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Research & Development DC or AC - quo vadis electrical connectors?

| Author / Editor: Jonas Bachmann * / Florian Richert

Continuous operation with direct current already serves a purpose today in data centers. Energy efficiency is its main advantage. New DC connectors are currently being standardized at the device-side. Here we take a look at current developments and standardization activities.

DC connectors: DC connectors are now standardized for data centers. The GP21 from Schurter is the first commercially available device connector for 400 V DC systems up to 2.6 kW according to IEC TS 62735-1.
DC connectors: DC connectors are now standardized for data centers. The GP21 from Schurter is the first commercially available device connector for 400 V DC systems up to 2.6 kW according to IEC TS 62735-1.
(Source: Schurter)

By the end of the 19th century, the "electricity war" was the first war of format in industrial history. The advocates of alternating current (Nicola Tesla and George Westinghouse) fought against Thomas Edison, who advocated direct current (DC). As a result, alternating current prevailed at the time. A brief look at present-day developments shows that in the digital age, entire flocks of DC-powered devices are in use.
Telecommunications and the data industry are powerful drivers for making direct current viable again. It makes sense to start supplying a data center with a DC voltage. If the servers are already fed with direct current, it would only make sense to process it in the same way across the entire network. From the grid to the chip. The DC architecture contains significantly fewer components. As a result, it is cheaper and less susceptible to faults.

Direct current with better energy efficiency

The omission of various transformations and conversions results in significantly improved energy efficiency for DC. Calculations and studies (ABB, Amstein + Walthert, Stulz) show an efficiency increase of 10% from the feed-in to the server. A reduction of 15% is assumed for the pure investments for the electrical infrastructure.
In addition to data centers, there is a wide range of applications that is not yet estimable. An obvious example would be building technology - or smart- and microgrids. Particularly the last-mentioned are dependent on their power generators or storage units, which supply direct current, due to the possibility of self-sufficient operation. Examples of microgrids would be critical infrastructures, hospitals or operations centers. In developing countries, microgrids will play a key role especially in regions with difficult infrastructure.

AC operation versus DC standards

In the classic power range around 250 V alternating voltage, the device connectors are standardized according to IEC 60320, mature and in almost all respects fully developed. Although there are attempts at improvements and additional functions, there is no reason to expect anything groundbreaking - although higher operating temperatures certainly show potential for renewal.

Since August 2015, the technical specification IEC TS 62735-1 for systems up to 2.6 kW has existed on the power distribution side. The IEC TS 62735-2 standard was adopted in December 2016 for powers up to 5.2 kW, which may no longer be separated under load (arc). Standardization on the device side is also in progress. An additional solution on the network side will, therefore, be added on the device's side.
The WG8 workgroup of the IEC Committee TC23, which is responsible for extending the standards for electrical installation material, includes more than 30 experts from Europe, North America, and Asia. Efforts are being made in this workgroup to develop solutions for DC connectors based on the previous AC standard IEC 60320.

Coexistence of AC and DC connectors

Table 1: Overview of compatibility between AC and DC plugs/sockets according to IEC.
Table 1: Overview of compatibility between AC and DC plugs/sockets according to IEC.
(Source: Schurter)

The availability of pure DC components is still in its early stages. For success, it would undoubtedly be helpful if parts of the existing infrastructure could continue to be used. A server should not care whether it is fed with the alternating or direct current because technically this is no magic on the device side. New territory is broken with the plug-in connection because DC connectors differ technically in some points from their AC counterparts. Especially concerning the dangers of mechanical separation under load, which will be discussed in detail later.
When switching from alternating current to direct current, it is therefore primarily important that the hardware used can be operated at both 230 V AC and 400 V DC. To ensure that the supply in the device is tapped correctly and reverse polarity is prevented, there are new mechanically coded connector systems. Only the hybrid device plug accepts both AC and DC sockets.

Direct current: Description of the DC connector

In the higher power ranges up to 5.2 kW, pulling under load is not permitted due to the resulting arc. IEC 62735-2 specifies components with automatic or mechanical interlocking. This means that either the circuit is automatically interrupted early by a switch or similar when disconnected under load, or a mechanical switch ensures locking/unlocking and at the same time that the circuit is closed or disconnected.
The common pull-out fuses known from AC products according to IEC 60320 (V-Lock, SecureLock, locking clip, etc.) have hardly anything in common.
Device-side components such as cable plug and connector are currently standardized in IEC TS 63236. Currently, this is in the draft phase and is expected to be published by the IEC in 2021.

Conclusion

The continuous use of direct current in the data center makes sense, as the conversion losses are massively reduced. On the power side, the IEC has defined the framework with TS 62735-1. IEC TS 62735-2 was developed for a higher power requirement of up to 5.2 kW. The appropriate plug connections with the necessary safety interlock (cold switch) are currently being developed.
Lifecycles of the supply infrastructure (PDU) are generally longer than those of the devices to be supplied, which are replaced more frequently by technological advancements. Regardless of this, the aim now is to provide an AC/DC device plug that allows the device to be operated on both an AC and a DC supply. This allows the changeover from today's AC power supply to direct current as required. The only thing that has to be changed is the supply cables.

This article was previously published in German on the Elektronikpraxis.

* Jonas Bachmann works as Head of Development of Connectors at Schurter in Lucerne, Switzerland

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