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.
Figure 1
A wind farm integrated into a transmission grid is subject to curtailment due to temporary or long-term insufficient capacity on the transmission lines. Maintenance outage of a nearby line, dispatch of competing wind farms and availability of other generators are examples of system events that may limit injection capacity. In general, events that increase transmission utilization present potential curtailment conditions for wind farms, and so the daily and seasonal load cycles, and changes to interchange and import/export patterns can influence injection capacity as well.
In measuring the potential curtailment of a wind farm for, say, the incoming year, it is important to take into account the wind availability as well. It may seem likely that curtailment will occur when the load is highest and transmission use is greatest; however, this condition may occur in summer when wind availability is low. Hence, we have the common situation that at summer peak, the available transmission is low, but the wind capacity is also low, resulting in no or minimal curtailment. Some operating wind farms have observed that most curtailments occur in the spring and fall periods where grid use may be relatively low but wind farm capacities are high.
One approach to estimating potential wind farm curtailment is to simulate the hourly chronological performance of the combined generation and transmission system taking into account outages, unit commitment, least cost dispatch and load variations. This method is widely known as production simulation. In addition to being data intensive and laborious to setup, the simulation duration can be significant, especially if one chooses to run multiple years in a Monte Carlo simulation. This Blog presents a methodology that is based on an analytical model that is generally much simpler to develop than production simulation models and provides some unique insight into how and how often curtailments come about.
If we think about wind turbines as induction generators, one would assume that these would be VAR (reactive power) sinks, demanding vars from the grid to be able to deliver watts. However, that may be true from the point of view of only the wind turbines themselves. In reality, wind farms are far more than a group of small generators. Electrically, wind farms that deliver at bulk power levels to the grid behave more like a small urban subtransmission grid with characteristics that are far removed from those of a large power facility such as a coal, oil, nuclear or natural gas plant.
In this Blog, we discuss the amazing story of how a lowly 2,000 MW HVDC line was able to support a transmission capacity increase of 3,200 MW.
(This Blog is an introductory discussion of the AC power flow at a beginner level. Other Blogs on this site discuss more advanced aspects of the power flow, including convergence and alternative solution methods.)
The Power Flow is a steady-state representation of a meshed three-phase electrical network. It is sometimes characterized as a “snapshot” of electrical operating conditions given a set of assumed electrical customers (loads) and supplies (generators) linked together through a transmission system (grid). A single-phase equivalent of the positive sequence network is used since balanced three-phase conditions are assumed.
On Nov 10, 2009, a massive power failure blacked out Brazil’s two largest cities and other parts of Latin America’s biggest nation leaving millions of people in the dark. Transmission connecting the large Itaipu dam to Brazil and Paraguay apparently tripped disconnecting some 17,000 megawatts of power. I was on Copacabana Beach years ago for a training course and can only imagine the disruption that the outage may have caused. A blackout in a major city is not a fun time.
But blackouts are interesting to study. More often than not, the initiating cause is something innocuous, such as the infamous overgrown trees in the 2003 Northeastern US-Canada blackout. (An announcement just came out that the 2007 Brazil blackout that was blamed on hackers was due to sooty insulators!) So when the news report says, “A storm near the hydro dam apparently uprooted some trees that caused the blackout,” I am inclined to consider that the trees hit some transmission lines which could have led to the isolation of Itaipu. That’s not so far-fetched. You never know what a failure-bunching event such as the major storm that hit Itaipu could do to redundancy and good planning practice. Reliability is only as good as the next blackout!
The answer, which we’ll try to explain in this blog, is “plus or minus” if the DC line is being built to overlay an existing AC system. In such a situation, the DC line may continually carry 2,000 MW but the incremental transfer will not necessarily equal 2,000 MW.
A power flow that doesn’t converge is annoying, to say the least. For one, any information you try to use from a non-convergent solution is moot and questionable (recall that a power flow is a solution of a set of equations representing Kirchhoff’s Laws for electric circuits) since the condition it represents may not be physically possible. So what then to do about it?