Knee Point Voltage of Current Transformer PS Class

Current Transformer PS Class

Before understanding Knee Point Voltage of Current Transformer and current transformer PS class we should recall the terms instrument security factor of CT and accuracy limit factor.

Instrument Security Factor or ISF of Current Transformer

Instrument security factor is the ratio of instrument limit primary current to the rated primary current. Instrument limit current of a metering current transformer is the maximum value of primary current beyond which current transformer core becomes saturated. Instrument security factor of CT is the significant factor for choosing the metering instruments which to be connected to the secondary of the CT. Security or Safety of the measuring unit is better, if ISF is low. If we go through the example below it would be clear to us.
Suppose one current transformer has rating 100/1 A and ISF is 1.5 and another current transformer has same rating with ISF 2. That means, in first CT, the metering core would be saturated at 1.5 × 100 or 150 A, whereas is second CT, core will be saturated at 2 × 100 or 200 A. That means whatever may be the primary current of both CTs, secondary current will not increase further after 150 & 200 A of primary current of the CTs respectively. Hence maximum secondary current of the CTs would be 1.5 & 2.0 A.
As the maximum current can flow through the instrument connected to the first CT is 1.5 A which is less than the maximum value of current can flow through the instrument connected to the second CT i.e. 2 A. Hence security or safety of the instruments of first CT is better than later.
Another significance of ISF is during huge electrical fault, the short circuit current, flows through primary of the CT does not affect destructively, the measuring instrument attached to it as because, the secondary current of the CT will not rise above the value of rated secondary current multiplied by ISF.

Knee Point Voltage of Current Transformer

This is the significance of saturation level of a CT core mainly used for protection purposes. The sinusoidal voltage of rated frequency applied to the secondary terminals of current transformer, with other winding being open circuited, which when increased by 10% cause the exiting current to increase 50%. The CT core is made of CRGO steel. It has its won saturation level.

The EMF induced in the CT secondary windings is

E₂ = 4.44φfT₂

Where, f is the system frequency, φ is the maximum magnetic flux in Wb. T₂ is the number of turns of the secondary winding. The flux in the core, is produced by excitation current I_e. We have a non-liner relationship between excitation current and magnetizing flux. After certain value of excitation current, flux will not further increase so rapidly with increase in excitation current. This non-liner relation curve is also called B - H curve. Again from the equation above, it is found that, secondary voltage of a current transformer is directly proportional to flux φ. Hence one typical curve can be drawn from this relation between secondary voltage and excitation current as shown below.

It is clear from the curve that, linear relation between V & I_e is maintained from point A & K. The point ′A′ is known as ′ankle point′ and point ′K′ is known as ′Knee Point′. voltage of current transformer" title="Knee Point Voltage of Current Transformer" class="alignright"/>

In differential and restricted earth fault (REF) protection scheme, accuracy class and ALF of the CT may not ensure the reliability of the operation. It is desired that, differential and REF relays should not be operated when fault occurs outside the protected transformer. When any fault occurs outside the differential protection zone, the faulty current flows through the CTs of both sides of electrical power transformer. The both LV & HV CTs have magnetizing characteristics. Beyond the knee point, for slight increase in secondary emf a large increasing in excitation current is required. So after this knee point excitation current of both current transformers will be extremely high, which may cause mismatch between secondary current of LV & HV current transformers. This phenomena may cause unexpected tripping of power transformer. So the magnetizing characteristics of both LV & HV sides CTs, should be same that means they have same knee point voltage V_k as well as same excitation current I_e at V_k/2. It can be again said that, if both knee point voltage of current transformer and magnetizing characteristic of CTs of both sides of power transformer differ, there must be a mismatch in high excitation currents of the CTs during fault which ultimately causes the unbalancing between secondary current of both groups of CTs and transformer trips.

So for choosing CT for differential protection of transformer, one should consider current transformer PS class rather its convectional protection class. PS stands for protection special which is defined by knee point voltage of current transformer V_k and excitation current I_e at V_k/2.

Why CT Secondary Should Not Be Kept Open?

The electrical power system load current always flows through current transformer primary; irrespective of whether the current transformer is open circuited or connected to burden at its secondary. If CT secondary is open circuited, all the primary current will behave as excitation current, which ultimately produce huge voltage. Every current transformer has its own non-linear magnetizing curve, because of which secondary open circuit voltage should be limited by saturation of the core. If one can measure the rms voltage across the secondary terminals, he or she will get the value which may not appear to be dangerous. As the CT primary current is sinusoidal in nature, it zero 100 times per second.(As frequency of the current is 50 Hz). The rate of change of flux at every current zero is not limited by saturation and is high indeed. This develops extremely high peaks or pulses of voltage. This high peaks of voltage may not be measured by conventional voltmeter. But these high peaks of induced voltage may breakdown the CT insulation, and may case accident to personnel. The actual open-circuit voltage peak is difficult to measure accurately because of its very short peaks. That is why CT secondary should not be kept open.

Precommissioning & Commissioning tests of Power Transformer

Common Pre-commissioning Tests of Transformer

Insulation resistance values of various winding should be checked. During measurement of insulation resistance, the transformer must not be connected with any external lines or lightning arrestor etc. The I R values obtained should be comparable with those indicated in the manufacturer's test reports. Oil sample from the top and the bottom of the tank, selector switch and diverter switch tank of the tap changer and radiator tank etc should be tested. The dielectric strength of the transformer oil should be more than 50 KV. Resistance measurement of winding should be done at all tap positions. The ratio test between the HV and LV at all tap positions should also be performed. For three winding transformers ratio between other pair of windings should also be taken.

The phase relationship test should also be carried out. The tap changer should be undergone the following tests a) Manual operation b) Local electrical operation c) Remote electrical operation d) Parallel operation e) The operation should be checked from one end of the range to the other to check for the limit switch operation also. f) IR values of the motor and control wiring should be checked by 500 V Megger.

Buchholz Relay Test

Buchholz relay operation for alarm and trip should also be checked by injecting air to the test pocket provided in the relay.

Low Oil Level Alarm Test

The low oil level alarm of the magnetic oil gauge should be checked.

Temperature Indicator Test

The contacts of the Oil Temperature Indicator and Winding Temperature Indicator for alarm trip and control should be checked and set at required temperature

Cooling Gear Test

a) The IR values and setting for operation of oil pumps and fans motor should be checked. b) Alarm trip contact settings of differential pressure gauge, oil and water flow indicators, where provided should be checked.

Marshalling Box

The wiring from various accessories to the Marshalling kiosk should be checked

Protective Relay Test

Trapping of associated circuit breakers is to be provided by actual operation of differential relay, over current relay, earth fault relay and other protective relays as applicable.

Magnetizing Current Test

The magnetizing current can be measured by feeding 400 V, three phase 50 Hz supply from HV side keeping the LV side open circuited and the value in different phases could be compared.

Additional checks during commissioning of power transformer

1. All oil valves are in correct position closed or opened as required. 2. All air pockets are cleared. 3. Thermometer pockets are filled with oil. 4. Oil is at correct level in the bushing, conservator tank, diverter switch tank etc. 5. Arcing horn of the bushing is set correctly 6. CT polarity is correct when bushing mounted CTs are provided.

If all the above tests/checks are found satisfactory, a settling time of at least 24 hours, should be allowed for the oil and air released from all points at six hourly intervals, before commissioning the transformer. Whenever possible, while commissioning, the voltage should be build up slowly and brought to the full level in about 4 to 6 hours. If the circuit breaker is tripped on differential, Buchholz or any other devices, the cause must be investigated before re-energizing the transformer. Sometimes tripping occurs maybe because of magnetic inrush current which depends upon the switching at a particular moment in the cycle. After commissioning magnetising current should be measured by connecting clip on meter in current transformer circuits and the reading should be compared with the test report value.

DGA testing of Power transformer

DGA or Dissolved Gas Analysis of Transformer Oil

Whenever electrical power transformer goes under abnormal thermal and electrical stresses, certain gases are produced due to decomposition of transformer insulating oil, when the fault is major, the production of decomposed gases are more and they get collected in Buchholz relay. But when abnormal thermal and electrical stresses are not significantly high the gasses due to decomposition of transformer insulating oil will get enough time to dissolve in the oil. Hence by only monitoring the Buchholz relay it is not possible to predict the condition of the total internal healthiness of electrical power transformer. That is why it becomes necessary to analyse the quantity of different gasses dissolved in transformer oil in service. From dissolved gas analysis of transformer Oil or DGA of transformer oil, one can predict the actual condition of internal health of a transformer. It is preferable to conduct the DGA test of transformer oil in routine manner to get prior information about the trend of deterioration of transformer health and life.

Actually in dissolved gas analysis of transformer oil or DGA of transformer oil test, the gases in oil are extracted from oil and analyze the quantity of gasses in a specific amount of oil. By observing percentages of different gasses present in the oil, one can predict the internal condition of transformer.

Generally the gasses found in the oil in service are hydrogen (H₂), methane(CH₄), Ethane (C₂H₆), ethylene(C₂H₄), acetylene (C₂H₃), carbon monoxide (CO), carbon dioxide (CO₂), nitrogen(N₂) and oxygen(O₂).

Most commonly used method of determining the content of these gases in oil, is using a Vacuum Gas Extraction Apparatus and Gas Chronographs. By this apparatus first gasses are extracted from oil by stirring it under vacuum. These extracted gasses are then introduced in gas Chronographs for measurement of each component.

Generally it is found that hydrogen and methane are produced in large quantity if internal temperature of power transformer rises up to 150°C to 300°C due to abnormal thermal stresses. If temperature goes above 300°C, ethylene(C₂H₄) are produced in large quantity. At the temperature is higher than 700°C large amount of hydrogen(H₂) and ethylene(C₂H₄) are produced.

Ethylene(C₂H₄) is indication of very high temperature hot spot inside electrical transformer.If during DGA test of transformer oil, CO and CO₂ are found in large quantity it is predicted that there is decomposition of proper insulation.

Furfural or Furfuraldehyde Analysis

Transformer core and winding have mainly paper insulation. Base of paper is cellulose. The Cellulose has a structure of long chain of molecules. As the paper becomes aged, these long chains are broken into number of shorter parts. This phenomenon we often observe in our home. The pages of very old books become very much brittle. In transformer, the aging affect of paper insulation is accelerated due to oxidation occurs in oil. When insulating paper becomes mechanically weak, it can not withstand the mechanical stresses applied during electrical short circuit and leads to electrical breakdown.

It is therefore necessary to monitor the condition of paper insulation inside a power transformer.

It is not possible to bring out a piece of paper insulation from a transformer in service for testing purpose. But we are lucky enough, that there is a testing technique developed, where we can examine the condition of paper insulation without touching it. The method is called Furfuradehyde analysis of in short Furfural test.

Although by dissolved gas analysis one can predict the condition of the paper insulation primarily, but it is not very sensitive method. There is a guide line in IEC-599, where it is told that if the ratio of CO₂ and CO in DGA results is more than 11, it is predicted that the condition of paper insulation inside the transformer is not good. A healthy cellulose insulation gives that ratio in a range of 4 to 11. But still it is not a very sensitive way of monitoring the condition of paper insulation. Because CO₂ and CO gases also produced during oil breakdown and sometimes the ratio may misleads the prediction.

When oil is soaked into paper, it is damaged by heat and some unique oil soluble compounds are realized and dissolved in the oil along with CO₂ and CO. These compounds belong to the Furfuraldehyde group. These are some times called Furfural in short. Among all Furfurals compounds 2- Furfural is the most predominant. These Furfural family compound can only be released from destructive heating of cellulose or paper. Furfuraldehyde analysis is very sensitive as because damage of few grams of paper is noticeable in the oil even of a very large size transformer. It is a very significant diagnostic test. It is best test for assessing life of transformer. The rate of rise of percentage of Furfurals products in oil, with respect to time, is used for assessing the condition and remaining life of paper insulation in power transformer.

ELECTRICAL EGG INTERVIEW QUESTIONS: INTERVIEW QUESTIONS

ELECTRICAL EGG INTERVIEW QUESTIONS: INTERVIEW QUESTIONS: Hai everybody,               I would like to share some informations from my knowledge and experience. Hopes it would help somebody who p...

INTERVIEW QUESTIONS

Hai everybody,
              I would like to share some informations from my knowledge and experience. Hopes it would help somebody who prepare for an interview. Here is some questions I faced on some interviews.

Transformer
1. What are the tests doing commissioning of a transformer?
2. What is BDV test?
3. What is PI value of  a transformer ?
4. What is DGA test ?
5. What are the protections using for a transformer?
6. What is differential protection?
7. In differential protection, either CT or VT measurements taking ?

Generator
1. What is excitation doing on a generator?
2. What is field flashing?
3. What are the protections using for generator?