ABB offers solar inverters for a wide range of rated powers and voltages. This extensive portfolio necessitates a tool for fast, accurate and customer-oriented device modeling. ABB’s Universal Framework simulation tool can be used in various simulation software packages applicable to power system analyses.
Piotr Mars, Miłosz Miśkiewicz, Paweł Błaszczyk and Tomasz Kuczek, ABB Corporate Research Cracow, Poland; piotr.mars@pl.abb.com, milosz.miskiewicz@pl.abb.com, pawel.blaszczyk@pl.abb.com, tomasz.kuczek@pl.abb.com
The very significant contribution that photovoltaic (PV) technology makes to renewable energy generation is set to continue in the years to come. Hence, delivery of a wide range of solar products is seen to be a crucial element of ABB’s future growth.
Amongst other PV-related products, ABB offers solar inverters for applications with a wide range of generated power at different voltage levels:
• Single- or three-phase string inverters rated between 2 kW and 60 kW. These find use in residential applications but can be combined into a larger setup for decentralized industrial- or utility-scale PV power plants.
• Central inverters rated at 100 kW to 2,300 kW and turnkey stations (inverters and related equipment), which are suitable for larger commercial- and utility-scale solar farms.
The Universal Framework simulation tool
The wide range of potential applications for ABB solar inverters raises a question: Is it possible to create a universal tool that allows fast, accurate and customer-oriented computer modeling of how these inverters will behave in all potential power system applications? The answer is, “yes,” and this article will describe just such a tool – the ABB Universal Framework simulation tool – as well as its use for the PVS980 central inverter in various simulation software packages that are utilized in power system analysis →1-2.
The ABB Universal Framework simulation tool is based on the generation of dynamic linked libraries (DLLs) that model the PVS980 central inverter in terms of its control algorithms and connection to a power system. Analyses show that universal black-box models generated by the tool provide the same quantitative results for all software packages considered – PSCAD, Matlab, DIgSILENT PowerFactory and PSS/E – which gives great flexibility in accommodating different customer design environments.
ABB’s Universal Framework simulation tool has been created over a number of years and now covers all requirements of power system design and analysis. The tool is simple to use and easy to adapt to new requirements →2.
Meet the grid codes
One critical aspect of PV inverter simulation covered by the tool is grid code compliance [1]. Inverters connected to a power grid must be compliant with requirements – so-called country grid codes – set out by the network operator that define the safe and proper operation of the entire power system. These compliance requirements can nowadays be reflected in a computer model that is accurate enough to represent the electrical characteristics of real devices under various working conditions.
The grid codes describe how devices and reference models should operate during normal network conditions and how they should behave in terms of active and reactive power provision when a fault occurs. The inverter model generated by the Universal Framework simulation tool should contain information about the quantities of active power delivered to the grid as a function of primary energy supply – both at a defined set point and in the event of network frequency change. The regulation of reactive power at different set points should be determined by a characteristic power quality curve.
The model must also follow inverter behavior regarding the withstanding of sudden grid voltage deviation from the nominal amplitude for a certain period and trip when the deviation duration exceeds the time given in the specification. Another aspect that must be covered by the model is the reactive current injected to provide grid support during a voltage dip. The satisfactory performance of the Universal Framework simulation tool under such line fault conditions has been validated by measurements. The certificate from that validation is very often required by the customer, so is of significant additional value.
Universal model principles
Primarily, the Universal Framework simulation tool must provide the functionality to simulate all grid code scenarios. The two most important components in such a simulation are the PV source and the inverter.
The simulation of the PV source component relies on a mathematical or physical model of the solar panel, with irradiance and temperature as basic parameters. Users can define the panel’s characteristics in the tool based on its datasheet and create, at will, virtual solar arrays to simulate various input power levels. This part can be included in a DLL, but this is not mandatory as a customer might want to connect an external PV farm model.
The simulation of the inverter component contains a mathematical implementation of the inverter and related control algorithms that covers maximum power extraction from the PV plant, grid synchronization and support (eg, reactive power injection), voltage control, active and reactive power provision control or fault ride through. The Universal Framework simulation tool supports electromagnetic transient and root-mean-square (RMS) input-output interfaces to accommodate different simulation types used by the customer.
As different customers use different simulation environments, it is desirable to have a way to transfer a single basic model into different functional modules. One way to achieve that functionality is to use a Matlab/Simulink root model to generate DLL blocks, which can then be used as separate, external modules in third-party software (such as DIgSILENT PowerFactory, PSCAD or PSS/E). Full automation of this process is provided by introducing a common framework that is responsible for functional code generation for the model. Interface code for the third-party simulation software (a so-called wrapper) is provided and integrated into the final model.
Additional functionalities required of the tool are: support of automated testing, report generation and model regression testing. These functions support developers in the model building, testing and bug-fixing process.
The tool also supports DLL back-testing in the Matlab/Simulink environment (so-called software-in-the-loop testing), which is crucial to prove that the code has been generated correctly. The DLL block can be tested in the same environment as the base model. Additionally, the generated models can be delivered as a black-box model to customers who use a Matlab/Simulink simulation environment.
The biggest advantage of black-boxing is that the internal ABB DLL intellectual property is protected. In this way, the entire model can be sent to the customer in PSCAD, PowerFactory or PSS/E without disclosing the internal algorithms. With the manuals provided, the customer can easily reproduce the results in order to validate the grid code behavior of the solar inverter. The whole process of model development and certification can be significantly improved by using the Universal Framework simulation tool as opposed to a conventional approach →3.
The DLL model implementation process usually depends on the simulation software employed and is not as straightforward as it may seem to be. Different simulation packages utilize different solvers, computational methods, signal interfaces and element libraries. The Universal Framework simulation tool overcomes all the inconveniences related to the implementation process by the creation of an intermediate DLL-specific environment that states a software-specific interface. Such a solution enables the DLL model to cooperate with the grid by controlling the software-specific network element based on the feedback measurements and user-configurable input signals. The DLL root model-specific parameters are configurable through the external parameter text file, which is common for all simulation software packages →4.
Model validation
To show that the model developed provides a correct response for various grid code scenarios, it was verified against laboratory experiments. Examples of solar inverter voltage and current during the fault test are illustrated in →5-6. A high level of curve convergence is visible in the prefault steady state and during the fault; transient regions are convergent, too. The differences occur after the fault phase due to the transformer inrush current, which was not taken into consideration during grid modeling. However, the error is within the tolerance margin.
05 Positive-sequence voltage RMS line-to-line value during a symmetric fault - measurements and simulation results comparison. 
06 Positive-sequence RMS line current during a symmetric fault - measurements and simulation results comparison.
Such validated models can be used for further studies in which the entire customer grid is included. Such a reference design is shown in →7. The grid part uses software-specific libraries and typically contains distribution and power transformers, cables and grid impedances at the point of common coupling (PCC).
A truly universal tool
The approach taken by the Universal Framework tool has been successfully verified for other solar inverter family products, which underlines its reuse credentials. The tool is also compatible with a range of power system analysis software packages and could also be reused for different product groups such as wind turbine inverters, STATCOMs (static compensators) and medium-voltage drive inverters. Moreover, the universal model approach can be extended to other simulation software packages that support a DLL interface, if such a customer demand appears. Compared to the conventional approach, the ABB Universal Framework tool for solar inverter modeling opens up the possibility of significant cost reduction through its simplicity, flexibility, adaptability and reusability.
Reference
[1] I. Romero, J. Daniel, D. Pereira, et al., ”Consulting the grid code: ABB and its power consulting experts are helping networks integrate renewables and meet grid code requirements,” ABB Review 4/2015, pp. 50-55.
