Milosz Miskiewicz, Glib Chekavskyy, Piotr Sobanski, Grzegorz Bujak, Marcin Szlosek ABB Corporate Research Center Kraków, Poland, email@example.com, firstname.lastname@example.org, email@example.com, firstname.lastname@example.org, email@example.com; Nikolaos Oikonomou, Marc Halbeis, Kai Pietilaeinen, John Eckerle, Jess Galang, Wim van der Merwe ABB System Drives Turgi, Switzerland firstname.lastname@example.org, email@example.com, firstname.lastname@example.org, email@example.com, firstname.lastname@example.org, email@example.com; Vahan Gevorgian NREL Golden, CO, United States; Torben Jersch IWES Bremerhaven, Germany
As the electric power sector transforms, an expanding wave of both variable and distributed energy resources are added to electric grids. Renewable energy resources are highly desirable as they reduce carbon emissions from the power source and yet the integration of these resources into the grid can be challenging for grid operators. To meet the swell of demands, today’s electric grids are increasingly complex; they must accommodate the continuous growth in their power capacity and the ever-increasing variety of installed equipment while maintaining safe and reliable operation.
To ensure the reliable and stable operation of all grid components, governments have established well-defined regulations that are specified in grid codes. Regulatory compliance is not easy to attain, however, as power generation and load must remain in balance without compromising power quality – an arduous task.
The penetration of new energy sources without mechanical-electrical inertia in the modern power grid necessitate new measures to assure total grid stability. This need has given rise to exacting directives that define the accepted behavior of the power source under abnormal grid events, eg, changes in voltage, frequency variations, or fault conditions like short circuit events.
These grid code requirements must be fulfilled by every energy generation plant, including variable energy resources such as photovoltaic systems and Wind Power Plants (WPP), thereby adding complexity to compliance. Due to the existence of a variety of structures in the power network and generation mix available, the applicable grid code varies significantly from country to country and can even vary from one utility to another. Before a renewable power source, eg, wind turbine, can be connected to the public power grid it must be certified by a specific applicable and possibly local grid code – a costly and protracted process that requires complex, exacting testing procedures to be performed. In light of this, ABB has introduced PEGS to the power grid market for use at testing and research facilities. This system emulates both healthy and faulty operating modes of MV power grids: end-customers can use PEGS at testing facilities to validate grid code compliance of their tested devices →01.
The need for power
In addition to fulfilling grid code compliance certification, tests need to be both repeatable and granular and allow the equipment’s behavior to be investigated at operational boundaries.
Previously, different approaches were used to investigate operational boundaries, eg, short circuit events were simulated by switching reactors onto the network. There are drawbacks to this approach, in terms of repeatability and granularity, because of its reliance on the mechanical properties of the reactor and the opening and closing times of the electric switching equipment.
Building an artificial grid with power electronic converters is another approach gaining prevalence. The grid can then be controlled to simulate changes in frequency and voltage. With careful design of the converter and adjunct system even asymmetrical grid conditions can be emulated. This standard approach uses many smaller converters, with fast switching semi-conductors, connected in parallel. Hence, power can be adjusted to the required level to simulate an event.
Nevertheless, emulation of short circuit behavior requires considerable effort due to the large current requirements associated with critical events. At lower power levels, the power electronic converter for emulation can be oversized to handle the current without damaging the semi-conductors or converter system. Nowadays, however, the power levels of large wind turbines, especially for off-shore use, obviate the feasibility of creating an artificial grid that employs many LV converters running in parallel. The new generation of off-shore wind turbines, such as the Haliade-X, manufactured by General Electric, exemplifies this. With a rated power of 12 MW, it would be prohibitively challenging to emulate short circuit events with currents in the region of 2 per-unit.
PEGS design and application
Enter ABB’s ACS6000 grid simulator: PEGS. Based on tried-and-proven technology, PEGS is adapted to the high power levels necessary for todays’ generation of power sources, like WPPs. The use of MV Integrated Gate-Commutated Thyristors, reduces the number of parallel connected semiconductors and converter systems necessary; this increases system reliability and reduces complexity.
At the power levels required by modern wind turbines, the available switching frequency of the semi-conductors is much lower than for lower voltage converters. ABB’s novel solution for testing modern wind turbines utilizes the flexible and modular structure of the ACS6000. The power converter consists of the active rectifier, DC link, and three-phase multi-level NPC inverters with a special output transformer that makes an increase in the output voltage possible. ABB’s converter allows multiple inverter units to be connected to the same DC-link.
Additionally, PEGS is equipped with a passive filter that increases voltage quality to high levels →02. Not only does voltage modulation achieve high accuracy, Total Harmonic Distortion value (THDv), a measure of voltage quality, is exceedingly low. The converter can emulate a grid with short circuit power levels in excess of 40 MVA while keeping the THDv below one percent →02 – a significant feat.
Due to the power converter’s modular design, PEGS allows an almost unlimited configuration potential and flexible control →03. ABB’s standard field-proven controller for MV applications, responsible for system initialization and protection, ensures the high reliability of PEGS operation. To further ensure the converter operates safely, PEGS-specific features are activated with a dedicated controller only after all protective conditions, ie, initialization procedures, have been fulfilled.
Newly created industrial protocols allow the operator to control PEGS output voltage according to the customer’s specific needs. Moreover, as part of a test laboratory, PEGS can operate in island mode as a controllable voltage source, or as part of Power Hardware In the Loop (PHIL) testing system with an interface between the physical and simulated systems.
Today, ABB’s ACS6000 MV converter, is found in many power-demanding industry segments, eg, mining, metals, and marine propulsion industries. With over 2000 units operating world-wide, this MV converter provides the broad range of power, voltages, and frequencies end-customers seek →03.
In addition to its practical testing application, PEGS can play an important role in research and development: grid infrastructure, electricity and energy systems integration. Moreover, PEGS can be successfully utilized for special test benches, eg, cables, motors, or transformers.
In the industrial world, PEGS installations vary in terms of voltage levels, power, and customary functionalities →04. ABB has, to date, delivered PEGS applications to research and testing facilities such as the National Renewable Energy Laboratory (NREL) in Colorado, USA and to the Fraunhofer Institute for Wind Energy Systems (IWES) in Germany →04.
Meeting the demanding requirements
PEGS has been designed with performance in mind to fulfill both practical certification tests and research purposes:
• High-quality output voltage, characterized by low THDv50 (< one percent) values. A modulation error of the first harmonic is small for a wide range of operating voltages. Consequently, the testing devices can be supplied with an almost pure sinusoidal voltage waveform in which the root mean square (RMS) voltage value can be precisely controlled according to the customer’s needs.
• Capable of generating repetitive and uninterrupted voltage drops from a nominal value to zero voltage rapidly, ie, ≤ 1 ms →04. This feature enables Under Voltage Ride Through tests to be performed; this is critical for eg, wind turbines, as they must validate their ability to stay online continuously in order to prevent major blackouts that can occur under faulty grid operation conditions →05.
• Capable of providing Rate of Change of Frequency (RoCof) events of connected power equipment.
• The ability to test two devices simultaneously at different voltage levels; thereby allowing the PEGS testing facility operator to examine two end-customers contemporaneously.
• Harmonic Injection (HI), which can be used for the estimation of grid impedance in order to monitor the grid condition eg, to detect island conditions. This feature allows certification tests to be conducted under artificially distorted PEGS output voltages tailored to the customers’ requirements →06.
06 The system allows artificially distorted output voltages for testing.
Further proven benefits
Research and testing facilities such as the NREL have been able to showcase numerous benefits gained from operating ABB’s PEGS system. PEGS creates the capability to test and validate advanced frequency response services by modern inverter-coupled generation for both high and low inertia grids. Advanced controls that can be tested by PEGS include: synthetic inertia, fast frequency response (FFR), primary frequency response, and power oscillations damping services.
Additionally, PEGS enables the capture of impedance characteristics of inverter-coupled generation in a wide range of frequencies (eg, 0–3 kHz for NREL controlled grid interface), crucial for an understanding of the nature and mitigation strategies for harmonic resonances, control interactions, and sub-synchronous oscillations. In addition, PEGS can integrate fast PHIL platforms for closed-loop testing of the impacts of variable power generation on power systems of different sizes (micro-grids, islands, larger power systems-coupled generation). Additionally, because PEGS is a grid forming inverter, it allows facilities to replicate and test different grid forming methods, eg, f-P droop, virtual synchronous machine, and virtual oscillator; as well as black start strategies for inverter-coupled generation.
For inverters undergoing examination, PEGS permits the testing and validation of features on real scales, eg, advanced protection schemes, fault detection with the use of injections of negative and zero-sequence-currents.
PEGS can also be used to test an actual grid-forming inverter within a controlled environment for various “grid-of-the-future” scenarios, eg, fixed-frequency micro-grids, zero-inertia, and 100 percent inverter-based power systems, among others.
Meeting the future energy needs
New electric energy systems bring new challenges →07. ABB not only provides customized products based on the PEGS platform to meet current demands, the company is also exploring PEGS functionalities and applications to meet future challenges. A new control concept will increase the power capacity of PEGS: the simultaneous operation of ACS6000 units. This will extend the power range of testing devices and make simulations of the behavior of micro-grid systems more possible. The integration of infrastructure and external control systems with a rapid PEGS response will allow operators to obtain the required reference voltage immediately, if external closed-loop control is needed. Consequently, PEGS can be used as part of a more complex application in accordance with the customer’s need. ABB is also developing the concept of a Power Electronics Load Simulator so that customers can emulate testing devices such as wind turbines; or enable specialized certification tests to be conducted, eg, unintentional islanding. ABB is also investigating a design of PEGS for testing and integration studies of DC high-power applications, eg, traction or PV. Moreover, hardware designers and software developers are working together to create control structures to improve platform utilization and increase the power of PEGS applications.
By working closely with national research laboratories, ABB can aptly predict the future needs of customers and end-customers who require their power source devices tested. ABB’s commitment to excellence, cross industry collaboration and the evolving renewable energy market drive their innovation and global offering.