Hector Pulgar-Painemal

Power system modal analysis considering doubly-fed induction generators

This paper presents a modal analysis of a two machine power system that includes a doubly-fed induction generator (DFIG). The DFIGs model considers rotor flux linkages as dynamic states and stator flux linkages as algebraic variables. Active- and reactive-power controllers are also modeled. Active power is tracked for optimal power extraction from the wind. Using the power system set of differential-algebraic equations, eigenvalue trajectories are obtained when the load is varied. The system dynamics show little interaction between the DFIG and the synchronous machine (base case). Thus, system behavior is compared when the DFIG is replaced by (i) a hypothetical synchronous generator and (ii) a negative load (NL). Results show that an NL model resembles very well a DFIG dynamic model in the power system. A high sensitivity with respect to the parameters of the fast loop of the power controllers is observed. Lowering the parameters of the reactive power controller can actually make the system more stable. With respect to the loading at the Hopf point, its estimation is obtained with a low error by the NL model. These results have been verified using a 39-bus, 10-machine system. Using an NL model for representing wind power generators in power systems analysis can provide reasonable results while reducing simulation time and model complexity.

Dynamic modeling of wind power generation

This paper presents a dynamic model appropriate for power system analysis. This article shows modeling assumptions, derivation of a third order model for a doubly-fed induction generator and its controller models. Due to the detail level, it can be used as a tutorial for students and engineers that are new in this area. A four-bus system with one synchronous machine and one wind turbine is used to perform a small signal stability analysis. No considerable difference is observed between the modal behavior of our test system and a small system with just one synchronous generator.

Flywheel Energy Storage Model, Control and Location for Improving Stability: The Chilean Case

A flywheel energy storage (FES) plant model based on permanent magnet machines is proposed for electro-mechanical analysis. The model considers parallel arrays of FES units and describes the dynamics of flywheel motion, dc-link capacitor, and controllers. Both unit and plant-level controllers are considered. A 50-MW FES plant model is tested in the Northern Chile Interconnected System (NCIS) when connected to the Argentinian Interconnected System (AIS). The FES plant provides transient support for primary frequency regulation and its impact on stability is studied using small-signal analysis and time domain simulations. To identify the best location to install the FES plant, eigenvalue sensitivity for the interarea mode is analyzed. The results are validated for different operation scenarios of wind and solar power. By installing the FES plant in the NCIS best location, the damping ratio of the interarea mode is increased from 0.53% to 13.1%. Moreover, the power transfer from the NCIS to the AIS can be augmented from 90 to 180 MW while still keeping the damping ratio above 9%. These findings are promising and may lead to useful planning criteria for energy storage deployment.

Emerging issues due to the integration of wind power in competitive electricity markets

The integration of wind resources into power systems entails new challenges at all levels of the electricity industry. In this paper, we discuss critical issues emerging due to the incorporation of wind power suppliers in existing electricity market designs. We discuss the impacts of wind power intermittency and uncertainty on the planning and operation of the system and markets. In particular, we demonstrate how wind resource integration induces higher degrees of variability in the net load served by the conventional generators and may result in loss of load situations. Our findings provide supportive evidence for the need to rethink, upgrade and redesign the current electricity markets structures to accommodate deep penetration of wind power.

On Inertia Distribution, Inter-Area Oscillations and Location of Electronically-Interfaced Resources

This paper explores the relationships between inertia distribution, inter-area oscillations, and location of electronically-interfaced resources that are enabled with either damping or inertia emulation controllers (EIRs). A two-machine system with an EIR is used for analytical derivations. Explicit analytical expressions are found for: (a) the location of the center of inertia (COI), which depends on the H-inertia constant and voltage set-points of the machines, and (b) the residue of the system transfer function, which is convex in terms of the EIR location. These expressions are validated using full-order models for machines, exciters, and governors; the results support the idea of placing EIRs further away from the COI, or equivalently, in areas with low inertia to attain the highest possible oscillation damping. In the case of large-scale systems, an inertia distribution index is proposed which allows estimating the distance from any bus to the COI location. The efficacy of the proposed index is tested in a real system. The system areas with less inertia are proved to be the best places to deploy EIRs. The proposed index does not require computationally expensive calculations as those from modal analysis and it seems promising to better reinforce power system dynamics through EIRs.

Hybrid Controller for Wind Turbine Generators to Ensure Adequate Frequency Response in Power Networks

Converter-interfaced power sources (CIPS) are hybrid control systems as they may switch between multiple operating modes. Due to increasing penetration, the hybrid behavior of CIPS, such as wind turbine generators (WTGs), may have significant impact on power system dynamics. In this paper, the frequency dynamics under inertia emulation and primary support from WTG is studied. A mode switching for WTG to ensure adequate frequency response is proposed. The switching instants are determined by our proposed concept of a region of safety (ROS), which is the initial set of safe trajectories. The barrier certificate methodology is employed to derive a new algorithm to obtain and enlarge the ROS for the given desired safe limits and the worst case disturbance scenarios. Then, critical switching instants and a safe recovery procedure are found. In addition, the emulated inertia and load-damping effect are derived in the time frame of inertia and primary frequency response, respectively. The theoretical results under critical cases are consistent with simulations and can be used as guidance for a practical control design.

Bifurcations and loadability issues in power systems

This paper deals with bifurcations and loadability issues in power systems. A single machine system with dynamic state space represented by a set of differential algebraic equations is considered. It is emphasized that this small system can give a first step in understanding the issues discussed in this paper. Using system loading as a bifurcation parameter and considering two methodologies for calculating system equilibrium points — specification of a fixed terminal voltage and a voltage-controller reference — the incidence of voltage imposed by the machine in the system stability is studied. The results of this paper reveal that careful attention must be paid to all methodologies for calculating equilibrium points, as some may obscure likely solutions in the state space. In addition, a proper adjustment of a machine voltage-controller reference can be exploited to provide relief to a system when it is operating close to collapse.

Impact of Residential Photovoltaic Generation in Smart Grid Operation: Real Example☆

Abstract This paper assesses the impact of residential photo-voltaic (PV) generation on the operation of a distribution smart grid in Chile. In particular, we focus on distribution losses and bus voltage regulation. This smart grid is composed of three hundred residential customers and belongs to the Central Inter-connected System (CIS) of Chile. A set of scenarios with different daily load profiles and generating levels are considered. The demand of each client is obtained by measuring the total demand at the distribution transformer and is disaggregating it using consumption profiles. The possibility of reactive power injection through the PV power converters is also considered. To estimate its maximum impact, the reactive power injection is calculated by minimizing network losses through optimal power flow. The location of the PV generation is a random variable with uniform distribution, and the expected losses and voltage profiles are determined using the Monte Carlo method. Three levels of PV generation are considered, 5%, 10% and 15% of the total distribution load. As was expected, the voltage in all the buses of the system increases when reactive power is injected by the power converters of the PV generation. In addition, losses compared to the case without PV generation, decrease by 44% in the most favorable scenario and 1% in the worst case. An estimation for the 40% of residential customers in Chile, considering only 5% penetration of PV generation, give as more than 170.000 MWh of savings in generation per year.

Estimating inertia distribution to enhance power system dynamics

The understanding of power system characteristics and their impact on system behavior can lead to improved dynamic performances. Based on the Center of Inertia (COI) concept, this paper presents a practical study on the inertia distribution estimation, which can be used to both planning in long time scale and operation in short time scale, to meet with increasing renewable integration level in the future power system. Two indices are created to calculate the inertia distribution over the grid. Specifically, it is found that the proposed indices are highly related to the grid structure, and also they are affected by different parameters such as inertia constant, line parameters, terminal voltage set point of synchronous machines and models of exciters and governors. With these characteristics, the proposed indices have the potential to be applied on problems such as the placement of phasor measurement units or energy storage systems and generators coherency detection. Simulation results in a radial system, a meshed system and the IEEE 39-bus test system verify the characteristic of the proposed indices and show their potential applications in modern power systems.

Parameterized Modal Analysis Using DIgSILENT Programming Language

A parameterized modal analysis is proposed to evaluate power system small-signal stability and a simulation procedure using DIgSILENT programming language (DPL) is presented for this purpose. Modal analysis, or small-signal stability analysis, refers to the ability of a system to withstand small perturbations around an equilibrium point without reaching instability or displaying sustained oscillations. This is an important problem in real systems as sustained oscillations may cause mechanical failures in generating units and make the system vulnerable to the point of losing stability. In order to define the most critical scenario, the standard approach employed by the industry is to consider the system at its maximum loading. This chapter proposes that critical scenarios should be further sought as in even lighter loading conditions, low-frequency oscillations may dangerously appear. Using DPL, the system modal analysis is parameterized in order to search for critical operating points. To validate the importance of employing a parameterized modal analysis, a real case is studied: the Northern Chile Interconnected System. The obtained results applying parameterized modal analysis show that the system in a real operation only requires minor deviations from the programmed operating points to incur in dangerous low-frequency oscillations. These results validate the importance of searching for critical scenarios and DPL scripting is used to program the steps of the proposed analysis. The scripts and applications presented in this chapter set the basis for additional development to identify critical scenarios for modal analysis in other real systems.