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# Electrician

## Principle of Tan Delta Test

A pure insulator when is connected across line and earth, it behaves as a capacitor. In an ideal insulator, as the insulating material which acts as dielectric too, is 100 % pure, the electric current passing through the insulator, only have capacitive component. There is no resistive component of the current, flowing from line to earth through insulator as in ideal insulating material, there is zero percent impurity.

In pure capacitor, the capacitive electric current leads the applied voltage by 90°.

In practice, the insulator cannot be made 100% pure. Also due to ageing of insulator the impurities like, dirt and moisture enter into it. These impurities provide conductive path to the current. Consequently, leakage electric currentflowing from line earth through insulator has also resistive component.

Hence, it is needless to say that, for good insulator, this resistive component of leakage electric current is quite low. In other way the healthiness of an electrical insulator can be determined by ratio of resistive component to capacitive component. For good insulator this ratio would be quite low. This ratio is commonly known as tanδ or tan delta. Sometimes it is also referred as dissipation factor.

In the vector diagram above, the system voltageis drawn along x-axis. Conductiveelectric current i.e. resistive component of leakage current, IRwill also be along x-axis.

As the capacitive component of leakage electric current IC leads system voltage by 90°, it will be drawn along y-axis.

Now, total leakage electric current IL(Ic + IR) makes an angle δ (say) with y-axis.

Now, from the diagram above, it is cleared, the ratio, IR to IC is nothing but tanδ or tan delta.

NB: This δ angle is known as loss angle.

## Method of Tan Delta Testing

The cable, winding, current transformerpotential transformer, transformer bushing, on which tan delta test or dissipation factor test to be conducted, is first isolated from the system. A very low frequency test voltage is applied across the equipment whose insulation to be tested. First the normal voltage is applied. If the value of tan delta appears good enough, the applied voltage is raised to 1.5 to 2 times of normal voltage, of the equipment. The tan delta controller unit takes measurement of tan delta values. A loss angle analyser is connected with tan delta measuring unit to compare the tan delta values at normal voltage and higher voltages, and analyse the results.

During test it is essential to apply test voltage at very low frequency.

### Reason of applying Very Low Frequency

If frequency of applied voltage is high, then capacitive reactance of the insulator becomes low, hence capacitive component of electric current is high. The resistive component is nearly fixed, it depends upon applied voltage and conductivity of the insulator. At high frequency as capacitive current, is large, hence, the amplitude of vector sum of capacitive and resistive components ofelectric current becomes large too.

Therefore, required apparent power for tan delta test would become high enough which is not practical. So to keep the power requirement for thisdissipation factor test, very low frequency test voltage is required. The frequency range for tan delta test is generally from 0.1 to 0.01 Hz depending upon size and nature of insulation.

There is another reason for which it is essential to keep the input frequency of the test as low as possible.

As we know,
$Dissipation\;factor\;\;=\;\frac{I_R}{I_C}\;\;=\;\frac{\frac{V}{R}}{V\;\times 2\pi fCR}\;\;=\;\frac{1}{2\pi fCR}$

That means, dissipation factor tanδ ∝ 1 / f.

Hence, at low frequency, the tan delta number is high, the measurement becomes easier.

### How to predict the Result of Tan Delta Testing

There are two ways to predict the condition of an insulation system during tan delta or dissipation factor test.
First one is, comparing the results of previous tests to determine, the deterioration of the condition of insulation due ageing affect.

Second one is, determining the condition of insulation from the value of tanδ, directly. No requirement of comparing previous results of tan delta test.

If the insulation is perfect, the loss factor will be approximately same for all range of test voltages. But if the insulation is not good enough, the value of tan delta increases in higher range of test voltage.

From the graph it is clear that, the tan&delta number non linearly increases with increasing test very low frequency voltage. The increasing tan&delta, means, high resistive electric current component, in the insulation. These results can be compared with the results of previously tested insulators, to take proper decision whether the equipment would be replaced or not.

## Principle of Tan Delta Test

A pure insulator when is connected across line and earth, it behaves as a capacitor. In an ideal insulator, as the insulating material which acts as dielectric too, is 100 % pure, the electric current passing through the insulator, only have capacitive component. There is no resistive component of the current, flowing from line to earth through insulator as in ideal insulating material, there is zero percent impurity.

In pure capacitor, the capacitive electric current leads the applied voltage by 90°.

In practice, the insulator cannot be made 100% pure. Also due to ageing of insulator the impurities like, dirt and moisture enter into it. These impurities provide conductive path to the current. Consequently, leakage electric currentflowing from line earth through insulator has also resistive component.

Hence, it is needless to say that, for good insulator, this resistive component of leakage electric current is quite low. In other way the healthiness of an electrical insulator can be determined by ratio of resistive component to capacitive component. For good insulator this ratio would be quite low. This ratio is commonly known as tanδ or tan delta. Sometimes it is also referred as dissipation factor.

In the vector diagram above, the system voltageis drawn along x-axis. Conductiveelectric current i.e. resistive component of leakage current, IRwill also be along x-axis.

As the capacitive component of leakage electric current IC leads system voltage by 90°, it will be drawn along y-axis.

Now, total leakage electric current IL(Ic + IR) makes an angle δ (say) with y-axis.

Now, from the diagram above, it is cleared, the ratio, IR to IC is nothing but tanδ or tan delta.

NB: This δ angle is known as loss angle.

## Method of Tan Delta Testing

The cable, winding, current transformerpotential transformer, transformer bushing, on which tan delta test or dissipation factor test to be conducted, is first isolated from the system. A very low frequency test voltage is applied across the equipment whose insulation to be tested. First the normal voltage is applied. If the value of tan delta appears good enough, the applied voltage is raised to 1.5 to 2 times of normal voltage, of the equipment. The tan delta controller unit takes measurement of tan delta values. A loss angle analyser is connected with tan delta measuring unit to compare the tan delta values at normal voltage and higher voltages, and analyse the results.

During test it is essential to apply test voltage at very low frequency.

### Reason of applying Very Low Frequency

If frequency of applied voltage is high, then capacitive reactance of the insulator becomes low, hence capacitive component of electric current is high. The resistive component is nearly fixed, it depends upon applied voltage and conductivity of the insulator. At high frequency as capacitive current, is large, hence, the amplitude of vector sum of capacitive and resistive components ofelectric current becomes large too.

Therefore, required apparent power for tan delta test would become high enough which is not practical. So to keep the power requirement for thisdissipation factor test, very low frequency test voltage is required. The frequency range for tan delta test is generally from 0.1 to 0.01 Hz depending upon size and nature of insulation.

There is another reason for which it is essential to keep the input frequency of the test as low as possible.

As we know,
$Dissipation\;factor\;\;=\;\frac{I_R}{I_C}\;\;=\;\frac{\frac{V}{R}}{V\;\times 2\pi fCR}\;\;=\;\frac{1}{2\pi fCR}$

That means, dissipation factor tanδ ∝ 1 / f.

Hence, at low frequency, the tan delta number is high, the measurement becomes easier.

### How to predict the Result of Tan Delta Testing

There are two ways to predict the condition of an insulation system during tan delta or dissipation factor test.
First one is, comparing the results of previous tests to determine, the deterioration of the condition of insulation due ageing affect.

Second one is, determining the condition of insulation from the value of tanδ, directly. No requirement of comparing previous results of tan delta test.

If the insulation is perfect, the loss factor will be approximately same for all range of test voltages. But if the insulation is not good enough, the value of tan delta increases in higher range of test voltage.

From the graph it is clear that, the tan&delta number non linearly increases with increasing test very low frequency voltage. The increasing tan&delta, means, high resistive electric current component, in the insulation. These results can be compared with the results of previously tested insulators, to take proper decision whether the equipment would be replaced or not.

## Sweep Frequency Response Analysis Test | SFRA Test

This is very reliable and sensitive method or tool for condition monitoring of the physical condition of transformer windings. The winding of transformer may be subjected to mechanical stresses during transportation, heavy short circuit faults, transient switching impulses and lightening impulses etc. These mechanical stresses may cause displacement of transformer windings from their position and may also cause deformation of these windings. Windings collapse in extreme cases, such physical defects eventually lead to insulation failure or dielectric faults in the windings.

Sweep Frequency Response Analysis Test or in short SFRA Test can detect efficiently, displacement of transformer core, deformation and displacement of winding, faulty core grounds, collapse of partial winding, broken or loosen clamp connections, short circuited turns, open winding conditions etc.

## Principle of SFRA Test

The principle of SFRA is quite simple. As all the electrical equipments theoretically have some resistanceinductor and some capacitance values hence each of them can be considered as a complex RLC circuit. The term ‘theoretically’ means some equipment may have very low or zero resistancecompared to their inductor and capacitance values again, some equipments may have very low or zero inductor compared to their resistance andcapacitance and again some equipments may have very low or zerocapacitance compared to their resistance and inductor but theoretically all of them can be considered as RLC circuit although may be R = 0, or L = 0 or C = 0. But in most cases the resistanceinductor and capacitance of an equipment have non zero values. Hence most of the electrical equipments can be considered as RLC circuit hence they response to the sweep frequencies and produce an unique signature. As in a transformer each winding turn is separated from other by paper insulation which acts as dielectric and windings themselves have inductor and resistance, a transformer can be considered as a complicated distributed network of resistance, inductance, and capacitanceor in other words a transformer is a complicated RLC circuit.

Because of that each winding of a transformer exhibits a particular frequency response.

In Sweep Frequency Response Analysis a sinusoidal voltage Vi is applied to one end of a winding and output voltage Vo is measured at the other end of the winding. Other windings are kept open.

As the winding is itself an distributed RLC circuit it will behave like RLC filter and gives different output voltages at different frequencies. That means if we go on increasing the frequency of the input signal without changing its voltagelevel we will get different output voltages at different frequencies depending upon the RLC nature of the winding. If we plot these output voltages against the corresponding frequencies we will get a particular patter for a particular winding.

But after transportation, heavy short circuit faults, transient switching impulses and lightening impulses etc, if we do same Sweep Frequency Response Analysis test and superimpose the present signature with the earlier patterns and observe some deviation between these tow graphs, we can asses that there is mechanical displacement and deformation occurred in the winding.

In addition to that, SFRA test also helps us to compare between physical condition of the same winding of different phases at the same tap position.

It also compares different transformers of the same design.

Analysis
Low frequency response
1) Winding behaves as a simple RL circuit formed by series inductor andresistance of the winding (At low frequencies capacitance cats as almost open circuit)

2) At low frequency winding inductances are determined by the magnetic circuitof the transformer core.

High frequency response
3) At high frequency winding behaves as RLC circuits

4) Winding exhibits many resonant points

5) Frequency responses are more sensitive to winding movement.

### Different Connection During SFRA Test

SIGNAL APPLIED ACROSS TRANSFORMER TERMINALSCONDITIONS
HV Red phase to NeutralLV Red Yellow Blue phases are open
HV Yellow phase to NeutralLV Red Yellow Blue phases are open
HV Blue phase to NeutralLV Red Yellow Blue phases are open
HV Red phase to NeutralLV Red Yellow Blue phases are shorted
HV Yellow phase to NeutralLV Red Yellow Blue phases are shorted
HV Blue phase to NeutralLV Red Yellow Blue phases are shorted
LV Red to Yellow phaseHV Red Yellow Blue phases and LV Blue phase are open
LV Yellow to Blue phaseHV Red Yellow Blue phases and LV Red phase are open
LV Blue to Red phaseHV Red Yellow Blue phases and LV Yellow phase are open

## Impulse Test of Transformer

Lighting is a common phenomenon in transmission lines because of their tall height. This lightning stroke on the line conductor causes impulse voltage. The terminal equipment of transmission line such as power transformer then experiences this lightning impulse voltages. Again during all kind of online switching operation in the system, there will be switching impulses occur in the network. The magnitude of the switching impulses may be about 3.5 times the system voltage.

Insulation is one of the most important constituents of a transformer. Any weakness in the insulation may cause failure of transformer. To ensure the effectiveness of the insulation system of a transformer, it must confirms the dielectric test. But the power frequency withstand test alone can not be adequate to demonstrate the dielectric strength of a transformer. That is whyimpulse test of transformer performed on it. Both lightning impulse testand switching impulse test are included in this category of testing.

Lightning Impulse

The lightning impulse is a pure natural phenomenon. So it is very difficult to predict the actual wave shape of an lightning disturbance. From the data compiled about natural lightning, it may be concluded that the system disturbance due to natural lightning stroke, can be represented by three basic wave shapes.

1) Full wave
2) Chopped wave and
3) Front of wave

Although the actual lightning impulse disturbance may not have exactly these three shapes but by defining these waves one can establish a minimum impulse dielectric strength of a transformer.

If lighting disturbance travels some distance along the transmission line before it reaches the transformer, its wave shape may approach to full wave.

If during traveling, if flash-over occurs at any insulator of the transmission line, after the peak of the wave has been reached, the wave may become in form of chopped wave.

If the lightning stroke directly hits the transformer terminals, the impulsevoltage rises rapidly until it is relieved by a flash over. At the instant of flash – over the voltage suddenly collapses and may form the front of wave shape.

The effect of these wave forms on the transformer insulation may be different from each other. We are not going here in detail discussion of what type of impulse voltage wave forms causes what type of failure in transformer. But whatever may be the shape of lightning disturbance voltage wave, all of them can cause insulation failure in transformer. So lighting impulse test of transformer is one of the most important type test of transformer.

### Switching Impulse

Through studies and observations reveal that the switching over voltage or switching impulse may have front time of several hundred microseconds and this voltage may be periodically damped out. The IEC – 600060 has adopted for their switching impulse test, a long wave having front time 250 μs and time to half value 2500 μs with tolerances.

The purpose of the impulse voltage test is to secure that the transformer insulation withstand the lightning overvoltage which may occur in service.

Impulse test

The impulse generator design is based on the Marx circuit. The basic circuit diagram is shown on Figure above. The impulse capacitors Cs (12 capacitorsof 750 η F) are charged in parallel through the charging resistors Rc (28 kΩ) (highest permissible charging voltage 200 kV). When the charging voltage has reached the required value, breakdown of the sparkgap F1 is initiated by an external triggering pulse. When F1 breaks down, the potential of the following stage (point B and C) rises. Because the series resistors Rs is of low-ohmic value compared with the discharging resistors Rb (4,5 kΩ) and the chargingresistor Rc, and since the low-ohmic discharging resistor Ra is separated from the circuit by the auxiliary spark-gap Fal, the potential difference across the spark-gap F2 rises considerably and the breakdown of F2 is initiated.

Thus the spark-gaps are caused to break down in sequence. Consequently thecapacitors are discharged in series-connection. The high-ohmic dischargeresistors Rb are dimensioned for switching impulses and the low-ohmicresistors Ra for lightning impulses. The resistors Ra are connected in parallel with the resistors Rb, when the auxiliary spark-gaps break down, with a time delay of a few hundred nano-seconds.

The arrangement is necessary in order to secure the functioning of the generator.
The wave shape and the peak value of the impulse voltage are measured by means of an Impulse Analysing System (DIAS 733) which are connected to thevoltage divider. The required voltage is obtained by selecting a suitable number of series-connected stages and by adjusted the charging voltage. In order to obtain the necessary discharge energy parallel or series-parallel connections of the generator can be used. In these cases some of the capacitors are connected in parallel during the discharge.

The required impulse shape is obtained by suitable selection of the series and discharge resistors of the generator.

The front time can be calculated approximately from the equation:

For R1 >> R2 and Cg >> C (15.1)

Tt = .R.C.123
and the half time to half value from the equation
T ≈ 0,7.R.C
In practice the testing circuit is dimensioned according to experience.

## Performance of Impulse Test

The test is performed with standard lightning impulses of negative polarity. The front time (T1) and the time to half-value (T2) are defined in accordance with the standard.

Standard lightning impulse
Front time T1 = 1,2 μs ± 30%
Time to half-value T2 = 50 μs ± 20%

In practice the impulse shape may deviate from the standard impulse when testing low-voltage windings of high rated power and windings of high input capacitance. The impulse test is performed with negative polarity voltages to avoid erratic flashovers in the external insulation and test circuit. Waveform adjustments are necessary for most test objects. Experience gained from results of tests on similar units or eventual precalculation can give guidance for selecting components for the wave shaping circuit.

The test sequence consists of one reference impulse (RW) at 75% of full amplitude followed by the specified number of voltage applications at full amplitude (FW) (according to IEC 60076-3 three full impulses). The equipment for voltage and current signal recording consists of digital transient recorder, monitor, computer, plotter and printer. The recordings at the two levels can be compared directly for failure indication. For regulating transformers one phase is tested with the on-load tap changer set for the rated voltage and the two other phases are tested in each of the extreme positions.

## Connection of Impulse Test

All the dielectric tests check the insulation level of the job. Impulse generator is used to produce the specified voltage impulse wave of 1.2/50 micro seconds wave. One impulse of a reduced voltage between 50 to 75% of the full testvoltage and subsequent three impulses at full voltage.

For a three phase transformer, impulse is carried out on all three phases in succession.

The voltage is applied on each of the line terminal in succession, keeping the other terminals earthed.
The current and voltage wave shapes are recorded on the oscilloscope and any distortion in the wave shape is the criteria for failure.

## Transformer Oil and Winding Temperature Rise Test

Temperature rise test of Transformer is included in type test of transformer. In this test we check whether the temperature rising limit of transformer winding and oil as per specification or not.

Temperature Rise Test for Top Oil of Transformer

1. First the LV winding of the transformer is short circuited.
2. Then one thermometer is placed in a pocket in transformer top cover. Other two thermometers are placed at the inlet and outlet of the cooler bank respectively.
3. The voltage of such value is applied to the HV winding that power input is equal to no load losses plus load losses corrected to a reference temperature of 75°C.
4. The total losses are measured by three watt-meters method.
5. During the test, hourly readings of top oil temperature are taken from the thermometer already placed in the pocket of top cover.
6. Hourly readings of the thermometers placed at inlet and outlet of the cooler bank are also noted to calculate the mean temperature of the oil.
7. Ambient temperature is measured by means of thermometer placed around the transformer at three or four points situated at a distance of 1 to 2 meter from and half-way up the cooling surface of the transformer.
8. Temperature rise test for top oil of transformer should be continued until the top oil temperature has reached an approximate steady value that means testing would be continued until the temperature increment of the top oil becomes less than 3°C in one hour. This steady value of top oil is determined as final temperature rise of transformer insulating oil.
9. There is another method of determination of oil temperature. Here the test in allowed to be continued until the top oil temperature rise does not vary more than 1°C per hour for four consecutive hours. The least reading is taken as final temperature rise of the oil.

During temperature rise test for top oil of transformer we make the LV winding short circuited and apply voltage to the HV winding. So for full load rated currentflows in the transformer, the supply voltagerequired will much less than rated transformer voltage. We know that core loss of a transformer depends upon voltage. So there will not be any considerable core loss occurs in the transformer during test. But for getting actual temperature rise of the oil in a transformer, we have to compensate the lack of core losses by additional copper loss in the transformer. For supplying this total losses, transformer draws currentfrom the source much more than its rated value for transformer.

Temperature rise limits of transformer when it is oil immersed, given in the table below

TEMPERATURE RISE LIMIT
FOR AIR AS
COOLING MEDIUM
TEMPERATURE RISE LIMIT
FOR WATER AS
COOLING MEDIUM
CONDITION
WINDING55oC60oCWhen oil circulation is natural
60oC65oCWhen oil circulation is forced
TOP OIL50oC55oCWhen transformer is sealed &
equipped with conservator tank
45oC50oCWhen transformer is neither sealed
nor equipped with conservator tank

NB: These temperature rises limits mentioned in the above table are the temperature rise above the temperature of cooling medium. That means these are the difference between winding or oil temperature and temperature of cooling air or water.

### Winding Temperature Rise Test on Transformer

1. After completion of temperature rise test for top oil of transformer thecurrent is reduced to its rated value for transformer and is maintained for one hour.
2. After one hour the supply is switch off and short circuit and supply connection to the HV side and short circuit connection to the LV side are opened.
3. But, the fans and pumps are kept running (if any).
4. Then resistance of the windings are measured quickly.
5. But there is always a minimum 3 to 4 minutes time gap between first measurement of resistance and the instant of switching off the transformer, which can not be avoided.
6. Then the resistances are measured at the same 3 to 4 minutes time intervals over a period of 15 minutes.
7. Graph of hot resistance versus time is plotted, from which windingresistance (R2) at the instant of shut down can be extrapolated.
8. From this value, θ2, the winding temperature at the instant of shut down can be determined by the formula given below-

Where, R1 is the cold resistance of the winding at temperature t1.

For determining winding temperature rise we have to apply the above discussed indirect method. That means hot winding resistance is measured and determined first and then from that value we have to calculate the winding temperature rise, by applying resistancetemperature relation formula. This is because unlike oil the winding of transformer is not accessible for external temperature measurement.

## Vector Group Test of Transformer

The vector group of transformer is an essential property for successfulparallel operation of transformers. Hence every electrical power transformermust undergo through vector group test of transformer at factory site for ensuring the customer specified vector group of transformer.

## The phase sequence or the order in which the phases reach their maximum positive voltages, must be identical for two paralleled transformers. Otherwise, during the cycle, each pair of phases will be short circuited.

The several secondary connections are available in respect of various primary three phase connection in a the three phase transformer. So for same primary applied three phase voltage there may be different three phase secondary voltages with various magnitudes and phases for different internal connection of the transformer.

Let’s have a discussion in detail by example for better understanding.

We know that, the primary and secondary coils on any one limb have induced emfs that are in time-phase. Let’s consider two transformers of same number primary turns and the primary windings are connected in star. The secondary number of turns per phase in both transformers are also same. But the first transformer has star connected secondary and other transformer has delta connected secondary. If same voltages are applied in primary of both transformers, the secondary induced emf in each phase will be in same time-phase with that of respective primary phase, as because the the primary and secondary coils of same phase are wound on the same limb in the core of transformer. In first transformer, as the secondary is star connected, the secondary line voltage is √3 times of induced voltage per secondary phase coil. But in case of second transformer, where secondary is delta connected, the line voltage is equal to induced voltage per secondary phase coil. If we go through the vector diagram of secondary line voltages of both transformer, we will easily find that there will be a clear 30o angular difference between the line voltages of these transformers. Now, if we try to run these transformers in parallel then there will be a circulating current flows between the transformers as because there is a phase angle difference between their secondary line voltages. This phase difference can not be compensated. Thus two sets of connections giving secondary voltages with a phase displacement can not be intended for parallel operation of transformers.

The following table gives the connections for which from the view point of phase sequence and angular divergences, transformer can be operated parallel. According to their vector relation, all three phase transformers are divided into different vector group of transformer. All electrical power transformers of a particular vector group can easily be operated in parallel if they fulfill other condition for parallel operation of transformers.

GROUPCONNECTIONCONNECTION
0
(0O)
YY0
DD0
1
( 30O)
YD1
DY1
6
( 180O)
YY6
DD6
11
( – 30O)
YD11
DY11

### Procedure of Vector Group Test of Transformer

Let’s have a YNd11transformer.

1. Connect neutral point of star connected winding with earth.
2. Join 1U of HV and 2W of LV together.
3. Apply 415 V, three phase supply to HV terminals.
4. Measure voltages between terminals 2U-1N, 2V-1N, 2W-1N, that means voltages between each LV terminal and HV neutral.
5. Also measure voltages between terminals 2V-1V, 2W-1W and 2V-1W.

For YNd11 transformer, we will find,

2U-1N > 2V-1N > 2W-1N

2V-1W > 2V-1V or 2W-1W .

The vector group test of transformer for other group can also be done in similar way.

## Transformer Ratio Test

The performance of a transformer largely depends upon perfection of specific turns or voltage ratio of transformer. So transformer ration test is an essential type test of transformer. The voltage should be applied only in the high voltagewinding in order to avoid unsafe voltage.

## Ratio Test of Transformer and Check of Phase Displacement

Actually the no load voltage ratio of transformer is equal to the turn ratio. Soratio test of transformer.

#### Procedure of Transformer Ratio Test

1. First, the tap changer of transformer is kept in the lowest position and LV terminals are kept open.
2. Then apply 3-phase 415 V supply on HV terminals. Measure the voltages applied on each phase (Phase-phase) on HV and induced voltages at LV terminals simultaneously.
3. After measuring the voltages at HV and LV terminals, the tap changer of transformer should be raised by one position and repeat test.
4. Repeat the same for each of the tap position separately.

The above transformer ratio test can also be performed by portable transformer turns ratio (TTR) meter. They have an in built power supply, with the voltages commonly used being very low, such as 8-10 V and 50 Hz. The HV and LV windings of one phase of a transformer are connected to the instrument, and the internal bridge elements are varied to produce a null indication on the detector.

Let’s have a discussion on transformer turns ratio (TTR) meter method of turn ratio test of transformer.

A phase voltage is applied to the one of the windings by means of a bridge circuit and the ratio of induced voltage is measured at the bridge. The accuracy of the measuring instrument is < 0.1 %.

This theoretical turn ratio is adjusted on the transformer turn ratio tested or TTR by the adjustable
transformer as shown in the figure above and it should be changed until a balance occurs in the percentage error indicator. The reading on this indicator implies the deviaton of measured turn ratio from expected turn ratio in percentage.

Out-of-tolerance, ratio test of transformer can be due to shorted turns, especially if there is an associated high excitation current. Open turns in HV winding will indicate very low exciting current and no output voltage since open turns in HV winding causes no excitation current in the winding means no flux hence no induced voltage. But open turn in LV winding causes, low fluctuating LV voltage but normal excitation current in HV winding. Hence open turns in LV winding will be indicated by normal levels of exciting current, but very low levels of unstable output voltage. The turn ratio test of transformer also detects high resistance connections in the lead circuitry or high contactresistance in tap changers by higher excitation current and a difficulty in balancing the bridge.

## Transformer winding resistance measurement

Transformer winding resistance measurement is carried out as a type test, routine test and also as a field test.

## تست ترانسفورماتورهای قدرت

در این قسمت بیشتر به تستهایی می پردازیم که در سایت ، پس از نصب ترانس و در مرحله پیش راه اندازی صورت می گیرند.

قبل از انجام تست ظاهر ترانس مورد بازبینی قرار خواهد گرفت و موارد زیر چک خواهد شد:

·        سفت بودن پیچها و سایر اتصالات

·        بررسی ترمینال ها

·        بررسی قسمت های چینی ترانس

·        عایق بندی بین سیم پیچی ها و تانک

·        عایق بندی بین تجهیزات و سیستم Earth

·        اتصال و سیستم Earth برقگیرها

·        اتصال زمین  تانک

·        اتصال زمین مربوط به نوترال اولیه یا ثانویه

·        بررسی تجهیزات خنک کنندگی مانند:

•        جهت چرخش موتورها

•        بازرسی مکانیکی فن ها

•        جفت بودن و صحت اتصال رادیاتورها

همچنین بررسی عملکرد:

·        تجهیزات ایمنی

·        مرحله اول رله بوخهلتس : آلارم توسط تزریق هوا

·        مرحله دوم رله بوخهلتس: تریپ

·        آلارم ترموستات + فرمان قطع (تریپ)

·        تپ چنجر(tap changer)

·        Limit switch

·        سنسورهای حرارتی سیستم خنک کنندگی ، راه انداز فن و سیستم گردش روغن و همچنین اندازه گیری مقاومت سیم پیچها در تپ های مختلف و تصفیه و تست دی الکتریک روغن ترانسفورماتورها تستها و چگونگی آنها

## آشنایی با ارت سنج

وجود چاه ارت برای حفاظ از کاربران و تخلیه بار الکتریکی روشی است که سال هاست اجرا می شود.اندازه گیری مقاومت این چاه ها از اهمیت ویژه ای برخوردار است با توجه به نیاز کاربر ارت سنج ه به سه دسته تقسیم بندی می شوند

## آشنایی با رله

رله یک سوئیچ الکترونیکی که تحت کنترل سایر مدارات الکترونیکی باز و بسته می شود. در اصل سوئیچ با یک آهنربای مغناطیسی برای باز و بسته کردن یک یا چند اتصال عمل می کند. این وسیله توسط “جوزف هنری” (Joseph Henry) در سال ۱۸۳۵ اختراع شد. چون رله می تواند مدار خروجی پر قدرتی را نسبت به مدار ورودی کنترل کند می توان آنرا به عنوان نوعی تقویت کننده در نظر گرفت.
وقتی جریان از سیم پیچ عبور می کند، میدان مغناطیسی حاصله یک میله فلزی را که به طور مکانیکی به یک اتصال متصل شده است، را جذب می کند. این حرکت موجب اتصال یا قطع یک اتصال با یک اتصال ثابت می شود. وقتی جریان قطع می شود، میله فلزی با نیروی تقریبی نصف قدرت میدان مغناطیسی به محل اولیه خود بر می گردد. معمولا این نیرو توسط یک فنر (spring) تامین می شود، البته از نیروی گرانش (gravity) نیز در موتورهای استارتر صنعتی ممکن است استفاده شود. اغلب رله ها برای عملیات سریع ساخته می شوند. در کاربردهای ولتاژ پائین، کاهش نویز دارای اولویت بیشتری است و در کاردهای ولتاژ بالا کاهش قوس الکتریکی اولویت بیشتری دارد.
اگر انرژی سیم پیچ توسط DC تامین شود، خیلی اوقات یک دیود به دوسر سیم پیچ متصل می شود تا انرژی حاصل از میدان مغناطیسی را به هنگام قطع مصرف و یا به عبارتی پراکنده کند، که می تواند یک ضربه ولتاژ باشد و به سایر قسمتهای مدار ضربه بزند. اگر سیم پیچ برای کار با AC طراحی شده باشد، یک حلقه مسی در انتهای سیم پیچ، تابیده می شود. این حلقه(Shading ring) یک جریان غیر هم فاز تولید می کند که کشش میله فلزی را در سیکلهای AC افزایش می دهد. یعنی هنگامی که جریان AC مقدار مینیمم خود را دارد این سیم با یک اختلاف فاز نسبت به آن دارای مقداری جریان است که می تواند میله را به سمت سیم پیچ نگه دارد و در غیر این صورت میله در هر سیکل از سیم پیچ جدا  و دوباره متصل می شود و موجب ضربه زدن به سایر قسمتهای مدار می شود.
به تشابه با عملیات کارکرد رله مغناطیسی، خواهید دید که در رله های حالت جامد از تریستور یا سایر سوئیچهای حالت جامد استفاده می شود. برای رسیدن به ایزولاسیون الکتریکی از(light-emitting diode) یعنی LED با یک ترانزیستور نوری استفاده می شود.

## فیبر نوری چگونه کار می کند؟

هرجا که صحبت از سیستم های جدید مخابراتی، سیستم های تلویزیون کابلی و اینترنت باشد، در مورد فیبر نوری هم چیزهایی میشنوید.
فیبرهای نوری از شیشه شفاف و خالص ساخته میشوند و با ضخامتی به نازکی یک تار موی انسان، میتوانند اطلاعات دیجیتال را در فواصل دور انتقال دهند. از آنها همچنین برای عکسبرداری پزشکی و معاینه های فنی در مهندسی مکانیک استفاده میشود.
یک رشته فیبر نوری

در این مقاله میخوانیم که این فیبرهای نوری چگونه نور را منتقل میکنند و نیز درمورد روش عجیب ساخت آنها !
فیبرنوری چیست؟

## آشنایی با روغن ترانسفورماتور و نکاتی در مورد نگهداری ترانسفورماتور

پیشگفتار

ترانسفورماتورها یکی از مهمترین تجهیزات، در سیستم های الکتریکی است. درجه اهمیت آن از اینجا مشخص می شود که این دستگاه در هر سه قسمت تولید انتقال و توزیع کاربرد اساسی دارد، به جرات می توان گفت که ایجاد خطا در این دستگاه و خارج شدن از سیستم باعث می شود ، بقیه سیستم نیز از کار بیافتند. وقابلیت اطمینان را پایین می آورد.

شناخت قسمتهای مختلف این دستگاه و هماهنگی با سایر تجهیزات با عث می شود از این دستگاه به طرز صحیح استفاده شود و طول عمر آن افزایش می یابد که به تبع آن قابلیت اطمینان سیستم نیز افزایش یابد.

یکی از مواد حیاتی که در ترانسفورماتورها مورد استفاده قرار می گیرد روغن است، پس لازم است تا ما از خواص و انواع این ماده شناخت داشته باشیم.

هیدروکربن ها که قسمت اعظم روغن ها را تشکیل می دهند به سه بخش مهم تقسیم می شوند:

الف – پارافین ها

ب – نفتالین ها

ج – ترکیبات آروماتیک

## تست شکست عایقی روغن

مختصری از دستگاه تست روغن (oil tester )
و روش انجام آزمایش استقامت الکتریکی روغن ترانسفورماتورهای قدرت

## سیرکولاسیون روغن

ترانسفورماتورهای قدرت غوطه ور در روغن با توجه به حجم وبزرگی ترانسفورماتور ها مقدار زیادی  روغن در تانک آنها وجود دارد که برای ترانسهای  با قدرت بالا  بیش از 500 بشکه روغن مصرف میگردد به دلایل مختلف روغن ترانسفورماتور احتیاج به تصفیه فیزیکی دارد که در بخش های قبلی توضیحات آن آورده شده بنا بر این در اینجا  فقط نحوه فیلتر اسیون ورعایت استانداردها  گفته شده است

## لینک کاتالوگ رله های SIEMENS

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