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 provided on an interconnection to stabilize frequency in response to a change frequency. Primary frequency control comes from an automatic generator governor response (also known as speed regulation) and a load response (load damping), typically from motors and other devices that provide an immediate response based on local (device‐level frequency responsive) control systems or device characteristics.
(2) Secondary frequency control, which is any action provided by an individual control area (CA), balancing authority (BA), or its reserve sharing group to correct the resource–load imbalance that creates the original frequency deviation, and restores both scheduled frequency and primary frequency responsive reserves. Secondary control comes from either manual or automated dispatch from a centralized control system to correct frequency error.
(3) Tertiary frequency control, which is any action provided by control areas on a balanced basis that is coordinated so there is a net zero effect on the area control error (ACE). Examples of tertiary control include the dispatching of generation to serve native load, economic dispatch to affect interchange, and the re‐dispatching of generation. Tertiary control actions are intended to restore secondary control reserves by reconfiguring reserves.
The US Federal Energy Regulatory Commission (FERC) requires newly interconnecting large and small generating facilities, both synchronous and non‐synchronous, to install, maintain, and operate equipment capable of providing primary frequency response as a condition of interconnection (FERC 2018). Since all SMs and VSMs can provide primary frequency control or PFR against frequency excursions, a SYNDEM smart grid naturally meets this FERC requirement. Moreover, the loads interfaced with power electronic converters are able to provide PFR as well. It is expected that this will be required by the regulatory commissions in the US and other countries in the near future.
It is envisioned that, eventually, no secondary or tertiary frequency control is needed for a SYNDEM grid because all players can autonomously take part in system regulation.
2.7.1 PFR from both Generators and Loads
In a SYNDEM smart grid, all non‐synchronous active participants, both generators and loads, are equipped with the intrinsic synchronization mechanism of synchronous machines. The generators are all turned into frequency‐responsive synchronous participants and can provide (virtual) kinetic balancing inertia to improve frequency stability, in the same way as conventional synchronous machines. This stops the trend of decreasing inertia due to the penetration of DERs. The loads interfaced through power electronic converters