HVDC Control System

HVDC control system is a hierarchially strctured control system:

• System Control
• Master Control
• Station Control
• Pole or Converter Control
• Valve base Control (VBC) / Valve Unit Control (VUC)

Each Control System is responsible for controling certain important Functions, whihc are listed below:

System Control:

The system function mainly includes the supervisory control and data acquisition (SCADA) functions listed below:

? Controls, indication and alarms

? Time synchronization and interlocking

? Sequential isolation using disconnect switch

? Voltage selection, resonance and phase detection

? Instrument transformer supervision

? Interface to special protection and operational tripping schemes

? Communications using standard grid protocols including IEC61850, IEC60870-5-101, IEC60870-5-104

? Metering including AC/DC voltage/current, active/reactive power, harmonics and other parameters

? Transient fault recording and cable monitoring

? Converter cooling and valve hall fire detection

? Auxiliary power supply and power supply supervision

Master Control:

Only one converter station shall have the active control of the HVDC transmission system at a time and is called the master station. The master station performs the dispatch controls as listed below:

? Converter control modes including constant active power, reactive power and voltage control

? Start-up and shutdown of HVDC transmission

? Frequency control

? Power ramping and power reversal

? Emergency power control/power demand override

? Reactive compensation and harmonic filter control

? Change over mechanism

? Automatic pole switch

Station Control:

The Station Control manages the equipment that is necessary to integrate the HVDC System into the customer’s power system and also those functions common to both poles. The main Station Control functions are:

? AC and DC Switchyard Control

? Active and Reactive power control

? AC voltage control

? On-Load transformer tap changer control

? DC current and voltage control

? Stability, Operational states and switching sequence control

? Valve firing control

Pole or Converter Control:

The Pole Controls are the heart of the HVDC control system. The DC power flow is controlled to the operator’s set point. Steady-state and dynamic performance of the AC systems is also enhanced by the Pole Controls. Features such as power swing stabilization, frequency limit control and sub synchronous resonance damping are some of the available features. The primary function of the Pole Control System is to maintain the transmitted power at the operator selected value. This is achieved with an optimal response during system disturbances and is robust and stable for all system configurations. During normal undisturbed operation, DC current control is active at the Rectifier and DC voltage control at the Inverter. A backup extinction angle control provides a safety margin to minimize commutation failures at the Inverter following disturbances in its AC system. An Inverter current control function becomes active should the Rectifier station be unable to provide the ordered DC current during AC System disturbances. Converter control coordinates the conversion of current order to a firing angle order or PWM duty cycle, tap changer control and other protection sequences.

Valve Base Control VBC (valve unit control):

The VBC has the functions to control the circulating current and capacitor voltage, to protect converter valves and to monitor converter valves. The valve control receives commands and signals from the pole (converter) control to control/monitors the conduction of power electronic devices within the valves. The key function is Interfacing the converter control system to many (hundreds or more) of the individual power electronics modules within the HVDC power converter. The Valve Base Electronics (VBE) communicates with a large number of power electronics modules using an industry standard passive optical communication technology that allows each module to be controlled and monitored in a tightly-synchronized, time-critical and reliable manner while minimizing the number of fibers needed without compromising redundancy. Some of the functions handled by VBC includes modulating reference voltage of bridge arm based on command received from converter control, Current balance control including the circulating current between upper and lower bridge arms and between different phases, Voltage balance control by calculating the right number of working submodules (SMs) ensuring the capacitors’ voltages within a reasonable range, Valve and bridge arm protection and monitoring.

Ref: Content from public document of TI

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