High Voltage Capacitors for DC-Link Applications

Transportable energy storage systems for grid applications serve several functions including integration and intermittency mitigation of renewables, improving grid stability and reliability by providing new capacity that can be deployed quickly and they provide a cost effective way to balance the load. Each energy storage unit includes a high power inverter that is used to convert DC voltage to three phase AC. A key component of the inverter circuit is the DC- link capacitor, used to minimize ripple current due to the inverter switching, voltage fluctuation and transient suppression. The DC-link capacitor is one of the largest, costliest and most failure-prone components in todays inverter systems. This holds true for all inverters used in applications that range from residential to automotive (HEV and EV) and grid based systems. This SBIR Phase I proposal addresses the development of high voltage DC-link capacitors using a hybrid polymer film, modified for higher breakdown strength, higher temperature, reduced dielectric absorption, superior self-healing properties and lower ESR. The hybrid film comprises a conventional polymer film such as polypropylene (PP), polyphenylene sulfide (PPS) and polyvinylidene difluoride (PVDF), converted into a film designed specifically for capacitor applications. The converting process is performed by adding on one or both surfaces of the base film a polymer dielectric that has superior properties when used in capacitor applications. This process is made possible by recent breakthroughs in material and processes technology, used to manufacture solid state Polymer Multi Layer (PML) capacitors that utilize acrylate polymer dielectrics. The acrylate dielectrics are electron beam cross linked, with a dielectric constant in the range of3.0 & lt; & lt;6.2, dissipation factor & lt;0.01, dielectric absorption & lt;0.5%, temperature withstand & gt;260 oC, operating temperature & gt;140oC, thickness as low as 0.2m, breakdown strength of the order of 1000V/m and excellent self-healing properties. Hybrid Acrylate/PP, acrylate/PET, acrylate/PVDF and acrylate/PPS films when used to produce metallized capacitors, have demonstrated superior dielectric properties when compared to the base polymer films, including higher breakdown strength, higher ripple and transient current carrying capacity, higher resistance to corona degradation and superior self-healing properties. The proposed development will explore the properties of metallized acrylate/PVDF and acrylate/PVDF/acrylate hybrid films using commercially available PVDF films. PVDF based polymers have a Category IIIb surface tracking rating, with an initiating voltage & lt;250V that limits capacitor self-healing properties. In contrast, PP that has good self-healing properties and high breakdown strength, is a Category I material with a tracking initiating voltage & gt;600V. Sigma has demonstrated that metallized acrylate/PVDF films have superior self-healing properties and improved breakdown strength, resulting in wound capacitors with energy densities greater than 2.5J/cc. This development will evaluate hybrid films with different acrylate:PVDF thickness ratios, by varying both the thickness of the acrylate layers and the gage of the PVDF films. The effect of grading the dielectric constant using high and low dielectric constant acrylates will be evaluated. A series of tests will be conducted to demonstrate the performance of hybrid films versus that of the base films. The choice of PVDF films and acrylate layer thicknesses will be designed to demonstrate dielectric performance and energy density & gt;1J/cc, for 1KV and 5KV metallized hybrid film capacitors. The ultimate objective of this development is to demonstrate that hybrid films have unique dielectric, thermomechanical and self-healing properties that cannot be obtained from the base polymer films. While development of new polymer films for capacitor only applications is economically prohibitive, converting existing films to capacitor specific hybrid films is both technologically and economically feasible. The proposed HV DC-link capacitors will result in lower electric power costs and will improve the life and reliability of distributed energy storage systems. When used in automotive inverters they will result in lower cost, smaller and more reliable automotive EV and EHV power control modules, which will improve the competitiveness of US OEMs and will help create new US jobs.