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

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

By Ketut Dartawan and Wei Yang

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 »

Integrating Solar PV Power with Existing Distribution Circuits; Part 2

October 23, 2012 : Photovoltaic Systems and Distributed Generation, Tech Blog

(This Blog is a continuation of an ongoing series on integrating inverter-based solar photovoltaic generation with existing electric distribution circuits. Link to Part 1)

Solar PV (shorthand for photovoltaic) generation is growing in support and implementation in part because of a supportive regulatory environment. Among the more common types of interconnection terms are NEM and FIT.

more »

Harmonics Limit Amount of PV on a Distribution Circuit

June 4, 2012 : Photovoltaic Systems and Distributed Generation, Tech Blog

Harmonics is a very specialized and not widely understood topic in the electric power field which can become a major issue when inverter-based photovoltaic (“PV”) generators, (popularly referred to as solar power), are added to existing distribution circuits. This Blog provides a quick overview of the phenomena, potential negative impacts, causal conditions, and mitigating measures associated with harmonics. The bulk of the material presented here is based on an oral presentation at the SOLAR 2012 Conference of the World Renewable Energy Forum (WREF 2012) held last May 13-17, 2012, at the Colorado Convention Center in Denver.

more »

The Future of Transient Analysis is Bright

April 25, 2012 : Tech Blog

Yes, or at least, it’s brightening.
We make this bold observation after attending the 2012 users’ group meeting for the PSCAD/EMTDC software, held March 27-20 at a little gem of a coastal town named Castelldefels in Spain. About 60 participants (eyeball count) from universities, manufacturers, utilities, sysops, sales reps and consultants gathered together for techno-talk on the decidedly geeky subject of power system transients and PSCAD applications.

more »

On Engineering Software

March 29, 2012 : Industry News, Tech Blog

As an analytical consulting firm, Pterra regularly uses about half a dozen engineering software, and about the same number on an occasional basis, to be able to conduct its services. The software are necessary to be able to simulate complex physics and market conditions and/or large scale databases. In addition, we try to use the same software that our clients use so that part of our deliverable is an updated system model or database.

more »

Banking on grounding of solar and wind farms

March 6, 2012 : Photovoltaic Systems and Distributed Generation, Tech Blog, Wind Power Integration

One more item to add to the list of technical items to consider when interconnecting solar and wind farms is grounding. The grounding issue often appears when (a) integrating solar power inverters or wind turbines to existing distribution circuits, or (b) designing collector systems for solar and windpower farms.

more »

Solar Photovoltaic Inverters and Ride-Through Capability

February 7, 2012 : Photovoltaic Systems and Distributed Generation, Tech Blog

In study after study, we (Pterra) are encountering this seemingly mounting issue of ride-through capability in solar photovoltaic (“PV”) inverters. For now, the matter is isolated to frequency ride-through in small grids such as those that may be found in the Hawaiian islands. However, there is potential for this to be critical in even larger systems as the number of inverter-based PV arrays interconnecting to existing distribution and transmission systems increases.

The crux of the present issue relates to the frequency ride-through settings and capability for commercial PV inverters. The industry standard that addresses frequency settings for solar PV is IEEE standard 1547, Interconnecting Distributed Resources With Electric Power Systems, which specifies that (for 60 Hz systems):

  • For inverters < 30 kW, trip at frequency < 59.3 Hz clearing in 0.16 sec
  • For inverters > 30 kW, trip at frequencies 57-59.8 Hz clearing in 0.16 to 300 sec (adjustable setpoint), or at frequency < 57 Hz clearing in 016 sec

Certain commercial PV inverters are equipped with extended frequency ride-through capability that will not trip until frequency drops below 55 Hz.

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