Effect of Time-constant on Performance of HVDC Contactor

What is a time-constant?

In an RL circuit, the time-constant T = L/R determines the rate of current change. The larger T is, the slower the current changes. The role of the inductor in the circuit suppresses the change in current. When the circuit is connected, the inductor inhibits the rapid rise of current, causing it to increase slowly; when the circuit is disconnected, the inductor continues to supply power, making the arc between the moving and stationary contacts of the electrical switch more persistent and harder to break, significantly shortening the electrical life of the switch.

In an RL circuit, the time-constant T = L/time-constant is agreed upon in GB or IEC electrical standards

Taking the Figure (1) EVQ100 product of ALQ Company (ALQ) as an example, this product belongs to the high-voltage DC contactor series. It is widely used in electric vehicles, AGVs, buses, golf carts, mining trucks, electric forklifts, electric stackers, electric excavators, electric loaders, electric ships, DC charging stations, chargers, shore piles, and other charging infrastructure, as well as in power battery packs, energy storage converters, high-voltage boxes, photovoltaic inverters, 5G, UPS, and other DC electrical equipment. It is used to connect and disconnect DC circuits, providing reliable isolation after disconnection.

Figure (1) EVQ100 product

For ALQ Company (such as ALQ high-voltage DC contactor products, which must comply with national standard GB14048.4 or international standard IEC60947-4 in design, certification, and type testing), product tests for connection and disconnection performance should be conducted according to the DC-1 standard. Please refer to Figure (2) EVQ100 product description, and Figure (3) the connection and disconnection conditions of different types of products. From the above figures, it is clear that when EVQ100 products undergo certification, the time-constant L/R = 1ms.

Figure（2）EVQ100 product description

Figure (3) the connection and disconnection conditions of different types of products

The convention of time-constant in the practical application of HVDC contactor

The load type of the DC-1 can be obtained by querying the IEC60947 standard，See Figure（4）Examples of utilization categories for low-voltage switchgear and controlgear. It shows that DC-1 belongs to Non-inductive or slightly inductive loads, resistance furnaces.

Figure（4）Examples of utilization categories for low-voltage switchgear and controlgear

Even so, under DC-1 conditions, L/R=1ms can have a severe impact on the electrical life of high-voltage DC contactor products. At this point, the electrical life will significantly decrease compared to pure resistive loads, potentially by about ten times. The key issue is that most application scenarios for high-voltage DC contactors involve resistive loads. This presents a problem: the time-constants used in certification tests differ markedly from those encountered in actual product use. To address this, distinct from IEC standards, an "industry standard" has emerged in industry applications, which uses pure resistive loads. Taking EVQ100 as an example, Figure (5) EVQ100 product manual shows, where the electrical life performance of the product is primarily calibrated using resistive loads. Considering the real-world conditions of test equipment at the laboratory, most devices inherently have some inductance, not pure resistance, thus determining a parameter like L/R≤0.1ms.

Figure (5) EVQ100 product manual

How to achieve the performance parameter of higher time-constant in the design of high voltage DC contactor?

With the wider application of HVDC contactors, some working conditions will require higher time-constant, which requires new products to take relevant technical measures to improve this performance in design.

Based on the study of time-constant, it has the following impact on products:

1. Influence on arc extinction: When the contactor breaks the circuit, the energy stored by the inductor will be released through the arc. When T is large, the current attenuation is slow, the arc energy is high and the duration is long, increasing the difficulty of arc extinction.

2. Influence on product contacts: Frequent operation in high T circuits tends to accelerate contact wear.

Therefore, in the design of high T products, the following aspects can be considered to strengthen:

1. Take stronger arc extinguishing measures such as magnetic blowing, vacuum arc extinguishing chamber or gas arc extinguishing.

2. Increase the opening distance of moving and static contacts.

3. Increase the space of the arc extinguishing chamber.

4. Connect the relevant components in series with the circuit inductor in the main circuit.

The technical development of HVDC contactors is iterated with the development of application industries and equipment. HVDC equipment has not only resistive loads, but also inductive, capacitive and mixed loads. For the corresponding industrial conditions, special types of HVDC contactors are required to match the HVDC equipment to work more long-term and reliably.