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Standing up to transmission reliability standards

By Drew Robb, GE Energy

New technology enables wind turbines to meet utility transmission reliability standards by maintaining system stability, reducing the risk of voltage collapse and minimizing grid disruptions.

Wind power has grown significantly in recent years. Currently, over 37 000 MW of wind capacity is in operation worldwide, an increase of over 6000 MW compared to 2002. During 2003 alone, Germany added 2000 MW, closely followed by the USA with about 1700 MW.

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With wind energy’s continuing popularity and growth worldwide, it has become a requirement by some utilities for wind turbines to comply with transmission reliability standards similar to those demanded of thermal generators. Transmission events, after all, are a fact of life on utility grids. Lightning strikes, equipment failures and downed power lines, for example, are not uncommon and generators are expected to cope with and, ideally, help recover from routine, and not so routine, system events.

“Until recently, wind turbines have had a single response to these instantaneous voltage drops: they trip off-line, protecting their functions until the grid recovers” said George Wakileh, a senior engineer at GE Energy. “If wind turbines keep running in the isolated part of the grid, it is very likely that the two separate grids will not be in phase after a short period of time.”

This immediate loss of generation, however, can impact system stability and lead to cascaded tripping. Perhaps even more importantly, tripping can result in lost generation.

In response, GE Energy recently introduced new power electronics which it calls “Low-Voltage Ride-Through” (LVRT). Developed to address transmission stability concerns caused by a wind turbine’s over-sensitive “tripping” response, LVRT allows wind turbines to “ride through” low voltage events in transmission systems without tripping off-line.

Interestingly, this advance was demanded by the energy mainstream. During a 2002 customer focus session initiated by GE, a customer suggested the concept of turbines that could ride through grid disturbances and remain on-line to provide added system support. That ultimately led the introduction of LVRT in 2003.

Figure 1. The difference between regular turbines and LVRT-enabled systems
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Grid disruptions

Grid disruptions can occur for many reasons and in some circumstances, recurring tripping can occur during the course of one day. “Islanding”, for example, is a situation that may occur if a section of the electrical grid becomes disconnected from the main grid, possibly because of accidental or intended tripping of a circuit breaker (due to lightning strikes or short circuits). Once the connection to the main grid is re-established it may cause huge current surges in the wind turbine generator. It also causes a large release of energy in the mechanical drive train (shaft, gearbox and rotor of the wind turbine).

LVRT was developed as a solution to such challenges and is the latest in a series of power electronics upgrades introduced by GE. It follows on the heels of Wind Volt-Amp-Reactive (WindVAR) electronics, a system that benefits rural and more isolated grids by maintaining system stability, reducing the risk of voltage collapse and minimizing the impact of grid disruptions. It achieves this by automatically maintaining defined grid voltage levels and power quality, compensating turbine output against grid voltage levels in fractions of a second as required.

LVRT goes further by feeding reactive power during grid disruptions, keeping wind farms online and generating power through these events. The electronic control system is designed to deliver ride-through capability at or below 15 per cent grid voltage for up to 500 milliseconds. In addition, beyond coping with the initial disturbance, LVRT remains engaged until after the fault is cleared, providing support to bring the system back to normal operating conditions.

“In developing this technology, we teamed with and fully utilized the talents of GE’s global research engineers. The team completely re-engineered the power electronics of our wind turbines to provide LVRT capability,” said Wakileh. “This entailed 38 alterations including changes to the main control cabinet of the turbine, its low voltage distribution panel, pitch system and power converter.”

As every system and sub-system has to be up during a grid event, it was not enough to develop a new box to add on to existing equipment. The development stage of the technology involved testing the entire system to verify that every component worked together and would remain on-line during a major disturbance.

Essentially, the LVRT-enabled turbine achieves this by optimizing all of the electronically controllable factors ” power converter, blade pitch system, motors, pumps, etc ” in order to have the turbine continue to operate properly and maintain high availability at low voltages.

The red line in Figure 1 indicates the response to a grid event of a normal turbine ” it cuts out immediately and stays off-line well after the event’s ending. The blue line, however, demonstrates how LVRT-based system responds. It instantaneously adjusts to the power disruption and recovers from it.

“Most wind turbines, with the exception of GE’s, require shunt capacitors to offset the hunger of wind generators to absorb reactive power and to provide power factor or voltage control at each turbine,” said Craig Quist, principal engineer, transmission planning at PacifiCorp. “As part of GE’s work with PNM at the New Mexico Wind Energy Center, it appears that the voltage ride-through capability has been significantly improved.”

Grid integration

As wind energy penetration increases, developers are going to have to face the reality that there will be little tolerance for a major source of energy that does not meet accepted transmission interconnection standards. With the advent of massive wind farms, utilities will not put up with turbines tripping off during fault instances. Apart from islanding and other grid situations, the potential revenue losses could be significant. “Wind farms are growing in size and the sector is rapidly commanding a larger share of the power industry’s supply portfolio,” said Wakileh, adding: “Transmission planners nowadays expect wind turbines to cope with faults in the high voltage grid, staying on-line, feeding reactive power, supporting system voltage, and helping the system to recover. LVRT successfully addresses these issues.”

For the power companies, fault ride-through improves system availability and reliability and ensures a dependable share of energy supply while preventing overloading of other parts of the network. At the same time it helps avoid system collapse, and enables the harnessing of wind to meet spinning reserve requirements. For operators, it increases the annual energy yield from a wind farm and thus overall profitability.

“Adding LVRT capability to our wind turbine product line is the latest in a continuing series of technology advancements that are helping to make wind power increasingly competitive in today’s energy environment,” says Steve Zwolinski, president of GE Energy’s wind segment, which developed the proprietary technology.

GE Energy is currently working to further improve LVRT to ride through 100 per cent loss of voltage for one second. Wind turbines currently running without this feature can be retrofitted.

Figure 2. The turbines in Fort Sumner are the tallest structures in New Mexico
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The proving ground

The first site to install and utilize the LVRT-enabled GE 1.5 MW turbines was recently opened at New Mexico Wind Energy Center. Now the world’s third largest wind generation facility, its 204 MW provides enough electricity to supply about 94 000 average US households. Owned and operated by FPL Energy, the $200 million wind farm comprises 136 turbines, each 64 m tall (making them the tallest structures in the entire state of New Mexico). The blades are over 33.5 m in length. The towers are sited along 24.1 km2 of land northeast of Fort Sumner. The project has been producing power since July 2003.

“Wind power is an attractive investment because the federal PTC and state incentives make it economically sound,” said Don Brown, a spokesperson for Public Services Company of New Mexico (PNM), an energy holding company based in Albuquerque, New Mexico that is selling the power from the project. PNM provides natural gas service to 441 000 gas customers and electric utility service to 378 000 customers in New Mexico. The company also sells power on the wholesale market in the western US. Combine the states’ 1 à‚¢/kWh credit with the 1.8 à‚¢ federal credit programme and you’ve got an economically viable operation.”

Brown also referred to a New Mexico Public Regulation Commission’s ruling that requires five per cent of investor-owned utilities’ retail sales to be generated through renewable energy by 2006. This increases to ten per cent by 2011.

“Our customers have asked us to be involved with renewable energy and we have responded with our PNM Blue Sky Program,” said Brown. This programme offers PNM retail customers the option of purchasing blocks of wind power for a slight premium.

Dozens of other sites across America also utilize LVRT-enabled GE turbines. The year 2003 saw GE carve out a 50 per cent-plus share of the US market, accounting for over 800 MW. FPL Energy, PPM Energy and Shell Wind Energy were the main companies developing and/or operating these sites primarily utilizing GE 1.5 MW with LVRT.

Outside of the USA, GE wind turbines are being used in new wind project developments from Ireland to Japan. A 15 MW wind farm on the coast of Hibikinada, Japan, for instance, came on stream in March 2003 using LVRT-enabled 1.5 MW turbines. The wind farm supplies power to Kyushu Electric Power Co. The project is located in the bay area on the northern tip of the island of Kyushu, within one of the major districts in Kitakyushu City, and is funded in part by the Japanese government to encourage renewable energy.

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