Mitigating the effect of low inertia and low short-circuit level in HVDC-rich AC grids

Lead Research Organisation: Cardiff University
Department Name: Sch of Engineering

Abstract

Renewable power, particularly offshore wind power, will be a major element of the UK's transition to meet its energy demands while reducing carbon emissions. HVDC will be the key technology for integrating offshore wind power into the UK AC grid and for interconnecting other AC grids in Europe. Line commutated converter (LCC) HVDC is particularly suitable for bulk power transfer while voltage source converter (VSC) HVDC is particularly suitable for connecting offshore power into AC grids with low inertia and low short-circuit level. Multi-terminal HVDC networks and DC grids based on VSC technology will be developed across the North Sea to form a future SuperGrid to increase the flexibility, redundancy and economic viability of offshore wind power transmission.

Conventional synchronous generators will be replaced increasingly by renewable generation with power electronic converters and HVDC transmission. This causes significant reduction of system inertia and short-circuit level. Particularly in the UK, large scale offshore wind power and interconnection with grids in other European countries will lead to an HVDC-rich AC grid. This will result in AC grids with low fault-circuit and low inertia which will present a series of challenges for AC system operation such as the potential impact on existing relaying protection; frequency instability and commutation failure of LCC HVDC.

This proposed project will look at the behaviour of low-inertia and low short-circuit level in HVDC-rich AC grids supplied through power electronic converters. The challenges will be that the capability of HVDC links to provide the system support could be (at the same time) adversely affected by these effects on the grids. LCC HVDC can provide artificial inertia but requires high short-circuit ratio of the grid to work properly. During AC fault and post-fault restoration, the inertia support capability of the LCC would be limited at the very time it is most needed. VSC HVDC control is less dependent to AC grids. However a DC fault can be easily propagated across whole HVDC grid due to the very low resistance of DC lines, which in turn affects the AC sides of all terminals. During the DC fault, the real power injected into AC grids as well as the inertia support from VSC HVDC grids would be lost at the very moment it is much needed to maintain the system frequency. At the same time, reactive power support to AC grids from VSC HVDC grids would also be lost completely or partially depending on converter topologies.

Investigations will be undertaken of the inertia support from HVDC converters, on the capabilities of the different types of converters to mitigate low-inertia effects and on their coordination through the AC side (for point-to-point HVDC links) or through the DC side (for converters within the same DC grid).

The hardware-in-loop (HIL) platform at Cardiff University, which consists of a HVDC grid test rig, a real time digital simulator (RTDS) and a power amplifier, will play a key role in the modelling and testing of HVDC-rich grids and HVDC converter control for mitigating low-inertia and low short-circuit level effects.

Through this project, in-depth understanding of operation characteristics of AC grids which are rich in HVDC links will be achieved. Solutions will be founded to enhance the system inertia and short-circuit level. More renewable power through HVDC can be integrated into AC grids without deteriorate the system performance. The research outputs of this project will be disseminated through industrial partners, international academic associations, conferences and journal publications.

Planned Impact

The proposed research programme will provide a significant technical advance in the areas of renewable energy sources and HVDC transmission. The provision of AC grid support (in terms of inertia and frequency response) will ensure safe development of an offshore DC grid without affecting the performance of the existing system. This, in turn, will reduce the need for further infrastructure reinforcements (due to low inertia and low short-circuit levels), allowing for a better grid integration of offshore renewable sources and, in the end, increasing the likelihood of meeting greenhouse gas emission policy directives by 2020 and 2050.

A significant economic investment has to be made in order to reinforce the existing transmission system to accommodate the power produced by renewables. However, the potential delays associated with planning and permission to build new AC lines would make it impossible to meet governmental targets on time. Therefore, investment in HVDC technologies is key and HVDC transmission is now a necessity in the UK. It is important to the government and UK citizens that public investment is kept to a reasonable level whilst significant accomplishments in increasing transmission capacity are achieved. Being able to achieve this without affecting system performance involves a deep understanding of the new technologies and their potential to provide system support. By providing ancillary services (control algorithms to provide inertia support), mathematical models and analysis tools, more effective system planning and operation may be achieved. This will lead ultimately to lower energy costs for the end user. It is clear that society can benefit directly from the improved performance of the transmission system operator.

The provision of ancillary services by HVDC converters, in terms of low inertia and low short-circuit level support, is not a standard industrial practice yet. National Grid, as transmission system operator in the UK, would benefit from this knowledge. A deep understanding of the operational issues associated with DC grids and with the future European SuperGrid is required. The research carried out in this project will allow National Grid to have a better understanding of the possibilities that HVDC technologies can provide for AC grid support and the operation of HVDC-rich AC grids. The simulation of case studies, analysis tools and the test platform provided by this proposed research project will complement National Grid's existing pool of resources so as to further its understanding of these possibilities. A strength of this proposal is its being supported by a manufacturer (Alstom Grid) and a consultancy company with an energy portfolio (PB Power). Cardiff University already works closely with both companies. Joint research projects have been carried out with Alstom Grid over a number of years; PB Power has provided Cardiff with a Visiting Professor, expert in HVDC technologies. Alstom can benefit directly from the novel ancillary services of HVDC converters and the understanding of potential control interactions within the converters, whilst PB Power may apply the knowledge generated by the proposed project and utilise the novel models when providing consultancy services to their clients. By making use of these unique and established relationships, outcomes of this project can be easily exploited.

The cooperation of the investigators of this project with other funded initiatives in this call, together with the organisation of workshops for students and researchers involved in the topic and other dissemination and engagement activities, will provide an excellent platform for national collaboration. This has the potential to influence industrial activities and international academia which will, in turn, maximise the potential and value of international activity in the field of HVDC as the need for improved transmission services becomes increasingly urgent.
 
Description a) Inertial Contribution from Large-Scale Variable-Speed Wind Turbines connected to the GB grid.
The research focuses on the reduction of inertia in grids due to more penetration of renewable energy sources and also the reduction of synchronous generation in the grid. Inertia determines how fast the system frequency will drop when there is a loss of generation or an increase in demand in the power system/grid. With higher wind penetration and HVDC connections, the total power system inertia will decline.
With more variable-speed wind turbines being connected to the GB grid and also the plan to have higher percentages of offshore wind turbines, it is necessary to see how these turbines can be controlled to provide inertial support when needed. Using the step torque scheme, when there is an increase in demand, the wind turbines detect the frequency drop and are controlled to provide a step increase in their power supply to the grid. In this research, the wind turbines frequency support capability was analysed for different wind speeds and also different wind power penetration levels.
It was discovered that, the amount of step increase in power supplied to the grid (in power imbalance situation), could help to reduce the frequency deviations, however when there is a large step increase in power from the turbines, it would later affect the frequency negatively when the wind turbines are in the recovery phase. This problem was studied and the optimum amount of step increase in power was found. This amount of power was lower than the maximum amount of power that the turbine could supply but was sufficient to reduce frequency deviations and rate of change of frequency without further dropping the frequency later in time.
b) An Auxiliary Dead-band Controller for the Coordination of Fast Frequency Support from Multi-terminal HVDC Grids and Offshore Wind Farms
From this research, an auxiliary dead-band controller was developed. High-voltage direct-current (HVDC) grids may provide fast frequency support to ac grids with the aid of supplementary control algorithms and synthetic inertia contribution from offshore wind farms. However, when all converters within the HVDC grid are fitted with these supplementary controllers, undesirable power flows and reduced power transfers may occur during a power imbalance. This is due to simultaneous frequency oscillations on the different ac systems connected to the HVDC grid arising during the support operation.
To prevent this adverse effect, an auxiliary dead-band controller (ADC) is proposed. The ADC modifies the dead-band set-point of the fast frequency controllers using measurements of rate of change of frequency and frequency deviation. A four-terminal HVDC integrated with an offshore wind farm is modelled to analyse and study the effectiveness of three different supplementary fast frequency control algorithms. Results show that the proposed ADC scheme improves the performance of fast frequency control algorithms. For completeness, a small-signal stability analysis is carried out to confirm that a stable system operation is maintained.

c) Experimental Validation of Fast Frequency Support from MTDC Grids
The three machine GB power system is modelled in a Real Time Digital Simulator (RTDS) and connected to a physical three terminal VSC-HVDC test rig via a power amplifier. This forms a hardware in the loop (HiL) system, which was used to validate the fast frequency controllers described in (b). Conventional power system stability studies are performed using analytical tools and offline simulations. To accelerate the advancements in this field and to validate control systems and operations hardware-in-the-loop (HiL) tests have to be developed. To contribute to this efforts, a real-time HiL (RT-HiL) platform is presented for ac/dc power system stability studies. Utilization of the RT-HiL platform for testing ancillary services in a voltage source converter (VSC) based high voltage direct current (HVDC) connected ac grid is considered for frequency support in ac/dc grids. The ac grid is modelled as a multi-machine network in the real-time simulator, while the dc network is implemented in a physical test-bed. The efficacy of the RT-HiL platform to emulate interactions and grid support services were tested and proved through a series of experiment tests. Finally, the experimental results were compared against the PSCAD simulation results and a good agreement was achieved.
Exploitation Route The developed ADC used with supplementary frequency controllers will aid a stable operation during frequency support. The findings can be used in the development of future multi-terminal dc (MTDC) grids. This research has industrial partners GE and National Grid. Some findings have been presented to GE grid solutions and they have found them interesting. Further research is been done and these may be used by the industrial partners in planning and design.
Sectors Energy

 
Description Findings were presented at Nigeria Energy Forum, April 2016. This forum focussed on improving the power industry in Nigeria and providing power to all. It encouraged the increase in renewable energy generation in Nigeria as a solution to power issues the country is facing. During the forum, a presentation on the "Impact of Change in generation mix on the inertia constant of the GB grid" was made to an audience of academics and engineers from the transmission and distribution companies in the country. The models and control strategies developed through this project has attracted the interests from the National HVDC Centre and National Grid. Two industrial grants from them have been awarded on AC grid stability with effects of HVDC stations.
First Year Of Impact 2016
Sector Energy,Environment
Impact Types Economic,Policy & public services

 
Description Feasibility study on multiform of DC grids for transnational interconnections
Amount € 600,000 (EUR)
Organisation State Grid Corporation of China 
Sector Public
Country China
Start 11/2017 
End 03/2019
 
Description Identification and mitigation of Sub and near-Synchronous Oscillations
Amount £150,463 (GBP)
Organisation National Grid Network Innovation Allowance (NIA) 
Sector Private
Country United Kingdom
Start 11/2019 
End 02/2021
 
Description Innovative tools for offshore wind and DC grids
Amount € 3,895,000 (EUR)
Funding ID 765585 
Organisation European Commission H2020 
Sector Public
Country Belgium
Start 09/2017 
End 08/2021
 
Description CIGRE B4.72 Working group DC grid benchmark models for system studies Dissemination Workshop 
Form Of Engagement Activity A formal working group, expert panel or dialogue
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Schools
Results and Impact The meeting was to discuss and plan for the development of new DC grid benchmark models for system studies and planning
Year(s) Of Engagement Activity 2018
 
Description HVDC Colloquium 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Study participants or study members
Results and Impact the colloquium enabled the dissemination of results from findings to an academic field with experts in power electronic devices, feedback was also received
Year(s) Of Engagement Activity 2016,2017
URL https://web.fe.up.pt/~hvdc/
 
Description HVDC Grids training course 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Schools
Results and Impact Received training on HVDC grids, why HVDC is favourable over AC technologies for power transmission; what the key technologies and challenges are for developing an HVDC grid; how an HVDC grid will be designed and operated; and how future HVDC grids will evolve.
Year(s) Of Engagement Activity 2016
 
Description IET ACDC Conference 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Industry/Business
Results and Impact Organised every 2 years on the UK, the conference brings together stakeholders in the AC and DC transmission field to showcase their research and industrial work in the form of oral and poster presentations. Findings were published in the form of a conference paper at the ACDC conference. A poster presentation was made and exhibited throughout the conference. This received a lot of questions and advice.
Year(s) Of Engagement Activity 2017
 
Description National Grid Dissemination Workshop 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach Local
Primary Audience Industry/Business
Results and Impact The purpose of this dissemination event was to share knowledge with the National Grid staff. State of the art on frequency support from HVDC grids, Subsynchronous resonance and HVDC breakers were discussed.
Year(s) Of Engagement Activity 2017
 
Description Nigeria Energy Forum 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Policymakers/politicians
Results and Impact The researcher of this project Khadijat Jose participated the event: Organising and delivering the forum was supported by a member of the research team. The forum addressed barriers to sustainable energy development in Nigeria and other African countries and highlighted opportunities for energy firms to deliver improved services to households, businesses and industries. A presentation on the impact of increase in renewable energy generation mix on the power system inertia was made and discussions on frequency support in these scenarios was discussed with transmission and distribution level engineers were made.
Year(s) Of Engagement Activity 2016
URL https://www.thenef.org/
 
Description Presentation to GE Power 
Form Of Engagement Activity Participation in an open day or visit at my research institution
Part Of Official Scheme? No
Geographic Reach Local
Primary Audience Industry/Business
Results and Impact Delivered an oral presentation titled 'Mitigating the effects of low inertia and low short-circuit level in HVDC-rich AC grid" in Cardiff University to GE Power - August 2016
Year(s) Of Engagement Activity 2016