Greg Zweigle

Synchrophasor-based power system protection and control applications

Synchrophasor data consist of analog and digital values with an associated precise time stamp. With precise time, these quantities are collected from various locations, time-aligned, and then processed as a coherent data set. Synchrophasors have generally been used for visualization and post-event analysis. However, new technologies allow synchrophasors to be processed in real time. Synchrophasor systems are now being used for realtime wide-area protection and control. This paper examines several ways synchrophasors are being used: · Voltage stability detection and correction · Load/generator shedding · Islanding control · Intermittent generation source control and grid interconnection Each application includes a discussion of how synchrophasors provided a unique solution and benefit over traditional solutions. Application performance, speed, data requirements, and equipment are also reviewed. We also discuss a future time-synchronized control solution.

Solar generation control with time-synchronized phasors

Solar-energy-based photovoltaic (PV) systems are a quickly growing source of distributed generation (DG) and connect to the power distribution system. PV-based DG poses challenges to grid reliability and power quality. One critical challenge is islanding control. Research is underway to devise best practice methods for anti-islanding for all power mismatch conditions. Synchrophasors provide an accurate means to detect islanding conditions by enabling precise, time-synchronized wide-area measurements. This paper presents an islanding detection system for PV-based DG. The system utilizes synchrophasor data collected from local and remote locations to detect the islanded condition. The paper shows how synchrophasors are used to control the DG during such conditions. It also discusses the power system modeling using a Real Time Digital Simulator (RTDS®) and closed-loop testing of the synchrophasor-based islanding detection system, which includes the PV-based inverter and the power distribution system. The effectiveness of the system was experimentally tested on a live power system.

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.

Interconnection control of distributed generation with time-synchronized phasors

Distributed generation (DG), such as solar-energy-based photovoltaic (PV) systems, wind generation, and other renewable resources, is a quickly growing source of energy for todays power system. These assets pose challenges to grid reliability and power quality. One critical challenge is islanding (inadvertent system separation) detection, separation, and eventual resynchronizing. Research is underway to devise best practice methods for appropriate islanding control for all load and generation conditions. Wide-area measurements such as synchrophasors provide an accurate means to detect islanding conditions by enabling precise time-synchronized measurements at diverse locations. This paper presents a real-time islanding detection system for PV-based generation that is representative of many types of DG. The system utilizes synchrophasor data collected at the PV location and elsewhere in the system to detect the islanded condition. The paper shows how synchrophasors are used to isolate the DG during such conditions. It also discusses the real-time modeling and closed-loop testing of the synchrophasor-based islanding detection system, which includes the PV-based inverter and the power distribution system. The effectiveness of the system was experimentally tested on a live power system. An evaluation is also presented for using synchrophasors to resynchronize the islanded system.

Adding shaft angle measurement to generator protection and monitoring

Traditional protective relays for generators have used electrical quantities (current and voltage) to measure the condition of the machine. It has long been recognized that information about the machine can also be used in protection. New technology makes it possible to combine mechanical and electrical inputs. This paper examines the use of rotor shaft angle measurement in a generator combined with the electrical angle of the output voltage. This provides for the direct measurement of system conditions that could only be estimated or approximated with earlier technologies. Some of the protection, control, and situational awareness applications now possible include the following: Subsynchronous resonance detection and mitigation : Out-of-step detection : Machine parameter estimation and validation : Transient stability control. One significant improvement over previous applications that provided these functions is that no physical connection or significant modification of the shaft is necessary. As power grids operate closer to critical stability limits, the ability to measure and control precise shaft angle will provide the high reliability necessary for electric power.

Model prediction based transient stability control

The paper proposes a transient stability control methodology for power systems that is based on model prediction control theory. Wide-area real-time measurements are combined with real-time model simulations to compute optimal control actions that are needed to stabilize a power system during large disturbances. Test results from Kundur two area test system and 39-bus New England test system illustrate the concepts.

Practical synchronized phasor solutions

Time-synchronized phasors provide solutions for power system analysis and control. This paper shares practical applications, using real-world implementations. Example systems include phase verification, measurement refinement, SCADA verification, communications channel analysis, frequency monitoring, improved state estimation, disturbance recording, black-start control, and protection.

Millisecond, microsecond, nanosecond: What can we do with more precise time?

Our industry moves energy at the speed of light, at the flick of a switch. A transmission line transporting 1500 megawatts delivers the equivalent of 250 pounds of coal per second, already converted into a convenient form of energy. For decades, we considered time in seconds or cycles: such as fuse curves and breaker clearing times. About three decades ago, our thinking moved into milliseconds because we needed to get better at quickly understanding wide-area events, protection was getting faster allowing for more power transfer, and the technology made it possible. Over the past decade, we have come to appreciate how synchrophasors can help us understand, control, and protect our power systems. One electrical degree at 60 hertz is about 46 microseconds, so measurements accurate to ten microseconds give us accurate synchrophasors. Traveling-wave technologies can put nanosecond resolution to good use. Achieving nanosecond absolute time is practical, affordable, and useful. In this paper, we explore how more-accurate time can improve the performance of electric power systems.

Synchrophasors for island detection

Detection and prevention of unintentional islands powered by distributed generators like photovoltaics remains a key concern of utility protection engineers, especially in high-penetration and grid-support environments. These evolving circumstances dictate that new island detection tools are needed. Synchrophasors are highly promising in this regard, but some in the community remain skeptical about them due to costs. This paper discusses synchrophasors, how they may be used to detect islands, their cost-reducing value-added features, and synchrophasor-based island detection methods.

Integrating synchrophasors and oscillography for wide-area power system analysis

The Hawaii Electric Light Company has successfully implemented an automated system to collect synchrophasor and intelligent electronic device (IED) oscillographic reports. They are realizing the benefits of analyzing these two data types together. This paper shares power system disturbances from their system. It also discusses the advantages of integrating time-stamped synchrophasor and IED oscillographic reports together in one power system analysis application. Modern IEDs provide high-accuracy, time-stamped power system measurements primarily in two formats—streaming synchrophasors and high-sample-rate oscillographic reports. Although historically these measurement classes have been treated separately, modern technology is bringing them together and enabling new applications. Synchrophasor measurements are a growing part of real-time operations at utilities during power system disturbances. They provide operators and engineers with a window into the present operating condition of the power grid that is not visible with the typical supervisory control and data acquisition (SCADA) measurement rates. Traditional post-disturbance analysis involves analyzing high- sample-rate oscillographic reports in order to understand the specific details of a disturbance and the IED operation during the disturbance. The advent of synchrophasors has driven the deployment of global timing signals to IEDs. Because of this, oscillographic reports are now time-stamped with the same high- precision timing source as synchrophasors. Combining synchrophasor measurements and IED oscillographic reports during post-disturbance analysis provides a wide-area context for a better understanding of the conditions leading up to and following a disturbance. Furthermore, integrating the collection of time-aligned IED oscillographic report summaries results in a system that automatically collects and displays these reports along with the synchrophasor data. These oscillographic report summaries can help further characterize the detected disturbance by providing IED information, fault location, fault current, and faulted phases to power system operations personnel.