Voltage Sourced Converter (VSC) HVDC Responses to Disturbances and Faults in AC Systems which have Low Synchronous Generation
The objective of this technical brochure is to consider the behaviour of High Voltage Direct Current (HVDC) Voltage Sourced Converter (VSC) transmission systems connected to AC systems where the short circuit ratio, on at least one end of a point-to-point system, could potentially be low. Whilst such operating conditions have been well covered in the literature for the conventional grid following control approach there is less understood about the behaviour of HVDC VSC when a synchronous grid forming control approach is used. A core focus of the Working Group has, therefore, been to consider what functionality is desirable from a synchronous grid forming controlled converter. This leads on to the important discussion of how the practical constraints of VSCs, designed and optimised for HVDC power transmission, impact on the capability, limits and constraints of such a control mechanism. Another important consideration when defining the functionality is that, for a HVDC transmission scheme to operate there must be, as a minimum, two converters, one sending active power and one receiving active power. Hence any change in the dynamic active power flow at one converter will be transferred via the HVDC system to the AC point of common coupling at the other converter.
Members
Convenor (UK)
C. Barker
Secretary (CA)
H. Suriyaarachchi
C. Cardozo (FR), E. Duggan (IE), P. Ferdinando (IT), X. Guillaud (FR), I. Huq (DE), R. Iravani (CA), S. Kabul (NL), S. Kodsi (CA), M. Martin Almenta (ES), F. Milano (IE), G. Olguin (CL), E. Prieto (ES), P. Rault (FR), P. Samuelsson (SE), G. Shafique (FR), M. Wang (DE), S. Wenig (DE)
Introduction
HVDC VSCs are sometimes considered as a generator, where active power is being imported into an AC system. The technical brochure therefore starts with a summary of synchronous machine behaviour in AC systems, specifically considering the behaviour of a synchronous machine dominated AC system in response to AC system faults. This description provides a platform of understanding on which to build the functional definition of what the desirable behaviour of a synchronous grid forming HVDC VSC would be.
Simple models along with mathematical descriptions are used to explain the fundamental concepts of, first, the grid following controller and then the synchronous grid forming controller. The Technical Brochure then reviews the practical constraints imposed by the power electronic converter and associated HVDC transmission circuit, giving vendor agnostic examples of practical ratings.
Having introduced the functional description of synchronous grid forming, its capabilities and constraints it is worth noting that not all applications of HVDC transmission would necessarily benefit from this method of control compared to the more conventional grid following control. The technical brochure has, therefore, provided a comprehensive analysis of different AC grids that could be interconnected via HVDC transmission and the controller needs. This can be used as a guide to future design studies.
With the introduction of SGFM control comes the need to define a platform that can be used to test the perform of the converter based on the requirements provided. An important point addressed is that a HVDC link is transmission infrastructure and, therefore, what happens in terms of active power exchange at one HVDC station must also impact on the other HVDC station. Therefore, the test circuit proposed considers both ends of a HVDC transmission system.
In order to better understand the behaviour of a synchronous grid forming controlled converter, along with how this compares with a grid following controlled converter, electro-magnetic transient simulations results are provided to demonstrate the converter behaviour under different dynamics cases.
Desirable features of SGFM when applied to VSC HVDC transmission
A number of desirable features of SGFM control for VSC HVDC are identified in the Technical Brochure, these include:
- Instantaneous active power response
- Voltage support for voltage amplitude jump
- Inertia contribution
- Fault current contribution
In addition, the HVDC converter should have:
- Robustness to large system strength changes
- Islanding operation capability
- (Re) synchronization capability (from islanded condition)
Description of the Technical Brochure
The Technical Brochure is structured as flows:
Section 2: offers a discussion of the dynamics of AC synchronous generation and the operation of a synchronous machine dominated AC system in order to provide a comparative context for HVDC transmission.
Section 3: introduces the GFL concept and describes the key elements of this controller as related to AC system strength.
Section 4: provides an overview of HVDC control methods with a particular emphasis on those requirements associated with SGFM as applied to HVDC transmission.
Section 5: discusses VSC HVDC transmission as compared to AC generation and discusses the techno-economic limitations of a VSC HVDC transmission scheme.
Section 6: discusses how the required behaviour of a HVDC transmission link may differ when comparing a HVDC transmission link connecting to asynchronous AC zones with one connecting two nodes within the same AC synchronous zone.
Section 7: presents a test circuit that can be used to assess the properties of a HVDC controller, in particular those properties that are given in Section 4 as being defining for a synchronous grid forming converter. This section also suggests definitions of measurements that can be used.
Section 8: provides some generic examples of controller responses to a range of performed tests.
Conclusion
The Technical Brochure considers the practical constraints of a VSC HVDC transmission scheme in terms of both converter current capability and the limited amount of stored energy available...
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