Power utilities around the world are rolling out smart meters in order to enable automated meter reading and optimized energy management. The X2 capacitors from the EPCOS B3293* heavy duty series of film capacitors are well suited for these meters because they operate reliably over a long service life under extreme environmental conditions.

Smart meters are continuing to enjoy strong growth rates as more and more power utilities implement automated meter reading (AMR) to save the expense of periodic trips to each physical location to read a meter. Moreover, information based on near real-time consumption can help both utilities and their customers better control the use and production of energy.

An estimated 40 percent of all power meters are installed outdoors, where they are exposed to high humidity, as well as extreme and rapidly changing temperatures. While outdoor mounting poses no serious problem for conventional meters with relatively simple electromechanical designs, the challenge is much greater for electronic smart meters.

Smart meters typically employ a capacitive power supply where the key component, the AC capacitor, is permanently.connected in series to the power line. This, combined with high humidity and temperatures, can significantly reduce the capacitance of conventional capacitors and thus severely limit their useful life in smart meters.

Historically, standard EMI suppression capacitors have been selected for this particular circuit position because of their safety approval (UL, ENEC). These capacitors, however, are not designed specifically to satisfy the long-term operation requirements of the power meter.

New heavy duty series

The challenge for the component manufacturer was to develop a capacitor that offers extremely stable capacitance values over a period of decades, regardless of the climatic conditions. These requirements defined the development targets for the EPCOS B3293* heavy duty series of AC film capacitors. They combine the high capacitance stability under AC load and severe environmental conditions within relatively small dimensions and maintain the safety certification for an X2 rating.

While there is no specific requirement for safety-certified capacitors, X2 capacitors are normally preferred by customers as they are approved to IEC 60384-14 and UL 60384-14. X2 capacitors, however, are designed specifically to operate in parallel with the power line for EMI suppression. In capacitive power supplies, however, the capacitor is connected in series with the load (Figure 1).

While standard X2 capacitors are designed to operate in parallel with the power line for EMI suppression (left), smart meters employ capacitive power supplies where the capacitor is connected in series.

Exposure to aggressive environments with high humidity levels and/or relatively high temperatures in combination with AC voltage can start a destructive process, causing the metallization edges of the electrodes to degrade, and resulting in a drop in capacitance (Figure 2).

Threefold threat to capacitors in smart meters

In smart meters the capacitors are exposed to a threefold threat:

• Permanent high AC voltage
• High temperature
• High humidity

The design challenge for the B3293* heavy duty series of film capacitors was, therefore, to offer high durability and very low capacitance drift under these conditions, which are lethal for conventional capacitors.

Designed to withstand high AC voltages and temperatures

In order to avoid the degradation of the electrodes due to high AC voltage, the B3293* series was designed with an alternating metallization pattern that in effect creates two capacitor circuits in series and thus reduces the AC voltage for each capacitor circuit by half (Figure 3). This effect is of high importance because it significantly reduces the speed of degradation. A further advantage of this design is the added safety: In the event of a short in one capacitor circuit, the other is not affected and the power supply can continue to function.

High humidity and temperatures in combination with AC voltage can cause the metallization edges on the film to degrade, resulting in a drop in capacitance. The light blue shaded area indicates where intact metallization was lost.

Metallized film capacitors are a very safe technology due to their self-healing property, which allows weak dielectric areas to be burned out when overvoltages appear. These capacitors represent the only suitable alternative for high capacitance values, high peak voltages, high temperature levels (105 °C), and X2 safety levels (the capability to withstand overvoltages of up to 1000 V). Thanks to their high dielectric strength, the B3293* heavy duty series thus eliminates the need to overdimension the capacitor in the circuit.

Polypropylene (PP) film is typically used for the majority of X2 class EMI capacitors used today. The B3293* series is based on polyester (PET) film, a dielectric material whose dielectric constant of 3.3 is 50 percent higher than that of PP film. PET also offers better resistance to high temperatures, enabling capacitors that are suitable for higher operating temperatures of up to even 125 °C.

Immune to high humidity

A further key requirement is to increase the capacitor’s resistance to humidity. In order to prevent moisture from entering the component, the B3293* series employs specially designed materials for the box, the resin filling, and the metallic spray for connecting the leads. The most important improvement, however, was gained by using a plastic film that is not metallized with zinc. This material, commonly used in X2 and AC capacitors, is very sensitive to humidity and, as a result, corrodes very easily when exposed to the severe conditions commonly encountered with outdoor meter installations. Moreover, the film metallization profiles of B3293* series have been optimized to be even more robust against humidity.

As a result of these design measures, the B3293* series of capacitors offers a very high capacitance stability under the most severe conditions and complies with the extremely high safety requirements that every X2 capacitor must meet. Long-term reliability proven with demanding tests Under standard testing conditions (85 °C, 85 percent relative humidity, 275 V AC), the B3293* heavy duty series exhibits a capacitance drift of less than 2 percent after 1000 h compared to a drift in excess of 10 percent after only 500 h for conventional X2 capacitors (Figure 4), confirming the significant improvement in performance. Indeed, tests under these same conditions confirm that capacitance drift remains below 2 percent even after 2000 h.

The internal structure of the EPCOS B3293* series creates a serial connection of two capacitors. As a result, each of the two capacitors must only handle half of the voltage.

The reliability of the series has also been proven in long-term endurance tests according to the rigorous requirements of the IEC 60384-14 standard and based on the demands of smart meters. IEC 60384-14 standard specifies a maximum capacitance drift of 10 percent after 1000 h at the maximum temperature for the climatic category and at 1.25 times the rated voltage. Under these extreme conditions, a 2.2 ¼F capacitor from the B3293* heavy duty series exhibits a capacitance drift that is well within the permissible 10 percent tolerance range (Figure 5).

The capacitance of the EPCOS B3293* heavy duty series remains extremely stable for 1000 h under standard testing conditions (85 °C, 85 percent relative humidity (RH), 275 V AC) with a drift of less than 2 percent.

Even at higher temperatures and AC voltage levels (105 °C, 305 V AC), capacitance drift is less than 6 percent for the EPCOS B3293* heavy duty series.

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