Power System Dynamic Simulation Research Articles

Optimal sizing and operating strategies for VSG based BESS in ancillary service market

In response to growing power demand and rising environmental awareness, renewable energy (RE) resources are becoming increasingly common. However, the power output of RE is intermittent, as it strongly depends on the weather conditions; this increases the difficulties for the system operator (SO) to maintain the system frequency. This paper proposes an optimal sizing and operating strategy for a battery energy storage system (BESS) which integrates with virtual synchronous generator (VSG) technologies to participate in ancillary service (AS) market. The proposed method improves the traditional dual-stage architecture of BESS capacity planning problem to speed-up computation. However, the dimension and complexity of the optimization problem would increase accordingly. Therefore, the Electron Drifting Algorithm (eDA) is proposed to solve the proposed optimization problem and improve the quality of solution. To verify the feasibility, the IEEE 30-bus test system is employed for dynamic analysis by using the dynamic power system simulation software DIgSILENT PowerFactory. This paper provides a new business model to promote investments in BESS, not only yielding economic benefits, but also effectively increasing the robustness of the grid and further accommodating additional RE.

Real-time co-simulation of adjustable-speed pumped storage hydro for transient stability analysis

Pumped storage hydro (PSH) based generation of electricity is a proven grid level storage technique. A new configuration i.e., adjustable speed PSH (AS-PSH) power plant is modeled and discussed in this paper. Hydrodynamic models are created using partial differential equations and the governor topology adopted from an existing, operational AS-PSH unit. Physics-based simulation of both hydrodynamics and power system dynamics has been studied individually in the past. This paper demonstrates a co-simulation of an AS-PSH unit between penstock hydrodynamics and power system events in a real-time environment. Co-simulation provides an insight into the dynamic and transient operation of AS-PSH connected to a bulk power system network. The two modes of AS-PSH operation presented in this paper are turbine and pump modes. A general philosophy of operating in turbine mode is prevalent in the field when the prices of electricity are high and in the pumping mode when prices are low. However, recently there is renewed interest in operating PSH to also provide ancillary services. A real-time co-simulation at sub-second regime of AS-PSH connected to the IEEE 14 bus test system is performed using digital real-time simulator and the results are discussed.

Accurate power-flow initialization of double-cage induction motors using unified Newton-Raphson method

Summary An initialization of power system dynamic simulation using conventional power-flow model is not suitable for induction motor load due to unavoidable mismatch between prespecified reactive powers from power-flow calculation and the ones actually required by motor dynamic model. To eliminate the undesired mismatch, this paper presents a unified method for simultaneously solving power-network and motor variables in single frame of reference using Newton-Raphson algorithm for getting a good rate of convergence. An advanced inverse-Γ model of double-cage induction motors is newly developed to perform the initialization with minimum extra power-flow variables. An ability of the unified Newton-Raphson algorithm is demonstrated using IEEE-300bus and 118bus systems with large numbers of groups of induction motors. The studied results show that the unified algorithm yields a correct steady-state initialization without presence of reactive-power mismatch and also retains a good quadratic convergence characteristic.

Comparative Implementation of High Performance Computing for Power System Dynamic Simulations

Dynamic simulation for transient stability assessment is one of the most important, but intensive, computational tasks for power system planning and operation. Several commercial software tools provide functionality for performing multiple dynamic simulations such as those in contingency analysis simultaneously on parallel computers. Nevertheless, a single dynamic simulation is still a time consuming process performed sequentially on one single computing core as the tools were originally designed. Modern high performance computing (HPC) holds the promise to accelerate a single dynamic simulation by parallelizing its kernel algorithms without compromising computational accuracy. Parallelizing a single dynamic simulation is a much more challenging problem than the contingency-type parallel computing. It requires a good match between simulation algorithms and computing hardware. This paper provides guidance for such a match so as to design and implement parallel dynamic simulation to maximize the utilization of computing hardware and the performance of the simulation. The guidance is derived through comparative implementation of four parallel dynamic simulation schemes in two state-of-the-art HPC environments: 1) message passing interface and 2) open multi-processing. The scalability and speedup performance of parallelized dynamic simulation are thoroughly studied to determine the impact of simulation algorithms and computing hardware configurations. Several testing cases are presented to illustrate the derived guidance.

A Numerical Approach for Hybrid Simulation of Power System Dynamics Considering Extreme Icing Events

The global climate change leads to more extreme meteorological conditions such as icing weather, which have caused great losses to power systems. Comprehensive simulation tools are required to enhance the capability of power system risk assessment under extreme weather conditions. A hybrid numerical simulation scheme integrating icing weather events with power system dynamics is proposed to extend power system numerical simulation. A technique is developed to efficiently simulate the interaction of slow dynamics of weather events and fast dynamics of power systems. An extended package for PSS/E enabling hybrid simulation of icing event and power system disturbance is developed, based on which a hybrid simulation platform is established. Numerical studies show that the functionality of power system simulation is greatly extended by taking into account the icing weather events.

Dynamics Analysis for Hydroturbine Regulating System Based on Matrix Model

The hydraulic turbine model is the key factor which affects the analysis precision of the hydraulic turbine governing system. This paper discusses the basic principle of the hydraulic turbine matrix model and gives two methods to realize. Using the characteristic matrix to describe unit flow and torque and their relationship with the opening and unit speed, it can accurately represent the nonlinear characteristics of the turbine, effectively improve the convergence of simulation process, and meet the needs of high precision real-time simulation of power system. Through the simulation of a number of power stations, it indicates that, by analyzing the dynamic process of the hydraulic turbine regulating with 5-order matrix model, the calculation results and field test data will have good consistency, and it can better meet the needs of power system dynamic simulation.

Efficient and Precise Dynamic Initialization of Induction Motors Using Unified Newton–Raphson Power-Flow Approach

An accurate initialization is very important for power-system dynamic simulations. Using conventional power-flow model to initialize induction motors is not straightforward because an unavoidable mismatch between pre-specified reactive powers from power-flow calculation and the ones, actually required by motors. To overcome this problem, this paper presents a unified method for incorporating a nonlinear model of induction motors into Newton–Raphson (NR) power-flow algorithm, allowing precise steady-state solutions of power networks with induction motors to be solved simultaneously. The power-flow results of the extended algorithm are then compared with benchmark results for model verification. The ability of the extended algorithm is also demonstrated using IEEE-30 bus and 118 bus test systems with large groups of induction motor loads. The studied results indicate that the presented algorithm not only gives exact steady-state initialization without existence of the reactive power mismatches but also preserves powerful Newton–Raphson's quadratic convergence characteristics.

Three-Phase Dynamic Simulation of Power Systems Using Combined Transmission and Distribution System Models

This paper presents a new method for studying electromechanical transients in power systems using three phase, combined transmission and distribution models (hybrid models). The methodology models individual phases of an electric network and associated unbalance in load and generation. Therefore, the impacts of load unbalance, single phase distributed generation and line impedance unbalance on electromechanical transients can be studied without using electromagnetic transient simulation (EMTP) programs. The implementation of this methodology in software is called the Three Phase Dynamics Analyzer (TPDA). Case studies included in the paper demonstrate the accuracy of TPDA and its ability to simulate electromechanical transients in hybrid models. TPDA has the potential for providing electric utilities and power system planners with more accurate assessment of system stability than traditional dynamic simulation software that assume balanced network topology.

Power System Dynamic Simulations Using a Parallel Two-Level Schur-Complement Decomposition

As the need for faster power system dynamic simulations increases, it is essential to develop new algorithms that exploit parallel computing to accelerate those simulations. This paper proposes a parallel algorithm based on a two-level, Schur-complement-based, domain decomposition method. The two-level partitioning provides high parallelization potential (coarse- and fine-grained). In addition, due to the Schur-complement approach used to update the sub-domain interface variables, the algorithm exhibits high global convergence rate. Finally, it provides significant numerical and computational acceleration. The algorithm is implemented using the shared-memory parallel programming model, targeting inexpensive multi-core machines. Its performance is reported on a real system as well as on a large test system combining transmission and distribution networks.

Dynamic modeling and control of DFIG-based wind turbines under balanced network conditions

The performance of wind power station is researched by utilizing a detailed model which includes a wind turbine (WT), doubly fed induction generator (DFIG) and power electronic devices. In the initial stage, a comprehensive review and definition of each part of this system are presented. Then dynamic modeling and simulation of a sample power system are carried out. The operation of a DFIG coupled with WT under balanced condition of a power grid is investigated and stationary reference frame is utilized for analysis of a wind energy conversion system. At the second step, a wind power station is connected to the power grid in order to test the performances of the wind power station controller. The control plan utilizes stator flux oriented control and grid voltage vector control for the rotor and the grid side converter, respectively. MATLAB simulations clearly confirm the effectiveness of the control strategies.

Interaction and Coordination among Nuclear Power Plants, Power Grids and Their Protection Systems

Nuclear power plants (NPPs) have recently undergone rapid development in China. To improve the performance of both NPPs and grids during adverse conditions, a precise understanding of the coordination between NPPs and grids is required. Therefore, a new mathematical model with reasonable accuracy and reduced computational complexity is developed. This model is applicable to the short, mid, and long-term dynamic simulation of large-scale power systems. The effectiveness of the model is verified by using an actual NPP full-scope simulator as a reference. Based on this model, the interaction and coordination between NPPs and grids under the conditions of over-frequency, under-frequency and under-voltage are analyzed, with special stress applied to the effect of protection systems on the safe operation of both NPPs and power grids. Finally, the coordinated control principles and schemes, together with the recommended protection system values, are proposed for both NPPs and grids. These results show that coordination between the protection systems of NPPs and power networks is a crucial factor in ensuring the safe and stable operation of both NPPs and grids. The results can be used as a reference for coordination between NPPs and grids, as well as for parameter optimization of grid-related generator protection of NPPs.

Smart application for inter-area oscillations monitoring based on Newton-type algorithm

Summary A new Wide-Area Monitoring System (WAMS) application for monitoring inter-area oscillations, based on the Newton-type algorithm is presented in this paper. The core of this novel WAMS application is a numerical algorithm for the real-time estimation of the dominant inter-area oscillation mode, which processes Global Positioning System synchronized information obtained from Phasor Measurement Units installed in the power system. The parameter model is a highly nonlinear function, making this a particularly difficult challenge. Two data sets were tested using the new algorithm, one based on simulated models and the other based on real-life data records. The computer simulated model was based on a known architecture with signals obtained using a dynamic simulation of a multi-machine power system. The real-life data records used information collected on the FlexNet Wide-Area Monitoring System (FlexNET-WAMS) installed in the Great Britain power grid. Copyright © 2016 John Wiley & Sons, Ltd.

An efficient approach to handle the problem of load flow divergence in phasor based simulators

Dynamic behavior and stability of power systems can be studied with phasor-based programs in a reasonable time. Dynamic simulation of power systems needs to be initialized by Load Flow (LF) solutions. However, LF solutions may diverge in some critical conditions. This paper proposes an efficient Transient Stability (TS) based method to overcome the problem of load flow divergence in phasor-based power system simulators. The main idea is using the capability of TS module in such packages in order to overcome the divergence problem in fast-decoupled load flow (FDLF) method. This article focuses on LF convergence in networks with high R/X ratios branches such as distribution systems and microgrids. Suggestions for speeding up the convergence time and preventing of angle instability during TS simulation are described. The proposed methodology can be utilized in TS programs that use FDLF method for solving LF problems. The obtained accurate simulation results in a reasonable time show the efficiency and practical applicability of the proposed method.

Application of a battery energy storage for frequency regulation and peak shaving in a wind diesel power system

This study presents the modelling and dynamic simulation of a high penetration wind diesel power system (WDPS) consisting of a diesel generator (DG), a wind turbine generator (WTG), consumer load, dump load and a battery energy storage system (BESS). First the WDPS architecture and the models of the WDPS components are described. The WDPS is simulated in wind-only (WO) mode where the DG is not running and the WTG supply active power and in wind-diesel (WD) mode where both DG and WTG supply power. The simulation results are given showing graphs of the main electric variables in the WDPS (system frequency and voltage and active power in each component) and main battery variables (current, voltage and state of charge). The results in the WO mode show how the BESS, under the command of a proportional-integral-derivative (PID) controller, supplies/stores active power to regulate the isolated system frequency. The WD control enables the BESS to smooth the load and wind power variations, so that the isolated system power quality is improved. Also it is shown in the WD mode a peak shaving application where the control orders the BESS to supply active power temporarily to support system frequency in a DG overload situation.

Dynamic modelling and simulation of an autonomous hybrid power system: non-conventional energy systems

This paper presents dynamic modelling and response of a small 500 W hybrid energy system based on solar PV, wind turbine and fuel cell. This system contains a solar PV module, a 400 W wind turbine and a PEM fuel cell as energy sources. Energy output access from solar PV module and wind turbine is given to electrolyser which converts water in hydrogen and oxygen. Hydrogen is used as fuel in fuel cell stack. Ultra capacitor bank is used to reduce variations of fuel cell stack output voltage. Power conditioner unit converts DC into AC and makes system suitable for standalone applications. PID controllers control flow rate in fuel cell controller, regulate voltages in boost converter and inverter. The whole system is simulated in Simulink of MATLAB. The systems behaviour is analysed for changing wind velocity, solar irradiation and load demand. The results are analysed and discussed. The presented hybrid system is capable to control voltage fluctuations during transients. It is found suitable for standalone applications.

Analysis and Initialization of GE Wind Turbine Control Model

In this paper the analysis of a General Electric Wind Turbine Control Model (GEWTCM) and comparison with a Generic Wind Turbine Control Model (GWTCM) is presented. The analysis is performed for the GEWTCM stationary state. Based on the analysis, a systematic method for the GEWTCM initializations as well as a methodology for calculation of the GEWTCM operating point in steady state are presented. It has been shown that, due to lack of limiting of the pitch controller and pitch compensation, uniqueness of solution for initial and steady state values of all GEWTCM state variables are ensured except for the state variables of the pitch controller and pitch compensation. Conclusions from the analysis can help in the implementation of the wind turbine control model in power system dynamic simulation software packages in applications with variable wind speed.

100 kJ/50 kW HTS SMES for Micro-Grid

A 150 kJ/50 kW conduction-cooled HTS SMES have been developed. This paper presents the design and configuration of the SMES and parts of important test results. The magnet of the SMES is a solenoid type coil made of Bi2223/Ag tapes, and cooled to about 20 K by conduction-cooled method. To evaluate the performance of the SMES, a series of tests are successfully carried out in an electric power system dynamic simulation laboratory. The current carrying ability of the magnet, the active and reactive power exchange capability between the SMES and power system, and the power compensating effect of the SMES are experimentally investigated and some test results are given. The designed SMES is key equipment for the micro-grid demonstration garden of Yunnan electric power grid of China, and will be installed for field tests.

A novel VSC-HVDC link model for dynamic power system simulations

This paper introduces a new RMS model of the VSC-HVDC link. The model is useful for assessing the steady-state and dynamic responses of large power systems with embedded back-to-back and point-to-point VSC-HVDC links. The VSC-HVDC model comprises two voltage source converters (VSC) linked by a DC cable. Each VSC is modelled as an ideal phase-shifting transformer whose primary and secondary windings correspond, in a notional sense, to the AC and DC buses of the VSC. The magnitude and phase angle of the ideal phase-shifting transformer represent the amplitude modulation ratio and the phase shift that exists in a PWM converter to enable either generation or absorption of reactive power purely by electronic processing of the voltage and current waveforms within the VSC. The mathematical model is formulated in such a way that the back-to-back VSC-HVDC model is realized by simply setting the DC cable resistance to zero in the point-to-point VSC-HVDC model. The Newton–Raphson method is used to solve the nonlinear algebraic and discretised differential equations arising from the VSC-HVDC, synchronous generators and the power grid, in a unified frame-of-reference for efficient, iterative solutions at each time step. The dynamic response of the VSC-HVDC model is assessed thoroughly; it is validated against the response of a detailed EMT-type model using Simulink®. The solution of a relatively large power system shows the ability of the new dynamic model to carry out large-scale power system simulations with high efficiency.

Implementation of Parallel Dynamic Simulation on Shared-Memory vs. Distributed-Memory Environments

Power system dynamic simulation computes the system response to a sequence of large disturbance, such as sudden changes in generation or load, or a network short circuit followed by protective branch switching operation. It consists of a large set of differential and algebraic equations, which is computational intensive and challenging to solve using single-processor based dynamic simulation solution. High-performance computing (HPC) based parallel computing is a very promising technology to speed up the computation and facilitate the simulation process. This paper presents two different parallel implementations of power grid dynamic simulation using Open Multi-processing (OpenMP) on shared-memory platform, and Message Passing Interface (MPI) on distributed-memory clusters. The difference of the parallel simulation algorithms and architectures of the two HPC technologies are illustrated, and their performances for running parallel dynamic simulation are compared and demonstrated.

Speed Control System Model Parameter Error Evaluation Based on PMU Data

Power system dynamic simulation is an effective way to fully grasp the power system dynamic response, and it is also the basis guiding power system planning, design and operation. The operation of the generator characteristics determines the dynamic characteristics of the power system to some extent, so enhancing the accuracy of generating set model parameters has great significance on improving the reliability of power system dynamic simulation. Due to the operation of the power system characteristics and special requirements on the safe and stable operation, we cannot understand its correctness of dynamic response by means of exerting disturbance test, which limits the simulation error evaluation work of the generating set. PMU data from effective disturbance scenarios is used in this paper to study the realization of the generator speed control system of hybrid simulation technology based on data measured by PMU. The corresponding hybrid simulation program is developed by using C++. The evaluation of speed regulation system error of the model parameters is realized on the basis of the simulation error index. The validity of speed control system model parameter error evaluation methods is proved by PMU data measured from disturbance scenarios of four Units in an area.