Scott Manson

Considerations for generation in an islanded operation

This paper discusses the conceptual design and operation of an isolated power system, recognizing the reality that generator or turbine trips will occur. The level of reserve generating capacity must be set with proper balancing of capital expenditures and operating costs against revenue lost in a production shutdown. The way that reserve capacity is provided is as important as the amount of reserve; seemingly adequate reserve can turn out to be badly insufficient if it is not well distributed across the available reserve sources.

Case Study: An Adaptive Underfrequency Load-Shedding System

Underfrequency (UF) schemes are implemented in nearly every power system and are deemed critical methods to avert system-wide blackouts. Unfortunately, UF-based schemes are often ineffective for industrial power systems. Traditional UF schemes are implemented in either discrete electromechanical relays or microprocessor-based multifunction relays. Individual loads or feeders are most commonly shed by relays working autonomously. The UF in each relay is set in a staggered fashion, using different timers and UF thresholds. Sometimes, dω/dt elements are used to select larger blocks of load to shed. Unfortunately, no traditional schemes take into account load-level changes, system inertia changes, changes in load composition, governor response characteristics, or changes in system topology. This paper explains an adaptive method that overcomes known UF scheme problems by using communication between remote protective relays and a centralized UF appliance. This method continuously keeps track of dynamically changing load levels, system topology, and load composition. The theory behind the improved scheme is explained using modeling results from a real power system.

Case study: Smart automatic synchronization in islanded power systems

Automatic synchronizing systems are used to reconnect multiple islanded power grid sections. These systems are required to function automatically with minimal human supervision because they must dispatch multiple generators simultaneously to reduce slip and voltage difference at the interconnection point. This paper describes a smart automatic synchronizing system that can connect multiple generators in an industrial power system during islanded and utility-connected modes via six different bus-tie and utility tie breakers. The automatic synchronizing system performs matching by controlling multiple governor and exciter interfaces within the facility. Controlling slip and voltage difference across any of the six different breakers in any combination is accomplished with up to six different and simultaneous power system islands. The power system studied is a large refinery containing six generators, totaling about 260 MW of generation. The functionality, operation, and validation of this automatic synchronizing system using real-time-digital-simulator tests are discussed.

Considerations for Generation in an Islanded Operation

This paper discusses the conceptual design and operation of an isolated power system, recognizing the reality that generator or turbine trips will occur. The level of reserve generating capacity must be set with proper balancing of capital expenditures and operating costs against revenue lost in a production shutdown. The way that reserve capacity is provided is as important as the amount of reserve; seemingly adequate reserve can turn out to be badly insufficient if it is not well distributed across the available reserve sources. The dynamic behavior of reserve capacity, as much as the amount of capacity that is ultimately available, is critical in determining how an isolated facility will behave in the wake of a unit trip or the loss of a grid connection. In this paper, experiences with detailed dynamic simulations of a range of isolated systems are described. These are related to test work and operational incidents that have provided practical calibrations. Based on simulation and experience, some guidelines are offered for configuring generation and selecting strategies for maintaining stability in large, isolated continuous-process facilities.

Solving turbine governor instability at low-load conditions

During routine commissioning of a steam turbine load-sharing system, serious low-load frequency instabilities were discovered. These instabilities were causing undamped oscillations in power and frequency to escalate until protective relays tripped a generator offline. Root-cause investigation led to a robust solution and some rather startling revelations about the implications of electronic governor controls and small (micro) grids. There are basically two ways to form an electronic governor control loop with droop: 1) speed control with a MW droop; or 2) MW control with a speed droop. The analysis in this paper shows one method to be superior under low-load conditions. The results of this analysis have implications for the frequency stability of the power grid today. Microgrids, green energy, distributed generation, and isolated industrial plants can all be susceptible to this instability. The authors estimate that approximately 60% of todays generation is prone to destabilize the power system frequency under low-load conditions.

Microgrid islanding and grid restoration with off-the-shelf utility protection equipment

Automatic islanding and reconnection are commonly required at the point of common coupling between microgrids and macrogrids. Islanding systems open the point of common coupling during short circuits, open circuits, and dangerous backfeed conditions in the macrogrid. Automatic synchronizing systems provide reconnection by dispatching multiple distributed energy resources to reduce slip and voltage differences at the point of common coupling. This paper explains how commercial, off-the-shelf protective relays can be used to automatically island microgrids from and reconnect microgrids to the macrogrid.

Current Transformer Selection Techniques for Low-Voltage Motor Control Centers

This paper provides a clear set of procedures and equations to follow in optimal current transformer selection for low-voltage motor control centers. Methods of using protective relay settings to minimize current transformer cost and size are also shared. The selection criteria are explained from the fundamental principles of operation of a current transformer and a protective relaying device. This paper shows how the current transformer ratio, voltage knee points, and relay protection elements can be selected together simultaneously to provide a low-cost, high-performance system. This paper describes a case study in which the authors developed a simplified set of current transformer selection criteria for compact IEC low-voltage motor-control center drawers at a large oil and gas field in Central Asia.

Analysis of fault-induced delayed voltage recovery using EMTP simulations

This paper focuses on testing the dynamic behavior of single-phase air conditioner motors on distribution power networks. The primary goal is to study the phenomenon of delayed voltage recovery by applying multiple instances of a custom-built single-phase induction motor model on a given distribution feeder. This model was developed in an Electromagnetic Transients Program (EMTP) simulation environment. The motors were subjected to voltage disturbances seen in feeders experiencing the fault-induced delayed voltage recovery (FIDVR) phenomenon. To study the FIDVR phenomenon, a range of voltage depressions were simulated for predetermined system conditions. This paper describes a point-on-wave model development and simulation study that supports a broader investigation of the effect of air conditioning and similar loads on the recovery of electric utility voltage after faults.

Steam Production and Electric Power System Stability: Striking a Balance Using a Comprehensive Electric Dispatch Control Strategy

Steam production and electric power system stability are often competing interests in an industrial refinery. Optimal control of steam production is required to meet the plant process operating requirements, and electrical grid stability is required to prevent power system blackouts. For many industrial plants connected to a utility grid, both of these operating criteria cannot be met simultaneously, placing the power system in serious jeopardy of a blackout.

Load modeling assumptions: What is accurate enough?

This paper presents an elegant method for determining the simplest model of a power system electrical/mechanical load that will suffice for dynamic frequency power system studies and closed-loop simulation work. The strategy behind this technique is to supply the simplest load model possible that gives sufficiently accurate results for the goals of each unique modeling effort. The paper identifies the frequency characteristics of several different load types. It also identifies the level of load model detail required for testing typical power management systems, contingency-based load-shedding systems, frequency-based load-shedding systems, governor control systems, island/grid/unit autosynchronization systems, and exciter control systems. The paper describes how to lump loads without loss of fidelity, when an induction motor needs to be modeled as a single-cage or double-cage motor model, what sort of mechanical load model is appropriate, when we can assume zero inertia for a direct-on-line type of load, and how to verify the turbine/generator inertia and load inertia from field tests. This paper concludes with a simple reference that engineers can use to specify the level of detail required when modeling industrial power system loads.