This video contains the different control methods of MTDC System and also introduced the protection of MTDC system

1. MTDC SYSTEMS
CONTROL & PROTECTION OF MTDC SYSTEMS
MR. ASIF L. JAMADAR

2. CONTROL OF MTDC SYSTEMS
 There are several methods of control in MTDC Systems. The various methods
are suggested as below:
 Current Margin Method
 Voltage Limiting Control
 Decentralized Current Reference Balancing
 Two ACR (Automatic Current Regulator) Method
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3. CURRENT MARGIN METHOD
 This method is natural extension of control philosophy in two terminal system.
 The current through the voltage setting terminal (say nth terminal) is given by,
 The current control is also provided at the Voltage Setting Terminal (VST) to
maintain same current
 The current reference is chosen at VST to satisfy the following equation,
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4. Fig. (a): Current Reference Balancer
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 The converter with the lowest voltage ceiling always acts as a voltage setting
terminal (VST).
 The central controller that regulates the current orders at all converter station is
called as Current Reference Balancer (CBR), refer fig. (a).
 This requires reliable two way communication between a central station and each
converter station.
 In this method, the change in voltage setting terminal requires the tap changing
converter transformer to modify the voltage margin.

5. VOLTAGE LIMITING CONTROL
 In the voltage limiting control, the rectifier and inverter characteristics are
arranged as shown in fig. (b).
 The effect of losses in rectifier station and inverter station on the system
operation
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Fig. (b): Operation with voltage limiting
method

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 The drawbacks of voltage control limiting scheme are,
1. The distribution in AC system connected to an inverter station can result in
other inverters getting unloaded due to drop in DC voltage.
2. Two or more terminals can operate in the voltage controlling mode forcibly.
3. Currents during DC line fault or communication failure are likely to be
higher without additional measures

7. DECENTRALIZED CURRENT REFERENCE BALANCING
 This method is without Current Reference Balancing (CRB).
 Decentralized Current Reference Balancing (DCRB) is designed to allow rapid
recovery of MTDC system operation following severe disturbances.
 The operation of the DCRB is illustrated with the help of a 3 terminal system with
two rectifier and one inverter [refer fig. (c)].
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Fig. (c): Decentralized current
reference balancing method

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 DCRB is to permit the system recovery without coordinated centralized
control of converter
 The objective of DCRB is to adjust the convertor control characteristics
around a preselected voltage level V0 as indicate in fig. (c).
 In DCRB the minimum and maximum current limits are preselected.
 The normal post disturbance characteristics are also shown in fig. (c) by solid
lines.
 The slope of balancing characteristics along with voltage V0 are chosen,
based on the consideration of voltage limit and magnitude of variations in DC
network voltages during balancing period

9. TWO ACR METHOD
 In this method, each converter is provided with two automatic current
regulator (ACR) and voltage regulator.
 The rectifier and inverter control characteristics are shown in fig. (d).
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Fig. (d): Static Vd-Id characteristics of a converter in two ARC

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 In normal operation, the rectifier station with the lowest voltage reference or
inverter station with highest voltage reference acts as voltage setting
terminal, refer fig. (e).
Fig. (e): Static Vd-Id characteristics of a 4 terminal system: operation with voltage control in
REC2 and operation with voltage control in INV 1

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 Without additional measures, the inverter may not be able to recover from
communication failure. To overcome this problem, the Voltage Dependent
Current Order Limit (VDCOL) is introduced.
 The operation of a 4 terminal system both during normal conditions and
when one of the converter station is shut down, is shown in fig. (f)
Fig. (f): 1. operating point under normal condition

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Fig. (f): 3. Operating point without REC 2
Fig. (f): 2. operating point without REC 1

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 The rectifier with highest voltage and inverter with lowest voltage are always
controlled by ACR, thus resulting in small power deviations during a
disturbance.
Fig. (f): 4. Operating point without INV
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Fig. (f): 5. Operating point without INV
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14. PROTECTION OF MTDC SYSTEMS
 Conceptually, the system can be shut down following a fault in DC line or
converter station and faulted component isolated using high speed disconnect
switches.
 In the consideration of reliability, the system requires fast clearing of fault with
minimum disturbance to healthy part of the system.
 A major problem is the large drop in the DC voltage even for distance faults.
This requires fast detection and clearing of fault to maintain power
transformers.
 DC switches would be required primarily for parallel MTDC systems and
particularly mesh system to utilize the improved security provided by such
configurations.
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Reference: K. R. Padiyar, “HVDC Power Transmission Systems”, Second revised edition, New Age
International Publishers

15. THANK
YOU
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