REO DC filter for protecting solar modules against premature aging. As a result of the ever more efficient energy conversion in modern power inverters, ever more high-frequency energy is being introduced into DC circuits. This energy is caused by the quick switching operations of modern power semi-conductors and could cause the premature aging of solar modules. REO-DC mains filters work in both directions. In practical usage, this means that the end devices are protected against unnecessary high frequencies. On the other side however, the power network is also protected against the spreading of self-generated high frequencies by the end devices.