On DC Injection to AC Grids from Distributed Generation
SAMUELSSON Olof
On DC Injection to AC Grids from Distributed Generation Lars Gertmar *,**, Per Karlsson *, Olof Samuelsson * *
**
LTH/IEA, Lund University, Lund, Sweden Phone: +46 46 222-9290/ -9841/ -7504 E-mail:
[email protected] URL: http://www.iea.lth.se
ABB AB, Corporate Research, Västerås, Sweden Phone: +46 21 32 31 31
Keywords Distributed power, Renewable energy systems, Generation of electrical energy, Converter grid interactions
Abstract Distributed Generation, DG, introduces multi-generator grids and new modes of operation. DG will likely introduce power electronic, PE, converters on a large scale at the low- and medium-voltage levels. DC injection into the AC grid is a threat from grid-connected PE converters that are badly structured. Low DC currents in AC current are difficult to measure at those low levels decided in IEEE™1547 and discussed in other international working groups. Transformers and PE can be codesigned to eliminate DC injection. Modern adjustable-speed drives, ASDs, especially, those with active rectifiers, form a basis to discuss and solve issues of DC Injection to AC Grids from DG also named Distributed Energy Resources, DER or even DR. DC components from power electronics, embedded in Solar power, Wind power, Energy storage, Smart houses, Smart-office buildings, Own generators, etc. will give rise to grid-aspects like AC components give frequencies around the fundamental one. FMEA, Failure Mode and Effects Analysis, is judged to be a tool for further synthesis.
Introduction Power electronics is today rare in electric power generation systems on large scale. Fig. 1 [1] depicts in principal the power flow in the traditional, cost-effective interconnected combination of generation, transmission, and distribution components that make up an electric power system. Today, this is based on a few central power stations, on AC transmission and distribution, T&D, with standards 50 or 60 Hz frequency and on electro-magnetic components with almost negligible power electronic conversion.
Fig. 1: Principal power flow in systems for traditional generation, transmission, and distribution, [1]
EPE 2005 - Dresden
Distributed Generation, DG [2] introduces multi-generator grids and new shapes, Fig. 2 [3], and new modes of operation. DG will likely introduce power electronic converters on a larger scale in the interconnected power systems, espy at the low- and medium-voltage levels. For grid-connected power electronic converters, inverters as well as active rectifiers, one foresees injected DC components in standard frequency AC electric power distribution grids to become an essential issue [4] [5] for IEEE standards and testing [6]. The phenomenon is named “DC Injection”, describing an act of forcing DC into the traditional AC grid.
ISBN : 90-75815-08-5
P.1
On DC Injection to AC Grids from Distributed Generation
SAMUELSSON Olof
Fig. 2: Traditional transmission lines for main-flow and dashed, tentative ways for “energy internet” flows with DG, [3] The power flows in the power systems today, shown in Fig. 1 and to the left in Fig. 2, are almost free from harmonics and nearly unidirectional, espy at the low- and medium-voltage levels. The popular press picture in Fig. 2, shows that a major part of the power flow will be the traditional transmission and distribution grid concept with thermal power stations. There are also embedded generators1, indicated at some buildings. Hydro power stations will of course also remain as a major source of electrical power generation. Large-scale electrical power generation is today based on electromechanics and constant-speed drives, with marginal power electronics. Thyristors are used in fieldcurrent control. Worldwide today, around 2/3 of the electrical power production is based on fossil fuels so that 5/6 of the electrical energy comes from thermal power stations, [7]. Gas is an energy source that is established on large scale all around the world. Gas is used for industry, transportation, heating and cooking and has a strong position in infrastructure and a potential to be utilized more for electrical power generation and better system efficiency. Gas is foreseen to be an expanded source of energy for combined heat and power, CHP, with distributed electrical power generation with tens to hundreds of kilowatt. It is obvious that power generation on a large scale, with distributed generation systems like new renewable electric energy sources, will need power transmission systems to avoid heavy costs and environmental stress from electro-chemical energy storage. Gas possesses its own transmission and distribution system which will be a cost-effective companion to the new renewables like wind and solar power for fungible electric power supplies in the future. Gas is a simple and efficient source of energy that can be stored as distributed units at embedded generating units (“own generators” in Fig. 2) for dispatched power production with energy storage as fuel as well as heat. So, there will be a clear long-term challenge to combine knowledge of electrical power generation, transmission, distribution/collection, utilisation, industrial automation and the like, in order to embody the dashed lines indicating the power and information flows in the central and right parts of Fig. 2. In this paper, it is challenging with power electronics and the issue of DC injection in the AC power network. 1 A large number of manufacturers make diesel-engine plants. These are by number the majority of plants made today. They are installed around the world, especially in small grids in the rest-of-the-world, ROW, sometimes appearing as DG combinations with new renewables and fossil fuel as energy sources, e.g., as wind-diesel or solar-diesel.
EPE 2005 - Dresden
ISBN : 90-75815-08-5
P.2
On DC Injection to AC Grids from Distributed Generation
SAMUELSSON Olof
The philosophy behind a synthesis of DC injection as an aspect of distributed (energy) resources, DR/DER, and, in particular, DG = distributed (electrical) generation, within EPE is:
Conversion from storable and non-storable energy sources into heat & cold and contemporary electrical power production for best utilisation of limited energy resources.
There is a growing awareness and a concern amongst students about environmental issues. R&D on distributed electrical generation is nourishing this awareness and the skills taught makes the student to realize that she or he actually can contribute to a sustainable world, as an electrical engineer. And, also very important, there are jobs.
The PE possibilities in the dashed, tentative ways for “energy internet” flows in Fig. 2. For cost-effectiveness and easy implementation, they are here assumed to be close to the pristine AC grids, improved with industrial automation to embody the dashed lines indicating the flows of not only power and energy but also flow of information.
There are other PE possibilities and aspects in the flows in Fig. 2. The ways of AC power can behave quite different when PE converters are directly connected in majority at a future AC grid and directly connected rotating electrical machines are in a minority at the same grid. This issue is covered in an accompanying EPE 2005 paper from LTH/IEA, [8]. Hosting capacity of future distributed generation with regards to levels of DC injection, harmonics and low-frequency variation in fundamentals, flicker, form thus issues within an area called Power Quality, PQ, [9] [10]. DC injection, the title of this paper, has so-far not been fully recognized in PQ, in spite of its PQ-position above harmonics and flicker in IEEE™1547, [4]. The actual DR/DER2 standard from 2003 prescribes that “the DR and its interconnection system shall not inject dc current greater than 0.5 % of the full rated output current at the point of DR connection”. According to an EU-DEEP, EUropean Distributed EnErgy Partnership, meeting [11], activities are ongoing to determine allowable grid penetration of DG/DER, espy for domestic units
