This vpresentation contains the introduction of MTDC transmission System, major additions of MTDC system as compare to two terminal HVDC system, Potential applications of MTDC system, types of MTDC System - Series and Parallel: Radial & Mesh.
2. INTRODUCTION
HVDC transmission systems designed and operated are point to point system
with two converter station (two terminal). MTDC system has more than two
converters & some of converters are operating as rectifiers and others are as
inverter.
Parallel operation of converters can also be viewed as multi terminal
operation.
In HVDC transmission, the extending two terminal to multi-terminal system is
not trivial as unlike in AC systems.
Considerably, control and protection of system become complex, therefore,
HVDC breakers are generally required in MTDC System.
MTDC system is best suited to provide the facilities for large power exchange
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3. MAJOR ADDITIONS FOR MTDC
The major additions for MTDC would be,
1. More DC switch gear is needed
2. Balancing of order between different stations so that sum of rectifier station
currents is always equal to sum of currents of inverter stations.
3. Ramping of DC power between stations on demand
4. Control of voltage profile on DC system
5. Managing of overload for different converters
6. Relocating of power between stations and terminals after a forced outage
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4. POTENTIAL APPLICATIONS OF
MTDC SYSTEMS
There are three specific areas of applications for MTDC systems.
These are listed below:
I. Bulk power transmission from several remote generating stations to several
load centres.
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5. POTENTIAL APPLICATIONS OF
MTDC SYSTEMS
2. Asynchronous interconnection between adjacent power systems.
3. Reinforcement of an AC network which is heavily loaded.
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6. TYPES OF MTDC SYSTEMS
There are two possible types of MTDC systems,
I. Series
II. Parallel
The Parallel MTDC Systems can be further subdivided into the following
categories:
a) Radial
b) Mesh
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7. SERIES MTDC SYSTEMS
A series connection system is shown in fig. (a). All converters are connected in series
and carrying same amount of current.
1 and 2 are acting as rectifier while 3 and 4 acting as inverter. Out of total supply
voltage, part of the voltage may be dropped across inverter 4 (20 to 30%) and 20 to
30% of power is only tapped at point 4.
Usually this type of arrangement is suitable for monopolar operation. The system is
grounded at only one point. Sometimes other points are also grounded through the
capacitor to improve the insulation coordination and system performance during
Fig. (a): Series Connection
System
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8. PARALLEL MTDC SYSTEMS
In this system, the principle of parallel connected AC system is extended to
DC system as well. The sending end converter acts as rectifier and more than
one converter (load) is connected to line.
It means the currents in all converter stations except one are adjusted
according to the power requirement.
Fig. (b): Parallel Connected Radial
MTDC System
All converters operates almost at constant
voltage. One of the terminal operates as a
voltage setting terminal at constant angle or
voltage.
An example of 3 terminal radial system is shown
in fig. (b). This shows the monopolar system
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9. Fig. (c): Parallel Connected Ring
MTDC System
A radial system is one in which the disconnection of
one segment of transmission would result in
interruption of power from one or more converter
stations.
In ring or mesh system, the removal of one link
would not result in disruption, provided the
remaining links are capable of carrying the required
power with increased losses.
Evidently, mesh (ring) system can be more reliable
than a radial system.
An example of a 4 terminal mesh system is shown
in fig. (c). The power reversal in a parallel MTDC
system would involve mechanical switching as
voltage cannot be reversed
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10. The operation of multiterminal system is hampered because of,
1. Lack of application targets
2. Complex requirement for control system
3. Reliable DC circuit breakers
But nowadays DC breaker is replaced by a control and protection
system in which disturbance is seen in the entire system and converters
are suitably blocked or bypassed
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