Automatic Load Frequency Control System

The Automatic Load-Frequency Control(ALFC) system regulates active power and frequency of the power units under its control to take part in the secondary (sometimes tertiary) frequency regulation. The system ensures that the group of automated power stations (power units) provides optimal frequency, according to the requirements and situation of the grid.

The system receives control signals from the centralized system for automatic power and frequency control and adjusts the load and frequency of the group. Operating parameters (secondary frequency regulation, rated primary frequency regulation,sustained turbine fast-valving signals) are communicated to the servers from the REDUL B600 PLC via ARTI3 protocol. Diagnostic information from the switches is communicated via the SNMP protocol.

The system is made as a cabinet with programmable logic controllers, servers, commutators, and HMI panel.

System Components

  • 2x REDUL B600 programmable logic controllers in hot-standby configuration:
    • 24VDC power supply module
    • B600 CU00061 central processor unit
    • Communication processor module with 4 x RS-485 ports with Modbus RTU and IEC 61870-5-101
    • Digital input module, 32-channel, 24VDC with group galvanic isolation
  • HMI panel in the cabinet door
  • 2 x servers (main and backup; fan-free)
  • 2 x switches (main and backup)
  • Power supply, lightning protection, cooling equipment


  • CPU modules of the PLC operate in hot-standby mode with redundant synchronization channel
  • Archiving servers are redundant with automatic data synchronization
  • Constant self-diagnostics and generation of alerts
  • The system constantly monitors status of the communication channels with the central system and with the power units control systems, logs errors, forms corresponding signals
  • Automatic selection of a functioning communication channel in case of restored operation
  • Suspension of data transfer between the power units and centralized system in case of failure of both channels in one direction


  • Simultaneous cyclic two-way data exchange between power units and ALFC system over RS-485 in Modbus RTU protocol, with the speed of at least 9,600 bit/s
  • Simultaneous cyclic two-way data exchange between ALFC system and central system for automatic power and frequency control over IEC 61870-5-104 protocol
  • Communication cycle of less than 1 s between the automatic power control systems of the power units and centralized system for automatic power and frequency control
  • Transmission of the data on load-frequency control and rated primary frequency regulation to the plant’s SCADA system

Turbine Control

  • The PLC in the system runs algorithms for the automatic load-frequency control and for the turbine sustained fast-valving
  • Signals from the emergency control system for fast-valving are received via “dry contact” (voltage-free) connection

Clock Synchronization

  • All measurements and signals from remote units are bound to the astronomical time with accuracy of maximum 100 ms
  • Internal clock of the system is synchronized with the astronomical clock via an NTP server in the power plant’s network

Data Archiving

  • Archiving of data on changes in the time to collect parameters for automatic load-frequency control, rated primary frequency regulation, turbine sustained fast-valving
  • Archiving of the events on appearing and disappearing of inaccurate data
  • Archiving of the events on system operation, including modification of the tools and software
  • All the archived data is stored for at least 3 months

Monitoring and Reporting

  • Included HMI visualizes operation of the system functions in form of tables, charts, reports
  • The system can provide reports on the hardware and software state of the components
  • Automatic or ad-hoc generation of the text files in the approved format with the changes in rated primary frequency regulation with 1 second steps
  • Transfer of the archived files to the dispatch office over main or backup communication channel via FTP protocol
Automatic Load Frequency Control System - Structure scheme

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