Experiences with wireless transformer monitoring system installed by the manufacturer and operated at strategically important locations of the Mexican grid

摘要

Several wireless on-line monitoring systems have been installed on 230 kV, 60 MVA transformers inside Mexico City, and one Mexican utility has operated a 33.6 MVA transformer at a high ambient-temperature location with similar wireless monitoring-system for more than one year. Besides, advanced sensors and dedicated monitoring equipment have been implemented on transformers at the nuclear power plant. Design of the monitoring system was based on perceived needs of the HV network operators, namely alarms or early warning of oncoming faults, and the transformer condition-assessment that allows the operator to make an optimum use of transformer capacity in terms of permissible overload. Besides, the monitoring system has to determine the number of oil pumps and ventilators for transformer operating conditions, i.e. the load current, hot spot temperature, moisture-content in cellulose and amount of gases dissolved in oil. The monitoring system has been composed of sensors that are intelligent electronic devices (IED) that record pertinent physical quantities, the modular intelligent electronic devices are connected to RS 485 communication bus to avoid the use of centralized equipment; specialized software that derives the required information from the sensor records, and wireless signal transmission that uses the mobile telephone system (employing General Packet Radio Service (GPRS) protocol) and makes the processed data available on Internet. In general, the operating experience gathered for more than one year of service in Mexican network has confirmed satisfactory performance of the monitoring system. Transformer overload has been weighted against the solid insulation loss of life. The cooling system control has saved energy by switching off the oil pumps and ventilators that were not needed during the low load periods. Such factors as a lower air density at the high altitude affect the transformer cooling efficiency, are taken into account when setting the signal processing algorithms. Degradation of HV bushing insulation has been indicated in time to prevent an in service failure. Planned improvements of the monitoring system include recording of short-circuit current in the transmission network and lightning as well as system-generated over-voltages. Such records will enable the transformer owner to schedule diagnostic off-line procedures, such as checking the winding mechanical integrity by Frequency Response Analysis, and the insulation condition by dielectric polarization methods, such as Frequency Domain Spectroscopy. The operating experience and description of the on-line wireless transformer monitoring system installed by manufacturer and operated on important locations of the Mexican grid is presented and some important aspects of on-line monitoring systems are discussed.