By Neil Thomson, SaskPower, and Dan Lawry, Shaw EDS
SaskPower has been able to increase the power capacity of a 72-kV overhead line serving a potash mine in Tantallon, Saskatchewan, by installing a real-time rating system. The customer served by the line required a fast but inexpensive increase in rating to accommodate a load increase. SaskPower recognized that the actual line rating was usually higher than the static rating and that monitoring the rating in real-time would allow the utility to harness this unseen capacity. As a means to that end, the ThermalRate real-time line rating system from Shaw Energy Delivery Services was installed in July 2007.
Potash production is a major Saskatchewan industry. Potash refers to potassium compounds, the most common being potassium chloride (KCl). Potassium is the seventh most abundant element in the earth’s crust, and is a major plant and crop nutrient. About 95 percent of world potash consumption is used in fertilizers and the remainder for industrial purposes.
SaskPower’s 72-kV Tantallon TA3 line feeds a potash mine from Tantallon Switching Station. The line is 4.1 miles of 4/0 Penguin ACSR conductor and was built in 1955. The TA3 line runs nearly north-south, with a 5-degree angle change near the middle. It is in a fairly flat area in the northern extension of the Great Plains.
The TA3 line had been scheduled for replacement with a new line with larger conductor to accommodate a planned load increase at the mine. The mine, however, had a need for an interim load increase prior to the line’s replacement, and a quick solution was needed. The interim load increase would exceed the conductor’s summertime static rating. The static rating is based on the traditional “semi-worst case” approach to establishing a line rating, which assumes simultaneous 40 degrees Celsius (104 degrees Fahrenheit) air temperature, 2 feet/second wind, and full sun. Given the weather conditions that typically exist in the area of TA3, it was expected that the line’s actual rating would be significantly higher than the static rating under most circumstances. In addition, because of the mine load’s highly variable nature, there was a high probability that the worst-case conditions for line rating and the worst-case conditions for line loading would not coincide.
The ThermalRate System was selected because of the ease of application. In particular, it does not make contact with the high-voltage conductor. It had a low cost of implementation and required only a short time to implement (from identification of need to being in service). For this particular application, a single ThermalRate Monitor was sufficient and was installed within the Tantallon Switching Station fence, contributing to the low cost and ease of installation.
SaskPower has utilized structure raises and nip-and-tuck re-tensioning as solutions to increase line rating when the rating is constrained by inadequate ground clearance. However, in this particular circumstance these were not viable solutions because the rating was being constrained by the allowable conductor temperature. Restringing the existing line structures with a larger conductor was not a viable solution, which left replacement with a new line with larger conductor as the only conventional solution.
The ThermalRate System uses patented ThermalRate Monitors (TRMs) to determine a line’s rating by measuring the heating and cooling effect of actual weather conditions. The TRM uses a conductor replica technique in conjunction with algorithms from the widely-used IEEE-738, “IEEE Standard for Calculating the Current-Temperature Relationship of Bare Overhead Conductors.”
ThermalRate Sensor mounted on unused structure at Tantallon switching station.
A ThermalRate System consists of one or more TRMs along the line route. Each TRM includes a sensor mounted at approximate line height and in the vicinity of the line–but not touching the line. The sensor is oriented in the same direction as the line section being monitored because wind direction is an important factor for the line rating. The sensor is wired down to the TRM controller, which is mounted in a convenient location below the sensor. The controller has a microprocessor which controls the sensor measurements, stores conductor and rating parameters, calculates the line rating, and communicates the ratings via DNP3 to the utility SCADA RTU. TRMs are installed at critical locations along the line and each reports the rating to SCADA. The lowest TRM rating is typically used as the rating of the line. For remote installations where AC power is not available, the TRM can be powered by a solar power supply.
The system has quite a bit of flexibility. The user is able to set up emergency ratings of any duration, enter a specific maximum conductor temperature (100 degrees Celsius in this case), assign a de-rating factor if desired, and obtain status information. For this application, ThermalRate calculated the conductor temperature based on the weather conditions and real-time line current and provided this to the customer.
The equipment was mailed to SaskPower, and SaskPower did the complete installation. Since the line is short, the TRM could be located anywhere along the line route. SaskPower chose to install the monitor at Tantallon Switching Station on a 138-kV line dead-end structure that is part of a normally de-energized emergency bypass. This dead-end structure had convenient access to 110 VAC, and it was easy to do the installation on an un-energized structure. A mast on the control building or a distribution pole could have been used, but this structure was most convenient.
The TRM sensor is mounted at a height of 9 meters, which is equal to the conductor’s average height at maximum sag condition (rating condition). This height allows the sensor to experience the same wind conditions as the line itself. Since the line takes a 5-degree turn, the sensor is oriented in the average direction of the TA3 line.
ThermalRate Sensor mounted on dead-end structure.
The last span into the customer site turns east-west. This is a slack span, which also uses Penguin conductor. Since it is at lower tension, this span does not sag as much as the rest of the line when the line temperature increases. Due to this, the span could be ignored and a monitor was not needed for this span.
The TRM controller was installed near the base of the structure for convenience and easy access. The controller includes an internal spread-spectrum radio for reporting the ratings to SCADA. Even though the TRM is in the switchyard, the radio was used to tie it back into the RTU. The distance between the antennae is about 70 feet. The radio was faster and cheaper than getting a crew out to pull wire through an existing duct (if space existed) or trench a new duct from the control building.
The only equipment mounted in the control building is the small radio. This radio’s RS-232 output is connected directly to the SCADA RTU. The radio is powered by 110 VAC. If there were multiple TRMs on a given line, SCADA would poll the ratings from each monitor via this single radio.
ThermalRate controller, mounted in a convenient location below the sensor.
For this particular customer, SaskPower established an operating procedure based on conductor temperature. The ThermalRate data is saved into a mysql database, and a secure web portal using the corporate website was made available to the mine operator (see below). This allows the customer to monitor the line loading, dynamic rating and the conductor temperature.
Under the interim operating agreement with this customer, they are allowed to operate the transmission line up to a conductor temperature of 95 degree Celsius (203 degrees Fahrenheit). ThermalRate calculates this temperature, and if the conductor temperature exceeds 95 degrees Celsius, there is an alarm and a set procedure for the SaskPower operators to follow if the temperature does not return to allowable levels. In this way, by using actual rather than assumed weather conditions, real-time rating can simultaneously increase both capacity and reliability. The recorded data shows that the conductor temperature has been nowhere near 95 degrees Celsius.
A table and trending graph (see page 52) is generated and included on the website. Data quality information from the acquisition process is embedded into the table to assess the believability/quality of the displayed data. SaskPower grid control center operators have limited involvement. It is expected that the customer operates the plant using the information from the website so that there are no violations.
The system was put into service in July 2007, and the line’s dynamic capability was found to be significantly greater than the line’s static rating. The most significant factor between the static rating and the dynamic rating is the assumption of wind speed. Data collected from the system indicates that the line rating is typically higher during the day when the wind picks up, even though there is solar input and the ambient air temperature is higher.
Figure 1 (above) shows one of the many times when the rating system revealed additional needed line capacity. The figure shows actual line flow, as well as static and dynamic rating. The actual line flow exceeded the static rating multiple times within 24 hours, but the dynamic rating showed that the line had the needed capacity. During the day, the dynamic rating could get quite high, and SaskPower chose to put a limit of 481 amps (60 MVA) on the displayed dynamic rating.
Given the ambient conditions that actually occurred during the summer of 2007, the conductor temperature never reached the limit of 100 degrees Celsius. For example, in the period between Aug. 1, 2007, and mid-September 2007, the conductor temperature only exceeded 70 degrees Celsius 0.6 percent of the time.
The new replacement line for TA3 is scheduled to go into service in spring 2008. SaskPower is currently reviewing the ThermalRate application on TA3 to determine whether it is going to move the system to another location or reconfigure ThermalRate to operate with the new line. One of the objectives of continuing to monitor the line’s operation is to allow a longer term evaluation of the difference between the dynamic rating and the static rating of the line.
Neil Thomson has worked for SaskPower since 1975 in various areas including transmission system planning, resource planning, communication and control system planning, and power plant construction. He is currently responsible for transmission, sub-transmission, communications, and control and protection system planning at SaskPower.
Dan Lawry worked for Power Technologies Inc. since 1993 in the area of thermal up-rating of overhead lines and other outdoor power equipment. He now works for Shaw EDS doing thermal rating-related work and supporting the ThermalRate line rating system.