关于CELEM电容选型指导
1. 根据需要选择电容器
在为应用选择电容器时,选择具有最大额定电压RMS、电流和无功功率的电容器,该电容器应与应用的最高工作电压和频率最接近。考虑操作过程中可能出现的任何纹波和任何DV偏置,因为即使是短暂的过压也会损坏电容器并缩短其使用寿命。每个电容器的数据表上都清楚地显示了最大额定值。
无功功率公式为:
Qc= V2rms x C x 2πf x 10-6
注:
Qc是无功功率,单位为kVAr
Vrms是以伏特为单位的RMS电压
C是以µF为单位的电容
f是以kHz为单位的频率
1. Choosing the Best Capacitor for Needs
When choosing a capacitor for an application, select a capacitor with maximum rated voltage RMS, current and reactive power that most closely matches the highest operating voltage and frequency of the application. Take into account any ripple and any DV bias that can occur during operation, as even momentarily over voltage damages the capacitor and reduces its life time. The maximum ratings appear clearly on the data sheet of each capacitor.
The reactive power formula is:
Qc= V2rms x C x 2πf x 10-6
where
Qc is reactive power in kVAr
Vrms is RMS voltage in volts
C is capacitance in µF
f is frequency in kHz
2. 功率电容器的适当冷却和端接
Celem电导冷却功率电容器以相对较小的体积处理大量功率。为了正确运行,电容器必须正确端接以确保所有内部元件的均匀冷却。
使用传导冷却电容器时必须满足四个关键要求:
1) 冷却。电容器的整个接触面必须使用导热膏与散热器接触,以确保最佳的热传导。当电容器以最大极限工作时,这一点尤其重要。切勿超过最大允许扭矩拧紧螺栓。
2) 传导损耗。大多数电力电容器能够提供数百安培的电流。当多个电容器连接到一个公共集电极时,由于集肤效应,可能会导致集电极表面积不足,导致收集电流的母线极度发热,即使适当冷却也会导致电容器过热。
3) 电容器的感应加热。当多个传导冷却电容器组装在两个汇流条之间时,那些离输出端子最近的电容器可能会受到感应加热。这种情况应通过如图B所示安装电容器或通过在母线之间构建低电感路径(如图C所示)来避免这种情况。C-Cap技术克服了这一缺陷。
4) 杂散电感。即使电容器组没有直接连接到工作线圈,也要避免电路中的杂散电感。电感与母线的长度成正比,与它们的宽度成反比,与母线之间的距离成反比。
2. Proper Cooling and Termination for Power Capacitors
Celem conduction-cooled power capacitors handle a large amount of power in relatively small volume. In order to operate correctly, capacitors must be terminated properly to ensure uniform cooling of all internal elements.
Four critical requirements must be addressed when using conduction cooled capacitors:
1) Cooling. The entire area of the capacitor’s contacts surface must be placed in contact with the heat sink using a thermal conductive paste to ensure optimal heat conductance. This is particularly critical when the capacitor is working at its maximum limits. Never tighten the bolts beyond the maximum allowed torque.
2) Conduction losses. Most power capacitors are able to supply hundreds of amperes. When several capacitors are connected to a common collector, insufficient collectors’ surface area may result, due to the skin effect, in extreme heating of the bus bar where the current is collected, and overheating of the capacitor despite proper cooling.
3) Induction heating of capacitors. When several conduction-cooled capacitors are assembled between two bus bars, those located closest to the output terminals may experience induction heating. This situation should be avoided by mounting the capacitors as illustrated in diagram B or by building a low inductance path between the bus bars as illustrated in diagram C. The C-Cap technology overcomes this deficiency.
5) Stray inductance. Even when a capacitor bank is not connected directly to a work coil, it is good practice to avoid stray inductance in the circuit. Inductance is proportional to the bus bars’ lengths, inversely proportional to their widths and inversely proportional to the distance between the bus bars.
3. 电容器组损耗
电容器组由电容器以及电容器与输出端子之间的连接组成。损失可能与这些要素中的任何一个有关。
电容损耗。电容器的损耗极低,通常约为无功功率的5 x 10-4 x。
连接损耗。在正确构造的电容器组中,连接损耗将与电容器损耗大致相同。
总电容器组损耗等于电容器损耗和连接损耗之和。 因此,它通常总计约为无功功率的10-3。
3. Capacitor Bank Losses
A capacitor bank consists of capacitors and connections between the capacitors and output terminals. Losses may be associated with either of these elements.
Capacitor losses. Losses in the capacitors are extremely low, generally about 5 x 10-4 x the reactive power.
Connection losses. In a properly constructed capacitor bank, connection losses will be approximately the same as the capacitor losses.
Total capacitor bank loss is equivalent to the sum of the capacitor losses and connection losses. As such, it generally totals about 10-3 of the reactive power.
4. 电容器组冷却所需的水流量
聚丙烯电容器可以在高达90°C的温度下安全运行。聚丙烯元件的最热点与Celem电容器的铜表面之间的温度梯度为40-45°C。因此,电容器的外表面不得超过45°C。在实际应用中,冷却回路出口处的冷却水温度不应超过40°C。
当电容器组与工作线圈串联冷却时,工作线圈的损耗至少是电容器组的十倍。因此,应先冷却电容器组,然后再冷却工作线圈。
4. Required Water Flow for Capacitor Bank Cooling
Polypropylene capacitors can safely run at temperatures of up to 90°C. The temperature gradient between the hottest points of the polypropylene element and the copper surface of Celem capacitors is 40-45°C. Therefore, external surfaces of the capacitors must not exceed 45°C. In practice, the cooling water temperature at the outlet of the cooling circuit should not exceed 40°C.
When the capacitor bank is cooled in series with the work coil, the losses of the work coil are at least ten times that of the capacitor bank. Therefore, the capacitor bank should be cooled first, followed by the work coil.