HVDC (High Voltage Direct Current) is a technology that transmits power in the form of high-voltage direct current (DC). It is distinguished from the general alternating current (AC) transmission method and is evaluated as a very effective transmission method in long-distance transmission or inter-country power grid connection.

The HVDC system is structured to convert alternating current power into direct current, transmit it as high voltage, and reconvert it back to alternating current at the receiving point.
This process takes place within the Converter Station, and the main components include a converter, transformer, and filter.

HVDC plays a key role in long-distance and undersea power transmission, inter-country grid connection, large-scale renewable energy access, and smart grid-based power grid construction, and is an important technology that contributes to improving the reliability and efficiency of future power systems.

For stable operation of the HVDC system and facility protection, a protection switchboard with various protection functions is applied, and the main protection panel consists of a converter transformer protection panel, DC transmission line protection panel, filter and reactor protection panel, grounding system protection panel, and control and communication protection panel.

These protection panels play key roles such as detecting and blocking HVDC equipment failures, monitoring abnormal conditions, and ensuring system stability, and are being applied as essential protection equipment in domestic and overseas HVDC construction projects.

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  The HVDC converter transformer protection switchboard is a protection system designed to precisely protect the converter transformer, which is one of the most critical facilities in the high-voltage direct current transmission system.
  This protection panel is structured around differential protection (87T), overcurrent protection (50/51), and ground fault protection (64/51N), and performs high-speed detection and blocking operations for various fault situations such as transformer internal failure, external short circuit, and ground fault.
  The applied protection relay is Siemens' SIPROTEC 7UT series, which integrates various protection elements such as primary and secondary winding protection of the transformer, neutral point protection, and tap changer interlocking monitoring to support clear identification of the fault range and stable operation.
  In addition, this protection switchboard is capable of complete communication linkage with the HVDC control system and SCADA, and is composed of an intelligent protection system that enables real-time status monitoring and failure history analysis during operation, including temperature monitoring function, event log storage, and test interface.

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  Double Tuned Filter (DTF) in an HVDC system is a key facility that improves power quality by removing specific harmonics
  These filters are very sensitive to harmonic currents, and when a fault occurs, they can have a negative impact on the entire HVDC system, so accurate and rapid protection is essential.
  The double tuning filter protection switchboard is based on Siemens' SIPROTEC 7SJ series protection relay and integrates the following multiple protection elements to respond to various failure situations such as harmonic abnormalities, overcurrent, and voltage abnormalities.
  First, the overcurrent protection (50/51) function enables immediate shutdown in the event of a short circuit or overload current within the filter circuit, and the asymmetric current protection (46) detects a three-phase unbalanced state and prevents damage to the filter equipment in advance.
  In addition, the harmonic abnormality protection (81H) function performs protective operation when the set harmonic component (e.g., 11th, 13th, etc.) exceeds the standard value, and the overvoltage and undervoltage protection (59, 27) function detects voltage abnormalities that affect filter operation and supports stable system operation.
  In addition, when an abnormality occurs in the electrical response of the reactor (R), condenser (C), and resistor (R) inside the filter through the filter characteristic abnormality monitoring (91) function, the filter protection relay performs a blocking or alarm operation to prevent quality degradation of the entire system.

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  In the HVDC system, the High Pass Filter (HPF) is a key device that maintains power quality and protects the stability and lifespan of equipment by removing harmonic components in the high frequency band, especially high-order harmonics of the 23rd order or higher.
  This filter suppresses high-frequency noise and voltage distortion occurring within the transmission system, contributing to preventing equipment malfunction and insulation breakdown.
  Considering the sensitive operation characteristics of the high-pass filter, the high-pass filter protection switchboard is constructed using Siemens' SIPROTEC 7SJ series protection relay and is designed to perform precise and rapid protection operations against various abnormal conditions.
  First, the overcurrent protection (50/51) function enables immediate blocking in the event of a short circuit or overload current in the filter circuit, and the asymmetric current protection (46) prevents damage to equipment by automatically operating the relay in the event of an unbalanced three-phase current.
  In addition, the harmonic abnormality protection (81H) function detects when high-order harmonics exceed the set standard value and assists the filter function by blocking the effect on the entire system.
  In addition, overvoltage and undervoltage protection (59/27) detects voltage abnormalities in the filter input terminal or parallel circuit to maintain normal operation of the equipment, and the filter characteristic abnormality monitoring (91) function detects conditions such as impedance changes, resonance abnormalities, and high frequency response abnormalities in the RLC circuit and blocks or generates an alarm when necessary.

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  Shunt capacitors used in HVDC systems are key equipment for compensating for reactive power and adjusting voltage in the system. In particular, they are installed in the alternating current filter section or converter input terminal to contribute to stabilizing voltage and improving power factor (PF).
  In this way, a capacitor bank, which is an essential facility for system operation, can lead to insulation breakdown, overheating, and reduced facility life when a failure occurs, so a protection system that can quickly and accurately protect it is required.
  The shutter capacitor protection switchboard is built around Siemens' SIPROTEC 7SJ series protection relay, and is designed to precisely detect various abnormal conditions such as overcurrent, harmonics, ground abnormality, and current imbalance, and quickly perform blocking or alarm operations.
  First, the overcurrent protection (50/51) function detects the charging current or fault current flowing into the capacitor bank, enabling quick shutdown operation to prevent device damage, and the asymmetric current protection (46) detects in real time when three-phase imbalance occurs, contributing to protecting internal devices.
  In addition, the ground fault protection (64N or 51N) function detects abnormal current flow through the ground path, and the harmonic protection (81H) performs blocking or alarm operation when overcurrent and voltage abnormalities occur due to resonance conditions or harmonic inflow.
  In addition, overvoltage protection (59) prevents system voltage rises from exceeding capacitor insulation limits, and contact failure monitoring (74TC) detects malfunctioning or non-operational conditions of circuit breakers or control contact devices to support operator response.

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  Shunt Reactor** used in the HVDC system is a key facility for absorbing reactive power and stabilizing voltage in the system. It is especially installed at the end of long-distance transmission lines or busbars to suppress voltage rises that occur at light loads and contribute to maintaining the stability of system operation.
  In this way, the reactor bank, which is an essential facility for system operation, can lead to damage such as winding insulation breakdown, overheating, and internal short circuit when a failure occurs, so a protection system that can quickly and accurately protect it is required.
  The shutter reactor protection switchboard is built around Siemens' SIPROTEC relay series, and the 7UT8 series is applied for 87R differential protection, and the 7SJ8 series relay is applied for other multi-function protection. It is designed to precisely detect various abnormal conditions such as overcurrent, overheating, ground abnormality, and current imbalance, and quickly perform blocking or alarm operations.
  First, differential protection (87R) plays a key role in protecting the reactor by comparing the input and output currents of the reactor, accurately determining internal failure situations such as internal short circuit or insulation breakdown, and performing quick blocking operations.
  This function is implemented through the SIPROTEC 7UT8 series.
  In addition, the overcurrent protection (50/51) function prevents damage to the device by detecting external fault current flowing into the reactor, the "overheat protection (49)" analyzes the current accumulation state based on a thermal model and detects a rise in winding temperature in advance, and the "asymmetric current protection (46)" prevents internal abnormalities through real-time detection when three-phase imbalance occurs.
  The ground fault protection (64N or 51N) function detects abnormal current flow through the ground path, and the “overvoltage protection (27/59)” prevents insulation breakdown due to excessive rise or fall of the grid voltage.
  In addition, "Harmonic Protection (81H)" detects current abnormalities when harmonic inflow or resonance conditions occur and performs blocking or alarm operations, and "Contact Failure Monitoring (74TC)" detects malfunctions and non-operational states of circuit breakers or control contact devices in real time to support rapid response by operators.
  These protection functions are implemented through the SIPROTEC 7SJ8 series and contribute to the long-term operating reliability of the reactor and protection of equipment life.