By Qing-Chang Zhong
Power systems are going through a paradigm shift, due to the addition of numerous non-synchronous distributed generators and active loads connected through power electronic converters. This situation imposes unprecedented challenges to the frequency stability of power systems. Power electronic converters can be controlled to behave as virtual synchronous machines (also called “cyber synchronous machines”). Thus, a novel system architecture occurs with interfaces unified according to the synchronization mechanism of synchronous machines, which realizes the paradigm shift for power systems from centralized control to democratized interaction. This architecture enables all active players on the supply side, inside the network and on the load side, to offer continuous primary frequency control (PFC), reconfigurable virtual inertia and flexible droop control without delay, which improves the frequency stability of future power systems.
Fundamental Challenge in Modern Power Systems
In conventional power systems, the frequency is controlled by regulating a small number of large generators, while most loads do not, usually, participate. However, the landscape of power systems is rapidly changing and various non-synchronous distributed energy resources (DER), including renewables, electric vehicles and energy storage systems, are connected to the power systems. The number of active players taking part in frequency regulation (accounting also for the potential of loads) can reach millions of control points, thus, imposing unprecedented challenges to the frequency stability of power systems. Apparently, the current centralized control paradigm is no longer feasible and a paradigm shift is required.
The fundamental challenge behind this paradigm shift is that future power systems will be based on power electronic interfaces, instead of directly-coupled electric machines, with millions of relatively small players. On the supply side, most DERs are connected to power systems through power electronic converters. In transmission and distribution networks, many applications built on power electronic topologies, such as HVDC links and FACTS devices, are introduced to control power systems in order to improve efficiency and reliability. On the load side, most loads will also be connected to the grid through power electronic converters. For example, motors, which consume over 50% of electric energy, are much more efficient when equipped with motor drives; Internet devices, which consume over 10% of the electric energy, have front-end power electronic converters; lighting devices, which consume about 20% of the electric energy, are replaced with LED lights, which have front-end power electronic converters as well.
Novel System Architecture with Unified Interfaces for Future Power Systems
Power electronic converters can be controlled to operate as synchronous machines (SM); such converters are called virtual synchronous machines (VSM) or cyber synchronous machines (CSM). A VSM may not need a dedicated synchronization unit, e.g., a phase-locked loop (PLL), to achieve synchronization. All conventional power plants can be integrated to the transmission and distribution network through SM as has commonly been the case without any major changes. All DER and loads that have power electronic converter interfaces with the grid, can be controlled to behave like VSM. For HVDC links, the power electronic converters at both ends can also be controlled as VSM. These lead to a novel system architecture with unified interfaces and a common governing mechanism for future power systems.
Primary Frequency Control (PFC) for Future Power Systems
This novel architecture enables all active players to make contributions to PFC, with the following distinctive features:
- PFC Contributions both from Supply and Load. All the non-synchronous active players can be operate as SMs and provide virtual balancing inertia in the same way as conventional SMs exhibit inertial response to all frequency changes. This approach tackles two major challenges in modern power systems, which pertain to the maximum frequency change. Firstly, the issue of inertia decrease, due to the penetration of DER and, secondly, the concerns over load damping decrease, due to the increasing use of loads controlled independently of frequency.
- Continuous PFC. A prominent feature of the future power systems described above is that many active players, both on the generation and load sides, can offer continuous PFC, i.e. they can adjust either the output or the intake in real time, according to the system frequency in an autonomous manner. The real power output may be controlled to change automatically based on the changing frequency. In this way, all active players could make a small, often negligible, amount of contribution in the event of frequency disturbances, which is a more democratized alternative of only a few players undertaking the “full responsibility” for the PFC response.
- Flexible Droop Control. Droop control plays an important role in PFC as it specifies the slope of the steady-state frequency control and the contribution of each individual player. The droop coefficients of active players, both supply and load, can be configured according to their nature and level of criticality, in order to provide the maximum PFC contribution without affecting the quality of service. Non-critical players could be configured to provide more contributions while critical players provide less or no contribution. For example, air conditioning and pumping systems can be easily configured to provide continuous PFC up to their full capacity (for minutes) without affecting user comfort or the overall performance.
- Fast Action without Delay. Due to the removal of phase-locked loops from power electronic converters, any frequency change can be acted upon without delay, reducing the amount of the balancing inertia required.
- Reconfigurable Virtual Inertia. The kinetic inertia of a conventional SM is fixed but the virtual inertia of a VSM is reconfigurable and can be changed if needed. Moreover, the virtual inertia of a VSM does not involve the estimation of system frequency or the rate of change of frequency, avoiding possible instability encountered in realizing synthetic inertia.
Qing-Chang Zhong, IEEE Fellow, is currently the Max McGraw Endowed Chair Professor in Energy and Power Engineering at Illinois Institute of Technology and the founding director of Syndem LLC. He is well recognized worldwide as a leading multidisciplinary expert in control, power electronics and power systems. An IET Fellow, he received a Ph.D. degree in control and power engineering from Imperial College London, U.K., in 2004 and a Ph.D. degree in control theory and engineering from Shanghai Jiao Tong University, China, in 2000.