In North America the three main bushing standards that are used are CAN CSA C88.1, IEEE C57.19.00/C57.19.01 and IEC60137. The CSA standard is in large part an adoption of the IEEE standard with some specific Canadian requirements (e.g. operational capability at -50°C is seen more as a standard requirement than a special requirement). However dimensional differences between the two standards for such key dimensions as the flange bolt circle diameters and below the flange lengths and diameters create an interchangeability issue for users. Over the years, CSA and IEEE have worked to harmonize their standards with IEC which is a global organization that prepares and publishes international standards. However, harmonization may not solve the interchangeability issue as IEC bushings do not have standardized dimensional requirements and are generally not interchangeable with different bushing brands. Therefore, users will continue to have to rely on their bushing suppliers to be able to supply their specified standard bushing designs for the foreseeable future.
So, what does the future hold? The demand for CSA bushings continues to decline with more and more Canadian utilities accepting IEEE and IEC bushings for their new power transformers. Harmonization will continue particularly in the area of test requirements with a harmonized IEEE standard continuing to dominate the North American bushing market. To help understand the differences between the testing requirements of the three different standards a comparison table has been prepared and is shown below.
In addition, asset managers are loath to stocking large quantities of spare bushings opting instead to stock a few critical ratings and using blanket agreements to supply their spare bushing needs. North American utilities will therefore be looking to partner with suppliers that can offer design flexibility and short lead times.
Comparison of CAN/CSA C88.1, IEEE C57.19.00/01 & IEC 60137 |
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| ITEM | CAN/CSA C88.1-96 | IEEE C57.19.00/01 | IEC 60137 | |||
| Requirement | Type | Requirement | Type | Requirement | Type | |
| Power factor (tanδ) & Capacitance Measurement | Clause 10.2. Table 2. C1 & tanδ at 10kV. The increase before and after the 1-hour low- frequency withstand voltage test to be ≤0.02 |
Routine /Type |
Clauses 7.4.1 and 7.4.2. Table 6. C1 & tanδ to be tested at 10kV, UST | Routine | Um ≤ 36 kV: 1.05 Um /√3; Um ≥ 52 kV: 1.05 Um /√3 and Um. Measurement of C1 & tan δ to be made between 2 kV and 20 kV |
Routine |
| Measure C2 power factor & capacitance at 2kV |
Routine | Measure C2 capacitance at 10kV, | Routine | —— | —— | |
| Dry 1-minute Power frequency withstand voltage test with partial discharge measurement | Clause 10.3. Test at values specified in Table1 for 1min. PD measurements to be made before and after for OIP and RIP bushings and after for resin- bonded paper and bulk-type bushings (as per Clauses 10.4.3 and 10.4.4 Insulation Integrity) |
Routine | Clause 7.4.3. Test at values specified in C57.19.01 Table 1 for 1min. Measure PD at 1.5 Um/√3 before and after. |
Routine | Clause 9.3. Measure at values specified in Table 4 for 1min. Measure PD after power frequency withstand test. | Routine |
| Insulation integrity | Clause 10.4 PD measurements to be performed. | Routine | —— | —— | —— | —— |
| Tap test | Voltage tap: 20kV for 1min | Routine | Voltage tap: 20kV for 1min | Routine | Voltage tap: twice the rated voltage of the voltage tap, at least 2kV for 1min | Routine |
| Test tap: >500V for 1min | Test tap: 2kV for 1min | Test tap: 2kV for 1min | ||||
| Full-wave & Chopped-wave lightning impulse withstand voltage test | —— | —— | —— | —— | Applicable only for transformer bushings with Um equal to or greater than 245 kV. 5 FW or by agreement 1 FW, 2 CW (at 110% of FW value), 2 FW | Routine |
| Full-wave lightning impulse withstand voltage test | Table 1, 1.2/50µs, ±15 impulses |
Type | 1.2/50µs, ±15 impulses | Type | 1.2/50µs, ±15 impulses. For bushings ˃ 72.5 kV: +15 full, -1 @ 110%full, -5 @ 121% chopped, -14 @ 110%full |
Type |
| Chopped-wave lightning impulse withstand voltage test | Table 1, ≥ －3 impulses, time to flashover 3µs | Type | ≥ －3 impulses, time to flashover 3µs | Type | As described above | Type |
| Switching impulse withstand voltage test | Table 1. 15 Positive Wet switching impulses. ≥ 950kV BIL. The front of wave shall be 200- 300 microseconds and the total duration shall be 1,000-4,000 microseconds. |
Type | 15 Positive Wet switching impulses. Above 900kV BIL. Standard 250 x2500 μs impulse | Type | Dry (indoor) or Wet (outdoor). Transformer bushings ≥ 245 kV also subject to a dry test. 15 impulses of positive polarity, followed by 15 impulses of negative polarity. ≥ 950kV BIL. 250 /2500 μs. For transformer bushings the dry, negative polarity test shall be at 110 % of the rated SIL. |
Type |
| Wet 1-minute Power frequency withstand voltage test | Table 1. ＜950kV BIL, 1 minute |
Type | ≤900kV BIL, 10 secs | Type | Clause 8.1. Table 4. Dry (all indoor bushings) or Wet (outdoor bushings ≤ 245 kV), 1 minute | Type |
| One-hour low- frequency withstand with partial discharge measurement | Clause 8.6. Tables 1 & 2. 1.7Uy for 1min and then 1.5Uy for 1h, PD ≤ 10pC, the increase before and after the low- frequency withstand voltage test ≤ 5pC. Uy - max. design voltage to ground | Type | 1.5Um/√3 for 1h, PD measurements taken every 5 minutes, PD ≤ 10pC, | Type | Clause 8.2.2. 1.1 Um/√3 held 5 min; 1.5 Um/√3 held 5 min; Um held 1 min; 1.5 Um/√3 held at least 60 min (Um ≥300kV) or 30 min (Um <300 kV); 1.1 Um/√3 held 5 min; 0 V; measure partial discharge every 5 minutes. PD ≤ 10 pC at Um and 1.5 Um/√3; PD ≤ 5 pC at 1.1 Um/√3. |
Type |
| Temperature rise test | Hottest-spot rise ≤75℃ @ oil rise 65±2℃ |
Type | Hottest-spot rise ≤75℃ @ oil rise 65℃, in contact with temperature index 105 insulation. For insulating material with temperature index greater than 105, the hottest-spot temperature rise should be chosen accordingly and agreed between the purchaser and the manufacturer | Type | Clause 4.8 and Table 3. There is no requirement for the hottest-spot rise | Type |
| Thermal stability test | 1.1Uy (for Uy ˃ 108 kV) and √3Uy for all other bushings; conductor losses simulated; DF measured every 2 hours until equilibrium (i.e. no appreciable changes in the DF over a 5- hour period) |
Type | 1.2Um/√3, rated continuous current applied or conductor losses simulated. PF measured periodically until equilibrium (i.e. PF changes no more than 0.02 over a 5- hour period) | Special | Um for bushings of Um equal to or less than 170 kV, 0.8 Um for bushings of Um greater than 170 kV. The conductor losses corresponding to Ir shall be simulated by appropriate means. |
Type |
| Cantilever test | 1330N for 1 minute | Type | Table 4 C57.19.01 (890N to 4000N depending on voltage and current ratings) for 1 minute |
Type | Table 1 (500N to 5000N depending on voltage and current ratings), 1 minute | Type |
| Draw-Lead Bushing Cap Pressure |
140kPa 1 hour | Type | 140kPa 1 hour | Type | —— | —— |
| Tightness test at the flange or other fixing device | —— | —— | —— | —— | Clause 9.9 1.5 bar for 15 minutes (air pressure) 1.0 bar for 12 hours (oil pressure) |
Routine |
| Pressure Test | Clause 10.6 280 kPa for 6 hours 140 kPa for 24 hours 70 kPa for 48 hours |
Routine | Clause 7.4.5 140 kPa for 1 hour and full vacuum for 1 hour |
Routine | Only required on gas-filled, gas- insulated and gas-impregnated bushings | —— |
| Prepared by: RHM International |
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