18-12-2012, 06:19 PM
HVDC Control System - Overview
HVDC Control System.pdf (Size: 286.22 KB / Downloads: 151)
Principles of HVDC Controls
Control of DC Voltage
• DC voltage is varied by means of a converter bridge
• In rectifier operation the power flow is from the AC system to the DC
system
• The power flow is changed from the DC system to the AC system by
reversing the DC voltage.
• The DC current does not change it’s direction.
• The operating range of the ideal converter - theory from 0° (+1.0 p.u. DC
voltage) to 180° (-1.0 p.u. DC voltage)
• The operating range of a real converter is from approx. 5° to approx. 160°
• In 90° operation the DC voltage of the converter is 0 V.
Converter Control Functions, Fixed Firing Angles α
• Firing angle is controlled by use of controllers and
the trigger set.
• Task of the trigger set - convert the firing angle in
appropriate firing pulses for each individual valve
and the synchronisation of these pulses to the AC
system
Converter Characteristics with Fixed Firing
Angles (no controller)
• Rectifier is operated with fixed firing angle of e.g. 15 degree.
• The slope of this characteristic depends on the commutation
impedance (AC System and transformer) of the converter.
• The weaker the AC system, the steeper the slope.
• Inverter is operated with fixed firing angle of e.g. 140 degree
• Operating point of the HVDC is intersection of the two
converter characteristics
• Operating point is strongly dependent on the AC voltage
DC Current Control at rectifier station
• The DC current control - to keep the DC current constant as
long as possible
• The DC current is controlled according to the reference value
• When the actual DC current becomes lower than the ordered
current, the current control increases the PI controller output
resulting in an increase of the rectifier DC voltage (UdRect) in
order to maintain the DC current.
• In case the actual current is too high the PI controller reacts in
opposite direction.
Extinction Angle Control (Gamma Control)
• The extinction angle control at inverter is provided in
order to maintain stability margins.
• The extinction angle reference value is the minimum
allowed value (e.g.17°)
• The extinction angle control becomes active when the
minimum limit is reached.
DC Voltage Limitation Controller at Rectifier
• A DC voltage limitation control at the rectifier is provided
in order to limit the DC voltage
• A margin of typical 0.03. p.u. is add to the inverter DC
voltage order.
• This prevents the DC voltage limitation controller to
become active during normal operation
• Backup control which becomes active to prevent from
excessive overvoltage in special fault situations
• It reduces the DC voltage to save valves by shifting the
firing angle in inverter direction of in order to maintain the
requested DC current