A portable monitoring system is used for on-line thermal modeling of critical operating parameters.
In the search for ways to increase revenues in a competitive market, ssoome electric utilities may decide to operate equipment beyond historic loading limits. Unless prudent engineering criteria are observed, thermal limits may be exceeded that, not only increase losses, but reduce the expected life of the equipment.
A realistic approach to the problem is to determine the theoretical load capability, based on thermal loading, rather than to restrict loading to the manufacturer's nameplate ratings. Another way to reduce risk is to employ real-time monitoring and data collection of critically loaded equipment to determine if thermal limits are being approached. In pursuit of such a system for real-time monitoring, a self contained instrumentation package was developed that included a PC user interface, which supported the monitoring function as well as database management and analysis.
Rating increases defined by the instrumentation package were not always realizable due to thermal limits of certain connected equipment such as transformers and air switches. To overcome these limitations, the instrumentation package was modified for application with transformers. The system, known as UPRATE for transformers, was used on several transformers to evaluate its real-time thermal modeling characteristics. In addition to its ability to provide real-time ratings, the technology proved to be a useful monitoring and diagnostic tool. Despite its usefulness, a monitoring program for an entire system would be difficult to economically and physically accomplish in the near term, which meant that there was substantial risk of exceeding thermal limits for certain crucial locations on a given system. By monitoring transformers that are approaching their thermal limits, corrective action can be taken based on the trending of their thermal loading. Once corrective action has occurred (for example, transformer changeout), the special monitoring is no longer required at that location and can be moved to another critically loaded unit. Based on this type of usage, a portable instrumentation package was developed to support the measurement of important transformer parameters.
The Portable Monitoring System A special application for the portable system is the remote monitoring of large transformers that are load-tested in service. The monitoring is continuous with automatic logging of load, temperatures, operational data and weather conditions. The data collected can be applied directly to graphical analysis for correlating thermal response to the applied load. Four major subassemblies, interconnected with prefabricated cables, consist of a remote terminal unit (RTU), a weather station, a load/relay box and a voltage transducer.
The RTU is the heart of the instrumentation package, housed in a NEMA 4 outdoor aluminum enclosure, and supports the monitoring of two banks simultaneously. The central microprocessor (CPU), memory, A/D converter, digital input and internal modem use the STD bus format and are inserted into a STD bus card cage. Custom firmware controls the operation of the RTU. The CPU runs continuously, sampling input data each time through its run cycle. Data are written to memory at predefined intervals. The RTU can store 30 days of data per memory card. The data can be downloaded locally to a laptop through an RS-232 port or via modem if a normal voice-grade phone line is available.
The RTU can also support leased line, hardwire, cellular, satellite and radio communications. Analog sensor inputs require signal conditioning and buffering prior to connection to the RTU's A/D converter. Parameters monitored can include top and bottom oil and tank temperatures, simulated winding hot spot temperatures, load, bus voltage, weather, dissolved gas, internal temperatures (measured by fiber optic sensors), load tap changer compartment temperatures, tap position and nitrogen blanket gas and supply pressures. Digital signals indicating pump/fan status and annunciator alarms are included in the baseline package as well as several digital outputs that can control fan and pump operation during thermal studies. The weather station is a three-piece subassembly that incorporates an anemometer, ambient air temperature sensor, pyranometer and rain detector.
The instrumentation package uses an "open" architecture approach to accommodate a variety of inputs by simply setting external DIP switches on the signal transmitter. Each sensor package is installed in a weather tight enclosure equipped with magnets for easy mounting and installation. The temperature sensors are contained in a cable bundle that is bound together with polyethylene spiral wrap so that the individual sensors can be separated to support the specific installation. Magnetic attachment is used for temperature sensors on flat surfaces.
Firmware and Software The programmable, read-only memory (prom)-based operating software is resident in the CPU and supports the various RTU functions. The run cycle strobes data from the A/D converter and stores it in memory. A serial port on the CPU supports local monitoring by a laptop equipped with the appropriate communications software. The firmware includes conversion factors to present the measured data in engineering units and is readily expandable to support additional analog, digital or serial inputs and outputs. Intelligence can be added to provide automatic call-out features to a designated computer or pager for critical alarms.
The PC user software supports data retrieval and database management functions, including analysis, display, time-based graphing and data export to ASCII files. The generic software package incorporates functions for monitoring data in real time, displaying and graphing historical data, exporting data to the ASCII file, unloading data, setting for nightly unload and defining custom formulas.
The RTU's hardware and firmware can be expanded to accommodate alarms, thermal models, ratings and diagnostics, SCADA/EMS interface, solar power and cellular phone or radio modem. With respect to the thermal modeling of the transformer, input data are used from weather instruments and load data to calculate the thermal state of the transformer.
Once correlated to measured data, the model can monitor the performance of the transformer to detect anomalies that arise due to loss of fans, pumps or clogged radiators. Any positive temperature discrepancy, that is, any temperature higher than expected, will activate an alarm. If the temperature is lower than expected due to rain, for example--the alarm will not be activated. Typical alarm sensitivities range from 3-5 degrees C (37.4-41 degrees F) over the full range of transformer operation.
The transformer's operating parameters, determined by the thermal model, can be used in rating algorithms to calculate and display real-time normal and emergency ratings. These data can be sent directly to Systems Operations using the station SCADA system. The most common interface has been analog voltage outputs and digital contacts monitored by a SCADA RTU.
Field Experience Twelve transformers at Duke Power have been monitored by four portable systems, configured as three two-bank installations and one single-bank system. The single-bank system was used on six dual secondary large power transformers during their special in-service load tests. Typical measured data from the portable system provided information on temperature, voltage and current. Major observations regarding the status of the transformers, based on the data collected were:
Detection of degraded cooling effectiveness due to infrequent cleaning and damage to fins of FOA coolers.
Discovery of mechanical temperature gages that lock up short of the actual temperature and fail to activate cooling equipment.
Determination of the effectiveness of adding fans to radiators to increase cooling.
Confirmation that the dynamic thermal response to loading of large FOA transformers closely tracks an algorithm based on the ANSI loading guides.
From the field data collected, it can be concluded that the portable monitoring and data acquisition system has been optimized for use in accruing and analyzing data from a broad range of transformer types for verification of loading algorithms. In addition, the portability feature has provided temporary monitoring of transformers during in-service load testing. Both applications provide the means for reducing the risk of loading beyond nameplate ratings. TDW
Robert S. Thompson received the BS degree in engineering from UNC at Charlotte in 1974 and is registered as a professional engineer in North and South Carolina. He has worked for Duke Power for 29 years in various engineering activities and is presently senior engineer in the Substation Engineering Group of Duke Engineering and Services, Inc. Thompson is a senior member of IEEE.
Frank J. Hoppe received the BSEE and MSEE degrees from SUNY at Stony Brook in 1969 and 1971, respectively. He has worked at Grumman Corp. as group head for the Flight Control System's Integration Section and for LILCO as head of the Underground Transmission Group. He is presently director of Hardware and Software Systems at Underground Systems, Inc. He is a member of IEEE and PES Insulated Conductors Committee.
The four sub assemblies are packaged in two carrying cases and can be assembled, installed and operating in less than three hours for a two-bank station. The RTU is mounted or clamped to an existing structure near the transformers and ac power is provided by extension cord or by cabling to a location with a 120 V ac or dc source. The weather station crossarm and mast are assembled and mounted or clamped to an appropriate structure that is clear of obstruction in order to avoid false indication of sun and wind conditions.
A cable connects the weather station to the RTU, using keyed connectors on both ends. The temperature sensor assembly is installed on the transformer bank and plugged into the RTU. Split core CTs are installed on the existing transformer instrument CT secondaries for load measurement, and the transmitter enclosure is mounted to the side of the transformer control cabinet. A dropping resistor within each CT assembly provides a voltage output to eliminate the danger of an open secondary. Cabling from the CTs to the transducer enclosure and from the enclosure to the RTU are then plugged in. The split-core CTs have low burden to allow live installation on the existing CT wiring near the transformer's CT terminal block. The bus voltage transducer enclosures are installed and connected to the secondaries of the station bus PTs. A cable connects each sensor package to the RTU.
The existing winding hot spot temperature bulb is removed and modified to incorporate a remote temperature sensor, which is connected to the temperature sensor cable via an in-line connector. The attachment of this sensor by connector allows the reuse of the cable, since the sensor installation is permanent and will remain in place when monitoring equipment is moved to another location. When the connections are completed, power is applied to energize the unit.