DC SPD Meaning

DC SPD, full name Direct Current Surge Protection Device, is a protection device designed specifically for DC power systems to defend against transient overvoltages (surges) caused by lightning strikes, switch operations, or other electrical disturbances. If these surges are not controlled, they may damage sensitive electronic devices in the DC system and even lead to system failures.

A DC surge protective device is designed to offer DC-powered systems and equipment protection from sudden spikes or surges voltage. DC SPDs suppress or divert voltage surges preventing damage to sensitive electronic components, system failures and even data loss.

Considerations for DC Surge Protection Devices in PV Installations

Inter-cloud and intra-cloud lightning flashes with magnitudes of 100kA can create related magnetic fields that trigger transient currents in PV system DC cabling. These transient voltages arise at equipment terminals and trigger important component insulation and dielectric failures.

These generated, and incomplete lightning currents are mitigated by placing SPDs at specific locations. The SPD is connected to the ground in parallel with the electrified wires. When an overvoltage occurs, it switches from a high-impedance to a low-impedance device. The SPD discharges the related transient current in this design, reducing the overvoltage that would otherwise exist at the equipment terminals.

This parallel device carries a no-load current. The SPD you choose must be designed, rated, and approved, particularly with DC PV voltages. The inherent SPD disconnect must be capable of interrupting the more severe DC arc that is not present in AC applications.

On large commercial and utility-scale PV systems operating at a maximum open-circuit voltage of 600 or 1,000V DC, connecting MOV modules in a Y configuration is a popular SPD setup.

A MOV module is linked to each pole and ground on each leg of the Y. There are two modules between each pole and both pole and base in an ungrounded system. Because each module is rated for half the system voltage in this configuration, the MOV modules do not exceed their rated value even if a pole-to-ground failure occurs.

The Function of DC Surge Protection Device

The core function of DC SPD is to absorb and release these sudden high-energy surges, limit the amplitude of overvoltage, and protect devices connected to the DC power supply from damage. They are typically installed at key nodes in DC power systems, such as the DC side of photovoltaic power generation systems, the power input of communication base stations, or the DC output end of electric vehicle charging piles to ensure stable operation of the system.

Compared with surge protective devices for AC (AC SPD), DC SPDs need to address the unique challenges of direct current, such as continuous unidirectional currents and potentially high voltage levels. Therefore, DC SPDs are designed with special components and technologies to meet the needs of a DC environment.

Working Principle

Proper selection, installation and maintenance of DC surge protective devices are necessary in ensuring effective voltage surge protection in DC systems. The performance effectiveness of a DC SPD varies with factors like surge rating, clamping voltage, response time, and the specific application.

You can break down the working of a DC surge protective device as follows:

- Surge Detection

A DC surge protective device will detect a voltage surge beyond its rating in a DC system. This device typically monitors the voltage level by utilizing special circuitry for detecting surge.

- Voltage Clamping

DC surge protective devices utilize components like metal oxide varistors (MOVs) or gas discharge tubes (GDTs) in achieving voltage clamping. These components display high resistance to the voltage within normal limits, allowing normal electrical current flow. Nevertheless, a voltage surge beyond the threshold decreases the component’s resistance significantly, creating a low-impedance path for the surge current. The threshold beyond which a voltage is considered a surge is referred to as clamping voltage or let-through voltage.

- Energy Absorption

The primary components of a surge protective device absorb excess energy when a voltage surge is diverted through the device. The design of metal oxide varistors (MOV) is such that they break down at high voltages dissipating the surge as heat.

In a DC circuit, the surge protector is in a high resistance state and does not work under normal voltage (Un). When it senses that the surge voltage exceeds the rated voltage (Uc), the SPD itself will quickly reduce its own resistance and conduct (within 25 nanoseconds), release the surge current, lower the voltage to a safe state, and then return to a high resistance state, completing protection for electrical equipment in the circuit.

The Key Features of DC Surge Protection Device

- High response speed: able to respond to surges in nanoseconds and quickly activate protection mechanisms.

- High energy absorption capacity: able to withstand and dissipate large amounts of surge energy, protecting backend equipment.

- Stable voltage protection level: ensuring that during surge events, the system voltage does not exceed the safe operating range of the equipment.

By installing DC Surge Protection Device, the reliability and safety of the direct current system can be significantly improved, extending the service life of equipment and reducing maintenance and replacement costs caused by surges. In various fields such as photovoltaic power generation, communication, transportation, etc., DC surge protection device has become an indispensable protective component.

How to install a DC Surge Protection Device

- Place the SDP as close to the panel to be protected as feasible.

- To decrease the length of the connecting wires from the surge protective device’s lugs to the next panel’s circuit breaker, drill and punch a hole in the surge protective device housing in an extraordinarily high place (or fused disconnect lugs).

- Use a close-nippled connection with wires traveling to the first breaker at the top of a panel whenever possible. This guarantees that all loads connected to the panel are adequately protected.

- Connect the SPD to the breaker panel with AWG #10 stranded wire or bigger (readily available and easy to install). In the wiring, avoid sharp bends and excessive length. The most successful installations aren’t usually the most aesthetically pleasing. The most effective encounters are short and direct.

- SPDs should be connected to a properly rated circuit breaker rather than the panel’s main lugs. A fused disconnect switch should be used to communicate with the lines and facilitate SPD servicing where circuit breakers are unavailable or impractical.

Comparing DC SPD with AC SPD

The major difference between DC and AC surge protective devices is based on the power system in use. As such, there are slight departures between the two concerning voltage ratings, surge handling capabilities, response times, and standards.

The following statements highlight some of the similarities and differences between DC and AC surge protective devices (SPDs):

- Frequency Handling

Surge protective device used in DC systems have no frequency specifications thanks to the constancy of DC voltage. On the other hand, those in AC systems have different frequency needs requiring different handling.

- Polarity Sensitivity

Surge protective devices in DC systems are polar sensitive requiring installation with correct terminal alignment. Due to the constantly changing voltage direction in AC systems, they have no specific terminal designations.

- Surge Detection and Clamping

Depending on the system design, both DC and AC SPDs will counter voltage surges by absorbing or diverting them to a safe level. However, the differing voltage characteristics can result in a change in the mechanisms applied in the detection and clamping.

DC SPD Types

Classified by Voltage Level

According to the voltage level of the DC system, DC surge protection device can be divided into the following categories:

- Low-voltage DC SPD: suitable for low-voltage DC systems, usually with a voltage range below 48V, commonly found in communication equipment, small photovoltaic systems, or low-voltage DC distribution systems.

- Medium-voltage DC SPD: suitable for medium-voltage DC systems, with a voltage range typically between 48V and 1000V, widely used in the direct current side of photovoltaic power generation systems, electric vehicle charging stations and other scenarios.

- High-voltage DC SPD: suitable for high-voltage direct current systems, with a voltage range above 1000V, mainly used in large-scale photovoltaic power plants, high-voltage direct current transmission systems etc.

Main Parameters of DC SPD

The parameters of a DC surge protective device define their performance and suitability in a particular DC system from voltage surges. Careful consideration of these parameters and the intended system for use is therefore vital for effective matching.

The main parameters provided for DC surge protective devices include:

- Leakage Current: When the DC surge protective device is operating normally, leakage current describes the minimal current flowing through it. Having a low leakage current is preferred as it results in reduced heat dissipation and loss of power.

- Maximum Continuous Operating Voltage: Defines the DC voltage beyond which the surge protective device is activated dependent on system’s rated voltage.

- Nominal Discharge Current: Describes the highest current value that a DC surge protective device can discharge when a surge event occurs.

- Operating Temperature Range: Defines the temperatures within which the DC surge protective device can perform optimally. This parameter is application specific especially where the DC system in need of protection is operated in extreme temperature conditions.

- Voltage Protection Level: Represents the maximum voltage across an activated DC surge protective device’s terminals. It is achieved when the current passing through the surge protective device matches that of the nominal discharge.

Application scenarios of DC Surge Protection Device

DC surge protectiton device is divided into two types:

- One is used in low-voltage DC, for protecting communication modules, monitoring, etc.

- The other is used in photovoltaics, for protecting photovoltaic systems, energy storage, etc.

Photovoltaic Power Generation System

- PV DC side protection: installed between the PV string and inverter to protect the PV modules and inverters from surge damage caused by lightning strikes or switch operations.

- PV AC side protection: installed at the output end of the inverter to protect AC side equipment.

Communication Base Station

- Power system protection: protects the DC power supply equipment of communication base stations, such as battery packs and rectifiers.

- Signal system protection: protects communication signal lines to prevent surges from interfering with or damaging communication equipment.

Electric Vehicle Charging Facilities

- Charging pile protection: installed at the DC output end of the charging pile to protect the charging pile and electric vehicle battery management system.

- Battery pack protection: used on the DC side of electric vehicle battery packs to prevent surges from damaging batteries.

Industrial Control System

- PLC and sensor protection: protects DC power supply devices in industrial control systems, such as PLCs, sensors, etc.

- DC motor protection: used for DC motor drive systems to prevent surges from damaging motors and drives.

In practical applications, when selecting a DC Surge Protective Device, consider the following factors:

- System voltage: choose a DC Surge Protection Device that matches the system voltage.

- Surge current rating: select appropriate nominal discharge current (In) and maximum discharge current (Imax) based on the surge risk level of the system.

- Installation environment: consider environmental factors such as temperature, humidity, etc., and choose a suitable protective level (IP rating).

Advantages of using a DC SPD

By employing DC SPDs, the vulnerabilities of DC-powered systems to voltage surges can be effectively mitigated, promoting equipment protection, system reliability, and overall operational safety.

A summary of the benefits of utilizing a DC surge protective device is discussed below:

- Equipment Protection: This is the primary benefit of configuring your DC system with a surge protective device. It diverts or suppresses excessive voltage surges safeguarding the equipment from damage.

- Extended Equipment Lifespan: Averting the damaging effects of surges by DC SPDs allows equipment to function for longer. Otherwise, unprotected equipment easily succumb to voltage surges resulting in damage or hampering of performance.

- Safety Assurance: When surge events occur, they pose safety hazards, especially in industrial setting utilizing DC sources with high energy. By absorbing or redirecting surge energy, these devices reduce the potential for electrical faults, fires, or other safety hazards.

- System Reliability: Surge protective devices contribute to the improvement of DC system reliability in their protection role. They reduce the risk of equipment failure helping to maintain continuous operation and minimize disruptions.

Can surge protectors for AC be used to protect DC circuits?

Some people may want to use surge protectors for AC to protect DC power supply systems. From a professional perspective, the voltage and current of AC electricity are periodically changing, 50 times per second (50 Hz) or 60 times per second (60 Hz). When the current changes from positive half-cycle to negative half-cycle, it will pass through the “zero point”, at which time the voltage and current will be “0”, effectively suppressing transient currents naturally.

                       Single phase AC signal                                              Three phase AC signal

But DC will not, it is a one-way continuous current voltage, there is no “zero point” option, so the surge current will not be suppressed, causing sustained impact on the equipment. If an AC surge protector is used to protect the DC line at this time, the continuous strong overvoltage and surge current will break through the AC surge protector, greatly shorten the service life of the surge protector, and cause a fire. Therefore, it is necessary to select reliable DC surge protectors for protection.

                                                                                   DC signal

Testing a DC Surge Protective Device

Testing a DC surge protective device verifies its functionality ensuring it can effectively offer equipment protection from voltage surges. When testing, compare test results with the specific response characteristics provided to which the SPD needs to adhere.

Commonly used tests include:

- Insulation Resistance Test: Here, you disconnect the SPD from the DC source, and measure the resistance between the device’s and ground terminals. It ensures paths of leakage or faults are absent.

- Voltage Drop Test: This test ensures the voltage drop is within the specified limits. You connect the device to a DC source before applying the rated voltage and measuring it.

- Surge Test: Here, you carry out a simulation of transient surges by applying surge impulses to the surge protective device. Thereafter, examine the waveforms comparing them with the test specifications.

Some misconceptions about surge protectors for direct current.

1. The idea that a simple DC system only requires single-stage surge protection to meet the requirements is incorrect. Surge protection is systematic, and different stages require different DC surge protectors for multi-level protection. Especially for communication systems, the more precise and sensitive the equipment, the more reliable surge protection it needs.

2. It is wrong to install DC surge protectors far away from devices as long as they are grounded. DC surge protectors should be close to the protected equipment. If a DC surge protector is too far from the device that needs protection, when a surging current hits, the DC surge protector must respond within microseconds to save electrical equipment. If the line is too long and all surging currents hit the device before reaching it, even if the DC surge protector reacts quickly, it will not have time to release the surging current. Therefore, DC surge protectors should provide “close protection” for electrical equipment.

3. In a direct current system where voltage remains stable without frequent fluctuations like alternating current voltage does not mean there is less risk of surges than in an AC system? Wrong – stable voltage does not equal no risk. In a direct current system, there is no “zero point” in terms of current or voltage but rather continuous flow which can easily attract lightning strikes making them more susceptible compared to AC systems. Taking solar panels as an example – outdoor devices like photovoltaic arrays are particularly prone to lightning strikes due to their large surface area and continuous flow of electricity which attracts lightning bolts causing powerful surges.

4. It’s wrong to have loose grounding requirements for low-voltage direct current systems; you cannot skip grounding or simply connect them near an enclosure with some distance between them. It’s essential to ground them properly because grounding plays a crucial role in protecting electrical devices using direct-current overvoltage protective devices. Connecting directly with enclosures doesn’t necessarily mean proper grounding; some enclosures may lack connections with earth or appear grounded but might be isolated by paint layers preventing effective grounding connection.If there’s slight leakage in equipment leading enclosure being charged then during arrival of power surges these would lead back through protective device causing fire hazards rendering overvoltage protective device useless.Therefore,it’s imperative that Direct Current Overvoltage Protective Devices are properly grounded

Conclusion

DC Surge Protection Device surge protectors, as the “safety guards” of DC power systems, play a crucial role in modern power protection. Whether it is photovoltaic power generation systems, communication base stations, or electric vehicle charging facilities, DC SPD can effectively resist the threats brought by surges, ensure the stable operation of equipment, extend its service life, and reduce maintenance costs.