Direct current systems that produce, consume or store energy are increasingly being used. As a result, the demand for accurate measurement of DC systems is increasing. This requires a specialized approach.
The need to deploy more renewable energy, the exponential growth in power consumption for data traffic and the Internet of Things, and rapid growth in the number of electric vehicles are causing a huge increase in the use of direct current (DC). This is further driven by, among other things, an increase in the number of Data Centers and 5G applications, UPS management in substations, and AC/DC rectifiers for rail networks.
There are two main types of DC generation:
- DC sources that produce ideal or near-ideal DC voltage, such as PV, DC generators, and battery storage.
- AC/DC conversion (single-phase or multi-phase rectification) that produces DC voltage with voltage ripple caused by pass-through from the AC input to the DC output.
Just like with low-voltage and medium-voltage AC distribution networks, Power Quality is essential for ensuring the performance of DC distribution networks and integrating new energy sources into the network. Poor Power Quality in the DC supply can lead to malfunctions or damage to electrical and electronic equipment, with serious economic and technical consequences.
A recent study, Power Quality in DC Distribution Networks, from the University of Cantabria (Department of Computer Science and Electronics), maps several specifically DC-related Power Quality phenomena. These largely correspond to the Power Quality phenomena we see in AC networks. According to the study, the committees responsible for the EN 50160 and IEC 61000-4-30 standards would recommend mapping the following Power Quality parameters in Low Voltage DC distribution networks:
- Voltage dips
- Voltage surges
- Voltage fluctuations
- Voltage ripples
In AC and DC networks, voltage dips, voltage surges and voltage interruptions are caused by interference, or sudden changes in voltage load. In DC networks, this can result in greater damage than in AC networks due to the lack of the reactive part of the impedance – in AC networks, this can mitigate the effect of voltage problems.
IEC 61000-4-29 defines test methods for immunity to voltage dips, short interruptions, and voltage variations on the DC power port of electrical or electronic equipment. This standard applies to low-voltage DC power ports of equipment with power supplied by external DC networks. However, no standard requirements have been defined (yet) for the tolerance of equipment to voltage dips, voltage interruptions or voltage surges in LVDC networks, except for electrical and electronic equipment used in railway, aircraft, and telecommunications applications
In AC as well as DC networks, Rapid Voltage Changes may occur (RVC). In DC-networks this can be caused by switching on equipment such as drives, generators, cutting off power, and voltage fluctuations. This can affect the performance of electrical and electronic equipment and possibly even cause flicker. In the case of PV generation, rapid changes in the sun’s intensity can result in variations in PV power right away, and therefore rapid voltage changes.
Voltage ripples, where DC voltage is not completely flat, are caused by a small AC voltage on top of the DC voltage. The main sources of voltage ripples in DC networks are AC/DC conversion, battery charging, and equipment that absorbs pulsating currents. Ripple in DC voltage should be measured and reduced to avoid interference of electrical and electronic equipment connected to the networks. Voltage ripples can cause heat and losses and interfere with screens and audio equipment.
The importance of measurement
We’re predicting a huge increase in the demand for accurate measurements in DC grids. But that’s only the first step – data needs to be interpreted in a meaningful way and followed up with practical advice. The emergence of direct current grids introduces several challenges. The NPR 9090 standard covers DC installations for low voltage and the international standard development at IEC, and NEN 1010 part 8 looks at prosumer installations. However, to date there no standardized measurement methods or Power Quality measurements for DC. Without extensive knowledge, measuring and analyzing makes little sense! HyTEPS can perform DC measurements, but also carry out analyses and suggest improvement opportunities. We can do this for railways, solar panels, battery systems, telecommunications, and for the maritime sector, where we perform, for example, AC + DC arc flash and selectivity calculations on yachts.
