WP1: Connection of offshore wind power to DC grids

This Work Package is led by UPC and has researchers, Marc Cheah, Muhammad Raza, Kevin Schönleber and Domenico Richiutto, based in Cardiff, UPC and ALSTOM.

The WP is advancing as expected and we are making progress on our objectives:

O1: Design and analyse the topologies of offshore DC grids. Some work has been done in the direction of designing, analysing and controlling offshore AC hubs comprising several wind farms and power converters. Next steps will include multiterminal HVDC systems and offshore HVDC grids.

O2: Determine steady state operation characteristics. The power converter operation characteristics have been identified and algorithms for analysis have been developed.

O3: Develop dynamic control systems for offshore DC grids. Appropriate models have been developed to analyse the dynamic behaviours of offshore AC hubs with offshore wind power plants.

WP1 Tasks

• Design of DC grid configuration – completed
• Economical/technical comparison – in progress
• Converter characteristics – completed
• Operating point determination – completed
• Steady state power flow programming – completed
• Wind turbine generation model – in progress (some relevants parts completed)
• Wind farm modelling – in progress (some relevants parts completed)
• Control system design – completed
• AC fault response – in progress
• Wind power reduction technology – in progress (some relevants parts completed)

WP1 milestones

• DC grids suitable for offshore wind determined – partially achieved
• Converter characteristics determined – achieved
• Algorithms for determining operating points developed – partially achieved
• Wind farm model available for use – partially achieved
• Wind power reduction method developed – in progress

WP1 Deliverables

• Various DC grid configuration; cost model technical index and comparison algorithm – deliverable pending, most of the work done
• Various operation modes of converters, V-I characteristics – deliverable pending, most of the work done
• Methods for determining the operating points – deliverable pending, most of the work done
• Controllers of wind farm converters of the DC grid – deliverable pending, important part of the work done
• Methods to reduce wind power output to avoid over DC voltage – in progress

WP2 Investigation of voltage source converters for DC grids

Cardiff leads this work package and WP members are Cinergia, KU Leuven and CEPRI, represented by researchers Jorge Gonçalves, Abel Ferreira, Robert Renner and Agustí Egea.

The main research objectives of the WP2 is to investigate the interaction of various voltage source converters (two level, half bridge MMC (modular multi-level converter), full bride MMC and mixed MCC) operating in the same DC grids.

3 ESRs and 1 ER are working under the WP2.

Jorge Gonçalves (Cardiff), whose individual project is “Operation of DC grids with various types of voltage source converter” has just returned to the UK from Beijing for his first secondment at China Electric Power Research Institute. He is currently working on the thermal response of MMC and the use of transient overload capability to improve AC system stability and frequency performance. An experimental test system is being design with a target to cover T2.9.

Abel Ferreira (Cinergia), whose individual project is “Power converter design and control for multiterminal DC grids” is currently on secondment at Cardiff University. He is investigating various types of MMC. Experimental tests are carried out using facilities of Cinergia.

Robert Renner (KU Leuven)’s individual project is “Design and implementation of VSC for DC grid”. His work has covered many tasks of WP2 on MMC and WP4 on ancillary service of AC systems.

Dr. Agustí Egea-Àlvarez (CEPRI) is carrying out a project on “Power flow control devices in DC grids”. He joined CEPRI since March 2015.

WP2 researchers are making good progress towards completing their tasks, milestones and deliverables.

Useful interaction with other MEDOW WPs include:

Agustí Egea-Àlvarez collaborated closely with WP4’s Joseph Tibin and WP2’s Jorge Gonçalves during their secondment in CEPRI. He has also identified possible collaborations with WP4 ESR Alejandro Bayo, regarding the power converter model linearization.

Jorge Gonçalves is working with WP4 researchers Tibin Joseph and Gen Li (ESR12) on use of MMC for offshore control and main AC grid interaction. He also worked with other PhD researchers at CEPRI during his secondment.

Robert Renner (ESR9) has regular collaboration with MEDOW researcher Alejandro Bayo Salas and other KU Leuven researchers, and has identified possible collaborations with other MEDOW researchers Jayachandra Naidu on Ancillary Services from WPP for HVDC grids, and Jorge Gonçalves on Applications of temporary VSC overload.

WP3 Relaying Protection

Universidade do Porto leads this work package and WP members are EFACEC, UPC and KU Leuven.

Researchers are: Mohammed Meraj Alam (UPORTO), Ataollah Mokhberdoran (EFACEC), Rodrigo Teixeira-Pinto (UPC) and Sahar Pirooz-Azad (KU Leuven).

One of the key objectives of this sub-program is to identify the specifications of the DC Circuit breakers, break current and fault clearance time, which are required to isolate the DC faults and to avoid collapse of DC Grids. Ataollah Mokhberdoran worked towards these objectives and has finished Tasks T3.1 (To identify the breaking current capability and DC fault clearance time required for DC circuit breakers) and T3.2 (DC circuit breaker design). Has has advanced a DC circuit breaker prototype and has accomplished the simulations in the software package SABER (software who allowed to study the behaviour of any particular semiconductor of the DC Circuit Breaker) and in PSCAD (software who allowed to assess the DC Circuit breaker in the power system network). He has been in his secondment at Universidade do Porto, where he has been a collaborative work with Mohammad Merak Alam on the integration and assessment of his newly developed model of the DC Circuit Breaker into PSCAD. As outcome of this close work both ESRs have submitted a conference paper. This work results in the achievement of M3.1 (DC protection requirement determined) and M3.4 (Post-fault restoration with various DC circuit breakers and isolators determined).

Mohammad Meraj has specified the impact in a step distance relay performance when a bulk energy comes from converts VSC based HVDC. This performance includes a transients approach. He is now working towards understanding the influence on the overall power system protection when energy is substituted from conventional big inertia generators to VSC based HVDC (T3.5). He has also obtained in his PSCAD model the different impedances of the step distance relay for the different fault types and fault location with different power injected from VSC based HVDC.

He has started modelling his VSC based HVDC onto RTDS simulator. This step is essential to test in a closed-loop commercial distance relay. This closed loop test will allow him to anticipate the real impact of the VSC based HVDC onto the actual AC Power System Protection.

Rodrigo Teixeira Pinto’s activities are focused on the power system restoration plans and security constraint optimal power flow algorithms for DC Grids embedded in AC networks. A combined AC-DC Optimal Power Flow Algorithm have been developed and assessed by commercial software (M3.4). He is demonstrating that DC networks can operate to provide support to the AC systems, increasing its supply, in case of outages (T3.4).

Sahar Pirooz Azad is working to achieve T3.3 (Develop relaying protection algorithm in the DC grid). At the moment, she is investigating possible protection algorithms suitable for DC grids (D3.3) and has made use of her secondment to EFACEC to work with both EFACEC and Universidade do Porto researchers on her work towards an algorithm to detect and restore faulted DC grids.

WP4 Relaying Protection

WP4 is led by KU Leuven and has members based at Cardiff, ELIA, DTU and Cardiff: Tibin Joseph, Gen Li, Alejandro Bayo Salas, Jayachandra Naidu and Qing Mu. ESR9 Robert Renner of WP2 also contributes to the Work package.

All ESRs are now working towards their goals, first results are being delivered. Also the first publications are realized.

* There is 1 journal paper accepted and 1 journal paper submitted for review. Several conference papers have been submitted, specifically the participation at the IET ACDC conference in Birmingham UK was impressive.

* The outreach activities run as planned

The work package members have held several interactions, mostly focussed around the MEDOW consortium meetings, with an intermediate delivereable on DC grid codes being worked on.

* First Secondments are starting

The following tasks, milestones and deliverables are currently pursued (a number of them will be achieved in the form of a publication when published):

T4.1, T4.2, T4.3, T4.4, T4.7, T4.8, T4.9

M4.1, M4.2, M4.3, M4.6

D4.1, D4.3, D4.4, D4.5, D4.6,

WP4 has a close relation with WP2’s ESR9, Robert Renner,  who works on the interface between WP2 and WP4.  This has resulted in a paper published in the IET journal Generation, Transmission and Distribution on “ancillary services in HVDC grids” (related to T4.5 and T4.6, M4.3 and D4.4. and 4.5)

Interaction with the ESRs of WP1 has started.

We are working on a common deliverable by all involved ESRs on HVDC grid codes (expected winter 2015) (D4.4 and D4.5)