| As a vital part of transmission and distribution systems, | | | | insulation problems. |
| transformers are built and expected to be unfailingly | | | | Some parameters of transformer monitoring, and the |
| reliable. Nevertheless, internal faults like partial | | | | sensors best applicable are discussed below: |
| discharges can occur, and the problem with such faults | | | | * Dissolved Gas Analysis (DGA) |
| is that if left un-corrected, they can eventually morph | | | | An established diagnostic method, gas-in-oil analysis |
| into catastrophic faults that can result in power | | | | involves analyzing the types, concentration and |
| outages and even end-user property damage. | | | | production rates of generated gases. Different types |
| Transformer Monitoring: What's Involved | | | | of gases are produced based on the types of faults; |
| * Data acquisition | | | | for example, overheated cellulose leads to the |
| * Sensor development | | | | formation of carbon oxides, but arcing leads to the |
| * Data analysis | | | | generation of acetylene. |
| * Development of links between measurements and | | | | Depending on the criticality of each unit, oil samples are |
| failures | | | | taken manually at regular intervals (between 12 and 24 |
| Easily Prevented | | | | months) and the gasses are extracted from these |
| Preventing disasters of this nature is actually quite | | | | samples. On-line gas sensors are typically the first |
| simple, and involves transformer monitoring. Monitoring | | | | choice in designing full time monitoring systems for |
| transformers and spotting problems before they turn | | | | DGA; simply because the technique of analysis is well |
| into unmanageable incidents can prevent faults that | | | | established and accepted, and the sensor is truly |
| are costly to fix and may result in a loss of service. | | | | capable of detecting a wide range of failure types. |
| Transformer monitoring mainly involves data acquisition, | | | | Partial Discharges (PD) |
| sensor development, data analysis, and the | | | | Partial discharge testing of de-energized transformers |
| development of causal links between measured | | | | is a valuable tool for evaluation of overall transformer |
| values and failures of transformers. | | | | integrity, however, on-line, realtime, partial discharge |
| Installing monitoring equipment on transformers is | | | | monitoring is, in general, more expensive and |
| usually done for two reasons: | | | | complicated. For example, glass fiber rods acting as |
| 1. Monitoring important transformer functions can help | | | | wave-guides inside the main tank have been applied to |
| detect developing faults before they lead to a | | | | large transformers, but the cost and complexity of |
| catastrophic failure | | | | installation has made such a system unsuitable for |
| 2. Monitoring transformer functions can allow for a | | | | online monitoring. |
| change from periodic to condition-based maintenance | | | | There are several advantages associated with |
| Monitoring Equipment | | | | electrical PD monitoring, but it has been difficult to |
| Monitoring equipment is permanently mounted on the | | | | design field applications thanks to the difficulties in |
| transformer and is online 24/7. Reliable, low-cost | | | | separating internal and external PD sources. Sensors |
| monitoring is thus a necessary condition. Failure rates | | | | that are being tested and developed currently include |
| of transformers are usually low (0.2 - 2% per | | | | externally fitted acoustical sensors, which are more |
| transformer/year), and high-cost failure prevention | | | | cost-effective but are susceptible to disturbances |
| systems cannot thus be justified, especially when | | | | from rough outdoor substation environments. |
| redundancy is available and the consequential costs | | | | * Temperature |
| are thus limited. To keep within this cost barrier, some | | | | The load capability of a transformer is limited by the |
| compromise on the functionality of themonitoring | | | | hot spot of the windings. The hot spot is typically |
| equipment is necessary. | | | | calculated indirectly from measurements of oil |
| Transformer Monitoring: Parameters | | | | temperatures and load current. An alternative method |
| * Oil temperature | | | | involves fiber-optic temperature sensors that are |
| * Moisture levels | | | | installed in the winding during the manufacturing |
| * Operation of cooling fans | | | | process. These sensors come in two varieties - fibers |
| * Electrical load levels | | | | which measure the temperature at a single point, and |
| In a majority of cases, it is enough to supply a reliable | | | | distributed fibers that measure the temperature along |
| warning signal without online analysis and diagnosis, | | | | their length. All of these systems involve high costs; in |
| provided that manual or automatic diagnostic methods | | | | particular, the distributed fiber sensor is the most |
| are available to follow up the alarm. Specifically with | | | | expensive to install and can only be applied to new |
| regard to power distribution networks in the US, a | | | | transformers. |
| majority of the transformer population is aging, and | | | | New Techniques |
| most emerging faults can be expected from these | | | | The condition of the insulation can be judged from |
| units.Monitoring equipment should thus be designed for | | | | other parameters as well, such as moisture levels and |
| field installation on operational transformers that might | | | | particulate content. Data interpretation with these |
| date back a few decades. | | | | parameters is not straightforward, but new techniques |
| Detection of Developing Faults | | | | are being developed using software to analyze the |
| The main transformer parts that need monitoring are | | | | large body of historical data available and identify |
| insulation quality, winding temperatures, oil quality, and | | | | patterns of progression towards failure. If similar |
| mechanical moving parts such as on-load tap | | | | deterioration is detected for a transformer in service, |
| changers (OLTC). Monitoring the windings and insulation | | | | remedial action can be taken. |
| systems for gas-in-oil and partial discharge are crucial; | | | | Other types of online sensors have also been |
| temperature and load monitoring on the other hand, is | | | | investigated. Examples of such systems are online |
| regarded as base information and should be included in | | | | measurements of the moisture content of the oil, static |
| any type of transformer monitoring. | | | | charge in oil, optical sensors and pump monitoring. |
| OLTC failures are typically caused by mechanical | | | | Also on-line measurements of the moisture in the |
| faults with bearings, springs, shafts and drive | | | | cellulose by optical fiber techniques are being studied. In |
| mechanisms, closely followed by electrical faults such | | | | general, these systems do not have a strong coupling |
| as burnt transition resistors, choked contacts, and | | | | to important and frequent failure modes. |