Читать книгу Power Electronics-Enabled Autonomous Power Systems - Qing-Chang Zhong - Страница 28

After homogenizing all heterogeneous players to achieve legal equality and equipping them with the synchronization mechanism of synchronous machines (SM) as a rule of law, the SYNDEM grid architecture is obtained and shown in Figure 2.5. All conventional power plants, including coal‐fired, hydro and nuclear power plants, are integrated to the transmission and distribution network through SM as normally done without any major changes. All DERs that need power electronic inverters to interface with the grid are controlled to behave as VSMs, more specifically, as virtual synchronous generators, to interact with the grid and all loads that have rectifiers at the front end are controlled to behave as VSMs, more specifically, as virtual synchronous motors. For HVDC links, the power electronic converters at both ends are controlled as VSMs, one as a virtual synchronous generator and the other as a virtual synchronous motor. This presents a unified, harmonized, and scalable architecture for future power systems. It is applicable to a system with one generation node and one load node or a system with millions of nodes.

Figure 2.5 SYNDEM grid architecture based on the synchronization mechanism of synchronous machines (Zhong 2016b, 2017e). (a) Electrical system. (b) Overall system architecture highlighting the relationship between the electrical system and the ICT system, where the dashed arrow indicates that it may not exist.

In such a power system, all power electronics based players, at the supply side, inside the network, and at the demand side, are empowered to actively regulate system stability in the same way as conventional power plants do, unlike in current power systems where only a small number of large generators regulate the system stability. Hence, this architecture is able to achieve the paradigm shift of power systems from centralized control to democratized interaction. Because the synchronization mechanism of SMs are inherently embedded inside all active players, they autonomously interact with each other via exchanging power through the electrical system. This paves the way for the autonomous operation of power systems, which means minimal human intervention is needed to maintain system operation within the designed frequency and voltage boundaries. For example, when a coal‐fired power plant is tripped off, the system frequency drops. All SMs/VSMs that take part in the autonomous regulation of system stability on the supply side would quickly and autonomously respond to the frequency drop and increase their power output in order to balance the power shortage. At the same time, all VSMs that take part in the autonomous regulation of system stability on the demand side would autonomously decrease power consumption to balance the power shortage. As a result, the frequency drop is reduced, which helps reduce the number of loads to be tripped off. If a new power balance cannot be reached after all generators reach the maximum capacity then some VSMs that serve non‐critical loads would further reduce the power intake until a new power balance is reached. Similarly, if a heavy load is turned off, all SMs/VSMs that take part in the autonomous regulation of system stability on the supply side would quickly and autonomously reduce the power output and all VSMs that can take part in the autonomous regulation of system stability on the demand side would autonomously increase power consumption to help reach a new power balance. The change of load power can be of short term or long term, depending on the types and functions of loads. Similarly, the variability of DERs can be taken care of by the players as well. As a result, a SYNDEM smart grid can prevent local faults from cascading into wide‐area blackouts, correcting the systemic flaw of power systems about local faults cascading into wide‐area blackouts.

The deployment of the SYNDEM grid architecture depends on the flexibility of power systems in generation and consumption. This is not a problem. Indeed, power systems worldwide are designed to be very flexible. For example, the UK Grid Code (National Grid, 2016) dictates that the system frequency shall be controlled within 49.5–50.5 Hz, i.e. around the nominal frequency, and that generators and apparatus should be capable of operating continuously when the system frequency is within 49.0–51.0 Hz. For a frequency droop slope, a 0.5 Hz change of frequency is equivalent to having additional reserve at the level of of the system capacity. Moreover, the SYNDEM grid architecture is able to release the inertia in wind turbines and large motors etc., which further increases the system inertia. If the reserve/inertia is still not enough, storage systems can be added. Note that the fast reaction of power electronic converters could also reduce the required level of inertia. Hence, it is envisioned that the flexibility of a SYNDEM smart grid is not a problem at all. Similarly, the normal operating range for voltage is for 400 kV and for 275 kV and 132 kV in the UK. There is plenty of flexibility in reactive power and voltage. Thus, the SYNDEM grid architecture offers a means to fully release the potential of the flexibility already in power systems, improving system stability and reliability.

The architecture shown in Figure 2.5(a) empowers all players to directly take part in the regulation of system stability, which enhances system autonomy. This is consistent with the worldwide trend of increasing autonomy and declining hierarchy in different areas (Anderson and Brown 2010; Friedman 2005; Moore 2011).

The architecture shown in Figure 2.5(a) is lateral rather than hierarchical, although it allows the addition of hierarchical management layers on top of it if needed. Hence, it offers a technical solution to realize the lateral power envisioned in (Rifkin 2011). It has been widely recognized that steam engines are the machines that powered the first industrial revolution and electric machines are the machines that powered the second industrial revolution. It is likely that virtual synchronous machines are the machines that will power the third industrial revolution because it is fair to say that steam engines are mechanical, electric machines are half‐electrical and half‐mechanical, while virtual synchronous machines are fully electrical. This is summarized as shown in Table 2.1.

Table 2.1 Machines that power the industrial revolutions.

Industrial revolution	Machines that power the revolution
The first	Steam engines (mechanical)
The second	Electric machines (electro‐mechanical)
The third	Virtual synchronous machines (electrical)?