another systemic flaw of power systems. Because the synchronization mechanism of synchronous machines is the key principle that has underpinned the growth and operation of power systems for over 100 years, the transition from today's grid into tomorrow's SYNDEM grid is evolutionary rather than revolutionary.
The SYNDEM grid architecture is scalable and can be applied to power systems at different scales, from single‐node systems to million‐node systems, from vehicles and aircraft to public grids. When there is a need, small systems can be connected together. When a part of the grid is faulty, it can be disconnected; after the fault is cleared, it can be re‐connected. If HVDC links are used to link AC systems at different frequencies, then the AC systems can be operated together or, if needed, independently. Hence, while the architecture allows small grids to merge and form large‐scale power grids, it also naturally allows large grids to break into small ones. Hence, this may offer the technical foundation to turn a move in China that broke up the Chinese Southern AC grid (Fairley 2016) into a natural trend worldwide.
The deployment of SYNDEM grids could considerably reduce infrastructure investment in generation, transmission and distribution networks. Take the UK power grid that has the capacity of about 70 GW as an example. If all loads contribute 2% in a contingency, which is within the tolerance for most individual loads, then the total contribution from all loads is about 1.4 GW. This is higher than the capacity of any current UK nuclear power plants, which means the UK grid is able to cope with the trip‐off of any nuclear power plant if all loads consumes 2% less electricity. The architecture also allows all inertia that already exists in the system, e.g., those in wind turbines, large industrial motors and electric vehicles, to be released, which helps considerably reduce the operational cost because of the fast reaction of power electronic converters. For example, if there are 10 million laptops plugged in a grid and each contributes 10 W when needed, then the equivalent reserve amounts to 100 MW, which is at the level of power generated by 50 wind turbines rated at 2 MW.
Because of the intrinsic synchronization mechanism embedded into each VSM, it is less likely for a VSM to disconnect from the grid under grid variations. This will increase the uptime of renewable generators and hence the yield, bringing more revenue to the owner.
2.6 Brief Description of Technical Routes
2.6.1 The First‐Generation (1G) VSM
Different options to implement VSMs are available in the literature. The VISMA approach (Beck and Hesse 2007; Chen et al. 2011) controls the inverter current to follow the current reference generated according to the mathematical model of synchronous machines, which makes inverters behave like controlled current sources. Since power systems are dominated by voltage sources, this may bring detrimental impact, in particular on system stability (Dong et al. 2013; Sun 2011; Wen et al. 2015). The approach proposed in (Gao and Iravani 2008) follows the mathematical model of SMs but it requires the measurement of the grid frequency, which is often problematic in practice (Dong et al. 2015). The approach proposed in (Karimi‐Ghartemani 2015) controls the voltage but it also requires the measurement of the grid frequency for the real power frequency droop control. The synchronverter approach (Zhong and Weiss 2009, 2011; Zhong et al. 2014) directly embeds the mathematical model of synchronous machines into the controller to control the voltage generated, even without the need for measuring the grid frequency or a phase‐locked loop (Zhong et al. 2014). The synchronverter has been further developed for microgrids (Ashabani and Mohamed 2012), HVDC applications (Aouini et al. 2016; Dong et al. 2016), STATCOM (Nguyen et al. 2012), PV inverters (Ming and Zhong 2014), wind power (Zhong et al. 2015), motor drives (Zhong 2013a), and rectifiers (Ma et al. 2012; Zhong et al. 2012b). The synchronverter technology offers a promising technical route to implement SYNDEM smart grids and is described in detail in Part II. Because it offers a basic and conceptual implementation of VSMs, it is classified as the first‐generation (1G) VSM.
2.6.2 The Second‐Generation (2G) VSM
It is well known that synchronous machines have inductive output impedances because of the stator windings. However, the output impedance of power electronic converters changes with the hardware design and the controller and could be inductive (denoted L‐converters), resistive (denoted R‐converters) (Guerrero et al. 2005; Zhong 2013c), capacitive (denoted C‐converters) (Zhong and Zeng 2011, 2014), resistive‐inductive (denoted
Since a droop controller structurally resembles an enhanced PLL (Zhong and Boroyevich 2013, 2016), it also has the intrinsic synchronization mechanism of synchronous machines and can provide a potential technical route to implement VSMs. The robust droop controller (Zhong 2013c), initially proposed for R‐inverters to achieve accurate power sharing and tight voltage regulation, has been proven to be universal and applicable to inverters with output impedance having an impedance angle between
2.6.3 The Third‐Generation (3G) VSM
There are many challenges during this paradigm shift of power systems. The iceberg of power system challenges and solutions is illustrated in Figure 2.11. On the surface, the challenges are seen as the change from centralized generation to distributed generation or democratization with many heterogeneous players, which are homogenized after operating power electronic converters as VSMs. However, the system stability is still a question because an interconnected system may become unstable even for individually stable systems. This relies on the synchronization mechanism (somewhat hidden). Both the first and the second generations of VSMs are equipped with the inherent synchronization mechanism, but it is not straightforward to rigorously prove the system stability. The third‐generation (3G) of VSMs are expected to guarantee the stability of each individual VSM and also the stability of a system with multiple VSMs interconnected together.
Figure 2.11 The iceberg of power system challenges and solutions.
2.7 Primary Frequency Response (PFR) in a SYNDEM Smart Grid
Maintaining the stability of frequency is a top priority in power systems operation. There are three types of overlapping frequency control (Illian H. et al. 2017):
(1) Primary frequency control, which is any action
