Supplying a growing consumer demand is the foundation of the electric power industry. In recent years, coal generation has become a major source of contention among environmental groups, government and some utility consumers. In an effort to respond to these issues, renewable energy sources have taken center stage in providing clean generation.

Starting in the 1980s, renewable energy started to become the topic of discussion to supplement and replace some of the country's baseline generation. Some of the early wind and solar installations did not provide the level of generation required to make them a viable part of the transmission grid. Another issue was the effect renewable energy would have on the reliability and power quality of the transmission grid.

Wind Farms and the Grid

Wind has emerged as a major player in the renewable generation field and has evolved to provide a viable amount of load that can be integrated into the transmission grid. In the 1980s, wind generators contributed 50 kW to 100 kW per tower. By the 1990s, the contribution provided by each tower increased to 300 kW to 500 kW. Today wind turbines can provide 1.5 MW to 3. 5 MW per tower.

One of the issues with wind as an emerging power source is that wind does not supply firm power to the transmission grid. What is firm power? Firm power is the ability to provide generation to the transmission grid without interruption. Firm power generation such as coal, natural gas and nuclear provide power at a low cost, whereas renewable energy like wind and solar require renewable energy credits to be cost-competitive. Wind generator manufacturers are trying to expand the wind generators operating range to allow it to operate in a larger envelope of wind conditions. Other efforts are being made to improve the consistency of wind power by finding ways to convert the energy produced by the wind generator turbines into a form of energy that can be stored and used during the times of energy need. Another effort is to build enough wind generation sites in different areas, including the ocean, to provide enough generation to meet the need of the transmission grid should some of the wind facilities be down because of maintenance or lack of an adequate wind operating envelope.

While wind technology is advancing, one of the issues of wind power is its effect on the power quality of the transmission grid and the installations attached to it. In most cases, the wind generation sites of today are large and interconnected to high-voltage transmission systems through a substation that takes the collection grid voltage and transforms it to the high or extra high voltage of the transmission grid. This allows the substation relay protection schemes, automation and substation equipment to control and protect against frequency and voltage fluctuations, and to provide a stable and synchronized output to the transmission grid. However, a trend has developed among wind farm developers to divide their generation farms into several small-scale wind generation farms that can be attached to the distribution systems of most utilities.

An Illustrative Installation

Some states will allow small wind generation farms that fall under a certain power level to connect directly onto a distribution system. One example of this is the Texas Public Regulatory Commission's Rule 25.211 Interconnection of On-Site Distributed Generation, which allows wind farms with a capacity of 10 MW or less to connect on electric systems less than 60 kV. Wind generation connections at this voltage level can cause power-quality issues that become significant problems for utilities and customers. Some examples of power-quality issues that are encountered are flicker, possible reactive power (VAR) swings and voltage issues.

An example of this situation is two identical 10-MW wind farms interconnected on the same 34.5-kV feeder 1 mile (1.6 km) apart and 11.38 miles (18.31 km) from the substation. The substation is an interchange that contains a 115-kV bus fed by two lines and also contains a 115-kV/69-kV substation with two parallel transformers and an 115-kV/34.5-kV substation. The 115-kV to 34.5-kV substation is the grid interconnection source for the two 10-MW wind farms. The tap changer on the 34.5-kV transformer is programmed with line drop compensation to meet the needs of all the circuits during summer loading. Both of the wind farms are on the same circuit and provide 15 MW to 20 MW of power to the feeder. The same feeder serves three commercial facilities, irrigation load, residential loads and another substation. The peak circuit load is 8 MW and the average load is 5 MW.

The wind generation facilities have a 34.5-kV collector system and are interconnected to the transmission grid through a vacuum recloser with a relay control that provides overcurrent protection. The generation farm's supervisory control and data acquisition system provides the ability to disconnect the wind farm from the transmission grid when the transmission system is down as a result of an outage or when other adverse conditions are being experienced by the utility. The wind turbines contain protective relays that provide for low-voltage ride through, reactive power control and time synchronization.

Power-Quality Issues

One of the first power-quality issues encountered with the wind turbines was the effect they had on the paralleling scheme of the 115-kV/69-kV transformers in the interchange substation. The reactive flow on the 115-kV bus was high enough to affect the stepping of the transformer tap changers, causing them to get out of step by more than three steps. The paralleling scheme, which is a circulating current scheme, caused the tap changers to block stepping when the tap changers are four or more steps apart. The tap changers would lock out on a weekly basis because of out-of-step blocking. After examining the operation of the tap changers, it was discovered that the reactive power from the wind turbines was traveling on the bus and through the 115-kV/69-kV transformers, at which point the tap changers were seeing enough reactance to affect the tap changer scheme. After much research, it was determined that the paralleling scheme should be changed to a master/slave scheme, otherwise known as a lock-step scheme. This change allows the master transformer to react to changes on the bus and then communicate when to step to the other transformer. Once this change was made, the transformers operated in parallel and tap changer lock-out was mitigated.

The other wind farm-related issue concerned the effect on customers who were attached to the same circuit. Two of the commercial facilities were affected by high voltage and flicker, which caused their relays to lock out. Power-quality recorders were installed at the point of interconnection of the wind farms and at the affected customer sites. The recorders showed that when the circuit was lightly loaded and the wind generation facilities were running, a high-voltage condition was present on the circuit, which caused some of the commercial facilities' equipment to be locked out by overvoltage settings within their relays. This occurred on a consistent basis, causing one of the facilities to run half of its plant on generators rather than the power system.

A study was done and the results provided information on solutions that could be taken to mitigate the problem that the consumers were experiencing. One action taken was to adjust the reactive power control relays to tighter tolerances, which resulted in the wind turbines running near unity power factor on a consistent basis. The other action taken was to adjust the settings of the tap changer control at the 115-kV/34.5-kV transformer. The adjustments included changing the line drop compensation settings and the bandwidth and reaction time of the tap changer control. After both actions had been taken, the power-quality recorders showed the voltage to be within ANSI voltage limits. The commercial customers who were contacted indicated that their recording equipment showed an improvement in the voltage, allowing them the ability to return their load to the power system.

Changing the World

Wind generation and other renewable energy sources are improving and can be a viable part of the distribution or transmission grid. They provide a supplement of power that can help alleviate stresses to the current transmission grid. However, it is important for utilities to work with these renewable facilities at the onset of the project and design the facilities as well as upgrade the power systems to meet the impacts of this future generation. Power systems are becoming more sophisticated and complex, so it is important to look beyond the standard practices that have served us well over the past several years and embrace the possibilities that face us with renewable energy and the smart grid.

Futurist and author Joel Arthur Barker states: “Vision without action is merely a dream, action without vision just passes time, vision with action can change the world.”

As utilities, our world is changing, but it provides us with an opportunity to improve the way we deliver power to our customers, including the quality of the power. For a long time, power systems operated without much change; however, utilities have the opportunity to change the power industry much the same way as Nikola Tesla and George Westinghouse did when creating the first power systems.

Mike Swearingen (mikeswearingen@tri-countyelectric.coop) received a BS degree from the Department of Computer Science and Mathematics at Eastern New Mexico University. He is the manager of engineering and operations at Tri-County Electric Cooperative in Oklahoma. Swearingen is a member of the IEEE Power & Energy Society, the Computational Intelligence Society, Nuclear and Plasma Sciences Society, and the Standards Association.