06-04-2011, 09:30 AM
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HVDC LIGHT, A TOOL FOR TRANSMISSION OF ELECTRIC POWER
INTRODUCTION
HVDC light is high voltage direct current transmission system based on solid state voltage source technology.
Utilizes most advanced power electronics and semiconductors.
Power is transmitted via underground/undersea cables.
More efficient way of transmitting power over distances above ~50km.
HVDC light installations
HVDC light converter station
Power transformer
The transformer is an ordinary single phase or three phase with a tap changer on the secondary side
The filter bus voltage will be controlled with the tap changer to achieve the maximum active and reactive power from the converter
The current in the transformer winding contains hardly any harmonics and is not exposed to any dc voltage
The transformer may be provided with a tertiary winding to feed the station auxiliary power system
Converter reactors
The main purposes of the converter reactors are
To provide low-pass filtering of the PWM pattern to give the desired fundamental frequency voltage
To provide active and reactive power control
To limit the short circuit currents
DC capacitors
The primary objective of the valve dc side capacitor is to provide a low inductance path for the turned off current and also to serve as an energy store
It reduces the harmonic ripple on the direct voltage
The ability to limit the dc voltage variations caused by the disturbances in the system depends on the size of the dc side capacitor
AC filters
Example of an HVDC light ac filter
Valves-The IGBT position
IGBT is the semiconductor used in HVDC light
To increase the power handling,six IGBT chips and three diode chips are connected in parallel in a sub-module
The IGBT has two,four or six sub-modules, which determine the current rating of the IGBT
A complete IGBT position consists of an IGBT,a gate unit,a voltage divider and a water cooled heat sink
The gate driving electronics control the gate voltage and current at turn on and turn off
The voltage across the IGBT is measured and the information is sent to the valve control unit through an optical fibre
The voltage divider connected across the IGBT provides the current needed to drive the gate
IGBT with four and six sub-modules
Valve function
To switch voltages higher than the rated voltage of one IGBT,several positions are connected in series in each valve
Each IGBT position can be individually regulated in the valve to the correct voltage level
The flexibility of the IGBT makes it possible to block the current immediately if a short circuit is detected
Mechanical design of valves
Valve cooling system
It consists of water cooled heat sinks which provides high efficiency cooling
The water is circulating through the heat sink in close contact with each IGBT,which efficiently transports the heat away from the semiconductor
The water passes continuously through a de-ionizing system to keep the conductivity of the water low
VSC technology based on PWM
HVDC Cables
Advantages of HVDC cables
HVDC light cables are installed close in bipolar pairs with anti parallel currents and thus eliminating the magnetic fields
They have no technical limitation for distance as in ac cables
Its strength and flexibility makes it well suited for several installation conditions
Control of active and reactive power
The fundamental base apparent power at the filter bus between the Converter reactor and the ac filter is ,
Sb= P+jQ= Ö3.Uf.IR
The active and reactive components are
P=UF.UC. sin d
wL
Q=UF.(UF –UC). cos d
wL
Active power flow
If UC is in phase lag,the
active power flows from
ac to dc side(rectifier)
If UC is in phase lead,the
active power flows from
dc to ac side
Reactive power flow
Features
Independent control of active and reactive power
Independent power transfer and power quality control
Power reversal
Reduced power losses in connected ac systems
Increased transfer capacity in the existing system
Fast restoration after blackouts
Flexibility in design
No relevant magnetic fields
Low environmental impact
Indoor design
Short time schedule
Applications
Wind power generation
Multi terminal dc grid
Interconnecting networks
Islanded operation
City infeed
Reliability and Quality
To assure high reliability and availability,the HVDC light principles include
1.Simple station design
2.Use of components with proven high reliability
3.Automatic supervision
4.Use of back up control systems and equipment such as measurements,pumps etc
Maintainability
Quality assurance/standards
Converter control
Current control
Current order control
PQU order control
Protective actions and effects
Transient current limiter
Permanent blocking
AC circuit breaker trip
Set lockout of the ac circuit breaker
Pole isolation
Start breaker failure protection
Conclusions
Opportunity to transmit small scale power long distances via cable
Opportunity to connect to passive load
Separate control of active and reactive power
No contribution to short circuit currents
No need of fast communication
Opportunity to operate without transformers
Small and compact