Robert J. Nelson

Reactive power performance requirements for wind and solar plants

A current challenge faced by the electric utility industry is to determine how variable generation plants (wind and solar) should contribute to the reliable operation of the electric grid, especially as penetration of these resources continues its upward trend. Traditionally, bulk system voltage regulation has predominately been supplied by synchronous generators, and this is reflected in the language of industry requirements. Where variable generation is concerned the requirements are vague and unclear. The technology used in variable generation plants are capable of providing voltage support, but will require a shift from how these plants are traditionally operated. This paper discusses the capability of wind and solar plants to provide voltage regulation. It also examines the deficiencies in existing standards and provides recommendations to improve upon existing requirements in order to clearly define the role of variable generation in providing voltage support to the bulk electric grid.

Models for Change

Not much more than a decade ago, the desired action for a wind generation facility was to shut down or disconnect electrically in the event of a disruption on the bulk electric system. This, of course, was a reflection of the novelty that was wind generation at the time, as well as prudent engineering judgment, as the loss of a small amount of generation-the wind plants that were in service at the time-posed little risk to system reliability.

Reactive power interconnection requirements for PV and wind plants : recommendations to NERC.

Voltage on the North American bulk system is normally regulated by synchronous generators, which typically are provided with voltage schedules by transmission system operators. In the past, variable generation plants were considered very small relative to conventional generating units, and were characteristically either induction generator (wind) or line-commutated inverters (photovoltaic) that have no inherent voltage regulation capability. However, the growing level of penetration of non-traditional renewable generation - especially wind and solar - has led to the need for renewable generation to contribute more significantly to power system voltage control and reactive power capacity. Modern wind-turbine generators, and increasingly PV inverters as well, have considerable dynamic reactive power capability, which can be further enhanced with other reactive support equipment at the plant level to meet interconnection requirements. This report contains a set of recommendations to the North-America Electricity Reliability Corporation (NERC) as part of Task 1-3 (interconnection requirements) of the Integration of Variable Generation Task Force (IVGTF) work plan. The report discusses reactive capability of different generator technologies, reviews existing reactive power standards, and provides specific recommendations to improve existing interconnection standards.

Review of existing reactive power requirements for variable generation

In order to reliably operate the bulk electric grid generators are expected to meet certain reactive power requirements depending on the system they operate in. In the past reactive power requirements were tailored to the capabilities of synchronous generators. Variable generators such as wind and solar plants were in the past small enough relative to the entire system that they were not required to supply voltage support to the grid. As the penetration of renewable resources have grown beyond insignificance, it is now the trend that variable generators connected to transmission and subtransmission grids should be required to provide reactive power support. The goal of this paper is to educate the reader on the current state of reactive power requirements for variable generation. This paper discusses reactive power requirements from various regions across the world with a focus on those in North America.

Fault Ride-Through trip curves

Over the past decade, new requirements have been proposed and implemented for Fault Ride Through, including a proposed NERC standard that would impose fault ride through requirements on all generation. Most existing fault ride through curves are used in the wind industry. Some have some very unusual-looking trip characteristics that are polygonal in shape, rather than the characteristic rectangular trip curves customarily used by protection engineers. This paper will examine some of these trip characteristics, increasingly common in the wind turbine industry, and show how a rectangular characteristic can be used to satisfy a polygonal trip requirement. The use of common trip curves such as that proposed by NERC in PRC-024-1 may require some changes in generator plant design, as occurred in the wind industry, to insure that equipment is designed for low-voltage operation.