DC Power Quality assessment on real MVDC and LVDC power systems

Development of HVDC standards started many years ago, but standardisation of MV and LV DC voltage and power quality issues is just being launched by several IEC Technical Committees coordinated by IEC System Committee on Low Voltage Direct Current. Therefore, only a few general standards and technical reports are currently available to guide actual MVDC or LVDC projects.

MVDC and LVDC systems are very different from HVDC systems: (i) HVDC systems have practically no customer load directly connected to DC, they feed loads only through DC/AC converter stations; (ii) while on MVDC and LVDC distribution grids, DC customers and DC/AC stations co-exist with common coupling points (PCC). Thus, the technical and economic issues arising in DC distribution grids are specific, in particular regarding the power quality (PQ) matters related to utilities, customers and equipment suppliers. On the end-user side, the power supply units of all the recent LV electric household appliances and IT devices have almost the same electronic structures. The difference between AC and DC fed equipment is roughly being with or without rectifier stage. Today, some DC appliances are already available on the market. Although these new devices are produced in small series or at a demonstration stage, DC PQ and electromagnetic compatibility (EMC) issues should be taken into account to support the fast development of local DC power distribution systems and various demonstration projects all over the world. The development of DC PQ standards needs relevant technical researches and assessment methods.

Power quality issues of AC power systems have been well covered by relevant international standards and grid connection contracts. Based on the AC PQ knowledge, this paper focuses on the researches of steady state DC PQ assessment methods, the recommendation of relevant DC PQ indices and computing methods. For power quality assessment in distribution grid, the integration of DC PQ indices in frequency domain simulation software is studied and verified by simulating equivalent grids of actual DC installations. Based on site recordings from 4 actual MVDC/LVDC projects, DC PQ indices are computed in order to quantify actual PQ levels and understand some recorded DC events.