On line Monitoring of Transformer can be described as all aspects of performance management concerned with tracking the system activities in order to gather the appropriate data needed for determining performance.
On-Line Monitoring implies aspects where data can be collected while the equipment is electrically energized and in service. Data (or samples such as oil) can be collected and /or measured while the equipment is energized (subject to local safety regulations).
Continuous On-Line Monitoring implies the two previous notions and also the application of intelligent electronic devices (IED). These are devices with a measurement mechanism of some type, together with internal signal and data processing capabilities, that can be described as ‘smart sensors’ or ‘smart systems’ providing measurement and control functions, while the subject equipment is energized and in service.
The basic goals of Continuous On-Line Monitoring are:
a)To generate early warnings in case of incipient faults, to reduce the risk of unexpected failure
b)To follow up the development of the diagnostic values on suspect or faulty units which cannot be taken out of service immediately
c)To reduce costs for periodic diagnostic testing and assign workers to other tasks
d)To archive measured and computed data in a database for future analyses
If applied adequately, Continuous On-Line Monitoring technology may have the following significant advantages:
a)It reduces the condition monitoring task interval nearly to zero (compared with off-line diagnostics)
b)It can detect the change of measured parameter(s) shortly after its occurrence
c)It can generate an automatic warning if preset limits or trends are exceeded
d)In case of unexpected failure, it performs a “black box” function (records the data before and after the failure and thus enhances the available evidence used for failure analysis)
Continuous DGA On-Line Monitoring
As DGA are proven to be a very good indicator for a large proportion of transformer incipient fault and because an early detection of an incipient fault may avoid major failure, Continuous DGA On-Line Monitoring is the most implemented online monitoring technology for transformers. Several sensors technologies are available in the market using different detection techniques (fuel cell, chromatography, semiconductor, photo-acoustic spectroscopy, thermal conductivity). Depending of the technology and its implementation, the systems are able to provide:
a)A single measurement of one specific gas
b)A single measurement of a composition of several gases with specific proportions and sensitivities
c)Multiple measurements of different gases
Gas-in-oil monitors often include a built-in moisture sensor to provide data on the moisture content in oil.
Other On-Line Continuous Monitoring Technologies
In addition to the Continuous DGA On-Line Monitoring systems described above, other discrete systems are available and can be combined with DGA monitors. Some of them include:
a)Systems supporting mathematical models designed to calculate additional information such as winding hot spot temperatures, rate of ageing of paper insulation, moisture content in paper or barriers, and effectiveness of the cooling systems
b)Monitoring of bushings by measuring the leakage current through capacitance tap (where capacitance taps are available)
c)OLTC monitoring including mechanical conditions of the drive system, contact wear, temperature differential, dissolved gas analysis, tap position tracking/counting
d)Partial discharge using electrical, acoustical, or UHF signals
