The future needs are driven by several factors:

• The primary energy mix are going from large hydro power plants designed for maximum energy production (as in Norway and Sweden) and large thermal units (continental Europe) to a less predictable and diversified energy mix from variable renewables, such as wind power and solar photovoltaics.
• More power interchange capacity to UK and continental Europe adds to the variability and the need for flexibility in operation.
• Increased end-user flexibility and energy-saving plans in industry and households may lead to more rapid load changes and larger differences between minimum power load and peak demand.
• Market-oriented operation of the power system imposes new requirements and demands on the power plants. The power output is more frequently adjusted, and this causes more dynamic changes in the entire production system operation. The power plant is not generally optimized for energy production, but to maximize the income in a commercial market.

By introducing power electronic frequency converters between the grid and the synchronous generators, turbine/generator speed becomes an additional degree of freedom in operation. This will strengthen the dynamic capability and several benefits can be envisioned, such as:

• Optimal speed control can increase power production efficiency at part load, making a larger range of operation feasible.
• Reversible pump-turbines will operate smoothly, ramping rates will be faster, efficiency is higher at part load and the operating range of Francis turbines is wider.
• On the system side, faster and more accurate response to control of active and reactive power can be achieved due the favourable performance of power electronic converters.
• Better utilization of the energy stored in the rotating masses (inertia) of the turbine/generator can provide additional system benefits. More power and energy can be released by adjusting speed (often referred to “synthetic inertia”).
• Power system stabilizers (PSSs) can be made more effective by utilizing active power to a certain extent.
• The fact that the frequency converter effectively decuples generator speed from the grid frequency can also improve transient stability properties (or the fault ride through capability).

However, there are challenges associated with the introduction of variable speed operation by frequency converters. The power electronics part by itself may give rise to unexpected and unwanted dynamics and harmonic disturbances. Moreover, by removing synchronous generators from “synchronous” operation, one may fear that important power system features like natural inertia and short circuit current contributions will be reduced.