Supercritical CO2 and the Allam-Fetvedt Power Cycle
April 29, 2020
Several states, including FCS’s home state of Maryland, have aggressive renewable energy goals. As more renewable energy sources come online, more conventional sources (e.g., coal, natural gas, etc.) will be taken offline. By nature, these renewable sources lack any sort of automatic frequency response mechanism like those found in large rotating thermal turbine-generators. Static Var Compensators (SVCs) mimic the action of rotating thermal turbines, providing the ability to respond quickly (within milliseconds) to reactive power transients on high-voltage electrical transmission lines1. These devices have been deployed worldwide and have seen use in the United States. Some successful SVS deployment is listed in a GE report.
Static Var Compensators (SVC)
Those readers unfamiliar or who need a primer on vars should read an excellent blog written by FCS Vice President Scott Hommel. Static Var Compensators provide voltage support to maintain the reliability and efficiency of power distribution networks. These devices are also part of the Flexible Alternating Current Transmission System (FACTS). Focus on reliability and efficiency is paramount with high levels of renewables connected to the grid. Reactive power swings are more likely to occur with renewables because of the rapid fluctuations experienced by wind and solar (photovoltaic) power.
Static Var Compensator Theory
In var theory, the reactive load is considered capacitive (leading) or inductive (lagging). The Static Var Compensator is connected to the grid. The connection to the grid is typically via a step-down transformer. Reducing voltage to the static var compensator also reduces the number of individual components in the device.
The static var compensator may be switched in as needed. When in use, the SVC switches in a bank of capacitors connected to a Thyristor-Controlled Reactor (TCR) to boost system voltage. This arrangement allows the SVC to provide a continuously variable var control to the network – adding or taking vars as required. The use of switched TCRs allows the system to be placed online or removed according to the instantaneous var demand. The capacitors provide “coarse” voltage control. The thyristor reactor provides “fine” voltage control.
The incorporation of thyristors leads to the production of significant heat. To remove this heat, industrial applications of thyristors require a dedicated cooling system of high-purity (deionized) water to maintain their operation. Since, by nature, thyristors also introduce harmonics, the Static Var Compensators also include filters to suppress these harmonic currents.
Static Var Compensator Advantages in Renewable-Heavy Grid
The construction of the Static Var Compensator is its biggest advantage when considering its ability to maintain grid voltage. These solid-state electronics provide a near-instantaneous response to system voltage fluctuations. This remains the most significant benefit to a renewable-heavy transmission network. On days where the wind may be blustery, wind turbines will have large voltage swings. On days with passing clouds, solar (photovoltaic) arrays will see similar issues.
Another benefit is that these systems are generally less expensive, react faster, and are more reliable than comparable systems. However, some other styles of systems like mechanically switched capacitor banks remain cheaper and are often used in conjunction with Static Var Compensator systems to provide the most economical solution to the customer.
Summary
Although the technology inside the Static Var Compensator is not new, its deployment has generally been hampered by the inherent ability of large steam turbines to regulate grid voltage. As more renewables join the grid, especially those with whose voltage may vary widely by the minute, these devices will become more common. In some countries, Transmission System Operators (TSO) have placed code requirements on renewables to ensure grid stability. In some cases, this includes the requirements of devices like SVCs.
Deb, Anjan K. (2000-06-29). Power Line Ampacity System
Padiyar, K. R. (1998). Analysis of Subsynchronous Resonance in Power Systems.
