Power electronics devices rely on complex dynamic control and high-frequency switching to perform their basic functions and meeting power quality and dynamic response requirements. However, the rapid dynamic control and fast switching of power electronics are introducing new system integration problems. A growing issue in the industrial community is the influence of these complex dynamic control systems and grid parameters (AC/DC) on the resonance and oscillatory behaviour of the entire system due to control interactions.
The two main goals of this research project are to:
- Establish a methodology to predict control interactions using impedance modelling and measurement.
- Develop impedance-based design guidelines for power electronic converter control systems to manage the risks of control interactions.
Objectives
The objective of this work is to develop a methodology for:
(1) Identifying the risks associated with the stability and control interaction before a new power electronic device (e.g. Windfarm, interconnector, STATCOM) is introduced to the network using impedance based stability analysis.
(2) Develop impedance-based design guidelines for power electronics converter control systems to manage the risks of control interactions.
Learnings
Outcomes
- In this project impedance estimation-based stability assessment method is developed. The approach takes care of the detail dynamic model of the most recently used MMC technology used in all dynamic reactive power support devices such as STATCOM.
- The approach developed has demonstrated to be effective to obtain the real network operability margin while taking care of various control block response inside STATCOM.
- The project has succeeded in developing an impedance based stability method able to capture and quantify the control interactions for a new power electronics connection to the GB network.
- The impedance-based design rules are formulated for power electronic converter control systems to mitigate control interaction issues.
- The method developed will help NGET set rules and design guidelines for power electronics converter control system to mitigate network instability which is measure of success of the project. The method is generic which can be applied to transmission system of different voltage level and complexity.
Lessons Learnt
The project accomplished all the tasks set in work packages 1 and 2, with the proposed methodology for stability analysis considering the admittance representation of converter-based devices. A detailed description of the impedance theory, measurement and modelling for the MMC STATCOM model was presented. An admittance stability assessment methodology was developed based on frequency sweep measurements. This methodology has demonstrated the ability to capture the dynamic characteristics of converters from a black box perspective. The methodology was tested and validated using dynamic simulations in PSCAD and considering the detailed switching model of a MMC STATCOM model. The validation was performed for several study cases, including sensitivity analysis for different operating conditions and control strategies. The key lessons from the work may be summarised as follows:
- There is a requirement to have detailed models of converter-based devices when performing grid connection studies in Electro-Magnetic Transient (EMT) time scales
- Stability assessment studies need to be performed for varying grid strengths/short circuit levels at the point of connection as well as at transmission voltage levels.
- Various other parametric sensitivity studies should be considered regarding key control schemes, interface PLL technologies etc.
It was found that the detailed MMC-based STATCOM model provided stability margin for varying short-circuit ratio of the Point of Common Coupling (PCC)[WG1] which was more realistic. It may be seen that the stability boundary predicted by the admittance stability assessment methodology changes significantly, depending on the operating condition considered for the admittance frequency sweep. It was also found that the stability margin reduced for the same grid condition when STATCOM control transits from capacitive to inductive mode. The control delay also had impacts on the stability margin.
When performing stability assessment studies in large grids, it might be impractical to model in detail the entirety of the network. It may not even be required, particularly that part of the network which does not contribute to nor is impacted by the characteristic and evolution of the dynamic phenomena. Depending on the phenomena being studied, different partition and reduction techniques should be used to simplify computation by paying attention to both the relative strengths between subsystems and their connectivity. Such an approach is necessary when performing the proposed linear time-invariant (LTI) stability assessment based on the state-space representation of the grid for control interactions studies. More specifically, devices in the subsystem under study must be modelled in detail using their LTI models, while the rest of the grid is aggregated into equivalent Thevenin models at the subsystem's boundary busbars.
For studies related to system-wide phenomena, such as frequency instability or inter-area modes of oscillations, a different modelling approach should be considered. In that case, power electronic devices in different subsystems might contribute to system instability or poorly damped modes of oscillation. If a large system is considered, this will translate into a high dimensional state-space representation, which can make using large, state-space matrices difficult.
Dissemination
1. Nicolas Cifuentes, Mingyu Sun, Robin Gupta and Bikash C Pal, “Black-Box Impedance-Based Stability Assessment of Dynamic Interactions Between Converters and Grid" IEEE Transactions on Power Systems Under review
