Medium voltage Vacume circuit breaker vs Medium Voltage switch fuse unit

Medium Voltage circuit breakers are favored when typical loads include transformers, capacitors, larger motors, generators or distribution feeders
(Ratings required exceed those of vacuum contactors 400A or 720 Amp up to 7.2 kV) Continuous load current is high (e.g., larger transformers, larger motors.

Switching is not very frequent (e.g., weekly or monthly):high endurance
(1,000s of operations) is satisfactory Process continuity is critical e.g., no time for fuse replacement Reduced-voltage (RV) starting is not needed (RV starting complicates switchgear bus arrangements)

Medium-voltage NEMA Class E2 controllers fused contactors are favored
when typical loads include motors or smaller transformers continuous load current is low or moderate (e.g., smaller motors or transformer)

Switching is very frequent (e.g., daily or several times per day); very high
endurance (100,000s of operations is needed).

Process continuity is compatible with fuse replacement time

Reduced-voltage starting is needed to reduce starting duty and voltage
fluctuation on system

Double Coil Syndrome In Plc Ladder logic and how to cure it

Double coil syndrome though sound like a human disease but it not . This disease is found in PLC programming.

Let us investigate what is this syndrome and how to eradicate the cause of it.

Before that let us discuss a logic in ladder Can we use same output coil for two more

than once in our ladder program? Yeah you herder right we can use the same output coil for more than once in our ladder program lets see how

Let us take an example. Suppose we have taken two toggles switches no- x01 and x02. If either x01 turns on or x02 turns on, the output coil named Q06 turn on which will turn on the motor. 

Let’s create a ladder program on this logic

x01             Q06

--| |-----------( )---

x02              Q06

--| |-----------( )---

So, If either toggle switch x01 or x02 turn on, then output Q06 should turn on which should start the motor. But after compiling/running the program you will see that it won't generate output as per our demand .Because of Double Coil Syndrome. Here as Output is taken twice hence it is called Double. If we will take it thrice then it will be called Triple Coil Syndrome.

Now let's analyze the logic

 by running the program we will see that by turning on the toggle switch x01  motor won't run. But when we turn on the toggle switch x02 then motor will run. as motor is running with toggle switch x02 that means motor is good .Now further checking revels that PLC card is also good and in working condition. So we have to get back to our program check it from top to bottom

x01                 Q06

--| a |-----------(c )---

x02                 Q06

--| b |-----------( d )---

Now take two cases

Case 1

lets take all switches that is a&b are turn off so output c&d will also be in off condition.

1- when we turn on the switch x01 so a is now on.

2- Since a is ON ,c(Q06) Will turn on.

3- as b is physically off ,so d(Q06) will also be off.

so when PLC will scroll down it will take the last command as its final command so output Q06 will stay off though we have turned on the x01.

Case 2

Let’s take all switches that is a&b are turn off so output c&d will also be in off condition.

now

1--a is off so c (Q06) is off.

2--b is on so d (Q06) is will turn on.

We can see here PLC will take the last command as its final command so output Q06 will stay on irrespective of the toggle switch x01.

This is what we call Double Coil Syndrome

Now let's cure the Disease

Just add an OR ladder to the logic.

As we want the motor to be on when x01 OR x02 turns on. Here's the situation.

|       x01             Q06

|------| |----|-------( )---|

|                |

|       x02    |

|------| |----|

|

Now, If switch x01 OR x02 turn on then Q06 will turn on .Double Coil Syndrome has been cured.

Liquid Resistance stater

Watch Full Operational Video Of Liquid Resistance Stater

Before going in detail about LRS we have to first know what a wound or slip ring induction Motor means and how it operates.

Wound/Slip Ring Motor Operation

Slip ring Induction motor is similar to that of conventional Induction motor but with high torque capacity.This torque I achieved by making some design changes in rotor. Its rotor has a resistance connected to its winding externally through a slip ring .this resistance can be anything from simple resistance box to liquid resistance box.

Wound Motor

What Isthe Need of Extra Resistance?

Squirrel cage induction motors draw 500% to over 1000% of full load current (FLC) during starting.While this is not a severe problem for small motors, it is for large (10’s of kW) motors Placing resistance in series with the rotor winding not only decreases start current, locked rotor current (LRC), but also increases the starting torque, locked rotor torque (LRT).

Wound Rotor With Slip Ring

Figure below shows that by increasing the rotor resistance from R0 to R1 to R2, the breakdown torque peak is shifted left to zero speed. Note that this torque peak is much higher than the starting torque available with no rotor resistance (R0) Slip is proportional to rotor resistance, and pullout torque is proportional to slip. Thus, high torque is produced while starting. This high torque is very much beneficial while dealing with heavy loads. The liquid rheostat allows for smooth stepless resistance change.

Torque Vs Slip Characteristics

A liquid rheostat consists of a tank of water mixed with a chemical (Sodium Carbonate) that alters the water’s conductivity in a specific way. It’s important to monitor the water on a regular basis and maintain the desired conductivity by adding water or chemicals as needed. Inserted in the tank are three conductive plates attached to the rotor winding of the induction motor. The motor has these winding brought out to three rings for carbon brush connections.

Liquid Resistance Stater Connection Diagram

The three plates are attached to a mechanism driven by a small reversible motor or manually driven wheel that can lower or raise the plates in the solution. When the plates are most of the way out, the resistance is maximized. When the plates are all they way in, (maximum surface area submerged) the resistance is minimized.

Transformer Testinf Part-3 (Ratio Test and vector group test)

(3) Turns Ratio / Voltage Ratio Test:

 Test Purpose:

• Turns Ratio Test / Voltage Ratio Test are done in Transformer to find out Open Circuited turns, Short Circuited turns in Transformer winding.
• The voltage ratio is equal to the turn’s ratio in a transformer (V1/V2=N1/N2). Using this principle, the turn’s ratio is measured with the help of a turn’s ratio meter. If it is correct , then the voltage ratio is assumed to be correct
• This test should be made for any new high-voltage power transformer at the time it is being installed.
• With use of Turns Ratio meter (TTR), turns Ratio between HV & LV windings at various taps to be measured & recorded.
• The turn’s ratio is measure of the RMS voltage applied to the primary terminals to the RMS Voltage measured at the secondary terminals.
• R= Np / Ns
• Where,
• R=Voltage ratio
• Np=Number of turns at primary winding.
• Ns= Number of turns at secondary Winding.
• The voltage ratio shall be measured on each tapping in the no-load condition.

 Test Instruments:

• Turns Ratio meter (TTR) to energies the transformer from a low-voltage supply and measure the HV and LV voltages.
• Wheatstone Bridge Circuit

 Method No1 Turns Ratio Testing:

 Test Procedure:

• Transformer Turns Ratio Meter (TTR):
• Transformer ratio test can be done by Transformer Turns Ratio (TTR) Meter. It has 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 (i.e. R-Y & r-n) are connected to the instrument, and the internal bridge elements are varied to produce a null indication on the detector.
• Values are recorded at each tap in case of tapped windings and then compared to calculated ratio at the same tap.
• The ratio meter gives accuracy of 0.1 per cent over a ratio range up to 1110:1. The ratio meter is used in a ‘bridge’ circuit where the voltages of the windings of the transformer under test are balanced against the voltages developed across the fixed and variable resistors of the ratio meter.
• Adjustment of the calibrated variable resistor until zero deflection is obtained on the galvanometer then gives the ratio to unity of the transformer windings from the ratio of the resistors.
• Bridge Circuit:

• 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 deviation of measured turn ratio from expected turn ratio in percentage.
• Theoretical Turns Ratio = HV winding Voltage / LV Winding Voltage
• % Deviation = (Measured Turn Ratio – Expected Turns Ration) / Expected Turns Ration
• 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 contact resistance in tap changers by higher excitation current and a difficulty in balancing the bridge.

 Test Caution:

• Disconnect all transformer terminals from line or load.
• Neutrals directly grounded to the grid can remain connected

 Method No 2 Voltage Ratio Testing:

•  This test is done to check both the transformer voltage ratio and tap changer.
• When “Turns Ratio meter” is not available, Voltage Ratio Test is done at various tap position by applying 3 phases LT (415V) supply on HT side of Power transformer. In order to obtain the required accuracy it is usual to use a ratio meter rather than to energies the transformer from a low-voltage supply and measure the HV and LV voltages.
• At Various taps applied voltage and Resultant voltages LV side between various Phases and phases& neutral measured with precision voltmeter & noted.

 Test Procedure:

• With 415 V applied on high voltage side, measure the voltage between all phases on the low voltage side for every tap position.
• First, the tap changer of transformer is kept in the lowest position and LV terminals are kept open.
• 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.
• After measuring the voltages at HV and LV terminals, the tap changer of transformer should be raised by one position and repeat test.
• Repeat the same for each of the tap position separately.
• At other taps values will be as per the percentage raise or lower at the respective tap positions.
• In case of Delta/Star transformers the ratio measure between RY-rn, YB-yn and BR-bn.
• Being Delta/Star transformers the voltage ratio between HV winding and LV winding in each phase limb at normal tap is 33 KV OR 33x√3 = 5.196 ,11 KV / √3 11
• At higher taps (i-e high voltage steps) less number of turns is in circuit than normal. Hence ratio values increase by a value equal to.5.196 + {5.196 x (no. of steps above normal) x (% rise per each tap)} 100
• Similarly for lower taps than normal the ratio is equal to 5.196 – {5.196 x (no. of steps above normal) x (% rise per each tap)}100

 Test Acceptance Criteria:

• Range of measured ratio shall be equal to the calculated ratio ±0.5%.
• Phase displacement is identical to approved arrangement and transformer’s nameplate.
• The IEEE standard (IEEE Standard 62) states that when rated voltage is applied to one winding of the transformer, all other rated voltages at no load shall be correct within one half of one percent of the nameplate readings. It also states that all tap voltages shall be correct to the nearest turn if the volts per turn exceed one half of one percent desired voltage .The ratio test verifies that these conditions are met.
• The IEC60076-1 standard defines the permissible deviation of the actual to declared ratio
• Principal tapping for a specified first winding pair: the lesser ±0.5% of the declared voltage ratio
• or 0.1 times the actual short circuit impedance. Other taps on the first winding pair and other winding pair must be agreed upon, and must be lower than the smaller of the two values stated above.
• Measurements are typically made by applying a known low voltage across the high voltage winding so that the induced voltage on the secondary is lower, thereby reducing hazards while performing the test .For three phase delta/wye or wye/delta transformer, a three phase equivalency test is performed, i.e. the test is performed across corresponding single winding.

 Test can detect:

• Shorted turns or open circuits in the windings.
• Incorrect winding connections ,and other internal faults or defects in tap changer

(4) Polarity / Vector group Test

 Purpose of Test:

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

 Test Instruments:

• Ratio meter.
• Volt Meter. A Ratio meter may not always be available and this is usually the case on site so that the polarity may be checked by voltmeter.

 Test Circuit Diagram:

 

 Test Procedure:

• The primary and secondary windings are connected together at one point.
• Connect neutral point of star connected winding with earth.
• Low-voltage three-phase supply (415 V) is then applied to the HV terminals.
• Voltage measurements are then taken between various pairs of terminals as indicated in the diagram and the readings obtained should be the phasor sum of the separate voltages of each winding under consideration.

 Condition:(HV side R-Y-B-N and LV Side r-y-b-n)

• R and r should be shorted.
• Apply 415 Volt to R-Y-B
• Measure Voltage between Following Phase and Satisfy Following Condition
  

Vector Group	Satisfied Following Condition
Dyn1	Rb=Rn+Bn
	Bb=By
	Yy<Yb
Dyn11	Ry=Rn+Yn
	Yb=Yy
	Bb<By
Ynd1	RN=Ry+Yn
	By=Yy
	Yy<Yb
Ynyn0	Bb=Yy
	Bn=Yn
	RN=Rn+Nn

Transformer Testing part-2 (Insulation resistance )

(A) Routine tests of Transformer

(1) Insulation Resistance Test:

 Test Purpose:

• Insulation resistance test of transformer is essential to ensure the healthiness of overall insulation of an electrical power transformer.

 Test Instruments:

• For LT System: Use 500V or 1000V Megger.
• For MV / HV System: Use 2500V or 5000V Megger.

 Test Procedure:

• First disconnect all the line and neutral terminals of the transformer.
• Megger leads to be connected to LV and HV bushing studs to measure Insulation Resistance (IR) value in between the LV and HV windings.
• Megger leads to be connected to HV bushing studs and transformer tank earth point to measure Insulation Resistance IR value in between the HV windings and earth.
• Megger leads to be connected to LV bushing studs and transformer tank earth point to measure Insulation Resistance IR value in between the LV windings and earth.
• NB: It is unnecessary to perform insulation resistance test of transformer per phase wise in three phase transformer. IR values are taken between the windings collectively as because all the windings on HV side are internally connected together to form either star or delta and also all the windings on LV side are internally connected together to form either star or delta.
• Measurements are to be taken as follows:

Type of Transformer	Testing-1	Testing-2	Testing-3
Auto Transformer	HV-LV to LV	HV-IV to E	LV to E
Two Winding Transformer	HV to LV	HV to E	LV to E
Three Winding Transformers	HV to LV	LV to LV	HV to E & LV to E

• Oil temperature should be noted at the time of insulation resistance test of transformer. Since the IR value of transformer insulating oil may vary with temperature.
• IR values to be recorded at intervals of 15 seconds, 1 minute and 10 minutes.
• With the duration of application of voltage, IR value increases. The increase in IR is an indication of dryness of insulation.
• Absorption Coefficient = 1 minute value/ 15 second value.
• Polarization Index = 10 minutes value / 1 minute value

 Tests can detect:

• Weakness of Insulation.

 (2) D.C. Resistance or Winding Resistance Test

 Test Purpose:

• Transformer winding resistance is measured
• To check any abnormalities like Loose connections, broken strands and High contact resistance in tap changers
• To Calculation of the I2R losses in transformer.
• To Calculation of winding temperature at the end of temperature rise test of transformer.

 Test Instrument:

• The Resistance of HV winding LV winding between their terminals are to be measured with
• Precision milliohm meter/ micro ohm meter / Transformer Ohmmeter. OR
• Wheatstone bridge or DC resistance meter.

 Method No: 1 (Kelvin Bridge Method for measurement of winding resistance)

 

Test Procedure:

• The main principle of bridge method is based on comparing an unknown resistance with a known resistance.
• When electric currents flowing through the arms of bridge circuit become balanced, the reading of galvanometer shows zero deflection that means at balanced condition no electric current will flow through the galvanometer.
• Very small value of resistance (in milliohms range) can be accurately measured by Kelvin Bridge method whereas for higher value Wheatstone bridge method of resistance measurement is applied. In bridge method of measurement of winding resistance, the error is minimized.
• All other steps to be taken during transformer winding resistance measurement in these methods are similar to that of current voltage method of measurement of winding resistance of transformer

 Method No: 2 (current voltage method of measurement of winding resistance)

Test Procedure:

• The resistance of each transformer winding is measured using DC current and recorded at a ambient temp.
• In this test resistance of winding is measurement by applying a small DC voltage to the winding and measuring the current through the same
• The measured resistance should be corrected to a common temperature such as 75°C or 85°C using the formula: RC=RM x ((CF+CT)/(CF+WT))
• where
• RC is the corrected resistance, RM is the measured resistance
• CF is the correction factor for copper (234.5) or aluminum (225) windings
• CT is the corrected temperature (75°C or 85°C)
• WT is the winding temperature (°C) at time of test
• Before measurement the transformer should be kept in OFF condition at least for 3 to 4 hours so in this time the winding temperature will become equal to its oil temperature.
• To minimize observation errors, polarity of the core magnetization shall be kept constant during all resistance readings.
• Voltmeter leads shall be independent of the current leads to protect it from high voltages which may occur during switching on and off the current circuit.
• The readings shall be taken after the electric current and voltage have reached steady state values. In some cases this may take several minutes depending upon the winding impedance.
• The test current shall not exceed 15% of the rated current of the winding. Large values may cause inaccuracy by heating the winding and thereby changing its resistance.
• For Calculating resistance, the corresponding temperature of the winding at the time of measurement must be taken along with resistance value.

 Required Precaution:

• According to IEC 60076-1, in order to reduce measurement errors due to changes in temperature, some precautions should be taken before the measurement is made.
• For Delta connected Winding: for delta-connected transformer, the resistance should be measured for each phase (i.e. R-Y , Y-B & B-R) .Delta is composed of parallel combination of the winding under test and the series combination of the remaining winding .It is therefore recommended to make three measurements for each phase to-phase winding in order obtain the most accurate results.
• For Delta connected windings, such tertiary winding of auto-transformers measurement shall be done between pairs of line terminals and resistance per winding shall be calculated as per the formula: Resistance per Winding = 1.5 X Measured Value
• For Star connected winding: the neutral brought out, the resistance shall be measured between the line and neutral terminal (i.e. R-N , Y-N,B-N) and average of three sets of reading shall be the tested value. For Star connected auto transformers the resistance of the HV side is measured between HV terminal and IV terminal, then between IV terminal and the neutral.
• For Dry type transformers: the transformer shall be at rest in a constant ambient temperature for at least three hours.
• For Oil immersed transformers: the transformers should be under oil and without excitation for at least three hours. In case of tapped windings, above readings are recorded at each tap. In addition, it is important to ensure that the average oil temperature (average of the top and bottom oil temperatures) is approximately the same as the winding temperature. Average oil temperature is to be recorded. Measured values are to be corrected to required temperatures.
• As the measurement current increases, the core will be saturated and inductance will decrease. In this way, the current will reach the saturation value in a shorter time.
• After the current is applied to the circuit, it should be waited until the current becomes stationary (complete saturation) before taking measurements, otherwise, there will be measurement errors.
• The values shall be compared with original test an result which varies with the transformer ratings.

 Test Acceptance criteria:

• DC Resistance Should be<=2% Factory Test.
• Test Current <10% Rated Current

 Test can detect:

• Short Turns
• Loose Connection of bushing
• Loose Connection or High Contact Resistance on Tap Changer.
• Broken winding stands