The power system control loops (including LFC systems) consists of control centers, electronic field devices, and communication networks working together, for the reliable and efficient generation, transmission, and distribution of power ^[3]. Sensors collect measurements of various physical parameters, like the terminal voltage, power flow, rotor speed, etc., from the field devices and the measurements are sent to the control center using dedicated communication protocols. The group of computational algorithms that processes and analyzes the measurements from sensors or terminal units is collectively called as energy management system (EMS) ^[5]. The decisions from the control center are then transmitted to actuators for the implementation of required changes through field devices or actuators. Primary control or governor control, secondary control scheme with the help of traditional supervisory control and data acquisition (SCADA), the secondary control scheme in smart grid/microgrid control using phasor measurement units (PMU), etc. have been developed for the LFC in the generation side of the power system ^[3]. The LFC scheme is basically implemented to ensure the balance between load and frequency in the power system and thus eliminating the non-zero frequency deviation ^[6]. A well-designed power system with LFC adjusts perfectly against the load variations and system disturbances while producing high-quality electric power and maintaining frequency within the tolerance limit ^[4].

The LFC scheme primarily starts with governor control, which is the control of the generation unit using speed regulation or droop characteristics. Droop characteristics represent the slope of the governor steady-speed characteristics curve ^[6]. From the control point of view, it can be viewed as a proportional controller that ends up with a steady-state frequency deviation ^[4].

The governor control (local control) of LFC system does not rely on the SCADA telemetry system, as the rotor speed measurements of the single generator are locally sensed ^[3]. In this case, the valve position of the prime mover is adjusted according to the sensed speed to reflect the corresponding change in the output power of the generator ^[4].

Even though this is a local control scheme, the control module/controller of this scheme does have a communication link with the control center of the plant as it defines the governor controller operating setpoint using this link. The attack surface of local control loops is limited due to the local sensing of measurements without using the SCADA network. Therefore, attacks like DoS, replay, integrity, timing, etc. are not applicable to this control loop. However, the malware can still compromise system cyber-security measures and enter substation LAN through entry points like USB keys. The malware then corrupts the control module settings and disrupts normal operation. The Modbus protocol is used by the controllers of modern digital governor control for the communication with control center computers via Ethernet ^[3][4].

 Different from governor control, the secondary control of the LFC scheme allows the frequency control of multiple generators that are operated in parallel, sharing large electrical loads. The secondary control provides a reset action for the steady-state frequency deviation and adjusts the generation automatically to re-establish the system frequency to the nominal value for the continuous load changes ^[7]. The secondary control system resets the frequency deviation at steady-state to zero value ^[6].

The LFC system configurations can be divided into single-area and multi-area schemes. In the multi-area or interconnected-area LFC system, the power exchange between the areas happens through connections called tie-lines ^[6]. The aim of the single-area LFC system is only restricted to the stabilization of operating frequency to the nominal value as the interconnected system adjustment is not needed ^[8]. In the multi-area LFC system, the generators of each area have to control local load and tie-line power variations from interconnected areas to attain load balances at local and global levels ^[6].

Traditionally, LFC of an area or interconnected areas involving multiple generators is done with the help of energy control centers that make use of on-line computers and remote data acquisition systems like SCADA ^[4]. In the modern electric grid and smart grid, PMU is used for real-time monitoring and control. The communication channels from RTUs to the control center and from the control center to governor control are the main attack points of secondary control loops both in single-area and multi-area LFC systems ^[1][4][9].

The typical LFC loop is given in Figure 1 and the attack points of the single-area LFC system are provided in Figure 2. The schematic diagram of the multi-area LFC system with attack points is provided in Figure 3.