۱- Introduction

۲- Method

۳- Performance of a future system with low inertia and damping

۴- Replacing inertia and damping with synthetic inertia and damping

۵- Synthetic inertia and damping with limited energy reserves

۶- Limit cycle analysis

۷- Response to n − ۱ disturbance

۸- Discussion

۹- Conclusion

References

Abstract

The power system inertia is decreasing in many electrical grids as the share of production from directly connected synchronous generators decreases. Lower inertia increases the frequency deviations in normal operation, which leads to increased wear and tear in hydropower turbines and other units providing frequency control to the system. The predominant concepts for synthetic inertia from for example wind power does not address the frequency quality in normal operation, only the acute problem of frequency stability during large disturbances. This paper investigates how the frequency quality and frequency controlling hydropower units are affected by decreasing inertia and damping, using the Nordic power system as a case study. A new type of synthetic inertia (SI), which is linear and continuously active, is suggested as a means to mitigate the impacts on these units. It is shown that the suggested linear SI controller can effectively replace synchronous inertia and damping, improving frequency quality and reducing hydropower wear and tear. The controller includes an energy recovery feedback loop, to avoid depletion of the energy source behind the controller. The power and energy needed to provide linear SI is quantified, and the impact of the SI energy recovery integration time constant is investigated.

Introduction

The electricity production from variable renewable energy (VRE) sources is increasing all over the world. In addition to having a weather dependent power output, these production units are normally connected to the grid through inverters, which means that they do not supply the grid with natural inertia like synchronous machines do. As VRE is replacing production from synchronous machines, the inertia of the power system decreases and the number of units that can provide frequency containment reserves, FCR (also called primary control) and frequency restoration reserves, FRR (also called secondary control), without curtailment decreases. In the European Union, this development has prompted the transmission system operators (TSOs) to include obligations for large generators to provide FCR and some type of inertia in the draft of the new grid code [1,2]. Hydro-Québec in Canada and EirGrid in Ireland have already specified requirements on inertia emulation from wind farms in their grid codes [3,4]. Decreasing inertia can have an impact on several aspects of grid stability [5]. The aspect that has drawn most attention so far is the impact on the rate of change of frequency (ROCOF) and the lowest frequency (the nadir) after a sudden disconnection of the largest unit in the system (the n − ۱ disturbance). Early grid code requirements on synthetic inertia (SI), like the ones by Hydro-Québec [3] and EirGrid [4], are clearly focusing on this aspect, and commercial implementations of SI, such as GE’s WindINERTIA [6] and ENERCON’s IE [7] are designed to support the grid only during large frequency events. Another aspect is the frequency quality during normal operation. Reduced inertia increases the amplitude and frequency of the grid frequency variations [8]. This changes the operational pattern of FCR, leading to more regulation and increased wear and tear of the units delivering frequency control reserves to the grid, for example hydropower [9,10], which may lead to increasing costs for frequency control. It also affects the worst case nadir for an n − ۱ disturbance, since the frequency may already be low when the disturbance occurs. The impact on normal operation has been overlooked in studies on synthetic inertia so far.