Innovative Thoughts

Unconventional thoughts offsite regular paths and according research can be helpful and are often even necessary for innovations. Whether new or resumed ideas can be developed to substance and eventual application is not clear in the beginning. This is the nature of R&D. Ideas which were developed and published in the emerging and growing field of renewable energy integration can become obsolete through newer, better ideas and solutions. The material of the present web site was permanently developed, also by stepping into unknown terrain, particularly so for the students doing their thesis work. The documents reflect these developments. Much work and results come out from reflection, discussion and trials. This way of work is shown throughout the documents. Besides text, figures and charts simulation files are included. They should be of value for own studies on the respective topics.

 

An essential portion of Grid Modernization Work relies on power electronic equipment as used in

• HVDC transmission

• and Grid-Tied IBR (Inverter Based Resources) for direct renewable energy infeed and battery energy storage systems (BESS)

HVDC systems transport bulk power, from offshore to onshore and over long distances between asynchronous networks or as embedded systems in parallel to existing High-Voltage AC transmission lines. AC lines are tapped with power transformers and switching substations. AC lines can collect power from various renewable resources distributed over the entire power grid. 

 

That is not possible with DC lines, except for some special case where the DC line is tapped with a converter substation. Power from wind farms or PV plants would then be collected in the AC grid and injected via a third converter station into the HVDC transmission. That can be a parallel tap - there exist such systems called multi-terminal systems - or a series tap, exists up to now only as an idea.

 

Another idea is to mix AC and DC current on a three-phase AC line. This had the advantage of simple RES connections at several places along the three-phase AC line while at the same time transmitting DC power over the longer distance. The advantage of such mixed current systems would be that the full thermal capability of the line can be harnessed, even at very long transmission distances. Investigations of such systems were from time to time pursued, but they never became a viable alternative despite the possible envisaged economic advantage. In countries with developing industries, population growth, vast distances and renewable resources like hydro power, wind and solar the mixed current transmission deserves certainly a closer look. The allocation of many RES along a mixed current transmission system could be both economic and technically expedient regarding supply/demand and its control. AC substations could provide for power infeed and for load coverage along the line while the DC transmission covers end-to-end power transfer. Fault handling is a major issue in such concepts not yet studied sufficiently. Which are the critical cases and can controls and protection systems be designed to operate the system reliably and safely?

 

Other ideas relate to transformerless Classic HVDC and normal AC power transformers equipped Classic HVDC. Both reducing installation costs.

 

Further ideas: 

Classic HVDC controls which could be altered and expanded to operate stable under weak grid conditions. Or embedded VSC HVDC equipped with automatic power sharing controls for maximum efficiency and transient stability.

 

Example for successful "new idea" realizations:

Instead of building filter circuits in Classic HVDC systems for short circuiting the 2nd order harmonic current on the converter transformer line side an efficient and cost minimum solution is to detect current of fundamental frequency in the DC current and to generate a firing angle control signal to compensate the fundamental AC component. This eleminates the 2nd harmonic content in the AC current on the converter transformer line side. This solution was applied in the Blackwater HVDC system. It saved the contractor about 2 Million US$ which he had otherwise to spend for low order harmonic filters and the associated switchgear.

 

The electrical engineer working in HVDC station design must understand the functions and impact of non-electrical components for maximum power transfer. E.g., a wet-surface cooler determines the inlet water temperature of converter valves. To know the effect of dry heat and wet heat is essential and to build a wet bulb temperature dependent characteristic for the DC current order can increase the energy transfer. This was done in the Blackwater Converter Station.

 

The installation of gapless ZnO-arresters for converter protection was an important step for reducing protection levels, and thereby, of insulation levels which have a considerable impact on converter transformer costs. 

 

Knowing the difference of capacitive voltage dividers and magnetic voltage dividers regarding their function in switchyards is essential for avoiding wrong designs. A magnetic voltage divider discharges capacitor banks and filter circuits when being switched off. Several breaker reclosures in a row without time for cooling down is not allowed. So, the correct switching sequence of several available filter arms must be observed while maintaining at the same time the filtering performance, filter component stress and reactive power balance. This is in itself no new idea, but must be observed when other new ideas like in Blackwater were implemented. That was the case when designing an estimator for minimizing converter valve losses through continuous reactive power control via the converter and filter circuit and reactor switching. 

 

HVDC history showing how ideas shape the future

 

Power transmission started actually with DC in the last decades of the 19th century. But transmission distances were restricted to some km since the voltage was only in the range of some kV generated with DC generators. AC transmission took then over due to the invention of the three-phase AC system permitting the construction of AC generators and use of power transformers to transform the voltage to a feasible transmission level of initially several tens of kV. This made long distance AC transmission possible.

 

All power systems in the world were then built as AC systems. For sea cable connections and for very long distance transmissions the AC system had limits due to its reactive power / voltage behavior. Therefore, the DC transmission was developed around the mid of the 20th century. It used mercury arc valve converters for rectifying from AC to DC at the sending end and inverting from DC to AC at the receiving end of the transmission line.

 

As described above, the idea to use direct current for high voltage long distance transmission was developed and could be realized already about seventy years ago. The HVDC technology had relatively fast advanced in the first two decades after building the first commercial converter stations to bridge water ways like the Cross-Channel Link between GB and France, to transport power over very long distances and to interconnect asynchronous networks.

 

Power semiconductor converters containing controllable thyristor valves replaced mercury arc converter valves of the first generation HVDC in the seventies. Thyristor and valve developments were further pursued to increase the power rating and to obtain robust thyristor and valve control circuits. In the eighties of the last century the DC voltage rating was already 500 kV, the DC current rating 3.6 kA. So a bipole could transmit 3600 MW. Nowadays the power rating goes up to 10 GW.

 

In parallel to the worldwide application of the now so called Classic HVDC equipped with thyristors, the voltage sourced converter (VSC) was developed. It fits better to difficult grid conditions (low system strength) and voltage control requirements. The VSC uses insulated gate bipolar transistors instead of thyristors. Initially the bipolar power rating was 400 MW, now in 2024 2000 MW are possible. The latest development brought the MMC converter (modular-multi-cell converter) as a special VSC type.

 

An advantage of VSC based HVDC as compared to thyristor technology is its controllability also under difficult grid conditions, and that there are no commutation failures. The most essential property of the MMC is that it does not need any filter circuits. This is cost relevant - less space, less switchgear, no filter components, measuring and protection circuitry - and avoids detuning and connected voltage distortion. The MCC technology seems sufficiently mature to justify its large scale application. Nevertheless, the growing number of HVDC installations has made their stable integration into the AC power grid a complex task. In addition the deployment of distributed inverter based generation (IBG) adds to the complexity.

 

Power system frequency is a critical quantity which must be kept within narrow limits, transiently and in steady state. Up to now synchronous generators provide the necessary frequency stability. Reducing this type of equipment and adding static generation equipment renders a hybrid generation system which must be intelligently controlled to emulate the rotating masses of machines. VSC based HVDC is promising in taking over this task. However, some doubts with regard to an equivalent behavior as provided by synchronous machines, exist. These doubts relate to the grid forming property. It is clear that Classic HVDC has no such property at all, and, therefore, it can not be used for grid forming without complex additional circuitry. VSC HVDC can in principle be confered this property. Only in principle, since the converter's current limit imposes a different initial response as compared to synchronous machines. Parameters to be observed are the system strength at the HVDC's grid connection points, the grid impedance structure and data, and the distribution of other generating equipment in the grid. Possible viable configurations and ratings must be found out and specified grid wide.

 

This web site's content rests on the author's industry experience in HVDC and power systems, both also in research and university teaching. Colleagues and project partners contributed to the know-how gathered over time. Also exchange with students contributed. Study material is offered on the web. The material under "instructors and students" is currently transferred to another server for further access using https security. A screen shot is shown under "Instructors and Students" on this site.

 

The material contains aside of theory also excercises and simulator lab work conducted with MATLAB/SIMULINK and PSCAD/EMTDC. MATLAB was brought from a visit to UCLA Berkeley home to Frankfurt in 1996 and played an important role for the education of students and research at UAS Frankfurt. Later the OPAL RT simulator at Stockholm KTH could be used for collaborative diploma thesis work.

 

Through these digital simulators new ideas could be studied and disseminated on conferences and in papers. Innovation became possible for researchers at smaller not so well funded institutes. From then on they did no longer depend on access to physically large and expensive RT simulators as the early ones of ABB in Turgi/CH and the FGH (in Mannheim/Germany).

 

Proposals and ideas on circuit concepts and controls belong to innovation work. I.e., in general they are not yet confirmed, validated or in some way taken as design specification. It is the feature of innovation work to start with ideas and then to consolidate the ideas, if necessary also against contraire opinions. This is the path which we must take to reach the goal of a clean green energy future. The new power and energy age is only possible by adopting and further developing technologies, one can even say by embracing technologies, and not by rejecting innovations as sometimes experienced with improper arguments or by being too shy or hesitant for new suggestions. Old arguments, as the one that wind energy must be used to the maximum possible extent without participation in load-frequency controls, hampers advances. Rotating reserve meant always to have some output margin for covering generation deficits and outages. New possibilities and chances must be discussed in the community. Information exchange barriers in technical groups between members from different companies need to be built down through IP agreements.

 

The money for innovations must, of course, be available. I.e., the innovative spirit of commercial project partners is needed too. The situation - in September 2023 - regarding new energy fonds was not satisfying. After COP28, in December 2023, there was hope that all partners have recognized the urgency of common work for mutual benefit. Cop29 in 2024 did not live up to promises made before.