The Voltage Response Curve (or for purposes of this article, the “VRC“) is what you get when you plot the voltage at or near a system node just before, during and immediately after an event involving a fault and subsequent clearing.  The VRC is a record of the dynamic response of the system.  It can be obtained from computer simulation, a well-isolated system test or a disturbance recorder.  In much the same way that planning and operations personnel look at swing curves (Note 1) to determine if a synchronous system is angularly stable, much can be deduced about the underlying system and its voltage stability by studying the VRC.

Shunt compensation, in the form of capacitor banks and static var devices (SVD), are commonly used to provide voltage support in heavily loaded systems.  Shunt devices offer a relatively cheap andeasy-to-implement solution to providing reactive power to load pockets or remote load areas of the grid.  In concept, one can add a combination of switched and controlled shunt compensation toincrease import capacity up to the thermal limit of the transmission system.  The savings from deferred investment in new transmission or congestion costs can justify the implementation of large shunt devices.  (The largest existing SVDs are a +500/-150 MVAR behemoth in the Allegheny Power service territory in Pennsylvania, USA, and the Chamouchouane SVC (actually two SVCs at one site) rated +330/-330 MVAR in Quebec, Canada.)

Ricardo Austria, “The Voltage Ledge,” California ISO Monthly Technology Seminar, April 27, 2007, Folsom, CA

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Moises Gutierrez, Ricardo Austria, “Developing Open Form User Models for Dynamic Simulation,” presented at PSLF Users Group Meeting, April 26-27, 2007, Chicago, IL

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DVRs, or dynamic voltage restorers, are a relatively new static var device that has seen applications in a variety of distribution and subtransmission applications.  DVRs are series compensation devices that protect electric load against voltage sags, swells, unbalance and distortion.  Though these devices may provide good solutions for customers subject to poor power quality, we caution regarding their application in systems that are subject to prolonged reactive power deficiencies (resulting in low voltage conditions) and in systems that are susceptible to voltage collapse.

The reason for the caution is that in many instances the main protection of networks against voltage collapse is the natural response of load to decrease demand when voltage drop.  The implementation of DVRs would act to maintain demand even when incipient voltage conditions are present thus reducing inherent ability to halt a collapse and increasing the risk of cascading outage and blackout.

Speculation over conversion of ac lines to dc goes back almost as far as HVDC itself. It’s an idea easily quashed by the fact that you pay for terminals rated at total DC MW and gain only the incremental capability over the ac case. Furthermore dc conversion idled a third of the ac transmission asset. And even if you’d cleared those hurdles, engineers were reluctant to take full advantage of dc’s control capability – a necessary step to make full use of dc’s capability in an ac system context. In any case construction of new ac lines were a vastly less inexpensive way to expand transmission capability.

So what’s new?

• New transmission is extremely expensive or can’t be built at all.
• FACTS, reconductoring, temperature monitoring are helping stretch ac capability but within limits  and often at a high cost in losses.
• Communication and control technology has made huge advances. Wide Area Management Systems (WAMS), coupled with very sophisticated FACTS options, are polite to talk about.
• An HVDC configuration has been introduced which makes full thermal use of all three phase positions of an ac line.

It’s time to look again at the conversion prospect; perhaps with a mind-set that considers dc as the ultimate FACTS device –  one that controls flow and boosts transfer capability of existing, in-place assets by up to 4:1.

Conventional wisdom says that the more motors connected to a feeder, the faster voltage will collapse when there is a reactive deficiency. This is true to the extent that voltages do drop faster, but the voltage may not fall all the way — so a voltage collapse does not occur. A different, and perhaps more problematic state is reached, when the feeder is in equilibrium point at a low per unit voltage.  This is operation on the Voltage Ledge.

Today’s popular commercial power system dynamics simulation (PSDS) software use explicit integration to handle the time-increment response of power system controls. The basis of this goes back to as early as 1960, when an integration technique known as the Fortran Analog Computer Equivalent, or FACE, was proposed. Explicit integration allows for simpler programming by assuming the response to stimulus can be represented in the next time step, thus avoiding iteration.  However, explicit integration leads to a slight numerical error that is cumulative.  This error leads to phenomena such as the “lie” and “drift” (illustrated in a case study below) encountered during dynamic simulation.

The renewables are here!  Whereas, power plants using renewable energy sources were not too long ago considered exotic, today they are the new face of energy — the wind mill replacing the smokestack as the symbol of electric power generation.  Spurred by governmental incentives, renewable energy sources are rapidly changing the nature and composition of power systems.  They are still a fraction of the overall energy portfolio, but the renewables’ level of penetration of energy markets is growing.  In most US RTOs and power pools, the queue for interconnection projects is dominated by renewables, primarily wind farms.

Our particular interest in renewables is their unique technical interconnection and operations impacts on existing power systems. Also, we are concerned about the long-term impact from the reliability planning perspective of integrating the system with high penetration levels of renewables.  Whereas renewables in practical systems were primarily hydro-based and geothermal, the new renewables are making their own statements.  These include wind farms (both onshore and offshore), solar and biomass.

The steel industry is just giddy on new and upgrades to steel mills.  Google the subject and you’ll find — the Minnesota Steel & Iron project in Itasca County, MN, the upgrade of the Pacific Steel Casting mills in Berkeley, CA, and Brazilian steel company CSN’s plans to build a steel-rolling mill in Kentucky, among others, in recent news.  For whatever the reason, mill developers see an increased demand for their product providing impetus for increased capacity.  A key economic factor for additional milling capacity is the availability of steady, low-cost power supplies for their mills.

This bodes well for the power industry in at least one respect — mills represent a high load factor customer, not a lot of MW installed required for the annual energy consumption.  However, they do pose new challenges to maintaining reliability and power quality.