断路器在输电电压下的快速开断对输电系统是明显有利的，快的开断时间不仅有利于系统稳定性，而且从降低故障期间设备损坏最小化以及对允许更快的重合间时间也是有利的。

“立即重合”表示已被自动跳开的线路立即恢复供电。在此情况下，断路器的重合无人为的时延。为了更有助于稳定性，断路器必须尽可能迅速地重合，这种要求已促使研制出具有不仅能快速分闸而且也能快速重合闸的断路器操动机构。