Behzad Kazemtabrizi

A New VSC-HVDC Model for Power Flows Using the Newton-Raphson Method

The paper presents a new model of the VSC-HVDC aimed at power flow solutions using the Newton-Raphson method. Each converter station is made up of the series connection of a voltage source converter (VSC) and its connecting transformer which is assumed to be a tap-changing (LTC) transformer. The new model represents a paradigm shift in the way the fundamental frequency, positive sequence modeling of VSC-HVDC links are represented, where the VSCs are not treated as idealized, controllable voltage sources but rather as compound transformer devices to which certain control properties of PWM-based inverters may be linked - just as DC-to-DC converters have been linked, conceptually speaking, to step-up and step-down transformers. The VSC model, and by extension that of the VSC-HVDC, takes into account, in aggregated form, the phase-shifting and scaling nature of the PWM control. It also takes into account the VSC inductive and capacitive reactive power design limits, switching losses and ohmic losses.

A New STATCOM Model for Power Flows Using the Newton–Raphson Method

The paper presents a new model of the STATCOM aimed at power flow solutions using the Newton-Raphson method. The STATCOM is made up of the series connection of a voltage-source converter (VSC) and its connecting transformer. The VSC is represented in this paper by a complex tap-changing transformer whose primary and secondary windings correspond, notionally speaking, to the VSCs ac and dc buses, respectively. The magnitude and phase angle of the complex tap changer are said to be the amplitude modulation index and the phase shift that would exist in a PWM inverter to enable either reactive power generation or absorption purely by electronic processing of the voltage and current waveforms within the VSC. The new STATCOM model allows for a comprehensive representation of its ac and dc circuits-this is in contrast to current practice where the STATCOM is represented by an equivalent variable voltage source, which is not amenable to a proper representation of the STATCOMs dc circuit. One key characteristic of the new VSC model is that no special provisions within a conventional ac power flow solution algorithm is required to represent the dc circuit, since the complex tap-changing transformer of the VSC gives rise to the customary ac circuit and a notional dc circuit. The latter includes the dc capacitor, which in steady-state draws no current, and a current-dependent conductance to represent switching losses. The ensuing STATCOM model possesses unparalleled control capabilities in the operational parameters of both the ac and dc sides of the converter. The prowess of the new STATCOM power flow model is demonstrated by numerical examples where the quadratic convergence characteristics of the Newton-Raphson method are preserved.

An Advanced STATCOM Model for Optimal Power Flows Using Newton's Method

This paper presents the optimal power flow (OPF) formulation of a recent power flow STATCOM model . The new model puts forward an alternative, insightful interpretation of the fundamental frequency operation of the PWM-controlled voltage source converter (VSC), in an optimal fashion. The new model makes provisions for the explicit representation of the converters internal ohmic and switching losses which in the context of an OPF formulation, yields an optimum operating point at which these power losses are at a minimum. The STATCOM model possesses unparalleled control capabilities in the operational parameters of both the AC and DC sides of the converter. Such control modeling flexibility is at its best when expressed in the context of an OPF solution using Newtons method. The STATCOM equations are incorporated into the OPF formulation using Lagrangian functions in quite a natural manner for efficient optimal solutions using a single frame-of-reference. The inequality constraint set of variables is handled equally well using the multipliers method. The prowess of the new model is demonstrated using two sample systems.

Pumped thermal electricity storage for active distribution network applications

This paper introduces a new model for Pumped Thermal Electricity Storage (PTES) devices as an emerging thermal storage technology. PTES devices are capable of reaching higher capacities than battery storage devices and therefore are suitable for grid-scale storage at the distribution voltage levels. The new model captures the inherent thermal characteristics, such as the variable efficiency, of the PTES device, yet it is not computationally burdensome for integration into non-linear optimisation problem formulations. It therefore makes it suitable for operational planning studies in active distribution networks. The new model uses a two-stage regression of a detailed thermodynamic model of PTES to capture the approximate behaviour. The salient feature of this reduced model is that the variable efficiency is a function of the energy content — the state of charge — of the device. The new model is tested on a medium-voltage 33-bus distribution network within a dynamic optimal power flow formulation for day-ahead operational planning. The main objective has been to minimize daily cost of buying energy from the external grid. The results have been compared with the same test network without any storage devices and with storage models with fixed round-trip efficiency. In both cases the results clearly show the suitability and prowess of the new model in producing accurate operational cycles for the device and its benefits in terms of significant savings in operational costs when using large-scale PTES devices.

A new voltage instability detection index based on real-time synchronophasor measurements.

This paper demonstrates the performance of the Reactive Power Loss Based Voltage Instability Detection Index (QLVIDI), previously introduced in [1], under various testing conditions applied to the New England 39 bus test system using time-based simulations in the Power System Analysis Toolbox (PSAT) in MATLAB. The proposed scheme calculates an instantaneous time series of reactive power losses taking into account the direction of active power flow in every node in the system by considering the reactive power limits of the generators. This time series data is then used to evaluate a cumulative reactive power loss and the degree of deviation from the base case loss for developing QLVIDI. The effectiveness of the proposed index in early detection of imminent voltage collapse scenarios has been tested in various cases and also compared with already existing Improved voltage instability monitoring index(IVIMI).

Reliability Evaluation of New Offshore Wind Farm Electrical Grid Connection Topologies

In this paper, a composite generation transmission approach has been chosen for adequacy evaluation of wind farms integrated with conventional generating units. It has been assumed that the failure characteristics of a turbine’s underlying subassemblies possess the required Markov properties to be modeled as Markov processes. The wind farm along with the offshore connection grid has then been modeled in a sequential Monte Carlo simulation which also contains sequential chronological models for wind speed and load level variations over a specific period of time. The reliability of the wind farm is then evaluated against a series of load- and energy-based indices as well as overall productivity of the farm with different connections. The results indicate a potential improvement in the reliability of wind power output when in cluster-type topologies, where the converters are grouped and maintained in a central offshore platform, in comparison to conventional string-type topologies. Academic researchers involved in reliability modeling and risk analysis of renewable sources of energy, particularly wind, as well as industry professionals both in wind turbine manufacturing and operation and maintenance of offshore wind farms would equally benefit from the degree of detail and accuracy provided by the models presented in this paper.Copyright

Coordinated control of DC voltage magnitudes and state of charges in a cluster of DC microgrids

This paper presents a modified approach for the control of DC voltages and State of Charges (SOC) of batteries in an interconnected cluster of DC microgrids. First, the model of a DC microgrid is reviewed, and then the proposed control method is presented. This method is composed of two compensation terms that are introduced in the secondary control level in the hierarchical control framework of the microgrids. These compensation terms are used to control the voltage throughout the cluster, and also to equalize the SOCs over the cluster. Even though this method is proposed as a centralized controller, it is shown that it is possible to implement it in a decentralized manner, using a consensus algorithm to estimate the average state of charge locally. Moreover, a modification is proposed that gets the reference SOCs from the tertiary control level, and tries to control storage units based on these references. Simulations ae carried out and the results are provided. These results show the effectiveness of the proposed methods and modifications.

Operational planning and optimisation in active distribution networks using modern intelligent power flow controllers

In this paper, a centralised control framework is introduced for day-ahead operational planning of active distribution networks which accommodate high levels of distributed generation resources. The purpose of the framework is to plan network operation in order to minimise power curtailment from distributed generation and maintaining acceptable levels of voltage regulation throughout the network. For this purpose, both power flow control and rapid network reconfiguration have been considered as various acceptable levels of control available to the network operator to provide required levels of operational flexibility. The power flow control within the network is promised by the application of fully controlled back-back voltage source converters placed in key points (both normally-open and normally-close) in the network. Meanwhile, the network reconfiguration constraints guarantee that radial topology is always maintained in order to avoid tremendous changes in the protection system coordination. The operation of a modified 33-bus system exemplar is analysed in three case studies namely, passive network (base case), active network using remote-controlled switches and active network using intelligent power converters. Results show a significant saving in terms of operational costs as well as transmission losses in active cases despite the radial constraint condition in place.

An advanced STATCOM model for optimal power flows using Newton's method

Summary form only given. This paper presents the OPF formulation of a recent power flow STATCOM model [1]. The new model puts forward an alternative, insightful interpretation of the fundamental frequency operation of the PWM-controlled Voltage Source Converter (VSC), in an optimal fashion. The new model makes provisions for the explicit representation of the converters internal ohmic and switching losses which in the context of an OPF formulation, yields an optimum operating point at which these power losses are at a minimum. The STATCOM model possesses unparalleled control capabilities in the operational parameters of both the AC and DC sides of the converter. Such control modeling flexibility is at its best when expressed in the context of an OPF solution using Newtons method. The STATCOM equations are incorporated into the OPF formulation using Lagrangian functions in quite a natural manner for efficient optimal solutions using a single frame-of-reference. The inequality constraint set of variables is handled equally well using the multipliers method. The prowess of the new model is demonstrated using two sample systems.

A New Simulation and Control Model for a Variable Speed Variable Pitch Direct Drive Large Offshore Wind Turbine Generator With Integrated Energy Storage

A comprehensive control and simulation model has been presented in this paper for a large multi-megawatt Variable Speed Variable Pitch Wind Turbine Generator (WTG) with Energy Storage System (ESS) integration. The generator is of a Permanent Magnet Synchronous Generator (PMSG) type which is then decoupled from the grid using a Fully-Rated back-to-back Converter (FRC) link. A Rotor Flux Oriented Control (RFOC) strategy has been adopted in order to extract the maximum output electromagnetic torque from the generator according to a Maximum Power Point Tracking (MPPT) profile for any wind speed lower than rated wind speed. The MPPT method presented in this paper relies on the turbine’s optimal output torque curve. The ESS makes provisions for enhanced performance especially during faults on the system. Dynamic performance of a super-capacitor makes it ideal as the integrated ESS in the WTG for satisfying such performance constraints. Results indicate that the WTG integrated with a super-capacitor ESS provides the required enhanced performance in terms of DC link voltage stability and limiting output power fluctuations during sudden variations in the wind speed as well as enhanced Fault Ride-Through (FRT) capability.Copyright