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Transformer Management Models
Power transformers are one of the most critical components of power systems. The cost of such units and the consequences of unexpected failure has always been a concern for system operators. It is no wonder that monitoring devices such as Buchholz gas relay and winding temperature indicators have been used for many years. More recently, the availability of microprocessors, ruggedized for high voltage substation environment, had made it possible to monitor a large number of parameters. With large memory capability and communication facilities it is tempting to record all these data for future usage.
However experience has shown that the monitored values are of limited interest by themselves. They gain there value when compared with rated values and combined among themselves to generated more significant information. This is the purpose of transformer models provided with Hydran M2 and Intellix MO150. In the following pages, these models are described along with the benefit to the user the user.

See detail :  Transformer Management Models

Source : GE

Sensible transformer maintenance.

Raymond, Charles T.

Following specific checking and maintenance guidelines as well as conducting routine inspections will help ensure the prolonged life and increased reliability of a dry-type transformer.

Because dry-type transformers are used so extensively in industrial, commercial, and institutional power distribution systems, their maintenance should be a top priority. As such, you should have a thorough knowledge of their maintenance requirements, which are similar to liquid-filled units in many ways but differ enough to warrant separate coverage. The following detailed discussion will help you attain the required knowledge.

Dry-type transformer classifications

Dry-type transformers are classified as ventilated, nonventilated, and sealed units, with each type detailed in the ANSI/IEEE C57.12.01-1989 standard, General Requirements for Dry-Type Distribution and Power Transformers. Because there are significant differences among these three groups and because some of these differences have an impact on maintenance procedures, it’s important that you know the key aspects of each transformer type.

A ventilated dry-type transformer is constructed so that ambient air can circulate through vents in the surrounding enclosure and cool the transformer core and coil assembly.

A nonventilated transformer operates with air at atmospheric pressure in an enclosure that does not allow ambient air to circulate freely in and out.

A sealed transformer is self-cooled, with the enclosure sealed to prevent any entrance of ambient air. These transformers are filled with an inert gas and operate at a positive pressure.

While construction varies per transformer type, inspection and maintenance guidelines are somewhat similar.

See detail : Sensible Transformer Maintenance

ABSTRACT
Recent surveys indicate that the average age of utility power transformers exceeds 30 years. Managing these critical assets requires monitoring the factors that cause transformer damage. Excessive heat and mechanical stress during through faults on transformers are recognized as the two major causes of damage. New technology in transformer protection relays provides for both thermal and through-fault monitoring.
This paper demonstrates how to use the transformer thermal damage and loss-of-life information from IEEE Std. C57.91-1995 to schedule proactive maintenance. It also presents through-fault recording and accumulated data and discusse  how these relate to transformer short-circuit standards.

INTRODUCTION
In the historical struggle between ac and dc power transmission, ac is generally preferred because it allows easy conversion of voltages to higher levels for long distance transport. Power transformers are a critical link in the ac path of electricity from the generating stations to end users. In terms of total investment, electric utilities invest at least as much in transformers as they do in generating stations. In many cases, because of the larger installed base, utilities invest more
in transformers.
Transformers are expected to last from 20–30 years, and in many cases, even longer. Because regulators and financial markets measure a utility’s ability to make efficient use of resources, utilities must maximize asset utilization. Based on transformer design and experience, we know that the amount of service a transformer “sees” is an indicator of serviceability. As they say, “it’s not the age—it’s the mileage.”
Measurable indicators of transformer serviceability include electrical load; top-oil, hottest-spot, and ambient temperatures; fault history; and dissolved gas analysis. Utilities that use these indicators can make intelligent profit/risk decisions and plan optimal transformer loading and maintenance.

See detail :  Transformer Maintenance Interval Management

See detail :  Power Transformer Maintenance and Acceptance Testing

Liquid-Preservation Systems
There are several methods to preserve the properties of the transformer liquid and associated insulation structures that it penetrates. Preservation systems attempt to isolate the transformer’s internal environment from the external environment (atmosphere) while understanding that a certain degree of interaction, or “breathing,” is required to accommodate variations in pressure that occur under operational conditions, such as expansion and contraction of liquid with temperature. Free-breathing systems, where the liquid is exposed to the atmosphere, are no longer used. The most commonly used methods are outlined as follows and illustrated in Figure 2.1.20.

• Sealed-tank systems have the tank interior sealed from the atmosphere and maintain a layer of gas — a gas space or cushion — that sits above the liquid. The gas-plus-liquid volume remains constant. Negative internal pressures can exist in sealed-tank systems at lower loads or temperatures with positive pressures as load and temperatures increase.
• Positive-pressure systems involve the use of inert gases to maintain a positive pressure in the gas space. An inert gas, typically from a bottle of compressed nitrogen, is incrementally injected into the gas space when the internal pressure falls out of range.
• Conservator (expansion tank) systems are used both with and without air bags, also called bladders or diaphragms, and involve the use of a separate auxiliary tank. The main transformer tank is completely filled with liquid; the auxiliary tank is partially filled; and the liquid expands and contracts within the auxiliary tank. The auxiliary tank is allowed to “breathe,” usually through a dehydrating breather. The use of an air bag in the auxiliary tank can provide further separation from the atmosphere.

“Buchholz” Relay
On power transformers using a conservator liquid-preservation system, a “Buchholz” relay can be installed in the piping between the main transformer tank and the conservator. The purpose of the Buchholz relay is to detect faults that may occur in the transformer. One mode of operation is based on the generation of gases in the transformer during certain minor internal faults. Gases accumulate in the relay, displacing the liquid in the relay, until a specified volume is collected, at which time a float actuates a contact or switch. Another mode of operation involves sudden increases in pressure in the main transformer tank, a sign of a major fault in the transformer. Such an increase in pressure forces the liquid to surge through the piping between the main tank and the conservator, through the “Buchholz” relay, which actuates another contact or switch.

Source : Electric power transformer engineering by James H. Harlow.

A Differential System can be arranged to cover a complete Transformer. The underlying principle of such a protection scheme is shown in the figure below.
The General Idea behind the Differential Protection is that the CT’s on the primary and Secondary side must transform the respective Line currents to the same value. For the Bias Coils in the relay to function without any damage, this transformed current may lie between 1A and 5A. Hence the function of the CT’s is to transform the line currents to the same magnitude and phase under normal operation of the Transformer.Should some imbalance occur within the Transformer, such as Interturn Faults within the Windings of the Transformer or Faults on the incoming or outgoing feeders to the transformer, the Line Currents are no longer at the balanced value. Thus, the transformed current at the relay coils is no longer the same at both ends, but different. This causes an imbalance within the differential relay, and as a result, some protection mechanism will be operated, so as to isolate the Transformer and protect it.

See detail : Transformer Differential Protection

Source : http://www.eng.uwi.tt/depts/elec/staff/alvin/ee35t/notes/Trans-Diff-Protect.html

Instrument transformers are used for measurement and protective application, together with equipment such as meters and relays. Their role in electrical systems is of primary importance as they are a means of “stepping down” the current or voltage of a system to measurable values, such as 5A or 1A in the case of a current transformers or 110V or 100V in the case of a voltage transformer. This offers the advantage that measurement and protective equipment can be standardized on a few values of current and voltage.

• Voltage transformers
• Current transformers
• Kappa has published a reference manual on instrument transformers

Voltage transformers

• Principle of operation
• Definitions
• Standards
• Tests
• Typical Specifications

Current transformers

• Principle of operation
• Definitions
• Standards
• Tests
• Typical Specifications