WP3: Multi-Terminal AC-DC Grids and Power Electronics


Innovation Challenges

The new distribution and transmission grid as described in the WP1 and WP2 sections, requires the deployment of innovative technologies that will enable the operation of the grid. Multi-terminal DC transmission is not yet commercially used, but it is anticipated to be a relevant option for the system expansion or reinforcement in the near future. Additionally, the principles and technologies for isolating and accurately detecting the faults clearly differ from existing AC solutions.

Three classes of High Voltage Direct Current (HVDC) breakers have been discussed in the literature: mechanical, static and hybrid. Fast measurement and fault detection systems for HVDC are not yet available and none of these principles has yet been demonstrated at the required ratings. Similarly, fault-tolerant HVDC converters have been proposed, but they are not used as current commercial products.

Further research activities are also required in modular power electronic converters while in mixed frequency insulation material, the effects of the dielectric stress distribution will become an increasing hindrance in the future grids. However, our current understanding of these effects is limited.


The main WP3’s objectives are related, but not limited to, High Voltage Direct Current (HVDC) line design, operation, fault location and optimization. Attention will be paid on the robustness of the deployment of Multi-terminal DC transmission in order to minimise technical risks and to ensure the interoperability of these systems. 
This WP comprises:
  • multi-terminal HVDC system design and operation;
  • fault detection and clearing in multi-terminal HVDC;
  • enabling technologies.
This work will involve the improvement of the fault behaviour and topology of HVDC converters to prepare them for use in multi-terminal DC grids along with the identification of the limit performances of existing alternative HVDC circuit breakers. A main deliverable of this WP will be the validation of a modular power conversion architecture along with its associated control structure for increasing flexibility and performance of the future Swiss electricity network.

WP3’s objective include:

  1. Standardisation work (including procedures) allowing to accelerate Switzerland’s connection to multi-terminal DC networks,
  2. Facilitation of the emergence of a common DC fault management concept for the European interconnected network,
  3. Investigation of key power electronic technology issues to be solved in order to establish cost efficient and reliable converters for transmission applications.


Storage system in MV for grid auxiliary services

Innovative interface of storage systems for MV applications, making use of modular converters and avoiding the 50Hz main transformer

Academic partner:   HES-SO VD, HES-SO FR   ( Mauro.Carpita@heig-vd.ch )   Industry partner:  Leclanché
Funding:  SCCER-FURIES; EOS Holdings; HES-SO; Leclanché   Project duration: 2015-2017 (1.5 years)
Utility grid system operation requires network services. Nowadays they rely heavily on large power plants. However, centralized production is gradually replaced by decentralized production facilities (IPDs).
This will result to additional costs for future network operations (DSO). IPDs capable of providing these services would increase the economic efficiency of distributed generation and would offer an opportunity to aggregate these services for the DSO.
The project has two main objectives:
1) to undertake theoretical studies on the most promising ancillary services
2) to develop and test a storage system based on a Modular Multilevel Converter (MMC) structure
For this purpose, the ancillary services selected include:
1a ) with/without less conventional generators (rotary) and more storage, without degradation of quality of service.
1b) including a large number of players without overloading the control centers and/or communication systems.
Also, for the tests a Statcom converter was developed based on a MMC in reduced scale, designed to interface a battery with the average voltage network without 50Hz transformer. Also, a suitable lay-out of the Gridlab was designed and configured to one or more scenarios consistent with the theoretical studies, including the Statcom MMC and possibly a serial compensator.
As a result, operational studies according to objectives a) and b) were finalized; a MMC converter was developed; and the serial compensator (LVSR) was adapted to the needs of the project. As a last step of this project, the relevance of these solutions to the revision of the Electricity Supply Act ( LApEL ) will be studied.
These results will support DSOs to develop a strategy for managing future network services in case ofa high penetration of IPD.
Next steps
Test of the serial compensator and its potential to solve current problems with DSOs.
Figure 1 – Case study: LV of District of  Corminboeuf  (FR)
Figure 2 – MMC laboratory set-up

Fault clearing in multi-terminal HVDC

Novel test source for flexible pulse-current creation for circuit breaker testing
Academic partner:     ETHZ (HVL, HPE) ( jbiela@ ethz.ch )     Industry partner:   ABB AG
Funding:  SCCER-FURIES;  private industrial funding     Project duration: 2014-2017 (3 years)


For the safe operation of future HVDC multi-terminal grids, HVDC circuit breakers are a key component. Advances in HVDC switching technology will enable the Swiss energy industry to take a leading role in the world-wide transition from fossil to renewable energy sources.

While first HVDC networks are already in operation, hybrid/DC circuit breakers are still in the prototype phase.
The presented project aims to identify the capabilities of different HVDC circuit breaker topologies and investigate optimization potential.
In the scope of this project, short circuit current interruption in multi-terminal HVDC systems were investigated over a broad range, from grid level to individual components, combining expertise in power system, power electronics and high voltage engineering. The conducted simulations showed the impact of grid topologies on the development of fault currents. Furthermore, different power electronic circuits for enabling a fast current breaking in a mechanical switch were simulated and compared. On the component level, a model gas circuit breaker was developed that is used to investigate the interaction of mechanical switches and power electronic devices.
In addition, a novel test source is developed to allow flexible pulse-current creation for circuit breaker testing.
The novel test current source is commercialized in cooperation with a Swiss industry partner. It is expected that leading test labs are interested in purchasing these test sources. Similar designs can be used to operate sources and magnets in accelerator technology of particle physics labs.
Figure 1 – Model gas circuit breaker to investigate the interruption behaviour in HVDC applications.
Figure 2 – Unipolar Arbitrary Current Source for Hardware in the Loop Tests 
Next step
In the future, further simulations regarding the optimization of HVDC circuit breaker topologies are planned and new concepts for hybrid circuit breakers based on power electronic converters and mechanical breakers are investigated. In parallel, the test bench for the built model gas circuit breaker is upgraded to investigate its interruption capability and its interaction with power electronic circuits.


Dry-Type Insulation Systems under Mixed-Frequency MV Stress

Alternative dry-type insulation material featuring high potential with respect to MF stress resilience
Academic partner:      ETHZ  (LEM, HVL) ( franck @ eeh.ee.ethz.ch );  FHNW (Prof. Schulz)     
Funding:  SCCER-FURIES;   NRP 70       Project duration: 2014-2018 (4 years)


The recent developments in Silicon Carbide (SiC) solid-state switches open up new possibilities in medium-voltage (MV) power electronic converter design and applications. Such solid-state transformers (SSTs) are promising alternatives to conventional low-frequency distribution transformers in certain applications, providing active grid-grid, grid-load or grid-source interfaces linking different voltage and/or frequency levels (including DC). However, comparatively higher power loss densities and the broadband medium-frequency (MF) voltage stress raise issues with respect to the endurance of the concerned insulation systems due to the temperature and frequency dependence of the (di)electric properties and the active degradation mechanisms.
This project aims at combining inputs from the physics of dielectrics, electro-thermal stress analysis simulation and experimental investigations to identify and quantify the active degradation processes and use this knowledge to develop design criteria for resilient insulation systems exposed to the stresses occurring in MV MF transformers.
As a results, a test bench was built up for laboratory investigations of the effects of MF voltage stress; a arbitrary waveform dielectric spectrometer was setup for offline aging diagnostics; and a virtual prototyping software for MF transformers (thermal, (di)electric, magnetic) was developed.
The innovation of this approach lies on the identification of an alternative dry-type insulation material featuring high potential with respect to MF stress resilience; the innovative insulation concepts for common/differential mode separation in MV insulation and electromagnetic shielding of MF transformers; and the accurate, fast, and comprehensive method for the evaluation of the dielectric losses of materials subject to fast transients.
Fig. 1 – MF/MV transformer used in a SST. 
Fig. 2 – Developed test bench for investigations of specimens under various thermo-electric stresses and electrode geometries. 
Next step
Test bench and dielectric spectrometer will be used to assess voltage endurance and aging of potential dry-type insulation materials in various electrode configurations.
Also, MV prototype of a 10 kV/50 kHz MF transformer with an insulation coordination will be designed for fast transients.