SAFETY PRECAUTIONS ON WORKING ON TRANSFORMERS AND CIRCUIT BREAKERS
What Are The Safety Precautions on Working on Power Transformers and Circuit Breakers
Take the following safety precautions when working on transformers and circuit breakers:
• Prevent moisture from entering when removing covers from oil-filled transformers.
• Do not allow tools, bolts, nuts, or similar objects to drop into the transformers. Tie tools or parts with suitable twine.
• Have workers empty their pockets of lose articles such as knives, keys, and watches.
• Remove all oil from transformer covers, the floor, and the scaffold to eliminate slipping hazards.
• Exhaust gaseous vapor with an air blower before allowing work in large transformer cases because they usually contain some gaseous fumes and are not well ventilated.
• Stay away from the base of the pole or structure while transformers are being raised or lowered.
• Ensure that anyone working on a pole or structure takes a position above or well clear of transformers while the transformers are being raised or supported with blocks.
• Ground the secondary side of a transformer before energizing it except when the transformer is part of an ungrounded delta bank.
• Make an individual secondary-voltage test on all transformers before connecting them to secondary mains. On banks of three transformers connected Y-delta, bring in the primary neutral and leave it connected until the secondary connections have been completed to get a true indication on the lamp tests.
• Disconnect secondary-phase leads before opening primary cutouts when taking a paralleled transformer out of service. Do not disconnect secondary neutral or ground connections until you have opened the primary cutouts.
• Do not stand on top of energized transformers unless absolutely necessary and then only with the permission of the foreman and after all possible precautions have been taken. These precautions include placing a rubber blanket protected with a rubbish bag over the transformer cover. Do not wear climbers.
• Treat the grounded case of a connected transformer the same as any grounded conductor. Treat the ungrounded case of a connected transformer the same as any energized conductor because the case may become energized if transformer windings break down.
• Ensure that the breaker cannot be opened or closed automatically before working on an oil circuit breaker and that it is in the open position or the operating mechanism is blocked.
• Ensure that metal-clad switching equipment is deenergized before working on it.
• Ensure that regulators are off the automatic position and set in the neutral position before doing any switching on a regulated feeder.
• Do not break the charging current of a regulator or large substation transformer by opening disconnect switches because a dangerous arc may result. Use oil or air brake switches unless special instructions to do otherwise have been issued by the proper authority.
• Do not operate outdoor disconnecting switches without using the disconnect pole provided for this
purpose.
• Ensure that all contacts are actually open and that safe clearance is obtained on all three phases each time an air brake switch is opened. Do not depend on the position of the operating handle as evidence that the switch is open.
• Do not operate switches or disconnect switches without proper authority and then only if thoroughly
familiar with the equipment.
• Remove potential transformer fuses with wooden tongs. Wear rubber gloves and leather over gloves.
• Do not open or remove disconnect switches when carrying load. First open the oil circuit breaker in series with the switches. Open disconnect switches slowly and reclose immediately if an arc is drawn.
Showing posts with label Transformers. Show all posts
Showing posts with label Transformers. Show all posts
TRANSFORMER TURNS RATIO TESTING BASIC INFORMATION AND TUTORIALS
What is Transformer turns ratio (TTR) testing? How is it done?
The voltage across the primary of a transformer is directly proportional to the voltage across the secondary, multiplied by the ratio of primary winding turns to secondary winding turns.
In order to ensure that the transformer was wound properly when it was new, and to help locate subsequent turn-to-turn faults in the winding, it is common practice to perform a TTR test.
The simplest method would be to energize one primary winding with a known voltage (that is less than or equal to the windingÕs rating) and measure the voltage on the other winding.
Since source test voltages can fluctuate, it is often more accurate to use a test set, designed for this purpose, that creates the test voltage internally, thus giving a direct read-out of the ratio measured.
The voltage across the primary of a transformer is directly proportional to the voltage across the secondary, multiplied by the ratio of primary winding turns to secondary winding turns.
In order to ensure that the transformer was wound properly when it was new, and to help locate subsequent turn-to-turn faults in the winding, it is common practice to perform a TTR test.
The simplest method would be to energize one primary winding with a known voltage (that is less than or equal to the windingÕs rating) and measure the voltage on the other winding.
Since source test voltages can fluctuate, it is often more accurate to use a test set, designed for this purpose, that creates the test voltage internally, thus giving a direct read-out of the ratio measured.
INSTRUMENT TRANSFORMERS TESTING BASIC INFORMATION AND TUTORIALS
What is instrument transformer testing?
There are two common designations of instrument transformers: CTs and voltage transformers (VTs) or potential transformers (PTs). The function of an instrument transformer is to reduce the level of voltage or current so that the protective relay (or metering) does not have to be rated for full line voltage or current.
The insulation resistance, transformer ratio, and polarity may be tested in both CTs and VTs. The ratio is the number of turns of wire in the primary winding divided by the number of turns of wire in the secondary winding.
The polarity is determined by which way the wire was wrapped around the iron core. This determines the relationship between the primary winding terminal (H1) and the secondary winding terminal (X1) so that X1 is positive with respect to X2 at the same time that H1 is positive with respect to H2.
The correctness of polarity is important to the correct operation of many relays and metering instruments. CTs often have two additional tests performed: "burden" and "saturation" tests. The burden on a CT is the amount of impedance connected to the secondary winding as a load, usually in the form of protective relays or metering.
The burden test consists of injecting a known current level (usually 1-5 A ac) into the load (usually from the shorting terminal block of the CT) and measuring the voltage at the point of injection. The impedance (or burden) of the circuit is the ratio of the voltage measured to the current injected.
A saturation test is performed to find out the voltage at which the iron in the CT saturates. A known voltage source is connected to the secondary of the transformer and is raised in steps, while the current value is recorded at each step.
When saturation is reached, the given voltage changes cause much smaller changes in current. The saturation test is used in conjunction with the burden test to make sure that the CT is capable of operating the load (usually protective relays) to which it may be subjected.
If the burden on the CT is too high, it may go into saturation and be unable to maintain its proper ratio. When this happens, protective relays may trip too slowly or not at all due to an insufficient level of current from the CT secondary.
There are two common designations of instrument transformers: CTs and voltage transformers (VTs) or potential transformers (PTs). The function of an instrument transformer is to reduce the level of voltage or current so that the protective relay (or metering) does not have to be rated for full line voltage or current.
The insulation resistance, transformer ratio, and polarity may be tested in both CTs and VTs. The ratio is the number of turns of wire in the primary winding divided by the number of turns of wire in the secondary winding.
The polarity is determined by which way the wire was wrapped around the iron core. This determines the relationship between the primary winding terminal (H1) and the secondary winding terminal (X1) so that X1 is positive with respect to X2 at the same time that H1 is positive with respect to H2.
The correctness of polarity is important to the correct operation of many relays and metering instruments. CTs often have two additional tests performed: "burden" and "saturation" tests. The burden on a CT is the amount of impedance connected to the secondary winding as a load, usually in the form of protective relays or metering.
The burden test consists of injecting a known current level (usually 1-5 A ac) into the load (usually from the shorting terminal block of the CT) and measuring the voltage at the point of injection. The impedance (or burden) of the circuit is the ratio of the voltage measured to the current injected.
A saturation test is performed to find out the voltage at which the iron in the CT saturates. A known voltage source is connected to the secondary of the transformer and is raised in steps, while the current value is recorded at each step.
When saturation is reached, the given voltage changes cause much smaller changes in current. The saturation test is used in conjunction with the burden test to make sure that the CT is capable of operating the load (usually protective relays) to which it may be subjected.
If the burden on the CT is too high, it may go into saturation and be unable to maintain its proper ratio. When this happens, protective relays may trip too slowly or not at all due to an insufficient level of current from the CT secondary.
SWITCHING TRANSIENT LOADING EFFECTS ON THE SYSTEM BASIC INFORMATION AND TUTORIALS
One of the primary uses of electricity
is for general lighting and the local DNO must ensure that its supply
is suitable for this purpose. Repeated sudden changes in voltage of a
few per cent are noticeable and are likely to cause annoyance.
The local DNO must ensure that these
sudden variations are kept within acceptable levels and this means
placing limits on consumers’ apparatus which demands surges of
current large enough to cause lighting to flicker.
In order to evaluate flicker in
measurable terms, two levels have been selected: the threshold of
visibility and the threshold of annoyance.
Both are functions of
frequency of occurrence as well as voltage change.
Since both these thresholds are
subjective it has been necessary to carry out experiments with
various forms of lighting and panels of observers to ascertain
consensus relationships between frequency of occurrence and
percentage voltage change for the two thresholds.
The DNOs have used this information in
setting the planning levels for flicker contained in Engineering
Recommendation P28, which govern motor starting currents, etc.
The network impedance from the source
to the point of common coupling between the lighting and the
offending load is of paramount importance and thus the local office
of the DNO should be consulted in cases where the possibility of
creating an annoyance arises.
Intermittently loaded or frequently
started motors, such as those on lifts, car crushers, etc., together
with instantaneous water heaters, arc welders and furnaces, are all
potential sources of disturbance.
Large electric furnaces present a
particular problem and it is frequently necessary to connect them to
a higher voltage system than is necessary to meet their load in order
to achieve a lower source impedance.
Fluctuations occurring about ten times
a second exhibit the maximum annoyance to most people, but even those
as intermittent as one or two an hour will annoy if the step change
is of sufficient magnitude.
CENELEC Standard EN61000-3-3, limits
voltage fluctuation emissions from equipment rated less than or equal
to 16 A and EN61000-3-11 limits emissions from equipment rated from
16 A to 75A.
K - RATED TRANSFORMERS BASIC INFORMATION AND TUTORIALS
What are K-Rated Transformers?
UL and transformer manufacturers have established a K-factor rating for dry-type power transformers to indicate their suitability for supplying nonsinusoidal load currents. The K-factor relates a transformer’s capability to serve varying degrees of nonlinear load without exceeding the rated temperature-rise limits.
The K-factor is the ratio of stray losses in the transformer winding for a given nonsinusoidal load current to the stray losses in the transformer winding produced by a sinusoidal load current of the same magnitude. These transformers are typically specially designed to handle the increased heating effects and neutral currents produced by nonlinear electronic load equipment. The following are some of the design features:
a) The neutral bus is rated at 200% of the secondary full load ampere rating to accommodate the large neutral currents that principally result from triplen harmonics and phase imbalance. The transformer neutral bus rated at 200% is capable of accommodating oversized or multiple neutral conductors.
b) The winding conductors are specially configured and sized to minimize heating due to harmonic load currents. Special configurations and sizing such as multiple, parallel conductors can reduce the skin effect of the higher frequency harmonics and accommodate the balanced triplen harmonics that circulate in the transformer primary (delta) windings.
c) Cores are specially designed to maintain flux core density below saturation due to distorted voltage waveforms or high line voltage. Standard K-factor ratings are 4, 9, 13, 20, 30, 40, and 50. The K-factor for a linear load is 1.
For any given nonlinear load, if the harmonic current components are known, the K-factor can be calculated and compared to the transformer’s nameplate K-factor (refer to
As long as the load K-factor is equal to or less than the rated K-factor of the transformer, the transformer is suitably rated and is considered safe to operate at rated load without overheating. Typical load K-factors for facilities containing large numbers of computers appear to range between 4 and 13.
Measured K-factor on the secondary of step-down transformers that serve almost exclusively nonlinear loads, such as personal computers, have been observed to range as high as 20, but this is extremely rare. In most cases, a transformer with a K-factor rating of 13 can be sufficient to handle typical nonlinear electronic load equipment.
UL and transformer manufacturers have established a K-factor rating for dry-type power transformers to indicate their suitability for supplying nonsinusoidal load currents. The K-factor relates a transformer’s capability to serve varying degrees of nonlinear load without exceeding the rated temperature-rise limits.
The K-factor is the ratio of stray losses in the transformer winding for a given nonsinusoidal load current to the stray losses in the transformer winding produced by a sinusoidal load current of the same magnitude. These transformers are typically specially designed to handle the increased heating effects and neutral currents produced by nonlinear electronic load equipment. The following are some of the design features:
a) The neutral bus is rated at 200% of the secondary full load ampere rating to accommodate the large neutral currents that principally result from triplen harmonics and phase imbalance. The transformer neutral bus rated at 200% is capable of accommodating oversized or multiple neutral conductors.
b) The winding conductors are specially configured and sized to minimize heating due to harmonic load currents. Special configurations and sizing such as multiple, parallel conductors can reduce the skin effect of the higher frequency harmonics and accommodate the balanced triplen harmonics that circulate in the transformer primary (delta) windings.
c) Cores are specially designed to maintain flux core density below saturation due to distorted voltage waveforms or high line voltage. Standard K-factor ratings are 4, 9, 13, 20, 30, 40, and 50. The K-factor for a linear load is 1.
For any given nonlinear load, if the harmonic current components are known, the K-factor can be calculated and compared to the transformer’s nameplate K-factor (refer to
As long as the load K-factor is equal to or less than the rated K-factor of the transformer, the transformer is suitably rated and is considered safe to operate at rated load without overheating. Typical load K-factors for facilities containing large numbers of computers appear to range between 4 and 13.
Measured K-factor on the secondary of step-down transformers that serve almost exclusively nonlinear loads, such as personal computers, have been observed to range as high as 20, but this is extremely rare. In most cases, a transformer with a K-factor rating of 13 can be sufficient to handle typical nonlinear electronic load equipment.
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