Increased renewable power generation, HVDC interconnections and geomagnetic effects all bias the AC grid with small direct currents which leads to two negative effects on transformers (theory of half-cycle saturation) increased noise and increased no-load losses. A unique solution is the DC compensation system, which is an add-on to a transformer which eliminates the DC effects. Furthermore, there are dedicated steps available, from pure detection of the problem, preparation of the transformer and measurement, all the way up to a full compensation system.
Keywords: DC bias, half-cycle saturation, geomagnetically induced currents, renewable grid effects
- The challenge
Power grid operators are required to constantly maintain and improve efficiency of their grid and power equipment. At the same time, growing public awareness and urbanization mean that noise pollution of the surrounding environment and neighborhoods has to be minimized. As a consequence, power grid operators are continuously specifying lower values of noise pressure and loss levels for power transformers. While these values are properly tested during the factory acceptance tests in a controlled environment, the actual situation in the grid may vary due to the impacts of direct current (DC). DC can occur from power electronics in the grid, HVDC interconnections, renewable power generation or small direct currents caused by geomagnetic effects.
- Reasons for DC bias in the grid
2.1 Renewable energy
In order to feed the electrical energy from renewable generation into the transmission grid, the application of converters is necessary. Usually, active switching elements such as thyristors are used for generating a sinus wave voltage by applying PWM (pulse-width modulation) inverters or multi-level PWMs. In Figure 1, it is highlighted that this can generate higher harmonic frequencies as well as DC offset (DC bias). Field studies were conducted jointly with utilities to prove the occurrence in the field.
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