Electrical Protections
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The following tutorials provide a brief overview of some of the key technical issues associated with power transformers. Whilst they are offered in good faith as introductory information, users should be aware that they are neither comprehensive nor complete. We hope you find them useful.For further information on any of the subjects covered please contact us at technical@transformerworld.co.uk
You are invited to submit suggestions for additional topics.
IndexT1 Percentage Impedance (Z%)
T2 Differential Protection
T3 Dissolved Gas Analysis (DGA)
T4 Moisture
T5 Buchholz Relays
T6 Vector Groups
T7 to be added
T8 to be added
Differential Protection
Differential protection is a unit scheme that compares the current on the primary side of a transformer with that on the secondary side. Where a difference exists (other than that due to the voltage ratio) it is assumed that the transformer has developed a fault and the plant is automatically disconnected by tripping the relevant circuit breakers. The principle of operation is made possible by virtue of the fact that large transformers are very efficient and hence under normal operation power-in equals power-out. Differential protection detects faults on all of the plant and equipment within the protected zone, including inter-turn short circuits.
Principle of Operation
The operating principle employed by transformer differential protection is the Merz-Price circulating current system as shown below. Under normal conditions I1and I2 are equal and opposite such that the resultant current through the relay is zero. An internal fault produces an unbalance or 'spill' current that is detected by the relay, leading to operation.
Design Objectives
An ideal scheme is required to be:
 | Extremely stable under through fault conditions |
| Very fast to operate for an internal fault |
Design Considerations
A number of factors have to be taken into account in designing a scheme to meet these objectives. These include:
| The matching of CT ratios |
| Current imbalance produced by tap changing |
| Dealing with zero sequence currents |
| Phase shift through the transformer |
| Magnetising inrush current |
Each of these is considered further below.
The Matching of CT Ratios
The CTs used for the Protection Scheme will normally be selected from a range of current transformers with standard ratios such as 1600/1, 1000/5, 200/1 etc. This could mean that the currents fed into the relay from the two sides of the power transformer may not balance perfectly. Any imbalance must be compensated for and methods used include the application of biased relays (see below) and/or the use of the interposing CTs (see below).
Current Imbalance Produced by Tap Changing
A transformer equipped with an on-load tap changer (OLTC) will by definition experience a change in voltage ratio as it moves over its tapping range. This in turn changes the ratio of primary to secondary current and produces out-of-balance (or spill) current in the relay. As the transformer taps further from the balance position, so the magnitude of the spill current increases. To make the situation worse, as the load on the transformer increases the magnitude of the spill current increases yet again. And finally through faults could produce spill currents that exceed the setting of the relay. However, none of these conditions is 'in zone' and therefore the protection must remain stable ie. it must not operate. Biased relays provide the solution (see below).
Dealing with Zero Sequence Currents
Earth faults down stream of the transformer may give rise to zero sequence current, depending upon winding connections and earthing arrangements. Since zero sequence current does not pass through a transformer, it will be seen on one side only producing spill current and possible relay operation for an out-of-zone fault. To prevent such occurrence, zero sequence current must be eliminated from the differential scheme. This is achieved by using delta connections on the secondary side of any CTs that are associated with main transformer windings connected in star.
Where CT secondaries are connected in star on one side of a transformer and delta on the other, allowance must be made for the fact that the secondary currents outside the delta will only be 1/√3 of the star equivalent.
Phase Shift Through the Transformer
Having eliminated the problem of zero sequence currents (see above) through faults will still produce positive and negative sequence currents that will be seen by the protection CTs. These currents may experience a phase shift as they pass through the transformer depending upon the transformer vector group. CT secondary connections must compensate to avoid imbalance and a possible mal-operation.
Magnetising Inrush Current
When a transformer is first energised, magnetising inrush has the effect of producing a high magnitude current for a short period of time. This will be seen by the supply side CTs only and could be interpreted as an internal fault. Precautions must therefore be taken to prevent a protection operation. Solutions include building a time delay feature into the relay and the use of harmonic restraint driven, typically, by the high level of second harmonic associated with inrush current.
Other Issues
Biased Relays
The use of a bias feature within a differential relay permits low settings and fast operating times even when a transformer is fitted with an on-load tapchanger (see above). The effect of the bias is to progressively increase the amount of spill current required for operation as the magnitude of through current increases. Biased relays are given a specific characteristic by the manufacturer.
Interposing CTs
The main function of an interposing CT is to balance the currents supplied to the relay where there would otherwise be an imbalance due to the ratios of the main CTs. Interposing CTs are equipped with a wide range of taps that can be selected by the user to achieve the balance required.
As the name suggests, an interposing CT is installed between the secondary winding of the main CT and the relay. They can be used on the primary side or secondary side of the power transformer being protected, or both. Interposing CTs also provide a convenient method of establishing a delta connection for the elimination of zero sequence currents where this is necessary.
Modern Relays
It should be noted that some of the newer digital relays eliminate the need for interposing CTs by enabling essentials such as phase shift, CT ratios and zero sequence current elimination to be programed directly into the relay.
Tutorial IndexBuchholz Relays
A Buchholz relay is a gas and oil operated device installed in the pipework between the top of the transformer main tank and the conservator. A second relay is sometimes used for the tapchanger selector chamber. The function of the relay is to detect an abnormal condition within the tank and send an alarm or trip signal. Under normal conditions the relay is completely full of oil. Operation occurs when floats are displaced by an accumulation of gas, or a flap is moved by a surge of oil. Almost all large oil-filled transformers are equipped with a Buchholz relay, first developed by Max Buchholz in 1921
Conditions Detected
A Buchholz relay will detect:
| Gas produced within the transformer |
| An oil surge from the tank to the conservator |
| A complete loss of oil from the conservator (very low oil level) |
Gas and Oil Flows
Buchholz relays are equipped with a number of gas and oil inputs and outputs, including test and sampling facilities
Functional Tests - a test petcock enables dry air to be admitted into the relay to check correct operation. A trickle of air is equivalent to a gradual accumulation of gas. A blast simulates an oil surge. These tests are sometimes referred to as 'blowing the Buchholz'. On completion it is important that the relay is bled to remove the air that has been introduced.
Draining - a valve in the bottom of the relay enables an oil sample to be taken or the relay to be drained. As with gas sampling, this facility can be brought down to ground level for enhanced operator safety and convenience.
Accessories
A range of accessories and services are available to assist with the safe and correct operation of Buchholz relays including:
| Ground level oil and gas sampling kits |
| Gas sampling devices - automatic |
| Gas sampling devices - manual |
| On-site gas testers - simple air/fault gas analysis |
| On-site gas testers - complex fault gas composition |
| Relay test kits |
| Recalibration |
For further information visit our Catalogue of products and services or contact us
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Dissolved Gas Analysis (DGA)DGA is one of the most valuable diagnostic tools available. It is a procedure used to assess the condition of an oil-filled transformer from an analysis of the gases dissolved in the cooling/insulating medium. It is a well established technique that is cost effective, providing essential information from a relatively simple, non-destructive test based upon oil sampling. Whilst the analysis is normally done in a laboratory, on-line devices are also available. The results reveal much about the health of the plant including its present condition, any changes that are taking place, the degradation effects of overload, ageing, the inception of minor faults and the most likely cause of major failures.
Oil sampling from transformers is equivalent to blood sampling from humans.
It should be noted that a severe fault may also produce free gases that collect in the Buchholz relay. This closely associated topic is dealt with in Tutorial T5
Taking an Oil Sample
It is important that oil samples are taken carefully to avoid contamination or the loss of gas. Techniques vary from the use of syringes to kits made up from bungs, tubes and sealed bottles. Opening a drain valve, filling a bucket and pouring the contents into a jar will not produce meaningful results.
In the Laboratory
In the laboratory the mixture of gases must be extracted from the oil, for example by the application of a vacuum. The mixture is then passed through a chromatograph where the individual components are separated, identified and quantified. The results are normally presented in tabular form with each gas listed together with the quantity found in parts per million (ppm) by volume.
Interpreting the Results
Interpreting the results is a specialist science. With knowledge and experience the results of a DGA test can be used to produce a detailed and accurate profile of an individual item of plant. This is made possible by the fact that different conditions within a transformer give rise to different quantities and types of gas. For example, acetylene is only produced by arcing.
The Gases Measured
The main gases that are measured and their sources are as follows:
| From the oil | |
| Hydrogen | H2 |
Methane | CH4 |
Ethane | C2H6 |
Ethylene | C2H4 |
Acetylene | C2H2 |
From the paper | |
Carbon Dioxide | CO2 |
Carbon Monoxide | CO |
The Application of DGA
DGA can be used in a variety of ways such as:
| On-line sampling for continuous monitoring - see Catalogue |
| One-off sampling with the results checked against statistical norms |
| Periodic sampling of a single item to establish trends |
| Selective sampling of large numbers with statistical predictions for the remainder |
| Mass sampling on a routine basis to collect detailed historical data |
| Fault analysis after a Buchholz alarm or trip |
Responding to Abnormal Results
Abnormal results are likely to require follow-up action in the form of more frequent sampling and closer monitoring. Internal conditions that produce gases include over-heating, partial discharges and arcing. Where discharges or arcing are taking place techniques that enable insulation defects to be located with accuracy have reached an advanced stage of development - see Catalogue (Field Services)
Abnormal results are likely to require follow-up action in the form of more frequent sampling and closer monitoring. Internal conditions that produce gases include over-heating, partial discharges and arcing. Where discharges or arcing are taking place techniques that enable insulation defects to be located with accuracy have reached an advanced stage of development - see Catalogue (Field Services)
Other Information Available from Oil Sampling
This tutorial deals briefly with the subject of DGA. However, transformer oil contains a great deal more information than is available from an understanding of the gases dissolved in it. Other parameters that need to be taken into account include moisture content, acidity, dielectric strength, the presence of furans etc. These are all important but outside the scope of this tutorial
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Overview
The following notes are based upon a seminar presented by Lars Arvidsson1 in the UK during March 2002 on the subject of Transformer Life Extension. They are written from the perspective of an asset manager and focus on the adverse effects of oxygen on the life of a transformer. It is Lars' view that oxygen has a far more detrimental effect on life expectancy than any other operational parameter.
It is important to be aware that these notes deal only with the effects of oxygen. Other important factors that affect the life of a transformer (such as temperature and moisture) will be added later as separate topics in this series.
Oxidation
The laws of chemistry state that oxygen has a strong affinity for electrons. An atom that has lost electrons to oxygen is said to be oxidised. In a transformer, oil and paper both degrade as a result of oxidation.
Oxidation of Oil
Transformer oil is produced to various specifications such as the paraffinic, naphthenic and ester types. Some oils possess a degree of natural protection against oxidation and are used without the addition of anti-oxidants. These are referred to as uninhibited oils. The more highly refined oils do not generally possess this natural protection and require anti-oxidants to be added. These are known as inhibited oils.
Ageing and oxidation are synonymous. The ageing of oil begins slowly as the anti-oxidants that are present work to neutralise the harmful peroxides and radicals as they are formed. However, with time the anti-oxidants decrease in quantity and the ageing process increases exponentially.
Ageing leads to the formation of acids, aldehydes, ketones, esters and eventually sludge (a mixture of long insoluble hydrocarbon molecules and particles). The process occurs in the presence of peroxides (unstable oxygen compounds) and free radicals and is accelerated by catalysts such as water and copper.
Oxidised oil presents a maintenance requirement that demands action. Left unattended, oxidised oil continues to deteriorate and transports contamination to the cellulose insulation within the transformer. Here the effects are much more serious. Whilst oil can be changed, cellulose cannot. If the oil is not maintained, sooner or later the condition of the cellulose will deteriorate to the point where it has to be accepted that the transformer has reached the end of its working life.
Oxidation of Cellulose (Paper)
Cellulose degrades (oxidises) much faster than oil because it contains oxygen within its molecular structure. The degradation process generates water, carbon dioxide and furfurals, and is accelerated by external sources of oxygen, high temperature and high levels of oil acidity. The water that is generated combines with moisture drawn in from the atmosphere to further accelerate the degradation process and set up a vicious cycle. The end result is broken molecular chains, a lower degree of polymerisation (DP) and loss of mechanical strength. In the absence of oxygen, decomposition occurs more slowly through the process of pyrolysis.
Action to Minimise Oxidation
Set out below are practical steps that can be taken to reduce the harmful effects of oxygen and extend the life of a transformer:
| Be aware that less oxygen means more life |
| Use sealed systems where practicable |
| Consider measures to prevent the ingress of oxygen (eg a bag or membrane in the conservator) |
| Eliminate the frequent cycling of forced cooling systems to avoid pumping in large quantities of air |
| Consider de-gassing treatments for the oil - either as a continuous process or as a maintenance exercise |
| Measure anti-oxidant concentrations in oil and add new inhibitor to old oil where appropriate |
| Measure the copper content in the oil (to reduce the catalytic acceleration of oxidation) |
| Test oil for sulphur (sulphur behaves in a similar way to oxygen) |
| Measure the peroxide number (PXN) of the oil |
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Oxygen has a seriously detrimental effect upon the life of a transformer. Oxidised oil can be maintained but once the effects have extended to the cellulose, the life of a transformer is at risk. Fortunately preventive measures are available that are cost effective and suitable for all asset management programmes.
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1 Lars Arvidsson MSc. Chemistry, is chief executive of Vasteras PetroleumKemi AB, an independent chemical analysis and consultancy company based in Sweden. His many published papers include Microbiological Activity within Hydrocarbon Bulks and his pioneering work in the transformer field includes the development of unique methods for assessing the condition of transformers. Email: lars.arvidsson@petroleumkemi.se
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