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2002 ABB ELECTRIC UTILITY CONFERENCE HVDC Technologies

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2002 ABB ELECTRIC UTILITY CONFERENCE HVDC Technologies 2002 ABB ELECTRIC UTILITY CONFERENCE PAPER IV – 3 POWER SYSTEMS HVDC Technologies – The Right Fit for the Application Michael P. Bahrman ABB Inc. 1021 Main Campus Dr. Raleigh, NC 27606 Abstract: Traditional HVDC transmission has often provided economic solutions for special transmission applications. These include long-distance, bulk-power transmission, long submarine cable crossings and asynchronous interconnections. Deregulated generation markets, open access to transmission, formation of RTO’s, regional differences in generation costs and increased difficulty in siting new transmission lines, however, have led to a renewed interest in HVDC transmission often in non-traditional applications. HVDC transmission technologies available today offer the planner increased flexibility in meeting transmission challenges. This paper describes the latest developments in conventional HVDC technology as well as in alternative HVDC transmission technologies offering supplemental system benefits. Keywords: HVDC, DC, CCC, VSC, PWM, RTO, Asynchronous, HVDC Light I. INTRODUCTION Economic signals arising from deregulation of the generation market have led developers and transmission providers alike to follow the path of least resistance much like the power flow over the network on which their mutual business interests rely. On the generation side, the developer has invoked a quick-strike strategy siting units where there is convergence of low-cost fuel supplies, relative ease of permitting, water supply, ready access to transmission and proximity to load. On the transmission side, the transmission provider has been preoccupied with cost reduction, compensation for stranded assets, potential under-utilization of assets and reacting to evolving regulatory mandates. Although such development may result in a short-term gain in new, economic power resources, the long term benefit is not all that clear. Over the last decade, the absence of clear financial incentives to invest in new transmission or diversified generation resources has skewed the economics of system development. New transmission construction or upgrades of existing lines have lagged load growth and generation development. This has led to transmission congestion, “land-locked” generation sites, and increasing dependence on one source of fuel supply. Traditional integrated generation and transmission planning has become more fragmented and provincial. Planning has often degenerated into a process of transmission assessment and simplistic generator interconnect studies rather than one of long-term, wide-area network optimization. When it comes to planning of new transmission, the old “can-do” attitude has for the most part been supplanted with one of “can’t-do.” If left unchecked, the result of this unbalanced development will be a higher overall cost of the power supply and increased market volatility when economic dispatch is curtailed due to congestion. The resulting increase in power supply costs will be born by the electric consumer downstream of the congestion. HVDC transmission offers an attractive means of bypassing interregional transmission congestion with a minimum of investment in new transmission. This is especially true where multiple ac lines with intermediate switchyards and reactive power compensation are required to achieve the desired stable transfer limit. Whereas, AC transmission will remain the primary solution for relieving congestion between immediate neighbors, HVDC is ideal for “leap-frogging” multiple network constraints. HVDC permits economical power exchange between distant high and low cost production areas and provides access to remote diverse power supply resources. Furthermore, the controllability of HVDC allows transfers to be made without increasing the burden on the underlying ac transmission system. 1 Figures 1 and 2 illustrate the complementary impact of transmission congestion between regions. Figure 1 depicts the physical impact of different transmission limits for an HVDC interconnection on the annual flow- duration curves. Figure 2 shows how these limitations affect the difference in regional spot prices between the two HVDC terminals over a given period wherein congestion constrains economic dispatch at the daily peak. PDCI(N2S) Power Flow Duration Curve Sorted Spot Price Differences Between Los Angeles (Rinaldi) Area When Price Cap is 250 $/MWH in 2004 and Pacific Northwest (Dalles) When PDCI Rating Changes in 2004 4000 PDCI-3100MW 250 PDCI-1650MW PDCI-3100MW 3000 PDCI-1100MW PDCI-1650MW 200 PDCI-1100MW 2000 1000 150 0 -1000 100 Power Flow (MW) -2000 Spot Price Difference ($/MWH) Difference Spot Price 50 -3000 -4000 0 1000 2000 3000 4000 5000 6000 7000 8000 0 Hours 0 100 200 300 400 500 600 700 800 900 1000 Hours Figure 1 Annual Flow Duration Curves for Different Figure 2 Regional Differential Spot Prices due to HVDC Capacities Congestion with Different HVDC Ratings One common concept regarding the economic benefit of HVDC transmission has been the so-called “break-even distance,” i.e., the distance where the savings in dc line costs over ac line costs pay for the increased cost of the HVDC terminals. For long distance transmission, however, this factor is often secondary since more than one ac line with intermediate switching stations, reactive power compensation and a number of other intermediate system reinforcements are required to equal the performance of a single bipolar HVDC link. Figures 3 and 4 illustrate this point for an existing HVDC project. Figure 3 shows the HVDC transmission required for delivering 500 MW of mine-mouth generation to a distant load center. The transfer was achieved using 464 miles of ± 250 kV bipolar HVDC transmission. Figure 4 shows the alternative AC transmission required to meet the same stability criteria. The equivalent AC transmission required two separate 345 kV AC transmission lines with an intermediate substation. Each line segment required 50% series compensation and increased ROW. The total transmission line length of the AC alternative was 940 miles. The HVDC alternative was selected as the more economic solution and clearly had less environmental impact. This example shows that the economic comparison goes far beyond the break-even-distance. Figure 3 HVDC Solution – 500 MW, ± 250 kV, Figure 4 AC Alternative – Two 345 kV lines, 50% 464 Mile Bipole Series Compensated, 940 miles With the right incentives, transmission providers, either traditional or merchant, should be able to develop strategic transmission assets for relieving congestion and reducing free market constraints. Congestion relief by means of transmission combined with increased emphasis on minimizing environmental impact by better utilization of existing transmission, by shared ROW, by underground transmission or by retiring “reliability-must- 2 run” generation has led to new applications for HVDC transmission. New HVDC technologies have been developed for these new traditional and non-traditional transmission applications. The following sections briefly describe three HVDC projects underway in the U.S. which each use a different HVDC technology. II. LONG-DISTANCE, BULK POWER APPLICATIONS Sylmar Replacement Project The Sylmar Converter Station is the southern terminal of the Pacific DC Intertie. The existing station consists of three different generations of converter equipment. The first generation, installed in the late ‘60’s, consists of the original converters with mercury arc valves. Each pole has three 133 kV Hg arc converters in series. The second generation of converter equipment consists of 100 kV series connected thyristor converters raising the transmission voltage from 400 kV to 500 kV to increase the power rating by 25 percent. The third generation of converter equipment consists of two parallel 500 kV, 550 MW thyristor converters, one on each pole bringing the total power rating up to 3100 MW. The Sylmar Replacement Project seeks to reduce the operation and maintenance costs, improve the reliability and seismic withstand capability, free-up real estate and replace vintage equipment with more environmentally friendly technology. Figure 5 shows a portion of the existing Sylmar Converter Station. The Project scope consists of replacing the two existing 550 MW converters with two new 1550 MW converters in the same valve halls, replacing the converter transformers with single phase, three winding units, replacing the valve cooling, replacing the control system and reusing the existing ac and dc filters with some minor revisions. The construction time is nine months. Once the new converters are in place, the mercury arc valves and other series converters can be retired. Figure 6 New Replacement 3100 MW Sylmar Figure 5 Overview of 2000 MW Portion of Existing Converter Station Using Existing 1100 MW 3100 MW Sylmar Converter Station Valve Hall The Sylmar replacement project will use much of the same high power converter technology used in the 3000 MW Three-Gorges HVDC Projects in China. This technology is ideally suited to bringing large blocks of power with minimal new transmission from remote, diverse resources. Such systems can be point to point or multiterminal, e.g., the Quebec – New England HVDC System. The attributes of conventional HVDC transmission systems are summarized below: Precise Power Flow Control, no inadvertent or parallel loop flows. No distance limitation due to instability. No reactive power demand of the transmission line. Reactive power demanded by converter stations and supplied by switched filters
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