HIGH VOLTAGE DIRECT CURRENT TRANSMISSION

(HVDC)PRESENTED BY:

WAJEE UL HASSAN 11053122-078

TRANSMISSION LINE

• A transmission line basically consists of 2 or more parallel

conductors used to connect a source to a load.

Uniform transmission line :

• Which has its electrical parameters remaining constant along

its length of the line.

• In which voltage to current ratio does not vary with distance.

• Electrical parameters of lines are Resistance, Inductance,

Capacitance and Conductance.

AC GENERATION AND TRANSMISSION

Advantages:

•Easy to transform voltage between different levels.

•Easy to handle voltages and is reliable in transmitting mechanism.

•Cost effective at small distance transmission.

•Generators are at distant places, away from the populated areas.

•They are either Thermal, Hydel or any other source.

DISADVANTAGES:

• Expensive at long distance transmission.

• Needs more conductors to supply power.

• Skin effect is present in AC transmission.

• Greater line losses.

• Not effective in under sea transmission of electricity.

ADVANTAGES OF DC TRANSMISSION

• It requires only two conductors for transmission.

• RMS is only about 71% of the peak voltage.

• No inductance, capacitance and phase displacement.

• There is no skin effect and hence full cross-section of the

conductor is utilized.

• Less insulation is required.

CONT’D……

• Underground cables can be used because of less potential

stress and negligible dielectric loss.

• For a given load and sending end voltage, voltage regulation

is better for D.C. transmission line.

• No stabilizer is required.

DC TRANSMISSION

• Electric power generators were quite at distant from load areas.

• Difficulties occur in AC transmission.

• Early HVDC systems used electromechanical conversion i.e. the

Thury system.

• All HVDC systems built since the 1940s have used electronic (static)

converters.

• Evolution in semiconductor industry.

STEP INVOLVE IN DC TRANSMISSION

The following steps are involved:

• stepping up the generated A.C.

• Converting the A.C. to D.C.

• Transmission through D.C.

• Converting the D.C. to A.C. at the load side

The main components of HVDC transmission are the converter

stations.

CONVERSION METHODS:

• Converters for HVDC are divided into three main categories:

1.Line-commutated converters (LCC)

2.Voltage-source converters (VSC).

3.Capacitor-commutated converters (CCC)

• In the above methods the LCC and VCS belongs to electronic

converter categories.

•  CCC has series capacitors inserted into the AC line connections.

LINE COMMUTATED CONVERTERS

• Most of the HVDC systems in operation today

•  2 types of valves are being used in LCC systems

1.Mercury arc valves

2.Thyristor valves.

• Our emphasis will be on thyristor valves converters.

LCC ARRANGEMENT

 6 pulses GRAETZ circuit

WAVEFORMS OF COMMUTATED CURRENT AND VOLTAGE

HOW THYRISTOR VALVES CONVERTER WORKS ?

• Thyristor works as an switch.

• Can be controlled to turn on.

• Thyristor valves are built using large numbers of thyristors in series.

• Additional passive components i.e Capacitors and resistors are connected in parallel.

• This arrangement ensure that the voltage across the valve is evenly shared between the thyristors. 

•  Thyristor along with its auxiliary equipment is known as a thyristor level.

THYRISTOR VALVE ARRANGEMENT

• Thyristor valve contain hundreds of thyristor levels.

• Each is operating at a different potential relative to the other.

Controlling the triggering of thyristor:

• The command to turn on the thyristors needs to be isolated.

• Two methods are adopted for triggering:

i.Magnetic

ii.optical

CONT’D….

• Optical method also comprises into two ways :

i.Direct optical triggering

ii.Indirect optical triggering

Indirect optical triggering:

• low-voltage control electronics send light pulses along optical fibers to the high-voltage side control electronics.

Direct optical triggering:

• It directly send the pulses to the high-voltage side control electronics

VOLTAGE-SOURCED CONVERTERS (VSC)

• IGBT have controlled turn on and turn off characteristics.

• They can be used to make self-commutated converters.

• The polarity of DC voltage is fixed.

• The DC voltage is considered to be constant.

• That’s why the such converters are called Voltage Sources Converters.

ADVANTAGES OF VOLATGE SOURCE CONVERTERS

• Ability to switch the IGBTs on and off many times per cycle improve the harmonic performance.

• Have dynamic voltage control ability.

• Independent control of reactive and active power

• Black-start capability

CONVERTER TRANSFORMERS

• AC side of each converter have transformers that isolate the station from the AC supply.

• These are often three physically separated single-phase transformers.

• Adjust the supplied ac voltage to the valve bridges to suit the rated dc voltage.

• The output of these transformers is then connected to the converter.

• Converter transformers for VSC HVDC systems are usually simpler and more conventional in design than those for LCC HVDC systems.

SPECIAL MODIFICATION OF TRANSFORMERS FOR LCC

• Converter transformers for LCC arrangements are specialized

• LCC have high levels of harmonic currents which flow through converter transformers.

• The secondary winding insulation experiences a permanent DC voltage, which affects the design of the insulating structure.

• In LCC systems, the transformer also need to provide the 30° phase shift needed for harmonic cancellation.

INHERENT PROBLEMS ASSOCIATED WITH HVDC

a) Expensive convertors

b) Reactive power requirement

c) Generation of harmonics

d) Difficulty of circuit breaking

e) Difficulty of voltage transformation

f) Difficulty of high power generation

EXPENSIVE CONVERTORS AND REACTIVE POWER REQUIREMENT

• Expensive Convertor Stations are required at each end of a DC transmission link, whereas only transformer stations are required in an AC link.

• Convertors require much reactive power, both in rectification as well as in inversion.

• The reactive power consumed may be as much at 50% of the active power rating of the DC link.

• The reactive power is partly supplied by the filter capacitance, and partly by synchronous or static capacitors.

GENERATION OF HARMONICS

• Convertors generate a lot of harmonics both on the DC side and on the AC side.

• Filters are used on the AC side to reduce the amount of harmonics transferred to the AC system.

• On the DC system, smoothing reactors are used.

• These components add to the cost of the convertor.

DIFFICULTY OF CIRCUIT BREAKING

• Due to the absence of a natural current zero with DC, circuit breaking is difficult.

• This is not a major problem in single hvdc link systems.

• Circuit breaking can be accomplished by a very rapid absorbing of the energy back into the AC system.

• The blocking action of thyristors is faster than the operation of mechanical circuit breakers.

• However the lack of HVDC circuit breakers hampers multi-terminal operation.

DIFFICULTY OF VOLTAGE TRANSFORMATION

• Power is generally used at low voltage.

• For efficiency must be transmitted at high voltage.

• The absence of the equivalent of DC transformers makes it necessary for voltage transformation to carried out on the AC side of the system.

• Prevents a purely DC system being used.

DIFFICULTY OF HIGH POWER GENERATION

• Due to the problems of commutation with DC machines, voltage, speed and size are limited.

• Thus comparatively lower power can be generated with DC.

ECONOMIC COMPARISON

• The hvdc system has a lower line cost per unit length as compared to an equally reliable AC system.

• Reasons are :

i.lesser number of conductors

ii.smaller tower size.

• DC system needs two expensive convertor stations which may cost around two to three times the corresponding AC transformer stations.

BREAK-EVEN DISTANCE FOR D.C. TRANSMISSION