Impact Recorder use for Power Transformer

In the above picture is shown image of impact recorder for reference which is mounted on power transformer body.

1. During transportation of Power Transformer from manufacturing company to installation site, it experiences mechanical vibrations and shocks.
2. Due to this there are chances of damage like winding and core damage, inter turn insulation damage which can cause short circuit, looseness of clamping, low clearance between tank and active part, loss of nitrogen from tank etc.
3. This type of damage can be identified by using the measured values of impact recorder before starting commissioning of Power Transformer.
4. Electronic impact recorder store the values digitally with date and time so it provide additional information.
5. Accepted values of impact recorder is multiplication of “g” where g= 9.8 m/s2 like 0.4g.
6. Values are measured in three axis like direction of placement, perpendicular direction of placement and vertical direction of placement of transformer.
7. Accepted impact values differ for different sized power transformers and impact recorder is generally used for high MVA power transformers.
8. If the measured values are more than acceptable limits than internal inspection and various testing need to done on transformer and it further goes to on site or factory repairs upon severity of damage.

Same content to understand by video on my YouTube channel at https://youtu.be/brpVGQi9imw

Testing of Switchgear - Part 3 (Breaker operating timing test )

In previous blog, I discussed for Breaker Contact Resistance Measurement (CRM test) for switchgear. We will discuss further test in this post.

The next test we will discuss is as below.

Breaker Operating Timing Test

Purpose of the test :  

Circuit breaker is a part in electrical system which separates faulty part in event of fault. The separation time of the faulty part must be minimum and circuit breaker should do this quickly. This timing is called breaker operating time in which it removes the faulty part from system. Inside the breaker, it operates through mechanical operation. If this mechanism has any abnormality, then it will not operate smoothly and the breaker operating time will be more. When there is fault in the system, there will be high currents in the power circuit. Every equipment is designed to sustain for large current for very short time considering that circuit breaker will operate and will remove the fault from the system. If the circuit breaker will not operate within its desired time during the fault, there will be heavy damage to electrical equipment due to high currents. So we are checking the breaker closing and opening time through this test to ensure normal breaker timings.

Breaker Operating Time: Breaker operation time is total time of breaker closing time and opening time.

Breaker Opening Time: Breaker opening time is time between charging of tripping coil to separation of fixed and moving contact of breaker.

Breaker Closing Time: Breaker closing time is time between charging of closing coil to joining of fixed and moving contact of breaker.

Testing of Switchgear - Part 2 (Breaker Contact Resistance Measurement test)

In previous blog, I discussed for insulation resistance test for switchgear. We will discuss further test in this post.

The next test we will discuss is as below.

Breaker Contact Resistance Measurement (CRM test)

• Why we are carrying this test?

We are conducting this test to find the resistance of breaker contact where moving contact and fixed contact meet each other. This contact resistance should be low because the circuit breaker is carrying high current. So if the resistance of contact will be high, there will be more energy loss (more I²R) losses and it can’t pass high currents. By carrying this test, we come to know about looseness in contact, corrosion present at contact surface or any other contamination in the contact. This test shows healthiness of the contact.

• How to measure contact resistance of breaker ?

We are using one kit called CRM 100 for the test. This test works upon method of 4 wire Kelvin DC voltage drop test. In this method 4 wires are used for resistance measurement. Two wires are used for current injection and two wires are used for voltage measurement. In this method the current amount is set to high like 100 Amps. Higher the current we inject, more accurate resistance we get. Two current leads and two voltage leads are connected across each breaker pole. The resistance here we will get in terms of 20 micro ohms. All three breakers pole resistance should be same. Here the resistance value we obtain, depends upon capacity/ voltage rating of the breaker.

We will discuss further test in next blog for Breaker Operating Timing Test.

Testing of Switchgear - Part 1 (Breaker Insulation resistance test)

What is Switchgear ?

Switchgear is one type of electrical equipment (called Panel) which receives power supply from upstream like generators, greater capacity switchgear, transformers etc and provides supply to down stream like lower capacity switchgear, transformers, motors etc. It consists of breakers, switches, MCBs, control circuits, heaters, relays, closing and tripping circuits etc.

What are the tests we need to do for testing Switchgear on site ?

Following are the tests required for testing switchgear on site.

1) Breaker Insulation resistance test

Suppose we conducting insulation resistance test on 6.6 kV switchgear breaker, so the DC voltage injection will be 5 kV. If the switchgear is of low voltage rating than DC voltage injection using insulation tester will be low. Breaker will have three poles for R,Y and B phase. 

Why we are injecting only DC supply for checking Insulation Resistance ?

We are injecting only DC supply because we are supposed to find Insulation Resistance (R) and not Impedance (Z). If we inject AC supply then inductance and capacitance of the insulation will also be added with resistance so impedance (Z) will be calculated. Other reason is that if we inject AC supply then the electrical equipment will start to function as it is intended for use. For example we are injecting 5 kV AC supply to motor of 6.6 kV rating than motor will be charged and will start to run.

So when the breaker is closed, conduct the test for 1 minute at 

R phase to Earth

Y phase to Earth
B phase to Earth

R phase to Y phase

Y phase to B phase

B phase to R phase 

When the breaker is open, the pole will have two part. One is moving contact and other is fix contact. 

Conduct the test between moving and fix contact for 1 minute at

R phase to R' phase

Y phase to Y' phase

B phase to B' phase

Thus we can ensure that there is no any other path for current to flow in the breaker and for checking the insulation of breaker pole circuit.

Here the value we will get in ohms. As per standard, the value should be more than or equal to (kV + 1) M-ohms. Where kV is the voltage rating of the switchgear. 

If you do not get the values within the range, check the power circuit for any material in between the circuit and clean the current carrying part through electrical contact cleaner spray. The current carrying part should be without any moisture to get the value of insulation resistance test.

For further test details i will post "Testing of Switchgear - Part 2" within few days.

What is CT and why we need small value for measurement and protection in electrical circuits ?

What is Current Transformer  ?

Current transformer is one type of instrument transformer which is useful for measuring AC current in any electrical equipment. Current transformer primary is connected in series with the circuit whose current is to be measured and it's secondary is connected to relay,  ammeter,  watt-meter,  etc. This current transformer measures current in primary by producing proportional current in secondary circuit.  All current transformer have particular ratio like 200/1, 400/5 etc upon it's requirement. So for ratio of 200/1, this current transformer will produce 1 ampere in secondary circuit if there is 200 ampere in primary circuit. 

Why we need small scale value for measuring the current in electrical circuits ?

Current transformer is converting high value of current to low value of current and we use low value of current for measuring and protection purpose. We need low value of current because If we will use large value current for measurement and protection, then the instruments like ammeter, wattmeter and relay rating will have to be larger to bear the high current and the  costing of this devices will also be high. Large currents are also not easy to handle without circuit breakers.

Insulation Resistance Test in Electrical Equipment

Note: This test is to be done only when there is no any power available in the equipment. The equipment must be in shutdown condition. This is because to prevent mixing of AC and DC supply for Equipment and personal safety.

What is Insulation Resistance (I.R.) Test ?

Insulation resistance is one type of Electrical Test which is commonly used for checking healthiness of Insulation of Electrical Equipment. Insulation testers are provided with different voltage rating like 500 V, 1000 V, 2500 V, 5000 V etc. We have to inject the particular voltage to equipment as per equipment's voltage rating. For example the cable is to be used for 430 V AC application than DC injection voltage will be 500 V and if the cable is to be used for 6600 V AC application than DC injection voltage will be 5000 V.

Why we need to check the Insulation Resistance

As the insulation of any electrical equipment is made, ageing process starts in the insulation and this process become faster depending upon the temperature and chemical reactions. So we need to check insulation resistance periodically. If insulation resistance of the electrical equipment becomes weak, there is risk of damage to electrical equipment and person.

How we check Insulation Resistance ?

We check the Insulation resistance of any electrical equipment by injecting DC supply according to the voltage level of equipment between the windings (or phase connection) and between windings (or phase connections) and earth connection for duration of one minute.

Why we are injecting only DC supply for checking Insulation Resistance ?

We are injecting only DC supply because we are supposed to find Insulation Resistance (R) and not Impedance (Z). If we inject AC supply then inductance and capacitance of the insulation will also be added with resistance so impedance (Z) will be calculated. Other reason is that if we inject AC supply then the electrical equipment will start to function as it is intended for use. For example we are injecting 5 kV AC supply to motor of 6.6 kV rating than motor will be charged and will start to run.

Connection provided for Insulation Resistance Test on Insulation Tester

There are three terminals provided on insulation tester.

1. Line

2. Earth

3. Guard

1)Line: This terminal is the DC voltage injecting terminal.

2)Earth: This terminal is provided to connect it to earth during testing for injection of DC voltage with reference to Earth.

3)Guard: This terminal is used for bypassing the resistance path that we don't want to calculate. For example, we are carrying IR test on bushing. During test there will be two currents flowing, first is the current through the bushing that we want to calculate and the second is the surface leakage current that we don't want to calculate. So to ignore this surface leakage we will connect guard terminal to this point.

Standard value for Insulation Resistance Test

The standard value of insulation resistance test should be (kV + 1 ) M-ohm. We consider 11 kV panel breaker for this example. As per standard its insulation resistance value should be 12 M-ohms. Suppose its insulation resistance is 15 M-ohms than it is acceptable but if it is less than 12 M-ohms than it is not acceptable.

What is Battery bank and Purpose of its use in Industry

What is Battery Bank  ??

Battery bank is one type of series connection of same type of batteries to achieve desired DC voltage as per requirement.

Series connection of Battery

Capacity of battery bank 

Capacity of battery is calculated in terms of Ampere-Hour (AH).

Suppose battery bank capacity is 500 AH and connected load ampere is 25 Amp then battery bank can supply DC power source up to 20 hours with 25 Amp load. 

25 Amp X 20 hours = 500 AH

Purpose of use in Industry

1) Useful in case of AC power supply fails for illumination and emergency pumps running purpose.

2) Useful for control of the plant by controlling machines through computer digital software like DCS and SCADA.

3) Useful for providing power supply to relays for electrical protection  purpose.

4) Useful for providing indication and annunciation purpose.

5) Providing power supply to tripping and closing coil of electrical circuit breakers.

Half yearly, Yearly and interval of 2 year check points for Transformer Maintenance

Checkpoint for interval of 6 months

1) Check transformer oil for acidity (<=0.03 mg KOH/g), sludge content (non detectable), flash point (>=140 degree), Dielectric dissipation factor (tan delta) (<=0.010) , Interfacial tension  (>=35 mN/m) and specific resistance (resistivity) (>=6x10^12 om cm at 90 degree).

OIL ACIDITY TEST

OIL SLUDGE TEST

OIL FLASH POONT TEST

OIL TAN DELTA TEST

OIL INTERFACIAL TEST

Yearly checkpoint of Transformer

1) Check mechanical function of Buchholz relay.

2) Marshalling box to be cleaned from inside. All illumination, space heaters, to be checked whether they are functioning properly or not. Check all the terminal connections of control and relay wiring and tighten.

3) Clean all the relays, alarms and control switches along with their circuit, in R&C panel (Relay and Control Panel) and RTCC (Remote Tap Changer Control Panel) by appropriate cleaning agent.

4) Check pockets for OTI & WTI (Oil Temperature Indicator & Winding Temperature Indicator) on the transformer top cover and replace oil if required.

5) Check proper function of Pressure Release Device.

6) Check insulation resistance and polarization index of transformer with battery operated megger of 5 KV range.

7) Check resistive value of earth connection and rizer with clamp on earth resistance meter.

8) Check DGA (Dissolve Gas Analysis) of transformer Oil annually for higher KV rating transformer and once in 2 years for lower KV rating transformer.

9) Check for proper sealing of marshalling box for cable holes.

10) Check all protections and alarm circuits by actual external initiation with relay operation.

11) Check IR value of cooling motor winding, noise and vibration of fan.

12) Check painting and surface finish.

13) Check tightness of bolts in gasket joints.

Checkpoint for interval of 2 years

1) Check calibration of OTI and WTI.

2) Check tan-delta measurement of bushings of transformer.

3) Check operation of gauge and healthiness of air cell.

Monthly and 3 months checks for Transformer Maintenance

Monthly checkpoint

1) Check oil level in oil cap under silica gel breather. If it is found below the specified level, oil to be top up as per specified level.

2) Check breathing holes in silica gel breather & clean properly if required, for proper breathing action. 

3) Silica gel breather should be of blue colour. If colour is pink then replacement or heating of silica gel is required.

4) Check oil level in buchholz relay.

Checkpoint for interval of 3 months

1) Check for dirt deposition on bushings and tightness of its oil filling plug & examine for any crack in porcelain discs.

2) Check for BDV (>=60 KV) and PPM (<10 PPM)of transformer oil and tap changer oil.

3) Check for cooling fan manual and auto function. Lubricate fan bearings if required.

4) Check step by step mechanism operation & end position limit switches in On load/off load tap changer. Lubricate all bearing points.