Operating Principle Of DC SPD

Name：CHT1-B40/2P Dc:1500V

Definition

This device is used to protect the DC circuits from damages caused by transient overvoltage such as lightning strikes, switching operations, and etc.. It works based on the principles of rapid response and high-energy discharge.

Core working principle

1. Detection of transient overvoltage: it will trip the protection when the monitoring voltage exceeds the threshold value (1.5 - 2 times of the rated voltage).
2. Discharge the surge energy: by using the components such as varistors and gas discharge tubes to conduct the energy to the ground or dissipated as heat energy.
3. Quick recovery: after the work of SPD, it will automatically returns to a high-impedance state, without affecting the power supply of the circuit.

Key protective components and their functions

1. Varistor (MOV): to absorb the medium-energy surges, its response time at the nanosecond level. Multiple impacts can cause aging.
2. Gas discharge tube (GDT): to discharge the big current (such as lightning strikes), which through-current capability can be up to several thousand amperes, and its response time can achieve the microsecond level, and it will possibly generate the arcs. 
3. Transient voltage suppressing diode (TVS): its response speed can achieve the picosecond range, precise clamping of low-voltage sensitive circuits, low through-current capacity, often equipped with the other components.
4. Decoupling inductor/resistance: it can coordinate the action sequence of multi-level protective components to ensure that the energy can be released step by step.

Typical application scenarios

Photovoltaic system (protecting the solar panels and inverters), electric vehicle charging stations, communication base stations (48V DC power supply), industrial DC power supply (for rail transit and energy storage system busbars)

Difference between the DC SPD and AC SPD

1. The AC SPD works by using nonlinear components, such as zinc oxide varistors, which present the high-resistance state and hardly conduct current under normal voltage. When a surge voltage occurs in the AC circuit, its resistance rapidly decreases, and it directs the surge current to the ground, thereby protects the back-end equipments. For example, in the household electricity utilization, when the lightning induction or the start/stop of electrical appliances generate a surge, the AC surge protector can act promptly.
2. The operating principle of DC SDP has resemblances as the AC SPD. But there are differences in the component selection and parameter design. It also uses the nonlinear components to realize the suppression of surges. However, due to the characteristics of the DC circuit, the long-term stability and tolerance of the components under the DC voltage need to be considered. For example, in the DC concentric circuit of a photovoltaic power station, the DC SPD can effectively protect the equipments from the impact of surges caused by lightning strikes or other reasons.

Selection Notes

1. Voltage grade: will match the system working voltage (such as the photovoltaic system usually is 1000V DC).
2. Through-current capacity (In): select according to the expected surge current (such as 20kA, 40kA).
3. Clamp voltage: the residual voltage should be lower than the withstand level of device.
4. Environmental adaptability: the outdoor needs to consider the dustproof, waterproof (IP grade) and temperature range.

Maintenance and failure modes

1. Regular inspection: after the varistor ages, the leakage current increases and it may generate heat.
2. Fault indication: some products have a visible window or remote signaling contacts to indicate the need for replacement.
3. Safety failure: when a high-quality product fails, it will automatically disconnec to avoid the risk of short circuit.