Data Center Power: How AC and DC Power are Used in Data Centers?

20 Apr 2020 by Technology

I think we can all agree when I say that power is one of the most important, if not the most important aspects of a data center when discussing colocation and hyperscale data centers. Today more than ever, topics such as power density, power usage effectiveness (PUE), and renewable energy dominate industry panels and discussions.

In this article, we will take a look at how Alternating Current (AC) and Direct Current (DC) power is used in the modern data center. We will also look at future power trends including the shift to making data centers more efficient using DC power. If that sounds interesting, you should definitely continue reading this article.

Difference Between AC and DC Power?

Before we get started, what is the difference between AC and DC power? There are many differences but to keep things simple, direct current is linear and alternating current alternates. Sounds straightforward enough. This means that with DC power, the current always flows in the same direction and does not oscillate between positive and negative terminals.

With AC, the current reverses 60 times per second (U.S.) or 50 times per second (Europe), and the voltage is easily transformed. This makes AC power easier to transport over several miles. With DC power, it’s not as easy to transport over long distances.

How Data Centers Use AC and DC Power?

Do most data centers use AC or DC power or both? To answer this question, lets first look at how power is used in the data center.

Data centers receive AC power from a utility provider or municipality’s electrical grid. Utility power is feed to the Automatic Transfer Switch (ATS) and into the switchgear. The switchgear is configured for critical supplies and non-critical supplies. It is used to Switchgear is also used for switching on and powering transformers. The transformers ensuring that the AC power from the grid is the right voltage and current type. This is referred to as a step up or step down in power.

Power from the transformer is transferred to the Main Distribution Board (MDB). MDBs are enclosures that house fuses, circuit breakers, and ground leakage protection units. The purpose of the MDB is to transfer low-voltage electricity and distribute it to various endpoints within a data center. This includes the Uninterruptible Power Supply (UPS) system.

UPS systems have several purposes within the data center. First, they distribute clean electricity by conditioning the AC power to ensure that electrical issues like power surges do not impact IT equipment. Clean power from the UPS is distributed to a number of circuit breakers. Individual circuit breakers are tied to individual power circuits that are delivered to specific colocation racks. Servers, storage devices, network hardware, and other IT equipment plug into rack-mounted power strips that connect to the power circuit.

In addition to power conditioning, UPS systems are primarily used for storing electricity in batteries. These backup batteries closely resemble car batteries. There can be tens or even hundreds of backup batteries within a single UPS depending on the size of the system. The electricity stored in the batteries is DC power. Inverters are used to convert AC power into storable DC power.

When a power outage occurs, the UPS system uses the power inverter to convert stored DC power from the batteries to AC power so the data center can remain operational. This includes powering servers and related IT equipment in colocation racks as well as mission-critical systems such as chillers, air conditioning units, fire suppression, lighting, and other systems. The amount of power to systems can be limited or turned off completely to save power.

How long can a UPS system power a data center? The answer is long enough to start up the backup diesel generators and initial a power transfer from the grid to the generators. We’re talking somewhere between 10-20 minutes in most cases. At that point, the ATS is responsible for sensing when there is a utility power failure and transfers the load from UPS to the backup diesel generators.

Diesel generators supply the data center with its power, delivering energy as alternating current (AC), just like the main power grid. Backup diesel generators serve a vital purpose and that is to keep the data center operational for a longer period of time. The length of time is dependent on the amount of onsite diesel fuel storage and the delivery of fuel which is referred to as fuel contracts. This can be hours, days or weeks.

Can DC Power Make Data Centers More Efficient?

Much of the industrialized world is based on AC power. However, there is a growing interest in using DC power sources in a variety of commercial applications. DC power is the product of sustainable energy sources such as photovoltaic solar panels, wind energy, fuel cells, and microgrids. That power is converted from DC to AC for use in our homes, commercial buildings and data centers.

Telecom carriers like AT&T, Verizon, CenturyLink, and others have long used DC power in their central offices. In fact, nearly every telco central office houses a 48V DC plant to provide power directly to its telecommunications equipment and UPS systems.

That’s not all. In 2016, Google announced the development and use of a 48V rack solution. Google also announced that it was working with Facebook and others to further the development of a DC power within the Open Compute Project. Could this be a sign of what’s to come? Can data centers increase their power efficiencies by using DC power?

Here’s where it gets interesting. With AC power, we know that it is possible to transmit power great distances with very little loss. With DC, it is not easy and there can be major losses during transport. However, advances in DC power technologies have made it easily regulated with compact integrated electronic circuits and power electronics which makes it more efficient and accurate. This is especially true with low <50V and industrial voltage up to 1000V.

In addition, energy transport in a data center is confined to the data center facility itself. This makes the concept of transport losses less critical. There is a lot of attention and discussion surrounding low and medium voltage power systems as well as ultra-low-voltage networks. The logic is that If the semiconductor losses in converters are reduced, the total system losses are decreased when DC is used and that DC power leads to better utilization of high and medium voltage transformers. This allows for an increase in demand without changing the transformer. This has the ability to remove sequential AC to DC and DC to AC power conversions.

Conclusion: Advancing Data Center Power Efficiencies

Colocation providers, cloud providers, and data center owners and operators will continuously explore different paths to make their facilities more efficient from a power perspective. It is in their best interest to do so as power is one of the largest costs for data centers.

Advancements in energy sources such as renewables will also have a profound impact on the industry as the demand for data centers surges. Could DC power play a greater role in powering the data center? I’ll leave that question up to you to answer.

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