First, we need to understand what an SST is. Traditional industrial frequency transformers are bulky and inefficient devices based on the principle of electromagnetic induction. Solid-state transformers (SSTs), also known as power electronic transformers, are novel power conversion devices that utilize advanced power electronics conversion technology and high-frequency transformers to achieve voltage conversion, electrical isolation, energy management, and intelligent control.

AC (medium voltage, such as 10kV) → AC/DC rectifier → HVDC busbar (such as ±375V, 750V, 1500V) → DC/AC inverter (high frequency) → high-frequency transformer → AC/DC rectifier → DC (low voltage) or AC (low voltage)

Small Size and Light Weight: High-frequency transformers are significantly smaller than industrial-frequency transformers.
Versatility: They enable active/reactive power regulation, harmonic suppression, and voltage sag compensation.
Bidirectional Power Flow: They naturally support the integration of renewable energy sources (such as photovoltaics and energy storage).
Provide DC Ports: They naturally create a stable HVDC busbar for direct power supply to DC loads and energy storage systems.

High Voltage Stress: Power electronic switching devices (such as SiC MOSFETs and IGBTs) withstand high voltages.
Complex Control: Multi-stage power conversion requires complex control strategies.
High Reliability Requirements: As a critical component in the power grid, any failure can cause a widespread power outage.
Fault Protection: When an HVDC busbar short-circuits, the fault current rises extremely rapidly. Without the zero-crossing point of the power-frequency AC, arc extinguishing is extremely difficult.

In the complex structure of an SST, high-voltage dc contactors play the role of “safety guards” and “energy routing commanders.” Their primary application scenarios and functions are as follows:

Scenario: When a short circuit occurs on the HVDC bus or downstream load side of an SST, the fault current rises dramatically.
Purpose: After receiving a signal from a protection system (such as a circuit breaker or relay), a high-voltage DC contactor must quickly and reliably interrupt fault currents up to several thousand amperes, physically isolating the faulted portion from the healthy system to prevent further expansion and protect expensive power electronic devices (such as IGBTs and SiC modules) and capacitors.
Requirement: Must possess extremely strong DC arc extinguishing capabilities.

Scenario: When the SST starts, the initial voltage of its DC-side support capacitor is zero. If directly connected, the capacitor acts as a short circuit, generating a huge inrush current that could damage main circuit components.
Purpose: A “pre-charge circuit + main circuit” solution is typically used. The pre-charge circuit consists of a pre-charge contactor and a current-limiting resistor in series. The pre-charge contactor is closed first, allowing the capacitor to be charged with limited current through the resistor. When the capacitor voltage approaches the bus voltage, the main contactor is closed, followed by the pre-charge contactor. This process smoothly builds the system voltage.
Requirements: Pre-charge contactors are usually selected with smaller rated current.

Scenario: When maintenance or repair work is required inside the SST, the equipment must be completely disconnected from the power supply.
Purpose: Installing DC contactors at key HVDC busbar nodes (such as energy storage and PV interfaces) provides a visible, physical disconnect point, ensuring the safety of maintenance personnel.
Requirements: Clear disconnect status indication (such as auxiliary contacts) and safe distances are required.

Scenario: In complex multi-port SSTs (e.g., those simultaneously connected to the grid, photovoltaics, energy storage, and DC loads), it is necessary to dynamically connect or disconnect certain energy sources or loads based on energy management strategies.
Purpose: High-voltage DC contactors act as controlled switches, executing the system’s energy scheduling commands. For example, when the energy storage battery is fully charged, it disconnects from the busbar; when the backup power source is needed, the corresponding contactor closes.
Requirements: Requires long mechanical and electrical life (typically over 10,000 operations).

To meet the stringent requirements of SST, the following parameters need to be focused on during selection:

Rated operating voltage: Must be higher than the maximum HVDC bus voltage of the SST system. Common ratings include 750VDC, 1000VDC, 1500VDC, and even higher such as 3000VDC.
Withstand voltage: This includes insulation strength between poles and between poles and ground (creepage distance).

Rated Operating Current: Selected based on the continuous current flowing through the circuit, such as 100A, 250A, or 500A.
Short-Time Withstand Current: The ability to withstand a brief fault current without damage before tripping.
Breaking Capacity: This is one of the most important specifications. It refers to the maximum fault current that can be safely interrupted at a specified voltage. SST applications typically require a breaking capacity of 5kA to 20kA or even higher.

Permanent Magnet Magnetic Blowout: This method uses a permanent magnet to generate a magnetic field, elongating and cooling the arc, ultimately extinguishing it. This is the most mainstream technology and offers high reliability.
Narrow Slit Arc Extinguishing: This method uses a narrow slit to confine the arc and increase arc resistance.
Vacuum Arc Extinguishing: This method extinguishes the arc in a vacuum environment, offering excellent performance but at a higher cost.

Typically, the coil is powered by a DC power supply (e.g., 24VDC, 110VDC).
Focus on both pull-in power and holding power. Modern contactors typically use permanent magnet holding technology. While high power is required for the instantaneous pull-in, the holding state consumes almost no power, making it energy-efficient and safe (self-holding upon power failure).

Sealing: Many high-voltage DC contactors use gas-filled seals (such as nitrogen) or vacuum seals to prevent environmental influences and oxidation, while also improving arc extinguishing performance.
Electrical Life/Mechanical Life: This is a critical performance indicator in scenarios with frequent switching.

Here are some leading brands in both global and domestic markets, along with their representative product series that are widely used in high-end applications such as new energy, rail transit, and SST.

In the revolutionary power grid equipment known as solid-state transformers (SSTs), high-voltage DC contactors have evolved from simple switching elements into critical protection and control components that ensure safe, reliable, and efficient system operation.
Their core value lies in:
Providing deterministic physical isolation, a powerful backup for soft switching in power electronics.
Enabling rapid fault clearing, protecting expensive core power components.
Enabling flexible system configuration and maintenance.
We hope the detailed explanation above helps you gain a comprehensive understanding of the application of HVDC contactors in SST systems. If you have any specific technical parameters you’d like to discuss, please feel free to continue the conversation.
Hotson is committed to providing efficient, safe high-voltage DC contactors for solid-state transformer (SST) solutions. For consultation or more information, please visit our website or contact our sales team directly. We look forward to working with you to promote industry development together!