The objective of this paper is to provide a review of distributed control and management strategies for the next generation power system in the context of microgrids. This paper also identifies future research directions. The next generation power system, also referred to as the smart grid, is distinct from the existing power system due to its extensive use of integrated communication, advanced components such as power electronics, sensing, and measurement, and advanced control technologies. At the same time, the need for increased number of small distributed and renewable energy resources can exceed the capabilities of an available computational resource. Therefore, the recent literature has seen a significant research effort on dividing the control task among different units, which gives rise to the development of several distributed techniques. This paper discusses features and characteristics of these techniques, and identifies challenges and opportunities ahead. The paper also discusses the relationship between distributed control and hierarchical control.

Distributed Control Techniques in Microgrids

A simple equivalent circuit to represent the thermal heat flow equations for power transformers is presented. Key features are the use of a current source analogy to represent heat input due to losses, and a nonlinear resistor analogy to represent the effect of air or oil cooling convection currents. The effect was first quantified in 1817. It is shown that the idea of "exponential response" is not the best way to think of the dynamics of the situation. It is also shown that one can consider ambient temperature to be a variable input to the system, and that it is properly represented as an ideal voltage source.

A fundamental approach to transformer thermal modeling. I. Theory and equivalent circuit

The aim of this paper is to introduce hot-spot and top-oil temperature thermal models for more accurate temperature calculations during transient states based on data received in a normal heat run test (i.e., the top oil in the tank of the transformer and the average winding-to-average oil gradient). Oil viscosity changes and loss variation with temperature are taken into account. The new thermal models will be validated using experimental (fiber-optic test) results obtained at varying load current on a 250-MVA-ONAF-cooled unit, a 400-MVA-ONAF-cooled unit and a 605-MVA-OFAF-cooled unit. The results are also compared with the IEEE-Loading guide (1995) Annex G method.

Dynamic thermal modelling of power transformers

This paper focuses on a comparative study of the modeling and simulation of the first CIGRE HVDC benchmark system using two simulation tools PSCAD/EMTDC and PSB/SIMULINK; an interface between them (PSCAD-SIMULINK) has also been implemented and used as a simulator. The CIGRE HVDC system and its controller has been carefully modeled in all three simulation environments so that the differences are minimal. Comparison of steady-state and transient situations have been carried out, and a high degree of agreement in most of the cases has been observed.

Detailed modeling of CIGRE HVDC benchmark system using PSCAD/EMTDC and PSB/SIMULINK

Due to the characteristics of electric power generation, transmission and distribution in the U.S., experts have identified local distribution as a likely part of the chain to be adversely affected by unregulated PEV (Plug-in Electric Vehicle) charging. This paper presents a study performed to assess the impact of PEV charging on a local residential distribution transformer.

Study of PEV Charging on Residential Distribution Transformer Life

For pt.I see ibid., vol.16, no.2, p.171-5 (2001). This paper has two main objectives. One is to show that the top oil rise thermal model proposed in part I is valid, for a large power transformer in service. The second is to show that there is a convenient way of estimating the parameters without removing the transformer from service. A Manitoba Hydro 250 MVA OFAF transformer was chosen and instrumented with data-gathering equipment. Two twenty-four hour test runs were performed, one in February of 1999 and the other in July of 1999. The most basic parameter to be determined was the rated top oil rise but also found were the top oil line constant and the nonlinearity exponent, commonly given the symbol n. The results are very positive.

A fundamental approach to transformer thermal modeling. II. Field verification

This paper discusses the digital simulation of HVDC transmission and ac machines by an electromagnetic transients computer program. Ease in interfacing user written control system models with the network solution is presented along with discussion of means to simplify dc valve group representation in simulation. A working digital simulator has been developed enabling flexible in-house studies to be undertaken. Test results of its performance are presented.

Digital Simulation of DC Links and AC Machines

Electromagnetic transient simulation programs (EMTP-type programs) are powerful tools for the study of a wide range of power system transient problems. This paper introduces a novel tool in which an emtp-type program becomes the objective function evaluator for a nonlinear optimization algorithm. In this approach, the nonlinear optimization program is given control to perform several consecutive runs with a view to minimize (or maximize) the desired objective function, which is computed from the results of each simulation run. Since the optimization algorithm strategically selects the parameters for the emtp run, the overall design process is orders of magnitude faster than that possible from sequential or random (Monte-Carlo-type) multiple-runs of the EMTP-type program. The paper discusses the mechanics of the interface as well as design of objective functions. The power of the proposed method is demonstrated through two examples for the design of power-electronic converters.

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Optimization-enabled electromagnetic transient simulation

Catastrophic equipment failures continue to occur today due to ferroresonance even though this phenomenon has been extensively studied over the past 90 years. This paper is concerned with comparing analytical nonlinear dynamics methods with a two-dimensional brute-force bifurcation diagram for displaying safety margins in a two-dimensional parameter space. A simplified single-phase model is used to represent the case of ferroresonance between a transformer and circuit breaker grading capacitor. Comparisons are made between the analytical method and EMTP simulations of an actual ferroresonant event.

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Stability domain calculations of period-1 ferroresonance in a nonlinear resonant circuit

A new technique is presented for interfacing electrical machine models with electromagnetic transients programs. The machine models with their associated controls, loads or turbines can be assembled as subroutines by the user and interfaced to the electrical network or other machine models directly. The machine models employ the standard state variable equations and may use any integration technique for solution. The technique does not require a Thevenin equivalent circuit of the electrical network.

R. W. Menzies

Papers

A fundamental approach to transformer thermal modeling. II. Field verification

Digital Simulation of DC Links and AC Machines

Optimization-enabled electromagnetic transient simulation

Stability domain calculations of period-1 ferroresonance in a nonlinear resonant circuit

Improved Interfacing of Electrical Machine Models to Electromagnetic Transients Programs