segunda-feira, 20 de maio de 2013

HVDC breakthrough heralds a new era for renewables

Em continuação a um post anterior que introduzia o conceito de HVDC (High Voltage Direct Current).

Retirado do site RenewableEnergyFocus:

Gail Rajgor

Power technology group ABB says it has made a breakthrough in the ability to interrupt direct current (DC), solving a 100-year oldelectrical engineering puzzle, and paving the way for a more reliable electricity system.

This article is taken from the March/April issue of Renewable Energy Focus magazine. To register to receive a digital copy click here.

It took years of research, ABB says, but what it hails as the world’s first circuit breaker for high voltage direct current (HVDC) is now available. It combines very fast mechanics with power electronics, and is capable of ‘interrupting’ power flows equivalent to the output of a large power station within 5milliseconds - that is thirty times faster than the blink of a human eye, the firm points out.

“The breakthrough removes a 100-year-old barrier to the development of DC transmission grids, which will enable the efficient integration and exchange of renewable energy,” the company says. “DC grids will also improve grid reliability and enhance the capability of existing AC (alternating current) networks.”

Joe Hogan, CEO of ABB says the firm – which has been in discussions with power utilities to identify the first projects for the new development (see box within article, Mar/Apr 2013 issue) – has written a new chapter in the history of electrical engineering. “This historical breakthrough will make it possible to build the grid of the future. Overlay DC grids will be able to interconnect countries and continents, balance loads and reinforce the existing AC transmission networks.”

Renewable energy lifeline

HVDC technology is needed to facilitate the long distance transfer of power from hydropower plants, the integration of offshore wind power, the development of visionary solar projects, and the interconnection of different power networks, explains the company. Hence, the Hybrid HVDC breaker development has been a flagship research project for ABB (which invests over $1bn annually in R&D activities).

The breadth of ABB’s portfolio and combination of in-house manufacturing capability for power semiconductors, converters and high voltage cables (key components of HVDC systems) were, it says, “distinct advantages” in the new development. The company pioneered HVDC nearly 60 years ago and continues to be a technology driver. Indeed, with over 70 HVDC projects, ABB accounts for around half the global installed base, representing an installed capacity of more than 60GW.

Deployment of HVDC has led to an increasing number of point-to-point connections in different parts of the world. The logical next step is to connect the lines and optimise the network. So ABB is already working on the construction of multi-terminal systems and the latest DC breaker innovation, it says, is a major step in the evolution of HVDC grids. In parallel to the new hybrid breaker development, ABB has an established HVDC grid simulation center developing solutions for future DC overlay grid operations.

Magnus Callavik, technology manager for ABB’s grid systems business, says the new breaker is a ‘game changer’. “It removes a significant stumbling block in the development of HVDC transmission grids where planning can start now. These grids will enable interconnection and load balancing between HVDC power superhighways integrating renewables and transporting bulk power across long distances with minimal losses,” he says. “DC grids will enable sharing of resources like lines and converter stations that provides reliability and redundancy in a power network in an economically viable manner with minimal losses.”

In simple terms, ABB's new Hybrid HVDC breaker will enable the transmission system to maintain power flow even if there is a fault on one of the lines, Callavik adds. Being able to interrupt power flow within milliseconds is critical. It “helps protect the DC transmission system and prevent power outages in new low loss compact power superhighways,” he says. “Existing mechanical HVDC breakers are capable of interrupting HVDC currents within several tens of milliseconds, but are too slow to fulfill the requirements of a reliable HVDC grid.”

Compared with high-voltage alternating current (AC) grids, active power conduction losses are relatively low and reactive power conduction losses are zero in an HVDC grid. So, Callavik notes: “HVDC transmission remains a technology of choice for bulk power transmission over long distances with minimum losses. HVDC lines also require less space and are capable of going underground or underwater. Voltage source converter based HVDC applications in embedded AC grids and for offshore connections have grown substantially, in line with quantum leaps in power ratings and significant loss reductions.”

Harmeet Bawa, Head of Communications for ABB's Power Products and Power Systems divisions, agrees. “The evolving grid needs to be increasingly flexible and interconnected, as well as more reliable and intelligent to address new challenges like large-scale integration of renewables.”

Countries like Germany could well be the incubator for such a vision and has many of the elements that call for the development of a more interconnected grid, Bawa says. “Several studies including the recent Network Development Plan and network study II by Dena (German Energy Agency) have examined the feasibility of using HVDC technology to connect renewable energy sources in the north with load centers in the south. Such a DC connection could later be integrated with an overlay grid to augment optimisation efforts and conventional attempts to expand the AC grid. In fact, ABB engineers first conceived a vision for the European grid in the 1990s.”

A major stumbling block or missing link previously has been the absence of an HVDC breaker that acts fast enough to interrupt current and isolate faults and at the same time keeps losses to a minimum, but this has now been overcome by ABB with its latest breaker. “Of course, in addition to closing technology gaps, we also need political and social consensus, standards, funding models and a regulatory framework to be put in place,” Bawa cautions.

New orders for ABB HVDC technology

ABB announced in February that it has won an order worth around $260mn from US utility Bonneville Power Administration (BPA), a power marketing agency within the US Department of Energy, to upgrade the existing Celilo HVDC converter station in Oregon. This station is an important part of the electricity link between the Pacific Northwest and southern California commissioned more than 40 years ago in 1970, ABB says. The order was booked in the fourth quarter of 2012.

The Celilo converter station is located at the north end of the Pacific DC Intertie, also known as Path 65, which has a capacity of 3100MW and originates near the Columbia River. The Intertie is 846 miles long and connects to the Sylmar converter station in the south.

The Pacific DC intertie transmits electricity from the Pacific Northwest to as many as three million households in the greater Los Angeles area. During the winter, the north consumes significant quantities of power for heating while the south requires less, but in the summer, demand is reversed with more power needed in the south for cooling. The Pacific Intertie allows power to flow between the Northwest and Southern California, helping to balance supply with demand.

Key components of the state upgrade include valves, controls, transformers as well as switchgear and cooling equipment. In addition to modernising the converter station, the upgrade will also make it feasible to boost capacity up to 3800 MW. ABB carried out a similar upgrade of the Sylmar converter station in 2004.
“The converter station upgrade will enhance the reliability of this important HVDC link, thereby reducing the risk of blackouts and helping to secure power supply in the region,” said Brice Koch, head of ABB's Power Systems division.

Baltic integration

Shortly after winning the BPA contract, ABB also secured an order worth about $110mn from Litgrid AB, the state owned electricity transmission system operator in Lithuania, to supply and install a high-voltage direct current (HVDC) converter station. The 500MW back-to-back HVDC converter station will help connect the 330kV Lithuanian grid to the 400kV Polish grid, thereby integrating the electricity networks of the Baltic states with the continental European power grid and contribute to the development of an EU electricity market. At present, the Lithuanian electricity system is connected only with the grids of Latvia, Estonia and the countries to the east. The new link will also bolster the NordBalt HVDC connection with Sweden, currently under construction by ABB.

“In addition to facilitating energy trading between Poland and Lithuania, this link will strengthen grid stability and security of power supply in the region,” Koch said. ABB will design, engineer, supply and install the converter station including high-voltage equipment, such as power transformers and thyristor valves. The station will be built near the city of Alytus in Lithuania and is expected to be operational in 2015. A 160 kilometre-long 400 kV alternating current (AC) overhead line will link the HVDC station in Alytus with a substation close to the city of Elk in Poland.

About: Gail Rajgor is magaing Editor of Renewable Energy Focus"

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