DESIGN OF AN ACTIVE DC FILTER FOR A HVDC SYSTEM: INTRODUCTION(part 1)

INTRODUCTION(part 1)

1. Why HVDC

In an AC system voltage conversion is simple with the help of the transformer which allows high power level and high insulation level within one unit and has low loss. Apart from that a 3 phase synchronous generator is superior to that of a DC generator. However HVAC links have disadvantages which may compel a change to DC technology. Inductive and capacitive elements put limits to the transmission capacity and also to the link to the transmission. Depending upon the required transmission capacity the system frequency and the losses evaluation can change. Direct connection between two AC system of different frequencies is not possible.

Merits of HVDC

A Dc link allows interconnection of two AC systems of different frequencies whose synchronization cannot be possible. Inductive and capacitive parameters don’t limit the transmission and also the is no loss due to skin effect(which is absent).Beyond a certain distance DC is less costlier than AC. Losses are less in HVDC transmission.

2. Types of DC links

2.1 Mono polar DC links Useful for very long distance transmission. Chances off breakdown are less It is more costlier compared to other transmission.

2.2 Bipolar link It is a combination in such a way that a common low voltage return path will only carry a small unbalance current during normal operation .This configuration is used if the capacity increased beyond that of a single pole. More than 50% of the transmission capacity can be utilized as well as there will be reduction in cost for high power transmission. Disadvantage is the unavailability of the return path will affect the operation of both the poles.

3. Converter operation

• Converters are basically the devices which converts AC to DC and vice versa.

• The devices doing the conversion from AC to DC are known as Rectifiers and the DC to AC conversion is done by the Inverters.

• Normally power electronics switches are connected in a combined fashion whose turn on and off made the total thing possible.

• Current flows through the valve when the anode is made more positive than cathode. The valves can made to turn on by applying proper gate pulses to the gate cathode terminal which are the pulses for short duration.

• In order to commutate a negative voltage is being developed across the conducting switches or a negative current is being applied to flow by turning on the other pair of switches in the bridge circuit.

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