What is the difference between AC and DC SPDs

What is the difference between AC and DC Surge Protective Device?

Created by: Glen Zhu | Updated Date: April 16th, 2024

What is the difference between AC and DC SPDs?

What is AC SPDs?

AC surge protective devices are designed to protect electrical equipment and appliances from voltage spikes or surges in alternating current power systems.

AC surge protective devices work by diverting the excess voltage from a surge towards the grounds or neutral wire, effectively limiting the voltage to a safe level. They could react rapidly to voltage fluctuations within nanoseconds, thereby prevent damage to connected electronic products and appliances.

There are three main types of AC SPDs – Type 1, Type 2, and Type 3 based on IEC 61643 – 11 standards. Type 1, Type 2, and Type 3 surge protectors feature different waveforms, application, installation location, and level of protection.

Types of AC surge protection devices

To understand what is AC surge protection devices, AC power system is the key point. AC surge protective devices should be applicable to different AC power supply system, mainly are TN, TT and IT system. The TN system is subdivided into TN-C, TN-S and TN-C-S.

What are these systems different from each other?

TN system

The TN system is typically a three-phase electrical grid system with a neutral point grounded. Its characteristic lies in the direct connection of the exposed conductive parts of electrical equipment to the system grounding point, creating a closed circuit through metal conductors when shell short circuits occur.

This results in a metallic single-phase short circuit that generates a sufficiently large short-circuit current to reliably trigger protective devices and cut off faults.

However, if the working neutral line N is repeatedly grounded, part of the current during shell short circuits may divert to the repeated grounding point, rendering protective devices unreliable or refusing to operate, thereby exacerbating faults.

In the TN system: The neutral point of the power transformer is grounded, and equipment shells are connected to the neutral point via the PE line (dedicated protection line), divided into TN-C, TN-S, and TN-C-S based on the connection position.

In TN-C configuration, the equipment shell is directly connected to the working protective neutral line (PEN), where the PEN line has no current voltage when the three-phase load is balanced. However, if it is unbalanced, there will be voltage on this line concerning the ground, posing a risk due to the charged shell.

TT power supply system

The TT system is a system where the neutral point of the power supply is directly grounded, and the exposed conductive parts of the electrical equipment are also directly grounded.

In the TT system, these two grounds must be independent of each other. The grounding of the equipment can be achieved by individual grounding devices for each device or shared grounding devices for multiple devices.

The characteristic of TT system as follow:

Both the neutral point of the power transformer and the equipment casing are grounded, with a neutral line leading out, providing both 220 and 380-volt power sources.

In the event of a phase-to-ground fault, the voltage is relatively low, which is safer for operators, but the current is small, making it difficult to trip circuit breakers, and poor grounding may lead to high voltage. Main circuits can install leakage protection devices to enhance protection capabilities.

It possesses a certain capability to dissipate lightning overvoltage; however, it consumes a significant amount of grounding steel, resulting in resource wastage. This system was commonly used in previous construction sites, but new standards primarily focus on retrofitting such outdated applications.

Because the equipment is grounded locally, a single equipment leakage fault will not affect the entire system via the protective line, making this system mainly suitable for voltage-sensitive precision electronic or data processing equipment. It is also suitable for users receiving low-voltage power from external sources without dedicated transformers.

IT system

The IT system is a system where the neutral point of the power supply is not grounded, and the exposed conductive parts of electrical equipment are directly grounded.

As there is no neutral wire brought out, this system only provides 380 volts for civilian electricity. Appliances requiring 220 volts must be connected through transformers or have dedicated power sources brought in separately.

The IT mode power supply system has high reliability and good security when the power supply distance is not long. It is generally used in places where no blackouts are permitted, or places where strict continuous power supply is required, such as electric power steelmaking, operating rooms in large hospitals, and underground mines.

However, it is not suitable for civilian or building areas requiring long-distance power supply or where equipment accessible to operators is located in close proximity.

The power supply conditions in underground mines are relatively poor and the cables are susceptible to moisture. Using the IT-powered system, even if the neutral point of the power supply is not grounded, once the device is leaking, the relative ground leakage current is still small and will not damage the balance of the power supply voltage.

Therefore, it is safer than the neutral grounding system of the power supply. However, if the power supply is used for a long distance, the distributed capacitance of the power supply line to the earth cannot be ignored.

When a short-circuit fault or leakage of the load causes the device case to become live, the leakage current will form a path through the earth and the protection device will not necessarily act. This is dangerous. Only when the power supply distance is not too long is it safer. This type of power supply is rare on the construction site.

When it comes to surge protection, AC SPDs can be installed to protect against transient surges on both the line and neutral conductors, providing comprehensive protection for the equipment connected to the system.

What is DC SPDs?

DC SPDs are normally used in solar power systems, telecommunications, automative and industrial automation. DC surge protective devices, serve a similar purpose to AC SPDs but are designed specifically for direct current (DC) electrical systems.

In solar power systems, DC SPDs are essential components for safeguarding photovoltaic (PV) panels, inverters, charge controllers, and other system components from voltage surges caused by lightning strikes, grid fluctuations, or switching operations.

These surges can pose a significant risk to solar installations, potentially causing damage to expensive equipment and interrupting power generation.

Similarly, in telecommunications networks, in automative electronics and industrial applications, DC surge protective devices play a vital role in protecting against voltage spikes and transient disturbances.

Types of solar surge protective devices

What is the difference between AC and DC surge protection devices?

The Frequency

AC surge protective devices are used to protect AC systems, where the voltage and currents vary sinusoidally, typically at 50Hz or 60Hz. While the voltage and current of DC surge protective devices are constant, with no frequency variation.

The Impedance

The impedance of AC systems consists of resistance, inductance, and capacitance, which are frequency-dependent.

DC systems only includes the source internal resistance and line resistance, without the frequency-dependent inductance and capacitance components, resulting in lower internal resistance.

Polarization Effect

The varistor chips in DC surge protectors can exhibit a polarization effect, where the forward and reverse breakdown voltages are asymmetric.

AC surge protectors do not exhibit this polarization effect, as the positive and negative half-cycles are symmetric.

Disconnection Mechanism

The advantage of low DC internal resistance in surge protectors is high efficiency, but the disadvantage lies in the potential danger of large discharge currents during short circuits. DC surge protectors lack the zero-crossing feature present in AC systems, making it difficult to cut off power, which can lead to fire accidents that are challenging to control.

This difficulty in DC breaking, along with the higher production process and technical requirements, contributes to the higher price of DC surge protectors, circuit breakers, and relays compared to their AC counterparts.

Moreover, the design of the disconnection mechanism in DC surge protectors is crucial for safety. It needs to respond quickly and have a larger disconnection distance. The mechanism must be capable of operating under minimal heat and ensure a sufficient creepage distance to reduce accident risks.

Essentially, DC surge protectors must act before reaching breakdown point to prevent fire incidents. However, the limited internal space poses a challenge for manufacturers in designing effective disconnection mechanisms, testing their technical capabilities. Our upgraded disconnection mechanism helps to isolate and extinguish arcs.

MOVs

The core component of either AC or DC SPD is the MOV, there are a few key differences in the choice of MOV.

AC SPDs need to handle bidirectional voltage surges, as the AC voltage alternates between positive and negative values. DC SPDs only need to handle unidirectional voltage surges, as the DC voltage is constant and unidirectional.

The MOVs in AC SPDs need to accommodate the AC grid voltages, which can range from 120V to 480V. The MOVs in DC SPDs need to withstand the higher continuous DC voltages found in solar PV systems, which can range from a few hundred volts up to 1500V.

DC SPDs may require MOVs with higher current handling capabilities due to the potential for larger short-circuit currents in DC systems compared to AC systems.

Applications of AC and DC surge protection devices

AC surge protective devices for power system

AC surge protection devices are essential for safeguarding electrical equipment and systems from damaging voltage spikes and surges, helping to ensure reliable performance and enhanced safety.

Figure 1 – Surge protection for local operating networks

Local operating networks (LONs) enables devices and systems to communicate seamlessly over a common network, fostering interoperability and efficient control.

However, electronic devices running operating systems are susceptible to power surge, a sudden increase in voltage will exceed their safe limits and damage or destroy the components.

Therefore, Type 3 AC surge protective device is mounted at the entry of power line and specific signal surge protector are required to protect the data transmission and control signal.

Figure 2 – Surge protection for industrial control panels

Industrial control panels connect with various electrical equipment, when lightning, and surges can easily cause equipment damage, surge disruption, and downtime losses.

A comprehensive protection strategy involves installing Type 1 AC surge protective devices at the main distribution box to handle the most severe overcurrents, Type 2 AC surge protectors for intermediate protection against surges, and Type 3 surge protectors to protect the terminals.

Figure 3 – Residential surge protection

Residential buildings are susceptible to surges caused by lightning strikes and power fluctuations, especially considering that modern houses heavily rely on electronics.

Various types of AC surge protective devices are utilized to safeguard the power supply system, electric charging infrastructure, telecommunications equipment, and lighting systems.

DC surge protective devices for solar system

Figure 4 – Surge protection for solar pump inverters

DC surge protection devices are installed in PV combiner boxes to ensure the operation of the solar pump inverter, avoiding the failure of water pumping due to sudden surges.

Solar energy is collected through PV panels and stored it as electricity in combiner boxes. The DC electricity is then transformed into AC electricity within the solar pump inverter to activate the water pump motor, thereby completing the irrigation process.

Figure 5 – Surge protection for battery storage system

Power storage systems are one of the key technologies of the energy revolution as they make it possible to store locally produced electricity on-site. The container battery storage systems store the power generated, e.g., by photovoltaic systems and wind turbines, and feed it back on demand.

DC SPDs specifically designed for BESS/ESS applications are required, with voltage ratings up to 1500Vdc. These DC SPDs are typically Type 2 or Type 1 SPDs that can handle the high DC voltages and discharge currents.

How to choose AC or DC SPDs?

Choosing the right surge protection devices involve a wide range of parameters related to types of surge protection devices.

Current type

The big difference between AC and DC surge protectors is working power systems. AC SPDs must accommodate the alternating positive and negative AC voltage, while DC SPDs must withstand continuous DC voltage and handle unidirectional surges.

Before selecting surge protection devices for your electrical system, it’s better to identify the type of power system you have and understand whether your system operates on a TN, TT, or IT system.

Voltage rating

For AC SPDs, typical voltage ranges span from 120V to 480V, making them suitable for connection to public utility grids. These devices are commonly deployed in residential buildings and industrial environments to safeguard against voltage surges.

DC SPDs are specifically engineered for solar systems, accommodating voltages ranging from several hundred volts up to 1500V, depending on the size and configuration of the system. These SPDs are tailored to the unique requirements of solar installations, providing effective protection against voltage transients induced by various factors such as lightning strikes and grid fluctuations.

Application

Determine the specific application and environment where the SPD will be installed. AC SPDs are commonly used in buildings, facilities, and traditional power distribution systems, while DC SPDs are often deployed in applications such as solar photovoltaic (PV) systems, battery storage systems, and telecommunications infrastructure.

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