Power Quality in DC Power Distribution Systems and Microgrids

Overview

The paper asks what DC power distribution demands in terms of power quality. The method is a literature survey: four sample DC architectures (data centres, residential and commercial buildings, telecom, high-voltage DC collector systems) first show why power quality matters, AC power quality is briefly summarised, and then the literature on DC-specific issues is compiled. There is no new measurement or experiment.

In the data-centre example, Intel's analysis concluded that facility-level 400 V DC distribution may be the best option, with end-to-end efficiency close to 73%. Reported advantages of 380 VDC over 48 VDC include 15% lower up-front capital cost, 33% less floor space, 36% lower lifetime cost and 20 to 100 times less copper; 23 installations worldwide were rated at 380 VDC at the time.

The DC-specific problems are these. DC bus fault current comes only from converters, storage and bus capacitance and is therefore limited by converter ratings, which makes overcurrent protection that separates faults from heavy load hard to set. Without current zero crossings, DC arcs extinguish more slowly, and series arcing faults can become self-sustaining and hard to detect because no current surge occurs. Converter EMI-filter capacitance can draw inrush large enough to weld connector contacts.

Limits: the article dates from 2015 and reflects the literature of that time; it does not cover high-voltage (800 VDC) rack architectures or current standards. The conclusion is clear: DC systems do not automatically avoid power-quality problems and need explicit design for inrush, fault detection and grounding. It also notes a shortage of technicians experienced with DC distribution above 60 V and a need for standardisation.

Key points

1. Intel's analysis concluded that facility-level 400 V DC distribution may be the best option and put its end-to-end efficiency close to 73%.
2. Reported advantages of 380 VDC over 48 VDC: 15% lower up-front capital cost, 33% less floor space, 36% lower lifetime cost and 20 to 100 times less copper; 23 installations worldwide were rated at 380 VDC at the time.
3. DC bus fault current comes only from converters, storage and bus capacitance, so it is limited, and protection that distinguishes faults from heavy load is hard to set.
4. Without current zero crossings, DC arcs extinguish more slowly; arcing series faults can become self-sustaining and hard to detect because no current surge occurs.
5. Converter EMI-filter capacitance can draw inrush large enough to weld connector contacts, so pre-charge or soft-start circuits are recommended; adding capacitance to raise fault current also raises inrush.
6. For 380/400 VDC, IT grounding with a high-resistance mid-point connection limits line-to-ground fault current and halves voltage to ground compared with TN-S; 48 V telecom systems ground the positive pole (−48 V/0 V).

Using it in design

• Make inrush control (pre-charge, soft start) an explicit design step in DC distribution, and assess contact-welding risk in connector and contact selection.
• Validate fault detection and protection settings on DC buses on the basis of converter-limited fault current, and include series-arc detection.
• Document the grounding choice for 380/400 VDC (TN-S or high-resistance mid-point IT) as a reasoned design decision.

Who should read it

Electrical designers evaluating DC distribution, microgrids and 380/400 VDC data-centre architectures.

Document contents

1. 1. Introduction
2. 2. Sample DC Architectures (2.1 Data Center Challenges and Approaches towards Improved System Efficiency, 2.2 The Role of DC Microgrids within Commercial Facilities and Residential Homes, 2.3 DC in Telecommunications Power Systems, 2.4 Transmission Level DC Grid Applications)
3. 3. AC Power Quality (3.1 Harmonics, 3.2 Disturbances)
4. 4. Power Quality in DC Distribution Systems and Microgrids (4.1 Harmonic Currents, 4.2 Inrush Current, 4.3 Fault Current, 4.4 Grounding)
5. 5. Conclusions

Related standards

• ETSI EN 301 605

Read the document

Publication details

Authors

Stephen Whaite; Brandon Grainger; Alexis Kwasinski

Publisher

Energies (MDPI AG)

Year

2015

Volume

8

Issue

5

Pages

4378-4399

Document language

English

Source type

Scientific

Access

Open licence — downloadable

DOI

10.3390/en8054378

Licence

CC BY 4.0 — share with attribution

Licence evidence

www.mdpi.com

Citation