April 2012

High-voltage ceramic capacitors

Detecting voltage fluctuations in smart grids

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As the share of electricity generated by photovoltaic and wind power plants grows, intelligent control systems are becoming ever more crucial in order to ensure the reliable and stable operation of the grids. Extremely low-tolerance TDK high-voltage ceramic capacitors are a key component in the sensitive automatic load break switches.

Until recently, nearly all electric power was generated in a relatively small number of large thermal or nuclear power plants and – after voltage adjustments at substations – distributed to factories and homes. The generation and provision of power was thus in one direction only: from power utilities to consumers. Today, with the fast-growing number of photovoltaic or wind power installations, a significant amount of power is being supplied by very many widely distributed smaller power plants from the consumer side as well.

One of the major challenges posed by such distributed power supply systems is that photovoltaic and wind power generation is dependent on the weather and thus unstable. Power companies must therefore constantly monitor and adjust the power levels (e.g. voltage, current, phase angle) flowing in each section of underground or above-ground power lines. In order to monitor the voltage fluctuations in the grid and support the advanced operation of the power distribution network, utilities install IT switches, which are intelligent automatic load break switches equipped with integrated voltage sensors (Figure 1). TDK high-voltage ceramic capacitors are a key component in the IT switch and are used as voltage or potential dividing capacitors.

Figure 1: Automatic load break switches in a smart grid

Traditionally, many switches are incorporated in power distribution networks to separate sections when problems occur and thus minimize blackout areas. IT switches as depicted also incorporate a sensor function in order to monitor voltage fluctuations in the grid.

Stable performance over a wide temperature range

The basic configuration of a voltage sensor is shown in Figure 2. The circuit consists of a high-voltage capacitor and a low-voltage capacitor connected in series. In order to be able to accurately detect voltage variations in the distribution lines, the electrical properties of high-voltage capacitors used in voltage sensors must be unaffected by surrounding temperature or voltage levels. The high-voltage capacitor must exhibit no capacitance changes at temperatures from −20 to +70 °C and up to distribution voltages of 3810 VRMS; low-voltage capacitors must also have similar characteristics. Additionally, the capacitors must be small enough that 6 each fit in both the input and output of the three-phase line of each switch.

Figure 2: Schematic diagram of a sensor circuit

IT switches consist of a high-voltage capacitor and a low-voltage capacitor connected in parallel. Both capacitors must exhibit a very low capacitance tolerance.

Based on the design of the FD series of TDK ceramic capacitors, a new ceramic material was developed that features a relative conductivity of over 90, while maintaining the C0G temperature characteristics (0 drift ±30 ppm/K). To achieve such outstanding properties, rare earth elements were added to the barium titanate composition. The result is a ceramic material with high conductivity and stable temperature characteristics that enables the compact dimensions needed for mounting in IT switches.

Robust design
The new TDK UHV ceramic capacitors feature a durable and robust design (Figure 3). Silver electrodes are sintered on the opposing end faces of the ceramic element, fitted with bracket screw terminals and soldered. The capacitor is then molded with epoxy resin which offers excellent insulation and moisture resistance. The capacitor features an AC endurance of 30 kV and impulse endurance of ±65 kV for 3 times, respectively (Table). It also supports specifications with higher insulation capacity requirements and is RoHS-compatible.

Figure 3: Basic structure of the TDK UHV ceramic capacitor

The new TDK UHV ceramic capacitors are extremely robust. Silver electrodes are mounted on the opposing ends of the ceramic element and fitted with screw terminals.

Table: Key technical data of TDK UHV ceramic capacitor series
AC voltage endurance for 60 s [kVRMS]30
Lightning impulse endurance at applied voltage for 3 times [±65 kV]Applied voltage for 3 times
Insulation resistance at 1 kV DC for 60 s [MΩ]105 or above
Capacitance ±5% at 1 kHz VRMS [pF]50 to 150
Dielectric loss tangent at 1 kHz VRMS [%]≤ 0.2
Temperature characteristics of capacitance [ppm/K]0 ±60
Partial firing voltage [at 3 pC 50 Hz kVRMS]≥15

Perfectly matched capacitor pairs

With existing switches, the temperature characteristics of capacitance in high-side and low-side capacitors were not a key factor. In automatic load break switches with built-in sensors, however, the impact of temperature changes must be minimized. To meet these requirements, TDK UHV ceramic capacitors in combination with leaded MLCCs of the TDK FK series possessing the same C0G temperature characteristics are well-suited to act as a voltage sensor. Prior to delivery, the capacitance values of both the low-side capacitors and high-side capacitors are matched and combined to ensure a consistent voltage output and a constant division ratio.

The combination of the TDK UHV series with the TDK FK series (Figure 4) delivers flat output signals in the temperature range from −20 to +70 °C. This enables an extremely accurate detection of voltage drops and rises in distribution lines and as a result the subsequent voltage stabilization by the voltage regulation devices installed in each section.

Figure 4: TDK ceramic capacitors of the UHV and FK series

TDK UHV ceramic capacitors in combination with MLCCs of the TDK FK series offer a constant ratio over a broad temperature range and thus enable precise detection of voltage fluctuations in the grid.

Designed for a variety of high-voltage applications
Based on over 40 years of experience developing capacitors for power distribution applications, TDK ceramic capacitors are available in a wide range of specifications for a variety of high-voltage applications. Conventional applications include power supply circuit breakers for power lines to homes, high-voltage devices for medical equipment (x-ray equipment), and charge and discharge devices for lasers and electron microscopes. Innovative TDK components are increasingly supporting new applications such as smart grids and thus further contributing to the improvement of energy efficiency.

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