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 (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 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 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 and shunt capacitor banks.  Simpler, less-expensive transmission lines, narrower ROW.  Requires termination at network locations with short circuit capacity of at least twice the converter rating.  Lower transmission losses.

3 III. ASYNCHRONOUS BACK-TP-BACK INTERCONNECTIONS There are five major asynchronous systems in the contiguous United States, Mexico and Canada. These consist of the Eastern System, the Western System, the ERCOT System, the main Mexican System and the Quebec . System. Existing interconnections between these systems are mainly by means of asynchronous back-to-back HVDC links at the network boundaries. Figure 7 illustrates the existing HVDC links in North America. The back- to-back ties between the networks can be clearly identified at the network boundaries.

Figure 7 Existing HVDC Links in North America

Rapid City Asynchronous Tie There are six existing back-back ties between the east and west systems in the US and Canada. The Rapid City Tie will be the seventh east-west Tie. As these HVDC links are located at the periphery of the ac networks, their ratings are relatively high compared to the available short circuit capacity at the point of interconnection. A new converter technology, however, allows higher converter ratings relative to the system strength at the point of connection thus opening up the possibility of transferring more power between networks with minimal need for new transmission lines. The Rapid City Tie will use capacitor commutated converters or CCC technology. This technology combines series compensation technique with classical line commutated converter technique. Integrated series compensation provides a simple means of compensating for the converter reactive power demand without switching filters or shunt banks. It also allows larger converters to be used in relatively weaker network locations, i.e., with short circuit levels as low as the converter rating itself.

The Rapid City Tie consists of two parallel 100 MW capacitively commutated converter back-to-back HVDC Links fed from a single radial 230 kV line on each side. The same technology is used on larger scale in the 4 x 550 MW Garabi interconnection between Brazil and Argentina. The Tie will enable its owners to exchange power from generation resources in the east to serve load in the west as well as exchange economy energy between the two systems. Figure 8 illustrates the CCC technology. Figure 9 illustrates the differences between conventional and CCC converters with respect to reactive power compensation

4 Smoothing Converter reactor

AC bus Figure 8 Capacitor Commutated Converter (CCC)

Contune AC filters Commutation capacitors

Control system

Conventional Q

converter filter

unbalance

Filter Figure 9 Reactive Power Balance I Filter d with Conventional and CCC CCC Stations Q

0.13 filter ConTune converter unbalance filter 1,0 Id

The attributes of CCC HVDC transmission are summarized below:  Precise Power Flow Control.  Improved stability at weak network locations.  Reactive power demand supplied by filters and series capacitor banks –no need for shunt bank switching.  Continuously tuned filters reduce the amount of shunt compensation required for filtering.  Lower dynamic overvoltage at load rejection  Can terminate at network locations with short circuit capacity as low as the converter rating.

IV. HVDC TRANSMISSION WITH DYNAMIC VOLTAGE SUPPORT Conventional HVDC transmission employs line-commutated, current-source converters which require an ac synchronous voltage source in order to operate. The conversion process demands reactive power from filters, shunt banks, or series capacitors which are part of the converter station. Any surplus or deficit in reactive power must be accommodated by the ac system. This difference in reactive power needs to be kept within a given band to keep the ac voltage within the desired tolerance. The weaker the system or the further away from generation, the tighter the reactive power exchange must be to stay within the desired voltage tolerance. Proper control of the converter and its associated reactive power compensation allows the ac system voltage to be held within a fairly tight and acceptable range. Unlike a generator or static var compensator, however, a conventional HVDC converter cannot provide much dynamic voltage support to the ac network. A new HVDC conversion technology

5 called HVDC Light, however, can not only control the power flow but also provide dynamic voltage regulation to the ac system.

HVDC Light conversion technology employs voltage source converters (VSC) with IGBT (insulated gate bipolar transistor) valves controlled with pulse width modulation (PWM). VSC converters permit independent control of real and reactive power. Reactive power control at each terminal is continuous and also independent of that at other terminal(s). Dynamic reactive power control can be used to support the interconnecting ac systems following disturbances and contingencies thereby increasing the transfer levels. Forced commutation with VSC even permits black start, i.e., the converter can be used to synthesize a balanced set of three phase much like a synchronous machine. Figure 10 illustrates the principle of PWM.

T1 I

T1o

T4o Figure 10 Voltage Source Converter

T4 U v with PWM U L

sustain only forward-blocking voltage + T  Valves :  conduct current in both direction 0  commutate between valves in the same phase T &T /T &T , T &T /T &T 1 1o 1o 4o 4 4o 1o 4o - T

Cross Sound Cable The Cross Sound Cable Project is an HVDC Light merchant transmission link with buried submarine cable which will interconnect the electric systems of New England and Long Island, NY. The CSC interconnection will provide additional bidirectional transmission capacity between New Haven, CT and Shoreham, Long Island. Transmission capacity is 330 MW at a HVDC transmission voltage of ± 150 kV.

1999 Light A 7-60 MW Up to ± 80 kV 1997 Helsjön Field p to Demonstration U to MW Up Project 60 MW 330 3 MW

5 converters under 1994 construction in 3 Start of Research projects with average Project rating of 250 MW 13 converters in commercial operation in 6 projects with average rating of 50 MW

Figure 11 Development of HVDC Light Transmission

6 The attributes of HVDC Light transmission are summarized below:  Independent real and reactive power control.  Continuous control of ac voltage  Dynamic voltage support  Superior stability at weak network locations.  Reactive power supplied or absorbed by converter.  PWM allows smaller filters.  No restriction on system strength  Black start capability.  No contribution to fault duties.

The attributes of HVDC Light transmission provide additional system benefits over and above those of getting around transmission constraints. The dynamic voltage control capability provides additional support for higher transfers on the interconnecting ac networks or allowing shutdown of uneconomic units run for local voltage support.

V. SUMMARY Transmission congestion drives up the cost of power to consumers. Recent generation development tends to lack diversity. Incentives to interconnect areas with significant differentials in power supply costs can improve the economy of system operation. HVDC transmission technologies available today offer the planner increased flexibility in meeting today’s transmission challenges. The latest developments in conventional HVDC technology as well as in alternative HVDC transmission technologies offer supplemental system benefits thereby increasing the potential for new applications.

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