Picture this: It’s the 21st century; the consumer electric market is expanding, needs are diversifying and George Westinghouse’s alternating current (AC) – previously thought to be the ultimate winner in the war of currents – is now making room for its bitter rival, Thomas Edison’s direct current (DC) systems.
We speak in allegory however, not to incite a continuation of the war but to set the scene for the fascinating and complex resurgence of what is called High Voltage Direct Current (HVDC) transmission. Benefitting from 137 years of additional development of both technologies, the electric power industry is beginning to realize the benefits of cooperation between the two.
AC transmission and distribution systems – being relatively simpler and less costly on the surface – have spent the last century expanding into the market, developing standardization, and building confidence with the public. DC technology on the other hand, has had a much more challenging path, requiring significant step-changes in technology to make it economically viable. As such, HVDC systems have not been as mature in application, understanding, or standardization.
In the last thirty or so years, developments in semiconductor technology and the ever-growing global demand for reliable and flexible power transmission have allowed HVDC technology to shine. Universities, utilities, and equipment manufacturers are looking more and more to HVDC to enable the interconnection of synchronous and asynchronous grids; transfer of bulk power; and connection of power storage solutions, as well as transmission of electricity from remote renewable generation. There are also growing efforts to utilize the same HVDC converter systems in grid-forming scenarios, dynamic support in the form of synthetic inertia, frequency control, and voltage support. Where traditional AC system components may fulfill one or two of these roles, HVDC converter systems are increasingly being asked to fulfill many.
Potential owners must identify which capabilities and what application they need from an HVDC system. Once decided, manufacturers must then develop a bespoke HVDC system that is fine-tuned for the AC system. Integrating a new HVDC system within existing, complex and critical AC power systems is a challenging engineering task. The high-speed control systems used to manage the rectification and inversion processes can have catastrophic consequences on the electrical stability of the connected grid, resulting in equipment damage and reduced reliability. Therefore, each HVDC system will undergo an extensive and rigorous multi-year iterative engineering design, modelling, and testing process.
The process for specifying and then verifying and validating each HVDC control and protection system is resource and time intensive for manufacturers and equipment owners alike. It can be a frustrating process; the complexity of the systems and desired functionality can lead to nerve-wracking questions about how the system will behave, and what happens if it fails? These questions can then pit owners against manufacturers in a battle where timelines for delivery are shrinking and expectations for testing are increasing. How does this get resolved?
One way to reduce this conflict is standardization – the holy grail for the electric industry. While bespoke solutions require bespoke testing which can be costly and time consuming, standards for equipment capabilities and reasonable testing scenarios allow manufacturers to build a basic level of trust with equipment owners.
However, the complexity of these control systems makes developing standard products a challenge. Therefore, the addendum to standardization is modularization and customization – control systems that are constructed with building blocks of known functionality. These building blocks can then be defined and documented for ease of understanding. They can also be rigorously tested to iron out behaviors and interactions. This type of testing and detailed documentation can promote familiarity and trust with equipment owners.
PSC is working with one equipment manufacturer to incorporate and propose standardized, modular and customizable functionality for their HVDC solution – which pairs well with the new generation of digital substations. The overall task being to assist in describing and documenting HVDC system behaviors including those that are commonly expected, but not always specified in contract documentation. These behaviors can be captured in detailed functional, operational, and physical requirements. These requirements then comprise part of an overall system model that is unambiguous in its purpose. The model can ultimately be used to develop standard system tests that can be executed, automated, re-used, and/or reproduced based on the specific functionality that is chosen.
This exercise will bridge the gap of knowledge and expectations around HVDC system capabilities – aimed at answering questions not yet asked. It will also align the manufacturer with the changing and increasingly complex needs of existing and potential owners. This will result in the ability to reuse engineering solutions, ultimately improving project delivery timelines and customer satisfaction.
On a broader scale, by meeting these goals, the manufacturer will be taking the steps within the wider HVDC industry toward maturing standardization, market applications and general acceptance of HVDC links as economic power system solutions.
Perry Hofbauer – P.Eng – PSC Principal Power Systems Engineer
Perry is a Principal Power Systems Engineer with experience focused on HVDC and FACTS control and protection systems. Specifically in design, design review, factory testing, on-site commissioning, operations, and maintenance. He has also had considerable experience as a Protection and Controls Engineer responsible for the design, implementation and maintenance of equipment and transmission lines from 25kV to 500kV AC.