BM Online Condenser Bushing Power Factor and Capacitance Monitors

Condenser bushings are accessories applied on high and extra-high voltage equipment, such as power transformers, shunt reactors and high voltage circuit breakers.  In general, despite the equipment’s individual cost being relatively low as compared to the overall cost of the equipment to which it is attached, a failure in the bushing’s insulation can bring about severe damage to the equipment.

Capacitance and dissipation factor (tangent delta) are important factors in the early detection of deterioration of the insulation in bushings, current transformers and other apparatus.  The Treetech Bushing Monitor BM allows these parameters to be monitored on-line during regular operation. This condition based assessment technique allows potentially catastrophic failures to be avoided, by detecting problems while still in the incipient phase.

Main Features

• Autonomous monitoring system, to be installed on the body of the equipment (transformer, reactor, etc.). Does not require installation of computers and/or special software for operation;

• Operating temperatures -40…+85ºC;

• Universal voltage input, 38 to 265Vdc/Vac;

• Spare bushings pre-programmed foroperation in case of single-phase transformer banks;

• Modular system: configurable for monitoring of 3, 6 or 9 bushings;

• Protection against bushing tap opening, with activation indicated by an alarm;

• Inputs for 2 Pt100Ù 0°C temperature sensors, allowing recording and correlation of insulation variation with changes in ambient temperature, oil temperature or other;

• Local indication  on displays:  current values for Capacitance  (C1), Tangent  Delta (C1), evolution trends for capacitance  and tangent delta, forecasted time to reach alarm levels, phase-to-ground  and phase-to-phase  voltages, leakage currents and temperatures;

• 8 output contacts, 1 fixed (NC) for auto diagnostics and 7 configurable (5 NO and 2 NC) for alarms by absolute values, high evolution trends or low leakage currents. Operation mode programmable NO or NC;

• 2 programmable analog outputs for remote readings ofcapacitances andtangent delta. Configurable output range: 0…1mA,0…5mA, 0…10mA, 0…20mA or 4…20mA;

• Internal clock and non-volatile memory for storage of history of capacitance and tangent delta data and alarm events;

• Serial communication port selectable RS485 or RS232 with Modbus RTU and DNP3.0 protocols;

• Communication via fibre optic cable, using external electrical-optical converter;

• Simplified programing divided into basic settings (sufficient for most applications) and advanced parameters. User password protected programming menu.

BM Operating Principle

The structure of condenser bushings gives rise to the appearance of a capacitance between the bushings central conductor and ground. Once the bushing has been energised, this capacitance allows the passage of a leakage current to ground, 90º ahead of the voltage. Due to losses in the dielectric, this leakage current also has a resistive component, in phase with the voltage.

Therefore, any change in these two parameters of the bushing insulation(capacitance and tangent) causes a corresponding change in the leakage current (capacitive and resistive components, respectively).

The Bushing Monitor BM operates by continually measuring the leakage currents of the three bushings in a three-phase set, through adapters connected to the test taps or bushing voltage taps. Processing the readings of the leakage currents allows the variations occurring in capacitance and tangent delta to be charted in relation to the initial values which allows on-line indication of their current values. Initial values can be obtained from factory tests for new bushings or from off-line tests performed at BM installation.

One of the techniques employed in processing the readings is using the sum of the leakage currents.  This technique is based on the fact that any change in one of the leakage currents is mirrored equally in the sum of the 3 currents. This allows detection of even small changes in the value of tangent delta, since the resistive portion of the leakage current, which is associated to the insulation’s tangent delta, has a very small participation in the overall leakage current. This way, variations that might not be detected directly in the leakage current are detectable in the sum of currents. Variations detected are always attributed to only one of the bushings in the three-phase  set, chosen according to the angle of the sum current Change Vector.

Topology of the BM System

The BM bushing monitoring system has a modular concept, and is composed of three basic parts

• Adapter for test tap or voltage tap provides the electric connection to the tap of the bushing, also ensuring its mechanical rigidity and weatherproof seal. It incorporates protection against over voltage and over current due to transient phenomena and also against accidental opening of the measuring circuit, preventing the tap from remaining   open. This protection provides a low impedance path for the leakage current for an unlimited time. In the case of an open circuit of the measuring path the Measurement Module closes its auto diagnostic contact and the Interface Module provides a visual alarm as well as closing the user programmed contacts.

• Measuring Module BM-MM receives the leakage currents from the three bushings in a three-phase  set, measures these currents and a first level of mathematical  and statistical processing of the information, making this data available to the Interface Module (BM-HMI) through a serial communication port RS485;

• Interface Module BM-HMI receives information from one, two or three measuring modules (BM-MM) and carries out their mathematical and statistical processing, making the results available as the current values for capacitance andtangent delta for each bushing displaying these on the front panel displays (also making the data available for serial communications, providing alarms and trips as set by the user).