Applying IEEE Std. 519-2014 for Harmonic Distortion Analysis of a 180 MW Solar PV Installation

April 19, 2017 : Blog, Industry News, Photovoltaic Systems and Distributed Generation, Power Quality, Tech Blog, Technical Papers

Pterra is presenting a paper on the above subject at the IEEE General Meeting 2017- Chicago 16~20 July.  Abstract of the paper follows:

IEEE updated its recommended practice and requirement for harmonic control in electric power system after more than two decades. The most updated version of the standard (IEEE Std. 519-2014) revised the 1992 version and its static harmonic voltage and current limits. Unlike the 1992 and the older versions of the standard, the 2014 version introduces a newer approach which considers the stochastic nature of harmonic distortions.  Furthermore, it recommends limits based on the number of times distortions may occur. For example, for the harmonic current distortion, it recommends three limits: daily 99th percentile, weekly 99th percentile, and weekly 95th percentile values. Applying the IEEE Std. 519-2014 for planning studies and for harmonic assessment of proposed projects can be very challenging because presently there is no known commercial tool which fully considers the stochastic simulations and limits required in the standard. This paper demonstrates the approach used by the authors in applying IEEE Std. 519-2014 to a harmonic study recently performed for a 180 MW solar farm.

Index Terms- harmonic analysis, harmonic filters, solar power generation, statistical analysis, time series analysis

Authors- Ketut Dartawan, Amin M. Najafabadi

Approaches to Complying with NERC Standard PRC-019-2 on the “Coordination of Generating Unit or Plant Capabilities, Voltage Regulating Controls, and Protection”

January 30, 2017 : NERC Compliance, Stability and Dynamic Modeling, System Operations, Tech Blog


The undesired outages of generating units during the July 1996 Outages in the Western Interconnection and the August 2003 blackout in the Eastern Interconnection have resulted in updates to reliability standards which secure, improve, and optimize generator response during power system disturbances. The North American Electric Reliability Corporation (NERC) has recently issued Standard PRC-019-2 which specifies reporting and review standards for generator protection coordination.  Because the skill requirements to conduct the review are not normally included in plant operations, outside experts are brought in that have a knowledge of what may be available in terms of information and data at the plant, the technical knowledge to conduct the coordination assessment and the experience to identify needs and deficiencies that are critical to presenting a credible review report.

In recent work, Pterra, acting as an external resource, developed approaches to conducting the review for compliance with PRC-019-2 for several legacy power plants.  Such power plants have been in operation for many years, but may have changed ownership at least once, and where test results and data may not be readily available.  This article discusses the general review approach, and applies this to a sample a 230-MVA Steam Turbine Generator Unit in a combined-cycle power station. more »

Ground Fault Overvoltage and Distributed Generation: Factors for Occurrence

January 18, 2017 : Power Quality, Renewable Energy, Tech Blog

In recent work performed by Pterra, the issue of ground fault overvoltage (GFOV) was raised in relation to integration of distributed generation (DG).  In particular, can inverter-based photovoltaic systems, connected in distribution feeders, induce GFOV on the high -side of the substation transformer?  And if so, under what conditions could this occur?  Pterra was engaged to conduct a research study by NYSERDA (the New York State Energy Research and Development Authority) to answer these very questions.  The resulting study and findings are documented and presented in the attached PowerPoint presentation.

Pterra ITWG – Phase I GFOV Study – 011717

For those who want to skip right to the answers: Yes, they can.  Subject to DG/load ratios, performance of surge arresters and interaction of inverter controls from different manufacturers.

Interharmonics Case Study: Nuisance Tripping in a 200 MW Wind Farm

October 18, 2016 : Distribution, Power Quality, Renewable Energy, Tech Blog, Uncategorized, Wind Power Integration

This topic was presented at the PSCAD Conference held October 6-7, 2016 in Houston, Texas.

Overview: A 200 MW DFIG wind farm is experiencing nuisance tripping.  These occur during switching of power factor correction cap banks comprising of 4×12 MVAR connected to the farm’s 34.5 KV collector buses.  Trip signals recorded by WTGs indicate power quality issues.  Harmonic distortion study of the Project did not indicate potential violations.

A more detailed analysis indicated presence of interharmonics.  The pdf file below presents the modeling, analysis and findings of the assessment.  Simulations were conducted using the PSCAD/EMTDC software, commercial product of Manitoba Hydro.

InterHarm_UGM16 (click to open pdf)

Siting of Power Plants: A Thermal Capacity Assessment for Grid Interconnection

July 13, 2016 : Blog, Power Flow Analysis, Power Flow Analysis, System Planning, Tech Blog, Technical Papers



For developers of power plants, one of the important factors to consider is where and how to interconnect a plant to an existing transmission network in order to reliably deliver its full output. For conventional power plants (i.e. coal, oil, natural gas, etc.), the availability of fuel supply and environmental permitting are the main considerations for siting. In the case of solar photovoltaic (PV) projects, given the availability of land area for mounting solar panels and sufficient solar irradiance, the point of interconnection (POI) to the grid can be the determining factor for siting. An assessment of the thermal capacity at potential POIs provides an effective screen for potential sites. Using transmission capacity injection analysis, developers can swiftly determine the capability of the existing network to support additional power from a new source such as a PV project. With this type of analysis, solar power project developers can know fairly early in the development process if the selected site and POI can support the plant’s output.
more »

Basics of Phasor Measurement Units

May 17, 2016 : phasors, System Operations, System Planning, Tech Blog, Technical Papers


State monitoring is of a great importance in power systems. Traditionally, Supervisory Control and Data Acquisition (SCADA) systems were used to monitor important system data such as voltage, current and line flows. SCADA updates measurement with a sampling frequency in the range of a few seconds. However, there is now a need for faster sampling and synchronization of measurements.
In 1988, Dr. Arun G. Phadke and Dr. James S. Thorp at Virginia Tech introduced the first Phasor Measurement Unit (PMU or synchrophasor). The core of the idea was to use an absolute time reference provided by the Global Positioning System (GPS) to synchronize data measured at different locations. This seemingly simple technical innovation soon became an important player in the power world. more »

Role of X/R Ratio in Circuit Breaker Short Circuit Duty Evaluation

January 30, 2015 : Blog, Distribution, Photovoltaic Systems and Distributed Generation, Tech Blog

Circuit breaker nameplates sometimes indicate only rating on symmetrical short circuit current. In such cases, the rating only reflects the AC component of the short circuit current. A common misinterpretation occurs when one compares the symmetrical short circuit current against the symmetrical short circuit current rating of the circuit breaker for the purposes of circuit breaker duty evaluation. This article provides pointers to avoid making the mistake.

Why is X/R Ratio Important?

Short circuit analysis is a critical piece of the engineering study for a power system. This analysis determines the maximum available fault current in the system, and hence the maximum level that the electrical equipment should be able to withstand.

When a short circuit occurs, the total short circuit current consists of:

  • ·        AC component (varies sinusoidally with time), also known as symmetrical current
  • ·        DC component (non periodic and decays exponentially with a time constant L/R;  L/R is proportional to X/R)
  • ·        The DC component makes the symmetrical current become asymmetrical.

The X/R ratio affects the dc component, and therefore, also the total current. The higher the X/R ratio of a circuit, the longer the dc component will take to decay (longer time constant). more »

Capacitor Bank Sizing for a Large Wind Farm Project

January 23, 2015 : Blog, New, Power Flow Analysis, Tech Blog, Technical Papers, Wind Power Integration

Pterra’s Amin Najafabadi, Senior Consultant, is presenting on the title subject. This is at the Electric Power Conference and Exhibition to be held in Rosemont, IL, April 23-25, 2015. Following is the abstract of the presentation.

Utilities generally require large wind power plants to meet power factor capability, similar to those of traditional generating facilities such as gas, steam, or coal power plants.  Reactive power compensation equipment such as capacitors, reactors, static VAR compensators, static synchronous compensators, and the like, are usually needed so that a large wind farm project can meet the ±0.95 power factor requirement at the point of interconnection. Switched capacitor banks serve as one of the more effective and economic solutions.

When sizing capacitor banks for a 200 MW wind farm consisting of Type III wind turbine generators, several challenges are encountered due to limitation of power flow programs used in industrial power system application. User’s customized functions and iterations may not be possible to be added to the programs. As alternative tools, Matlab and OpenDSS power flow engines are used in order to meet the requirement and to properly utilize reactive capability from each wind farm unit as well as to update main transformer impedance according to manufacturer specification. The results of the study showed capacitor banks needed for the project are about 30 percent less than the results from traditional power flow program. Simple switching schemes are tested and the voltage change due to capacitor switching is insignificant.

The full text of the presentation will be available at the conference or upon request via

Transient, Temporary and Ground Fault Overvoltages at Wind Farm Installations

March 4, 2013 : Tech Blog, Wind Power Integration

Wind farms by the nature of their design and operating characteristics are susceptible to a variety of overvoltages.  Hence it is always important to conduct studies and tests of the various levels of overvoltages and how the equipment at the wind farm are able to withstand with or without mitigation measures.  In this Blog, we will provide an overview of the issues, the analytical approach and potential mitigation.  Then, we demonstrate how these are applied to a sample wind farm. more »

Flicker Trouble Ahead for Solar PV Inverters?

February 22, 2013 : Photovoltaic Systems and Distributed Generation, Tech Blog
(Updated March 7, 2013 with additional text shown in red.)

The seemingly innocuous flickering of lamps could be a new technical battleground for the further growth and spread of photovoltaic (“PV”) electric power. On one side of the impending conflict is the flicker standard, a venerable reference that could very well trace its roots back to the advent of the electric age. On the other side are the new darlings of the power industry — environment-friendly, renewable solar power. The one thing about solar power is that in bulk amounts, its units need to be connected to existing electrical systems, and a side effect of this integration is the production of flicker. The more PV devices connected to the same electrical circuit, the more flicker is produced and the closer the level of flicker is to the allowable limit defined by the flicker standard. more »

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