T & D - 41) Primary Distribution Voltage Levels

Primary Distribution Voltage Levels

Most distribution voltages are between 4 and 35 kV. In this article, unless otherwise specified, voltages are given as line-to-line voltages; this follows normal industry practice, but it is sometimes a source of confusion.

The four major voltage classes are 5, 15, 25, and 35 kV. A voltage class is a term applied to a set of distribution voltages and the equipment common to them; it is not the actual system voltage.

For example, a 15-kV insulator is suitable for application on any 15-kV class voltage, including 12.47 kV, 13.2 kV, and 13.8 kV. Cables, terminations, insulators, bushings, reclosers, and cutouts all have a voltage class rating. Only voltage-sensitive equipment like surge arresters, capacitors, and transformers have voltage ratings dependent on the actual system voltage.

Utilities most widely use the 15-kV voltages as shown by the survey results of North American utilities in Figure 1. The most common 15-kV voltage is 12.47 kV, which has a line-to-ground voltage of 7.2 kV.

The dividing line between distribution and subtransmission is often gray. Some lines act as both subtransmission and distribution circuits. A 34.5-kV circuit may feed a few 12.5-kV distribution substations, but it may also serve some load directly.

Some utilities would refer to this as subtransmission, others as distribution.

Figure 1 - Usage of different distribution voltage classes (n = 107). (Data from IEEE Working Group on Distribution Protection, 1995)

The last half of the 20th century saw a move to higher voltage primary distribution systems. Higher-voltage distribution systems have advantages and disadvantages (see Advantages and disadvantages of higher voltage distribution below).

The great advantage of higher voltage systems is that they carry more power for a given current.

Higher Voltage Distribution

Advantages

Voltage drop - A higher-voltage circuit has less voltage drop for a given power flow.
Capacity - A higher-voltage system can carry more power for a given capacity.
Losses - For a given level of power flow, a higher-voltage system has fewer line losses.
Reach - With less voltage drop and more capacity, higher voltage circuits can cover a much wider area.
Fewer substations - Because of longer reach, higher-voltage distribution systems need fewer substations.

Disadvantages

Reliability - An important disadvantage of higher voltages: longer circuits mean more customer interruptions.
Crew safety and acceptance - Crews do not like working on higher-voltage distribution systems.
Equipment cost - From transformers to cable to insulators, higher-voltage equipment costs more.

Information above shows maximum power levels typically supplied by various distribution voltages.

Less current means lower voltage drop, fewer losses, and more power-carrying capability. Higher voltage systems need fewer voltage regulators and capacitors for voltage support. Utilities can use smaller con-ductors on a higher voltage system or carry more power on the same size conductor.

Table 1 - Power Supplied by Each Distribution Voltage for a Current of 400 A

System Voltage (kV)	Total Power (MVA)
4.8	3.3
12.47	8.6
22.9	15.9
34.5	23.9

Utilities can run much longer distribution circuits at a higher primary voltage, which means fewer distribution substations. Some fundamental relationships are:

Power – For the same current, power changes linearly with voltage.

when I₂ = I₁

Current – For the same power, increasing the voltage decreases current linearly.

when P₂ = P₁

Voltage drop – For the same power delivered, the percentage voltage drop changes as the ratio of voltages squared. A 12.47-kV circuit has four times the percentage voltage drop as a 24.94-kV circuit carrying the same load.

when P₂ = P₁

Area coverage – For the same load density, the area covered increases linearly with voltage: A 24.94-kV system can cover twice the area of a 12.47-kV system; a 34.5-kV system can cover 2.8 times the area of a 12.47-kV system.

where:
V₁, V₂ = voltage on circuits 1 and 2
P₁, P₂ = power on circuits 1 and 2
I₁, I₂ = current on circuits 1 and 2
V_%1, V_%2 = voltage drop per unit length in percent on circuits 1 and 2
A₁, A₂ = area covered by circuits 1 and 2

The squaring effect on voltage drop is significant. It means that doubling the system voltage quadruples the load that can be supplied over the same distance (with equal percentage voltage drop); or, twice the load can be supplied over twice the distance; or, the same load can be supplied over four times the distance.

Resistive line losses are also lower on higher-voltage systems, especially in a voltage-limited circuit. Thermally limited systems have more equal losses, but even in this case higher voltage systems have fewer losses.

Line crews do not like higher voltage distribution systems as much. In addition to the widespread perception that they are not as safe, gloves are thicker, and procedures are generally more stringent. Some utilities will not glove 25- or 35-kV voltages and only use hotsticks.

The main disadvantage of higher-voltage systems is reduced reliability. Higher voltages mean longer lines and more exposure to lightning, wind, dig-ins, car crashes, and other fault causes. A 34.5-kV, 30-mi mainline is going to have many more interruptions than a 12.5-kV system with an 8-mi main-line. To maintain the same reliability as a lower voltage distribution system, a higher-voltage primary must have more switches, more automation, more tree trimming, or other reliability improvements.

Higher voltage systems also have more voltage sags and momentary interruptions. More exposure causes more momentary interruptions. Higher voltage systems have more voltage sags because faults further from the substation can pull down the station’s voltage (on a higher voltage system the line impedance is lower relative to the source impedance).

Cost comparison between circuits is difficult (see Table 2 for one utility’s cost comparison). Higher voltage equipment costs more – cables, insulators, transformers, arresters, cutouts, and so on. But higher voltage circuits can use smaller conductors. The main savings of higher-voltage distribution is fewer substations.

Higher voltage systems also have lower annual costs from losses.

Table 2 – Costs of 34.5 kV Relative to 12.5 kV

Item	Underground	Overhead
Subdivision without bulk feeders	1.25	1.13
Subdivision with bulk feeders	1.00	0.85
Bulk feeders	0.55	0.55
Commercial areas	1.05–1.25	1.05–1.25

Source: Jones, A.I., Smith, B.E., and Ward, D.J., “Considerations for Higher Voltage Distribution,” IEEE Transactions on Power Delivery, vol. 7, no. 2, pp. 782–8, April 1992.

As far as ongoing maintenance, higher voltage systems require less substation maintenance, but higher voltage systems should have more tree trimming and inspections to maintain reliability. Conversion to a higher voltage is an option for providing additional capacity in an area. Conversion to higher voltages is most beneficial when substation space is hard to find and load growth is high.

If the existing subtransmission voltage is 34.5 kV, then using that voltage for distribution is attractive; addi-tional capacity can be met by adding customers to existing 34.5-kV lines (a neutral may need to be added to the 34.5-kV subtransmission line).

Higher voltage systems are also more prone to ferroresonance. Radio inter-ference is also more common at higher voltages.

Overall, the 15-kV class voltages provide a good balance between cost, reliability, safety, and reach. Although a 15-kV circuit does not naturally provide long reach, with voltage regulators and feeder capacitors it can be stretched to reach 20 mi or more. That said, higher voltages have advantages, especially for rural lines and for high-load areas, particularly where substa-tion space is expensive.

Many utilities have multiple voltages (as shown by the survey data in Figure 1). Even one circuit may have multiple voltages. For example, a utility may install a 12.47-kV circuit in an area presently served by 4.16 kV. Some of the circuit may be converted to 12.47 kV, but much of it can be left as is and coupled through 12.47/4.16-kV step-down transformer banks.

Resource: Electric distribution equipment and systems by T. A. Short (Buy the book at Amazon)

T & D - 11) Measurement of insulation resistance (IR) – Part 2

Insulation Resistance (IR) Values – Index

1. IR Values For Electrical Apparatus & Systems
2. IR Value for Transformer
3. IR Value for Tap Changer
4. IR Value for Electric motor
5. IR Value for Electrical cable and wiring
6. IR Value for Transmission / Distribution Line
7. IR Value for Panel Bus
8. IR Value for Substation Equipment
9. IR Value for Domestic /Industrial Wiring
0. Required Precautions

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.

Insulation resistance testing: HV - Earth and HV - LV

Transformer 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 windings)
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

Insulation resistance (IR) value for electric motor

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

11kV and 33kV Cables between Cores and Earth

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
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.

1 Phase Wiring

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.

3 Phase Wiring

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.