How can control power factor in power plant ?

When the synchronous generator is pushing reactive power into the electrical system we say the machine is over-excited. When the synchronous generator is absorbing reactive power from the electrical system, it is under-excited. So, the reactive power output of the generator is associated with the generator field current, provided by the excitation system. 

But note that we do not control reactive power output of a generator, at least in terms of automatic control. Synchronous machines are usually operated on automatic voltage regulator (AVR) mode, so we are actually controlling voltage. The reactive power output of the generator is thus automatically adjusted as needed to maintain the specified voltage.

Conversely, if you try to control reactive power output or power factor in a generator, you will not be able to control voltage.

And we should operate the synchronous generators on AVR because of stability, the AVR is a proven solution to several stability issues (steady state stability and transient stability).

Advantages of Electrical Drives

Electrical drives are readily used these days for controlling purpose but this is not the only the advantage of Electrical drives. There are several other advantages which are listed below – 

1) These drives are available in wide range torque, speed and power.
2) The control characteristics of these drives are flexible. According to load requirements these can be shaped to steady state and dynamic characteristics. As well as speed control, electric braking, gearing, starting many things can be accomplished.
3) The are adaptable to any type of operating conditions, no matter how much vigorous or rough it is.
4) They can operate in all the four quadrants of speed torque plane, which is not applicable for other prime movers.
5) They do not pollute the environment.
6) They do not need refueling or preheating, they can be started instantly and can be loaded immediately.
7) They are powered by electrical energy which is atmosphere friendly and cheap source of power.

Parts of Electrical Drives

The diagram which shows the basic circuit design and components of a drive, also shows that, drives have some fixed parts such as, load, motor, Power modulator control unit and source. These equipments are termed as parts of drive syatem. . Now, loads can be of various types i.e they can have specific requirements and multiple conditions, which are discussed later, first of all we will discuss about the other four parts of electrical drives i.e motor, power modulators, sources and control units.

Electric motors are of various types. The dc motors can be divided in four types – shunt wound dc motor, series wound dc motor, compound wound dc motor and permanent magnet dc motor. And AC motors are of two types – induction motors and synchronous motors. Now synchronous motors are of two types – round field and permanent magnet. Induction motors are also of two types – squirrel cage and wound motor. Besides all of these, stepper motors and switched reluctance motors are also considered as the parts of drive system.

So, there are various types of electric motors, and they are used according to their specifications and uses. When the electrical drives were not so popular, induction and synchronous motors were usually implemented only where fixed or constant speed was the only requirement. And for variable speed drive applications, dc motors were used. But as we know that, induction motors of same rating as a dc motors have various advantages like they have lighter weight, lower cost, lower volume and there is less restriction on maximum voltage, speed and power ratings. For these reasons, the induction motors are rapidly replaced the dc motors. Moreover induction motors are mechanical stronger and requires less maintenance. When synchronous motors are considered, wound field and permanent magnet synchronous motors have higher full load efficiency and power factor than induction motors, but the size and cost of synchronous motors are higher than induction motors for the same rating.

Brush less dc motors are similar to permanent magnet synchronous motors. They are used for servo applications and now a days used as an effcient alternative to dc servo motors because they don’t have the disadvantages like commentation problem. Beside of these, stepper motors are used for position control and switched reluctance motors are used for speed control.

Power Modulators - are the devices which alter the nature or frequency as well as changes the intensity of power to control electrical drives. Roughly, power modulators can be classified into three types,
i) Converters,
ii) Variable impedance,
iii) Switching circuits.

As the name suggests, converters are used to convert currents from one type to other type. Depending on the type of function, converters can be divided into 5 types –

i. AC to DC converters

ii. AC regulators

iii. Choppers or DC-DC converters

iv. Inverters

v. Cycloconverters

AC to DC converters are used to obtain fixed dc supply from the ac supply of fixed voltage. The very basic diagram of ac to dc converters is like.

AC to DC Converter

Ac Regulators are used to obtain the regulated ac voltage, mainly auto transformers or tap changer transformers are used in this regulators. 

AC to AC Converter

Choppers or dc-dc converters are used to get a variable DC voltage. Power transistors, IGBT’s, GPO’s, power MOSFET’s are mainly used for this purpose. 

DC to DC Converter

Inverters are used to get ac from dc, the operation is just opposite to that of ac to dc converters. PWM semiconductors are used to invert the current. 

DC to AC Converter

Cycloconverters are used to convert the fixed frequency and fixed voltage ac into variable frequency and variable voltage ac. Thyristors are used in these converters to control the firing signals.

AC to AC Converter

Variable Impedances are used to controlling speed by varying the resistance or impedance of the circuit. But these controlling methods are used in low cost dc and ac drives. There can be two or more steps which can be controlled manually or automatically with the help of contactors. To limit the starting current inductors are used in ac motors.

Switching circuits in motors and electrical drives are used for running the motor smoothly and they also protects the machine during faults. These circuits are used for changing the quadrant of operations during the running condition of a motor. And these circuits are implemented to operate the motor and drives according to predetermined sequence, to provide interlocking, to disconnect the motor from the main circuit during any abnormal condition or faults.

Sources may be of 1 phase and 3 phase. 50 Hz ac supply is the most common type of electricity supplied in India, both for domestic and commercial purpose. Synchronous motors which are fed 50 Hz supply have maximum speed up to 3000 rpm, and for getting higher speeds higher frequency supply is needed. Motors of low and medium powers are fed from 400V supply, and higher ratings like 3.3 kv, 6.6 kv, 11 kv etc are provided also.

Control unit – choice of control unit depends upon the type of power modulator that is used. These are of many types, like when semiconductor converters are used, then the control unit consists of firing circuits, which employ linear devices and microprocessors.
So, the above discussion provides us a simple concept about the several parts of electrical drive.

Classification of Electrical Drives

The classification of electrical drives can be done depending upon the various components of the drive system. Now according to the design, the drives can be classified into three types such as single-motor drive, group motor drive and multi motor drive. The single motor types are the very basic type of drive which are mainly used in simple metal working, house hold appliances etc. Group electric drives are used in modern industries because of various complexities. Multi motor drives are used in heavy industries or where multiple motoring units are required such as railway transport. If we divide from another point of view, these drives are of two types:
i) Reversible types and
ii) Non reversible types.
This depends mainly on the capability of the drive system to alter the direction of the flux generated. So, several classification of drive is discussed above.

Safety Clearance for Transformer

Clearance from Outdoor Liquid Insulated Transformers to Buildings (NEC):

Liquid	Liquid Volume (m3)	Fire Resistant Wall	Non-Combustible Wall	Combustible Wall	Vertical Distance
Less Flammable	NA	0.9 Meter	0.9 Meter	0.9 Meter	0.9 Meter
	<38 m3	1.5 Meter	1.5 Meter	7.6 Meter	7.6 Meter
	>38 m3	4.6 Meter	4.6 Meter	15.2 Meter	15.2 Meter
Mineral Oil	<1.9 m3	1.5 Meter	4.6 Meter	7.6 Meter	7.6 Meter
	1.9 m3 to 19 m3	4.6 Meter	7.6 Meter	15.2 Meter	15.2 Meter
	> 19 m3	7.6 Meter	15.2 Meter	30.5 Meter	30.5 Meter

 Clearance between Two Outdoor Liquid Insulated Transformers (NEC):

Liquid	Liquid Volume (m3)	Distance
Less Flammable	NA	0.9 Meter
	<38 m3	1.5 Meter
	>38 m3	7.6 Meter
Mineral Oil	<1.9 m3	1.5 Meter
	1.9 m3 to 19 m3	7.6 Meter
	> 19 m3	15.2 Meter

 Dry Type Transformer in Indoor Installation (NES 420.21):

Voltage	Distance  (min)
Up to 112.5 KVA	300 mm (12 in.) from combustible material unless separated from the combustible material by a heat-insulated barrier.
Above 112.5 KVA	Installed in a transformer room of fire-resistant construction.
Above 112.5 KVA with Class 155 Insulation	separated from  a fire-resistant barrier not less than 1.83 m (6 ft) horizontally and 3.7 m (12 ft) vertically

 Dry Type Transformer in Outdoor Installation (NES 420.22):

Voltage	Distance  (min)
Above 112.5 KVA with Class 155 Insulation	separated from  a fire-resistant barrier not less than 1.83 m (6 ft) horizontally and 3.7 m (12 ft) vertically

 Non Flammable Liquid-Insulated Transformer in Indoor Installation (NES 420.21):

Voltage	Distance  (min)
Over 35KV	Installed indoors Vault (Having liquid confinement area and a pressure-relief vent for absorbing any gases generated by arcing inside the tank, the pressure-relief vent shall be connected to a chimney or flue that will carry such gases to an environmentally safe area
Above 112.5 KVA	Installed in a transformer room of fire-resistant construction.
Above 112.5 KVA (Class 155 Insulation)	separated from  a fire-resistant barrier not less than 1.83 m (6 ft) horizontally and 3.7 m (12 ft) vertically

 Oil Insulated Transformer in Indoor Installation (NES 420.25):

Voltage	Distance  (min)
Up to 112.5 KVA	Installed indoors Vault (With construction of reinforced concrete that is not less than 100 mm (4 in.) thick.
Up to 10 KVA & Up to 600V	Vault shall not be required if suitable arrangements are made to prevent a transformer oil fire from igniting
Up to 75 KVA & Up to 600V	Vault shall not be required if where the surroundingStructure is classified as fire-resistant construction.
Furnace transformers (Up to 75 kVA)	Installed without a vault in a building or room of fire resistant construction

 Transformer Clearance from Building (IEEE Stand):

Transformer	Distance from Building  (min)
Up to 75 KVA	3.0 Meter
75 KVA to 333 KVA	6.0 Meter
More than 333 KVA	9.0 Meter

 Transformer Clearance Specifications (Stand: Georgia Power Company):

Description of Clearance	Distance  (min)
Clearance in front of the transformer	3.0 Meter
Between Two pad mounted transformers (including Cooling fin)	2.1  Meter
Between Transformer and Trees, shrubs, vegetation( for unrestricted natural cooling )	3.0 Meter
The edge of the concrete transformer pad to nearest the building	4.2 Meter
The edge of the concrete transformer pad to nearest  building wall, windows, or other openings	3.0 Meter
Clearance from the transformer to edge of (or Canopy) building (3 or less stories)	3.0 Meter
Clearance in front of the transformer doors and on the left side of the transformer, looking at it from the front. (For operation of protective and switching devices on the unit.)	3.0 Meter
Gas service meter relief vents.	0.9 Meter
Fire sprinkler values, standpipes and fire hydrants	1.8 Meter
The water’s edge of a swimming pool or any body of water.	4.5 Meter
Facilities used to dispense hazardous liquids or gases	6.0 Meter
Facilities used to store hazardous liquids or gases	3.0 Meter
Clear vehicle passageway at all times, immediately adjacent of Transformer	3.6 Meter
Fire safety clearances can be reduced by building a suitable masonry fire barrier wall (2.7 Meter wide and 4.5 Meter Tall) 0.9 Meter from the back or side of the Pad Mounted Transformer  to the side of the combustible wall
Front of the transformer must face away from the building.

Clearance of Transformer-Cable-Overhead Line (Stand: Georgia Power Company):

Description of Clearance	Horizontal Distance (mm)
	to pad-mounted transformers	to buried HV cable	to overhead HV Line
Fuel tanks	7.5 Meter	1.5 Meter	7.5 Meter
Granaries	6.0 Meter	0.6 Meter	15 Meter
Homes	6.0 Meter	0.6 Meter	15 Meter
Barns, sheds, garages	6.0 Meter	0.6 Meter	15 Meter
Water wells	1.5 Meter	1.5 Meter	15 Meter
Antennas	3.0 Meter	0.6 Meter	Height of Antenna + 3.0 Meter

Safety Clearance for Electrical Panel

 Working Space around Indoor Panel/Circuit Board (NES 312.2):

Voltage	Exposed live parts to Not live parts( or grounded parts )	Exposed live parts to Grounded parts (concrete, brick, and walls).	Exposed live parts on both sides
Up to 150 V	0.914 Meter (3 Ft)	0.914 Meter (3 Ft)	0.914 Meter (3 Ft)
150 V to 600 V	0.914 Meter (3 Ft)	1.07 Meter (3’6”)	1.22 Meter (4 Ft)

 Clearance around an Indoor electrical panel (NES 110.26):

Description of Clearance	Distance  (min)
Left to Right the minimum clearance	0.9 Meter (3 Ft)
Distance between Panel and wall	1.0 Meter
Distance between Panel and Ceiling	0.9 Meter
Clear Height in front of Panel>480V	2.0 Meter
Clear Height  in front of Panel <480V	0.9 Meter (3 Ft)
Clearance When Facing Other Electrical Panels < 480V	0.9 Meter (3 Ft)
The width of the workingspace in front of the Panel	The width of Panel or 0.762 Meter which is Greater.
Headroom of working spaces for panel boards (Up to 200Amp)	Up to 2 Meter
Headroom of working spaces for panel boards (More than 200Amp &Panel height is max 2 Meter)	Up to 2 Meter( If Panel height is max 2 Meter)
Headroom of working spaces for panel boards (More than 200Amp &Panel height is more than 2 Meter)	If Panel height is more than 2 Meter than clearance should not less than panel Height
Entrance For Panel (More than 1200 Amp and over 1.8 m Wide)	One entrance required for working space (Not less than 610 mm wide and 2.0 m high )
Personal Door For Panel (More than 1200 Amp)	Personnel door(s) intended for entrance to and egress from the working space less than 7.6 m from the nearest edge of the working space
Dedicated Electrical Space.	Required Space is width and depth of the Panel and extending from the floor to a height of 1.8 m (6 ft) above the equipment or to the structural ceiling, whichever is lower
The door(s) shall open in the direction of egress and be equipped with panic bars, pressure plates, or other devices that are normally latched but open under simple pressure
the work space shall permit at least a 90 degree opening of equipment doors or hinged panels

 Clearance for Conductor Entering in Panel (NES 408.5):

Description of Clearance	Distance  (min)
Spacing between The conduit or raceways(including their end fittings) and Bottom of Enclosure	Not rise more than 75 mm (3 in) above the bottom of the enclosure
Spacing Between Bottom of Enclosure and Insulated bus bars, their supports,	200 mm
Spacing Between Bottom of Enclosure and Non insulated bus bars	200 mm

 Clearance between Bare Metal Bus bar in Panel (NES 408.5):

Voltage	Opposite PolarityMounted on Same Surface	Opposite PolarityWhere Held Free in Air	Live Parts to Ground
Up to 125 V	19.1 mm	12.7 mm	12.7 mm
125 V to 250 V	31.8 mm	19.1 mm	12.7 mm
250 V to 600 V	50.8 mm	25.4 mm	25.4 mm

 Clearance of Outdoor electrical panel to Fence/Wall (NES 110.31):

Voltage	Distance  (min)
600 V to 13.8 KV	3.05 Meter
13.8 K V to 230  KV	4.57 Meter
Above 230 KV	5.49 Meter

 Working Space around Indoor Panel/Circuit Board (NES 110.34):

Voltage	Exposed live parts to Not live parts( or grounded parts )	Exposed live parts to Grounded parts (concrete, brick, and walls).	Exposed live parts on both sides
601 V to 2.5 K V	0.914 Meter (3 Ft)	1.2 Meter (4 Ft)	1.5 Meter (5 Ft)
2.5 K V to 9.0 K V	1.2 Meter (4 Ft)	1.5 Meter (5 Ft)	1.8 Meter (6 Ft)
9.0 K V to 25 K V	1.5 Meter (5 Ft)	1.8 Meter (6 Ft)	2.5 Meter (8 Ft)
25 K V to 75 K V	1.8 Meter (6 Ft)	2.5 Meter (8 Ft)	3.0 Meter (10 Ft)
Above 75 KV	2.5 Meter (8 Ft)	3.0 Meter (10 Ft)	3.7 Meter (12 Ft)

 Clearance around an Outdoor electrical panel (NES 110.31):

Description of Clearance	Distance  (min)
Clear work space:	Not less than 2.0 Meter high(Measured vertically from the floor or platform) or not less than 914 mm (3 ft) wide (Measured parallel to the equipment).
Entrance For Panel (More than 1200 Amp and over 1.8 m Wide)	One entrance required for working space (Not less than 610 mm wide and 2.0 m high )
Entrance For Panel: On Large panels exceeding 1.8 Meter in width	One Entrance at each end of the equipment.
Nonmetallic or Metal-enclosed Panel in general public and the bottom of the enclosure is less than 2.5 m (8 ft) above the floor or grade level	Enclosure door or hinged cover shall be kept locked.

 Elevation of Unguarded Live Parts above Working Space (NES 110.34E):

Voltage	Elevation  (min)
600 V to 7.5 KV	2.8 Meter
7.5 K V to 35  KV	2.9 Meter
Above 35 KV	2.9 Meter + 9.5 mm/KV

 Working Space for Panel (Code Georgia Power Company):

Voltage	Exposed live parts to Not live parts( or grounded parts )	Exposed live parts to Grounded parts (concrete, brick, and walls).	Exposed live parts on both sides
Up to 150 V	3.0 Meter	3.0 Meter	3.0 Meter
150 V to 600 V	3.0 Meter	3.5 Meter	4.0 Meter
600 V to 2.5 KV	3.0 Meter	4.0 Meter	5.0 Meter
2.5 KV to 9 KV	3.0 Meter	5.0 Meter	6.0 Meter
9 KV to 25 KV	5.0 Meter	6.0 Meter	6.0 Meter

Insulation Resistance (IR) Values

Introduction:

The measurement of insulation resistance is a common routine test performed on all types of electrical wires and cables. As a production test, this test is often used as a customer acceptance test, with minimum insulation resistance per unit length often specified by the customer. The results obtained from IR Test are not intended to be useful in finding localized defects in the insulation as in a true HIPOT test, but rather give information on the quality of the bulk material used as the insulation.

Even when not required by the end customer, many wire and cable manufacturers use the insulation resistance test to track their insulation manufacturing processes, and spot developing problems before process variables drift outside of allowed limits.

 Selection of IR Testers (Megger):

• Insulation testers with test voltage of 500, 1000, 2500 and 5000 V are available.
• The recommended ratings of the insulation testers are given below:

Voltage Level	IR Tester
650V	500V DC
1.1KV	1KV DC
3.3KV	2.5KV DC
66Kv and Above	5KV DC

 Test Voltage for Meggering:

• When AC Voltage is used, The Rule of Thumb is Test Voltage (A.C) = (2X Name Plate Voltage) +1000.
• When DC Voltage is used (Most used in All Megger), Test Voltage (D.C) = (2X Name Plate Voltage).

Equipment / Cable Rating	DC Test Voltage
24V To 50V	50V To 100V
50V To 100V	100V To 250V
100V To 240V	250V To 500V
440V To 550V	500V To 1000V
2400V	1000V To 2500V
4100V	1000V To 5000V

 Measurement Range of Megger:

Test voltage	Measurement Range
250V DC	0MΩ to 250GΩ
500V DC	0MΩ to 500GΩ
1KV DC	0MΩ to 1TΩ
2.5KV DC	0MΩ to 2.5TΩ
5KV DC	0MΩ to 5TΩ

 Precaution while Meggering:

Before Meggering:

• Make sure that all connections in the test circuit are tight.
• Test the megger before use, whether it gives INFINITY value when not connected, and ZERO when the two terminals are connected together and the handle is rotated.

During Meggering:

• Make sure when testing for earth, that the far end of the conductor is not touching, otherwise the test will show faulty insulation when such is not actually the case.
• Make sure that the earth used when testing for earth and open circuits is a good one otherwise the test will give wrong information
• Spare conductors should not be meggered when other working conductors of the same cable are connected to the respective circuits.

After completion of cable Meggering:

• Ensure that all conductors have been reconnected properly.
• Test the functions of Points, Tracks & Signals connected through the cable for their correct response.
• In case of signals, aspect should be verified personally.
• In case of points, verify positions at site. Check whether any polarity of any feed taken through the cable has got earthed inadvertently.

Safety Requirements for Meggering:

• All equipment under test MUST be disconnected and isolated.
• Equipment should be discharged (shunted or shorted out) for at least as long as the test voltage was applied in order to be absolutely safe for the person conducting the test.
• Never use Megger in an explosive atmosphere.
• Make sure all switches are blocked out and cable ends marked properly for safety.
• Cable ends to be isolated shall be disconnected from the supply and protected from contact to supply, or ground, or accidental contact.
• Erection of safety barriers with warning signs, and an open communication channel between testing personnel.
• Do not megger when humidity is more than 70 %.
• Good Insulation: Megger reading increases first then remain constant.
• Bad Insulation: Megger reading increases first and then decreases.
• Expected IR value gets on Temp. 20 to 30 decree centigrade.
• If above temperature reduces by 10 degree centigrade, IR values will increased by two times.
• If above temperature increased by 70 degree centigrade IR values decreases by 700 times.

How to use Megger:

• Meggers is equipped with three connection Line Terminal (L), Earth Terminal (E) and Guard Terminal (G).

• Resistance is measured between the Line and Earth terminals, where current will travel through coil 1. The “Guard” terminal is provided for special testing situations where one resistance must be isolated from another. Let’s us check one situation where the insulation resistance is to be tested in a two-wire cable.
• To measure insulation resistance from a conductor to the outside of the cable, we need to connect the “Line” lead of the megger to one of the conductors and connect the “Earth” lead of the megger to a wire wrapped around the sheath of the cable.

• In this configuration the Megger should read the resistance between one conductor and the outside sheath.
• We want to measure Resistance between Conductor- 2To Sheaths but Actually Megger measure resistance in parallel with the series combination of conductor-to-conductor resistance (R_c1-c2) and the first conductor to the sheath (R_c1-s).
• If we don’t care about this fact, we can proceed with the test as configured. If we desire to measure only the resistance between the second conductor and the sheath (R_c2-s), then we need to use the megger’s “Guard” terminal.

• Connecting the “Guard” terminal to the first conductor places the two conductors at almost equal potential. With little or no voltage between them, the insulation resistance is nearly infinite, and thus there will be no current between the two conductors. Consequently, the Megger’s resistance indication will be based exclusively on the current through the second conductor’s insulation, through the cable sheath, and to the wire wrapped around, not the current leaking through the first conductor’s insulation.
• The guard terminal (if fitted) acts as a shunt to remove the connected element from the measurement. In other words, it allows you to be selective in evaluating certain specific components in a large piece of electrical equipment. For example consider a two core cable with a sheath. As the diagram below shows there are three resistances to be considered.

• If we measure between core B and sheath without a connection to the guard terminal some current will pass from B to A and from A to the sheath. Our measurement would be low. By connecting the guard terminal to A the two cable cores will be at very nearly the same potential and thus the shunting effect is eliminated.

(1) IR Values For Electrical Apparatus & Systems:

(PEARL Standard / NETA MTS-1997 Table 10.1)

Max.Voltage Rating Of Equipment	Megger Size	Min.IR Value
250 Volts	500 Volts	25 MΩ
600 Volts	1,000 Volts	100 MΩ
5 KV	2,500 Volts	1,000 MΩ
8 KV	2,500 Volts	2,000 MΩ
15 KV	2,500 Volts	5,000 MΩ
25 KV	5,000 Volts	20,000 MΩ
35 KV	15,000 Volts	100,000 MΩ
46 KV	15,000 Volts	100,000 MΩ
69 KV	15,000 Volts	100,000 MΩ

One Meg ohm Rule for IR Value for Equipment:

• Based upon equipment rating:
• < 1K V = 1 MΩ minimum
• >1KV = 1 MΩ /1KV

As per IE Rules-1956:

• At a pressure of 1000 V applied between each live conductor and earth for a period of one minute the insulation resistance of HV installations shall be at least 1 Mega ohm or as specified by the Bureau of Indian Standards.
• Medium and Low Voltage Installations- At a pressure of 500 V applied between each live conductor and earth for a period of one minute, the insulation resistance of medium and low voltage installations shall be at least 1 Mega ohm or as specified by the Bureau of Indian Standards] from time to time.

As per CBIP specifications the acceptable values are 2 Mega ohms per KV

(2) IR Value for Transformer:

• Insulation resistance tests are made to determine insulation resistance from individual windings to ground or between individual windings. Insulation resistance tests are commonly measured directly in megohms or may be calculated from measurements of applied voltage and leakage current.
• The recommended practice in measuring insulation resistance is to always ground the tank (and the core). Short circuit each winding of the transformer at the bushing terminals. Resistance measurements are then made between each winding and all other windings grounded.

• Windings are never left floating for insulation resistance measurements. Solidly grounded winding must have the ground removed in order to measure the insulation resistance of the winding grounded. If the ground cannot be removed, as in the case of some windings with solidly grounded neutrals, the insulation resistance of the winding cannot be measured. Treat it as part of the grounded section of the circuit.
• We need to test winding to winding and winding to ground ( E ).For three phase transformers, We need to test winding ( L1,L2,L3 ) with substitute Earthing for Delta transformer or winding ( L1,L2,L3 ) with earthing ( E ) and neutral ( N ) for wye transformers.

IR Value for Transformer (Ref: A Guide to Transformer Maintenance by. JJ. Kelly. S.D Myer)
Transformer	Formula
1 Phase Transformer	IR Value (MΩ) = C X E / (√KVA)
3 Phase Transformer (Star)	IR Value (MΩ) = C X E (P-n) / (√KVA)
3 Phase Transformer (Delta)	IR Value (MΩ) = C X E (P-P) / (√KVA)
Where C= 1.5 for Oil filled T/C with Oil Tank, 30 for Oil filled T/C without Oil Tank or Dry Type T/C.

•  Temperature correction Factor (Base 20°C):

Temperature correction Factor
OC	OF	Correction Factor
0	32	0.25
5	41	0.36
10	50	0.50
15	59	0.720
20	68	1.00
30	86	1.98
40	104	3.95
50	122	7.85

• Example: For 1600KVA, 20KV/400V,Three Phase Transformer
• IR Value at HV Side= (1.5 x 20000) / √ 1600 =16000 / 40 = 750 MΩ at 20⁰C
• IR Value at LV Side = (1.5 x 400 ) / √ 1600= 320 / 40 = 15 MΩ at 20⁰C
• IR Value at 30⁰C =15X1.98= 29.7 MΩ

Insulation Resistance of Transformer Coil

Transformer Coil  Voltage	Megger Size	Min.IR Value Liquid Filled T/C	Min.IR Value Dry Type T/C
0 – 600 V	1KV	100 MΩ	500 MΩ
600 V To 5KV	2.5KV	1,000 MΩ	5,000 MΩ
5KV To 15KV	5KV	5,000 MΩ	25,000 MΩ
15KV To 69KV	5KV	10,000 MΩ	50,000 MΩ

 IR Value of Transformers:

Voltage	Test Voltage (DC)  LV side	Test  Voltage (DC) HV side	Min IR Value
415V	500V	2.5KV	100MΩ
Up to 6.6KV	500V	2.5KV	200MΩ
6.6KV to 11KV	500V	2.5KV	400MΩ
11KV to 33KV	1000V	5KV	500MΩ
33KV to 66KV	1000V	5KV	600MΩ
66KV to 132KV	1000V	5KV	600MΩ
132KV to 220KV	1000V	5KV	650MΩ

 Steps for measuring the IR of Transformer:

• Shut down the transformer and disconnect the jumpers and lightning arrestors.
• Discharge the winding capacitance.
• Thoroughly clean all bushings
• Short circuit the windings.
• Guard the terminals to eliminate surface leakage over terminal bushings.
• Record the temperature.
• Connect the test leads (avoid joints).
• Apply the test voltage and note the reading. The IR. Value at 60 seconds after application of the test voltage is referred to as the Insulation Resistance of the transformer at the test temperature.
• The transformer Neutral bushing is to be disconnected from earth during the test.
• All LV surge diverter earth connections are to be disconnected during the test.
• Due to the inductive characteristics of transformers, the insulation resistance reading shall not be taken until the test current stabilizes.
• Avoid meggering when the transformer is under vacuum.

Test Connections of Transformer for IR Test (Not Less than 200 MΩ):

• Two winding transformer:

1. (HV + LV) – GND
2. HV – (LV + GND)
3. LV – (HV + GND)

• Three winding transformer:

1. HV – (LV + TV + GND)
2. LV – (HV + TV + GND)
3. (HV + LV + TV) – GND
4. TV – (HV + LV + GND)

• Auto transformer (two winding):

1. (HV + LV) – GND

• Auto Transformer (three winding):

1. (HV + LV) – (TV + GND)
2. (HV + LV + TV) – GND
3. TV – (HV + LV + GND)

For any installation, the insulation resistance measured shall not be less than:

• HV – Earth 200 M Ω
• LV – Earth 100 M Ω
• HV – LV 200 M Ω

 Factors affecting on IR value of Transformer

The IR value of transformers are influenced by

• surface condition of the terminal bushing
• quality of oil
• quality of winding insulation
• temperature of oil
• duration of application and value of test voltage

(3) IR Value for Tap Changer:

• IR between HV and LV as well as windings to earth.
•  Minimum IR value for Tap changer is 1000 ohm per volt service voltage

 (4) IR Value for Electric motor:

For electric motor, we used a insulation tester to measure the resistance of motor winding with earthing ( E ).

• For rated voltage below 1KV, measured with a 500VDC Megger.
• For rated voltage above 1KV, measured with a 1000VDC Megger.
• In accordance with IEEE 43, clause 9.3, the following formula should be applied.
• Min IR Value (For Rotating Machine) =(Rated voltage (v) /1000) + 1

As per IEEE 43 Standard 1974,2000
IR Value in MΩ
IR (Min) = kV+1	For most windings made before about 1970, all field windings, and others not described below
IR (Min) = 100 MΩ	For most dc armature and ac windings built after about 1970 (form wound coils)
IR (Min) = 5 MΩ	For most machines with random -wound stator coils and form-wound coils rated below 1kV

• Example-1: For 11KV, Three Phase Motor.
• IR Value =11+1=12 MΩ but as per IEEE43 It should be 100 MΩ
• Example-2: For 415V,Three Phase Motor
• IR Value =0.415+1=1.41 MΩ but as per IEEE43 It should be 5 MΩ.
• As per IS 732 Min IR Value of Motor=(20XVoltage(p-p/(1000+2XKW))

IR Value of Motor as per NETA ATS 2007. Section 7.15.1

Motor Name Plate (V)	Test Voltage	Min IR Value
250V	500V DC	25 MΩ
600V	1000V DC	100MΩ
1000V	1000V DC	100MΩ
2500V	1000V DC	500MΩ
5000V	2500V DC	1000MΩ
8000V	2500V DC	2000MΩ
15000V	2500V DC	5000MΩ
25000V	5000V DC	20000MΩ
34500V	15000V DC	100000MΩ

IR Value of Submersible Motor:

IR Value of Submersible Motor
Motor Out off Well (Without Cable)	IR Value
New Motor	20 MΩ
A used motor which can be reinstalled	10 MΩ
Motor  Installed in Well (With Cable)
New Motor	2 MΩ
A used motor which can be reinstalled	0.5 MΩ

 (5) IR Value for Electrical cable and wiring:

• For insulation testing, we need to disconnect from panel or equipment and keep them isolated from power supply. The wiring and cables need to test for each other ( phase to phase ) with a ground ( E ) cable. The Insulated Power Cable Engineers Association (IPCEA) provides the formula to determine minimum insulation resistance values.

• R = K x Log 10 (D/d)

• R =IR Value in MΩs per 1000 feet (305 meters) of cable.
• K =Insulation material constant.( Varnished Cambric=2460, Thermoplastic Polyethlene=50000,Composite Polyethylene=30000)
  D =Outside diameter of conductor insulation for single conductor wire and cable
• ( D = d + 2c + 2b diameter of single conductor cable )
  d – Diameter of conductor
  c – Thickness of conductor insulation
  b – Thickness of jacket insulation

HV test on new XLPE cable (As per ETSA Standard)

Application	Test Voltage	Min IR Value
New cables – Sheath	1KV DC	100 MΩ
New cables – Insulation	10KV DC	1000 MΩ
After repairs – Sheath	1KV DC	10 MΩ
After repairs – Insulation	5KV DC	1000MΩ

11kV and 33kV Cables between Cores and Earth (As per ETSA Standard)

Application	Test Voltage	Min IR Value
11KV New cables – Sheath	5KV DC	1000 MΩ
11KV After repairs – Sheath	5KV DC	100 MΩ
33KV no TF’s connected	5KV DC	1000 MΩ
33KV with TF’s connected.	5KV DC	15MΩ

IR Value Measurement (Conductors to conductor (Cross Insulation))

• The first conductor for which cross insulation is being measured shall be connected to Line terminal of the megger. The remaining conductors looped together (with the help of crocodile clips) i. e. Conductor 2 and onwards, are connected to Earth terminal of megger. Conductors at the other end are left free.
• Now rotate the handle of megger or press push button of megger. The reading of meter will show the cross Insulation between conductor 1 and rest of the conductors. Insulation reading shall be recorded.
• Now connect next conductor to Line terminal of the megger & connect the remaining conductors to earth terminal of the megger and take measurements.

IR Value Measurement (Conductor to Earth Insulation)

• Connect conductor under test to the Line terminal of the megger.
• Connect earth terminal of the megger to the earth.
• Rotate the handle of megger or press push button of megger. The reading of meter will show the insulation resistance of the conductors. Insulation reading shall be recorded after applying the test voltage for about a minute till a steady reading is obtained.

 IR Value Measurements:

• If during periodical testing, insulation resistance of cable is found between 5 and 1 MΩ /km at buried temperature, the subject cable should be programmed for replacement.
• If insulation resistance of the cable is found between 1000 and 100 KΩ /km, at buried temperature, the subject cable should be replaced urgently within a year.
• If the insulation resistance of the cable is found less than 100 kilo ohm/km., the subject cable must be replaced immediately on emergency basis.

 (6) IR Value for Transmission / Distribution Line:

Equipment.	Megger Size	Min IR Value
S/S .Equipments	5 KV	5000MΩ
EHVLines.	5 KV	10MΩ
H.T. Lines.	1 KV	5MΩ
LT / Service Lines.	0.5 KV	5MΩ

(7) IR Value for Panel Bus:

• IR Value for Panel = 2 x KV rating of the panel.
• Example, for a 5 KV panel, the minimum insulation is 2 x 5 = 10 MΩ.

 (8) IR Value for Substation Equipment:

Generally meggering Values of Substation Equipments are.

.Typical IR Value of S/S Equipments
Equipment		Megger Size	IR Value(Min)
Circuit Breaker	(Phase-Earth)	5KV,10 KV	1000 MΩ
	(Phase-Phase)	5KV,10 KV	1000 MΩ
	Control Circuit	0.5KV	50 MΩ
CT/PT	(Pri-Earth)	5KV,10 KV	1000 MΩ
	(Sec-Phase)	5KV,10 KV	50 MΩ
	Control Circuit	0.5KV	50 MΩ
Isolator	(Phase-Earth)	5KV,10 KV	1000 MΩ
	(Phase-Phase)	5KV,10 KV	1000 MΩ
	Control Circuit	0.5KV	50 MΩ
L.A	(Phase-Earth)	5KV,10 KV	1000 MΩ
Electrical Motor	(Phase-Earth)	0.5KV	50 MΩ
LT Switchgear	(Phase-Earth)	0.5KV	100 MΩ
LT Transformer	(Phase-Earth)	0.5KV	100 MΩ

IR Value of S/S Equipments As per DEP Standard
Equipment	Meggering	IR Value at Commissioning Time (MΩ)	IR Value at Maintenance Time(MΩ)
Switchgear	HV Bus	200 MΩ	100 MΩ
	LV Bus	20 MΩ	10 MΩ
	LV wiring	5 MΩ	0.5 MΩ
Cable(min 100 Meter)	HV & LV	(10XKV) / KM	(KV) / KM
Motor & Generator	Phase-Earth	10(KV+1)	2(KV+1)
Transformer Oil immersed	HV & LV	75 MΩ	30 MΩ
Transformer Dry Type	HV	100 MΩ	25 MΩ
	LV	10 MΩ	2 MΩ
Fixed Equipments/Tools	Phase-Earth	5KΩ / Volt	1KΩ / Volt
Movable Equipments	Phase-Earth	5 MΩ	1MΩ
Distribution Equipments	Phase-Earth	5 MΩ	1MΩ
Circuit Breaker	Main Circuit	2 MΩ / KV
	Control Circuit	5MΩ
Relay	D.C Circuit-Earth	40MΩ
	LT Circuit-Earth	50MΩ
	LT-D.C Circuit	40MΩ
	LT-LT	70MΩ

 (9) IR Value for Domestic /Industrial Wiring:

• A low resistance between phase and neutral conductors, or from live conductors to earth, will result in a leakage current. This cause deterioration of the insulation, as well as involving a waste of energy which would increase the running costs of the installation.
• The resistance between Phase-Phase-Neutral-Earth must never be less than 0.5 M Ohms for the usual supply voltages.
• In addition to the leakage current due to insulation resistance, there is a further current leakage in the reactance of the insulation, because it acts as the dielectric of a capacitor. This current dissipates no energy and is not harmful, but we wish to measure the resistance of the insulation, so DC Voltage is used to prevent reactance from being included in the measurement.

 1 Phase Wiring:

• The IR test between Phase-Natural to earth must be carried out on the complete installation with the main switch off, with phase and neutral connected together, with lamps and other equipment disconnected, but with fuses in, circuit breakers closed and all circuit switches closed.
• Where two-way switching is wired, only one of the two stripper wires will be tested. To test the other, both two-way switches should be operated and the system retested. If desired, the installation can be tested as a whole, when a value of at least 0.5 M Ohms should be achieved.

3 Phase Wiring:

• In the case of a very large installation where there are many earth paths in parallel, the reading would be expected to be lower. If this happens, the installation should be subdivided and retested, when each part must meet the minimum requirement.

• The IR tests must be carried out between Phase-Phase-Neutral-Earth with a minimum acceptable value for each test of 0.5 M Ohms.

IR Testing for Low voltage
circuit voltage	Test voltage	IR Value(Min)
Extra Low Voltage	250V DC	0.25MΩ
Up to 500 V except for above	500 V DC	0.5MΩ
500 V To 1KV	1000 V DC	1.0MΩ

•  Min IR Value = 50 MΩ / No of Electrical outlet. (All Electrical Points with  fitting & Plugs).

• Min IR Value = 100 MΩ / No of Electrical outlet. (All Electrical Points without fitting & Plugs).

 Required Precautions:

• Electronic equipment like electronic fluorescent starter switches, touch switches, dimmer switches, power controllers, delay timers could be damaged by the application of the high test voltage should be disconnected.
• Capacitors and indicator or pilot lamps must be disconnected or an inaccurate test reading will result.
• Where any equipment is disconnected for testing purposes, it must be subjected to its own insulation test, using a voltage which is not likely to result in damage. The result must conform with that specified in the British Standard concerned, or be at least 0.5 M Ohms if there is no Standard.