Xcel Energy has Significantly Reduced Wind Farm Curtailments in the Buffalo Ridge Region by monitoring a thermal line constraint through the use of real-time line rating. Previously, wind farm production had been frequently curtailed on windy days, because the static (assumed) rating of a nearby 115-kV line was too low to support the flow that would transfer to the line if a particular contingency occurred in the region.

Xcel Energy (Minneapolis, Minnesota, U.S.) recognized that the actual line rating was typically much higher than the static rating and that using real-time ratings would significantly reduce needed curtailments. Xcel Energy installed the ThermalRate real-time line-rating system from Shaw Energy Delivery Services (Charlotte, North Carolina, U.S.) in May 2005 and has completed three summers of successful operation.

POST-CONTINGENT FLOW

Buffalo Ridge runs along the border of southwestern Minnesota and eastern South Dakota. Wind generation development at Buffalo Ridge has grown dramatically during the past 10 years (Fig. 1). The current transmission outlet capacity in the region is not large enough to support this rapid wind-generation growth.

Xcel Energy operators rarely have to curtail Buffalo Ridge generation due to actual line flow. However, on windy days, operators frequently have to curtail generation due to post-contingent flow, which is the line flow if the worst-case single contingency occurs somewhere on the system. The transmission system must be operated such that the post-contingent flow does not exceed the rating of the line. Therefore, wind generation was being curtailed whenever the post-contingent flow on a line was expected to exceed the static line rating.

The 115-kV line between the Lyon County and Marshall substations was identified as the line that most frequently constrained wind farm generation. The Lyon County-Marshall 115-kV line is 4.1 miles (6.6 km) of Penguin T2 397.5 kcmil conductor owned by the town of Marshall, Minnesota. Rebuilding the Lyon-Marshall 115-kV line was not a good option, since there are only two sources feeding the city of Marshall and the risk of taking one out for a long period of time was considered too great. Similarly, building a new line was also not considered a good option because of the cost and time needed to complete such a project.

Marshall is located within 20 miles (32 km) of Buffalo Ridge. Historical weather data shows that high winds at Buffalo Ridge correspond to increased winds at Marshall. The post-contingent flow for the Lyon County-Marshall 115-kV line is only a concern during periods of high Buffalo Ridge output. However, at times when wind generation is high because of strong winds, those same winds are typically providing line cooling and therefore increased rating. Conversely, the periods of low rating occur when wind generation is low, so full-line capacity is not required. Therefore, using the real-time line rating was considered an ideal solution for this situation. Use of real-time ratings for the Lyon County-Marshall 115-kV line would allow the Buffalo Ridge wind generation to be more fully utilized.

THE THERMALRATE SYSTEM

Shaw Energy Delivery Services' ThermalRate System was selected as the real-time rating system for three main reasons:

• Ease of installation — the line did not need to be taken out of service

• Simplified maintenance — it does not make contact with the live conductor

• Accuracy — it functions equally well at both high and low load and at any wind speed.

The system uses patented ThermalRate Monitors (TRMs) to determine a line's rating by measuring how actual weather conditions heat and cool a conductor. The TRM uses a conductor replica (Fig. 2) technique in conjunction with algorithms from the widely used IEEE 738 Standard for Calculating the Current-Temperature Relationship of Bare Overhead Conductors.

A ThermalRate System consists of one or more TRMs along the line path. Each TRM includes a sensor mounted at approximate line height, a controller mounted somewhere below the sensor, an antenna for radio communication and an optional solar power supply for installations where ac power is not available. TRMs are installed at critical locations along the line, and the lowest TRM rating is used as the rating of the line.

The TRM controller has a microprocessor that controls the sensor measurements, stores parameters of the actual conductor, calculates the line rating and supports DNP3 communications so the line ratings can be polled by a standard supervisory control and data acquisition (SCADA) remote terminal unit (RTU).

SYSTEM INSTALLATION

Personnel from Marshall Municipal Utilities, Xcel Energy and Shaw Energy Delivery Services selected three TRM locations based on the line orientation, ease of installation and availability of ac power. TRMs were installed at Marshall substation, Lyon County substation and the approximate line midpoint. The TRMs at the substations are powered by dc station power, while the midpoint TRM is solar powered.

Each TRM includes a spread-spectrum radio for reporting the ratings to SCADA. Each of the three TRMs responds to SCADA requests from an RTU at the Lyon County substation. The RTU includes logic to detect a communication failure as well as logic to report the lowest of the three TRM readings. Figure 3 shows the communications equipment installed in an existing cabinet at the Lyon County substation.

Each TRM is installed on a wood distribution pole set for this purpose. The TRM controllers are installed near the base of the pole for easy access. The TRM sensors are installed at a height of approximately 30 ft (48 km), so that they experience the same wind conditions as the line itself (Fig. 4). The 30-ft elevation was chosen because it is the average height of the lowest conductor under maximum sag conditions.

The TRM sensors are oriented in the same direction as the line section being monitored because wind direction is an important factor when determining line capacity. The TRMs at Marshall and mid-line are east-west to monitor the main length of the line. The Lyon County sensor is north-south to monitor a single span from the Lyon County substation.

OPERATING RESULTS

The ThermalRate System provides both normal and emergency ratings to Xcel Energy's SCADA system. Xcel Energy control room operators monitor actual flow and a continuously calculated estimate of post-contingent flow, and compare these values with the real-time ratings provided by the TRMs.

The actual (real-time) rating of the Lyon-Marshall 115-kV line during the summer of 2006 is shown in Fig. 5. Notice that the actual rating exceeded the static rating more than 96% of the time. Use of the real-time rating harnesses unused capacity by monitoring the cooling conditions at the line and calculating the actual ratings in real time. By using actual, rather than assumed, weather conditions, the real-time rating can simultaneously increase both reliability and wind output.

Figure 6 shows one of the many times when the rating system revealed additional line capacity and prevented wind curtailment. Similar situations typically occur 10 to 20 times per month. The figure shows actual line flow, calculated post-contingent flow, and both static and actual line ratings. The line is constrained by the post-contingent flow, so even if the actual load is below the static rating, Xcel Energy will reduce generation if the potential load (due to loss of some other line in the system) would go above the rating.

The actual rating determined by the ThermalRate System to date has never dropped below the actual or post-contingent flow. This is because the load is strongly related to the wind. Therefore, the operators have never had to intervene to reduce generation due to this line's rating. If there is a time in the future that the line rating drops below the load, ThermalRate would signal the operators to reduce wind generation.

Any time the post-contingent flow is greater than the static rating (and less than the TRM rating), a wind farm curtailment is avoided by the rating system. The value of this wind generation is determined by first calculating the energy that is between the post-contingent flow curve and the static rating.

Real-time rating is particularly effective in wind-generation applications. This has allowed Xcel Energy and Marshall Municipal Utilities to increase the Lyon County-Marshall 115-kV line rating and recognize previously unused capacity. The recognition of this capacity has allowed higher steady-state and post-contingent flows and greatly reduced the need to curtail Buffalo Ridge wind generation.

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Pam Oreschnick is a registered professional engineer in the state of Minnesota and has a BSEE degree from the University of Minnesota. She has worked for Xcel Energy since 2005 and has nine years experience in the electric utility industry. pamela.j.oreschnick@xcelenergy.com

THERMAL LINE RATING

The rating of an overhead transmission line is the level of electrical loading that the line can support through the daily demand cycles without either excess sag or loss of strength. A thermal line rating is a function of the weather conditions seen along the line, including wind speed, wind direction, air temperature, sun and other secondary influences such as precipitation and reflected sun from the ground and clouds.

The thermal rating of most lines is limited by line sag. As electrical current increases through an overhead conductor, the temperature increases, the conductor elongates and the line sags. The thermal rating is the maximum current that can be transferred on the line without causing the line to sag past the minimum clearance to ground.

The sag, and therefore the thermal rating, is a function of weather conditions, which in most cases are very conservatively chosen. Common weather assumptions are full sun, 40°C (104°F) air temperature and 2-ft/s (1.4-mph) wind speed perpendicular to the conductor. Conservative assumptions must be used for safety reasons, but in reality, the actual line rating is usually much higher than the assumed (static) rating. Therefore, using a static rating generally underutilizes a line's thermal capacity.