Power generation has been an integral industry that generates electricity to be distributed to all sectors of activity, both residential and industrial. It uses many sources of energy to generate electricity: water, wind, solar, nuclear, fossil fuels, and more. Many of these energy sources are called "prime movers", which means a mechanical input is involved. For example, the gravity of water helps move turbine propellers. Depending on the configuration of generators, they may also require a DC field source to "excite" the rotor windings of the generators. This field source is often referred as the exciter or excitation system.
There are many configurations for exciter systems, and they are often categorized as static excitation or dynamic excitation. These DC sources are often sourced by step-down transformers from the main stator system.
Rotor windings are highly-stressed components in modern electric generators, as the rotor itself is the rotating part of the generator or alternator. Their insulation degrades over time via their dielectric elements. If a rotor needs to be replaced, the labor costs, unexpected downtime, and revenue losses all make significant financial deterrents of letting the rotor fail.
The companies that construct and manage hydroelectric generating stations need to measure the insulation resistance of the exciter systems and detect when the rotor insulation has degraded past a certain point. These companies often refer to this insulation measurement as "field ground relays". These hydro companies have independently tested many suppliers, and their results have shown that Bender's ISOMETER® Insulation Monitoring Device (IMD) is the most accurate and reliable field ground relay on the market. For many years now, this industry has mandated all exciter systems to be equipped by the ISOMETER IRDH series, and more recently the iso685.
The IMD measures the insulation resistance of the whole exciter system, from the step-down transformer, through the AC/DC rectifier bridge and all the way to the rotor windings. The wide range of measurement allows hydro-generation companies to monitor the insulation resistance in the hundreds of kΩ and even MΩ, while maximizing the generator production uptime to its very limit by selecting two alarm setpoints of 40 kΩ and 10 kΩ, for example. Once the insulation drops below the second alarm threshold, it signifies to the maintenance team to plan a shutdown to clean the rotor.