In various Blogs on this site, we have talked about unique technical issues associated with integrating wind farms into existing grids. This Blog now addresses the matter of transient overvoltages, or TOV, specifically with respect to potential risks to customers and any required mitigation associated with wind farms installations.

Unlike base load power plants such as nuclear and some coal plants which operate near full capacity for days at a time, solar photovoltaic (PV) and wind farms are variable resources whose output is dependent on the minute-by-minute change in weather conditions. For solar PV arrays, clouds and atmospheric interference are the sources of variability. While for wind power installation, gusts and weather patterns are the main culprits. This difference in operating characteristic for variable resources requires a novel approach to determining the impact of transmission capacity on the size of the plant.

Pterra conducts training in power technology subjects, not as a primary line, but in response to a perceived need. Occasionally, work in analytical consulting leads to knowledge and skills that clients and associates desire to acquire. And we are more than happy to oblige, if only to break the stream of days spent talking to computers (instead of people). Plus there is something strangely attractive in speaking to minds that are just exploring this lifetime field, electric power. We hope that most will stay on and help the industry. And we hope that some new insight will consolidate our own understanding of how electrons move. This is not to say that these courses are aimed for Gen X’ers alone. But a noticeable percentage who attend do come from that demo.

Yesterday was the 7th anniversary of the founding of Pterra, LLC.   The original team of 5 who started this journey remain, with some worthy additions.   All have grown somewhat older, hopefully wiser, and after all the contingencies encountered through the years, more resilient and united as ever.

Our core competencies remain the same: power engineering analysis, new technologies, modeling and simulation.   But service applications have grown, from the initial focus on transmission planning and interconnection of new generation, Pterra now offers distributed generation studies, solar photovoltaic and wind power modeling, applications training, assessment for high voltage direct current transmission, expert witness, among others.

No seven-year itch here.   Just some wistful reminiscing and cautionary tales for the next 70 years.   Overall, one can say that it is possible to follow the dream, to have a workplace adopted to family, health, faith, other life situations.   Or, to use an electric power analogy: to be like a lightning arrester, withstanding the normal and continuous challenges and allow all other extraordinary surges to flow.

If only for this one new feature, the trip to attend the meeting (held April 28-29 in sunny Orlando, Florida) was worth it. The new feature is …

PSLF now allows “continuous” tap solutions for phase angle regulators, or PARs. Why does this matter? It matters a lot to those who work in the U.S. Eastern Interconnection (EIC) where most utilities use the competing software package, PSS/E.

Distributed generation (DG) has become a viable option and is gaining wider acceptance to utilities, customers, and independent power producers. While DG offers many advantages, the interconnecting utility typically requires a system impact study for interconnecting DG to the existing electric grid to ensure it would not adversely impact the operation, reliability and safety of the grid. By its nature, DG would interconnect to lower voltage systems generally classified as “distribution”. The studies can range from relatively quick feasibility assessments to comprehensive studies involving extensive equipment and power system modeling, measurements, and detailed simulations. Specific topics for such studies include: islanding, steady state power flow, voltage regulation, short-circuit, protective relaying, power quality (flicker and harmonic), power factor, system stability, grounding, and ground fault overvoltage.

Distributed generators (DGs) are small generating units that are connected to the distribution network at voltages below 69 kV. DG units usually have capacities of 10MW or less, and are based on different energy sources, such as wind, solar and diesel. The distribution network is generally a radial system and designed for one direction of load flow, i.e. from the electric grid to the load. The unidirectional flow assumption is no longer valid when DG is interconnected at the customer or load side since the flow of power can now go in either direction: from the load side to the grid or from the grid to the load side. This fundamental change affects how an impact study, generally required to identify and mitigate any changes to reliability of the distribution system, for DG interconnection is conducted. Reference 1 summarizes several typical tasks required in an interconnection impact study.

by K. Dartawan, R. Austria

What is Distributed Generation (DG)? Unlike big generation stations connected directly to the utility’s transmission grid, DG is typically smaller, about 10 MW or less connected to the distribution network or customer side. The DG could be fueled by renewable sources such as photovoltaic (solar), wind, bio mass or could be non-renewable energy such as diesel or gas.

Figure 1

Wind farms are unique to power systems in that the construction and development time is much shorter than that of transmission lines and other bulk system facilities.  Wind farms can be placed into service well ahead of any planned upgrades, or even proposed non-wind power plants.  In these situations, the wind farms may be allowed to interconnect on a conditional basis or an energy basis; i.e., if congestion is present, they may be first to lose transmission access or have to share the available capacity with other generators, including other wind farms.  Hence, it is important to be able to estimate potential curtailment subject to transmission congestion.  In a previous article, we introduced the raw elements of the methodology for estimating curtailment of wind farms due to transmission congestion.  (See A Methodology for Estimating Potential Curtailment of Wind Farms, Pterra Tech Blog, September 2010).   We now look at the overall methodology applied for the purposes of making annual or seasonal projections of curtailment. 

by E. Cano, K. Dartawan, R. Austria
One potential impact of interconnecting distributed generation (DG) is the potential sympathetic tripping of overcurrent (OC) protection devices, where a healthy feeder trips unnecessarily for a fault on another feeder. The sympathetic tripping comes from DG with high short-circuit current contribution (typically rotating machines such as Diesel or Gas Turbine units) and can be observed in radial feeders that are fed from a common source.

Also, this issue applies to DG on a lateral-backfeed from the DG to the adjacent lateral circuit.