O Plus Dry ™ bushings Technical guide Transformer components Standards organization Designations IEEE C57.19.00-2004 C57.19.01-1991 C57.19.01-2000 C57.19.100-2012 IEC 60137 Figure 1 - Applicable standards General information ABB has been producing dry condenser bushings for more than 30 years. The technology involved in designing and producing this type of bushing has been subject to continuous improvement and has matured over this time. The result is the next generation of transformer bushings - O Plus Dry™ bushings. O Plus Dry bushings are designed and manufactured encompassing the latest materials, techniques, and processes. O Plus Dry bushings use a resin impregnated synthetic (RIS) technology and do not contain any oil, paper, or porcelain. The condenser is cast into a solid body comprising an epoxy resin system with foils. Silicone rubber weather sheds are then extruded onto the air-side of the condenser. The result is a family of bushings that increase safety and improve reliability with the the following benefits: – Improved availability – Reduced maintenance needs – No oil and no porcelain – Explosion resistant – Fire retardant – Hydrophobic/self-cleaning Applicable standards O Plus Dry bushings are designed for AC application and meet or exceed all requirements of the applicable IEEE standards, as well as withstand voltages for routine test levels required by IEC standards. If you would like more information, please contact your ABB representative. Specifying O Plus Dry bushings – Bushings will have weather sheds that are formed using a filled, high-temperature vulcanizing (HTV) silicone rubber that is helically extruded on to the air-side of the bushing. – Bushings must have a test or voltage tap allowing for power factor testing without disconnecting the winding leads. – Bushings must not contain any paper. – Bushings shall not contain oil nor share oil with the transformer. – The partial discharge for bushings must be below 5 pC at 2 times maximum line-to-ground (L-G) voltage. – The power factor for bushings must not exceed 0.50 percent at 20 °C.
Transcript
O Plus Dry™ bushingsTechnical guide
Transformer components
Standards organization Designations
IEEE C57.19.00-2004
C57.19.01-1991
C57.19.01-2000
C57.19.100-2012
IEC 60137
Figure 1 - Applicable standards
General informationABB has been producing dry condenser bushings for more than 30 years. The technology involved in designing and producing this type of bushing has been subject to continuous improvement and has matured over this time. The result is the next generation of transformer bushings - O Plus Dry™ bushings.
O Plus Dry bushings are designed and manufactured encompassing the latest materials, techniques, and processes. O Plus Dry bushings use a resin impregnated synthetic (RIS) technology and do not contain any oil, paper, or porcelain. The condenser is cast into a solid body comprising an epoxy resin system with foils. Silicone rubber weather sheds are then extruded onto the air-side of the condenser. The result is a family of bushings that increase safety and improve reliability with the the following benefits: – Improved availability – Reduced maintenance needs – No oil and no porcelain – Explosion resistant – Fire retardant – Hydrophobic/self-cleaning
Applicable standardsO Plus Dry bushings are designed for AC application and meet or exceed all requirements of the applicable IEEE standards, as well as withstand voltages for routine test levels required by IEC standards. If you would like more information, please contact your ABB representative.
Specifying O Plus Dry bushings – Bushings will have weather sheds that are formed using a filled, high-temperature vulcanizing (HTV) silicone rubber that is helically extruded on to the air-side of the bushing.
– Bushings must have a test or voltage tap allowing for power factor testing without disconnecting the winding leads.
– Bushings must not contain any paper. – Bushings shall not contain oil nor share oil with the transformer.
– The partial discharge for bushings must be below 5 pC at 2 times maximum line-to-ground (L-G) voltage.
– The power factor for bushings must not exceed 0.50 percent at 20 °C.
2 O Plus Dry technical guide
ConstructionBushing construction begins with a central conductor. The conductor is made from an appropriate metal alloy and can be hollow or solid depending on the specific application requirements. Using the conductor, a condenser is built that consists of a finely graded insulation system that is impregnated with an epoxy resin and then cured. The condenser is precisely wound using a synthetic material and strategically placed aluminum foils that provide capacitive grading layers necessary to control and shape the electric field. The field is controlled in such a way as to optimize the bushing size, mass, and electrical performance based on voltage class and other parameters of the bushing.
Once the condenser is wound, it is placed in a mold and a filled epoxy resin is cast around and throughout the condenser. When the condenser body is removed from the mold, it is post cured to fully harden the resin. Out of the mold, the condenser is already in its final shape; no machining is needed to shape the condenser body.
Before adding weather sheds to the bushing, the flange is installed. Attaching the flange after the condenser molding process allows ABB to use one condenser design through a broad range of end-bushing mechanical configurations. The weather sheds are formed using a filled, high-temperature vulcanizing (HTV) silicone rubber that is helically extruded on to the air-side of the solid condenser. After the extrusion process, the bushing is again heated in an oven in order to cure the silicone weather sheds. By applying HTV silicone weather sheds in-house, ABB is able to eliminate porcelain, which often contributes to the long lead-time for conventional bushings.
Weather shedsThe shed profile has been carefully designed to optimize its performance as a weather shed. The shed profile alternates small and large sheds and is self-cleaning. The angle of the shed was carefully designed to ensure water will run off the edge of the shed rather than around the helical profile. The bottom surface as well as the top surface of each shed is tilted at a downward angle. This technique optimizes the protected areas of the weather shed, resulting in the lowest possible leakage currents even in severe environments. The tip of each shed is well-rounded minimizing the amount of dielectric stress and mitigating the risk of erosion.
The specially formulated HTV silicone rubber weather sheds are applied to the epoxy condenser body using a void-free helical extrusion process. ABB has developed a patented method of extruding the sheds with a helical pattern. This extrusion method allows the weather sheds to be applied in one continuous process regardless of the length of the air-side of the bushing. The tapers at the beginning and end of the weather sheds are carefully formed by hand. The silicone rubber sheds are completely sealed via chemical bonding that ensures the best possible interface between the silicone and condenser body. The result is a seamless silicone coating with no molding lines. This reduces the chance of tracking, since the non-uniform material of a molding line is not present for pollutants to build up.
Figure 3 - Electrical stress on a typical shed tip | Figure 4 - Electrical stress on an O Plus Dry bulbous shed tip
Figure 2 - O Plus Dry weather sheds
Because of the solid, epoxy impregnated condenser, very few steps are required to complete the bushing. For higher voltage bushings rated above 69 kV, a high-dielectric, flexible material is injected between the flange and condenser body. The test or voltage tap is installed. Then, the appropriate end terminals are added according to IEEE or customer specific requirements.
O Plus Dry technical guide 3
O Plus Dry bushings use only high-quality filled HTV silicone rubber from the best available suppliers. The use of HTV silicone rubber ensures the highest possible durability of the sheds, as well as outstanding tracking and erosion resistance. This material is stable beyond the temperature conditions required by IEEE standards.
The HTV silicone rubber has the property of hydrophobicity. This property helps break up water films on the surface into individual droplets. This phenomena reduces leakage current along the surface, helps prevent flashover, and elevates the voltage withstand capability during wet and highly-contaminated conditions. The low leakage current minimizes discharge activity on the surface and minimizes erosion. In effect, hydrophobicity acts as a self-cleaning property that extends service life and significantly reduces the need for insulator cleaning maintenance.
The mechanism behind the hydrophobic property of the HTV silicone rubber is the diffusion of low molecular weight (LMW) silicone from the body of the material to the surface. The LMW silicone forms a layer on the surface that is hydrophobic, and tends to encapsulate contamination. This layer is extremely thin, ie, only a few molecules thick, and is distributed over the entire surface of the weather shed. The LMW silicone in the rubber diffuses to the surface over the entire life of the bushing. The amount of silicone lost through this process is negligible and does not affect the life expectancy of the bushing.
A second property of the HTV silicone rubber is its ability to recover quickly from contaminated conditions. Under heavy and prolonged pollution, the hydrophobic characteristic may be reduced temporarily but it recovers quickly as soon as the conditions are normalized. Testing carried out by ABB shows the loss of hydrophobicity during the extreme conditions of a 1000-hour salt fog test to be recoverable within a few days if allowed to dry, and even faster if exposed to UV light and high temperatures.
Figure 5 - Test tap Figure 6 - Voltage tap
Test tapO Plus Dry bushings rated 69 kV and below have a small housing containing a test tap as part of the mounting flange. The test tap provides a convenient means for making power factor and capacitance measurements by the ungrounded specimen test (UST) method.
Top/Bottom terminalsO Plus Dry bushings are equipped with silver-plated, top terminal studs that comply with IEEE C57.19.01-1991 and bottom terminals that comply with IEEE C57.19.01 - 2000. The top terminal studs specified in the year 2000 version of the standard were not always adequately sized for the loads seen on today’s grid. This small change helps ensure that O Plus Dry bushings perform flawlessly throughout their life.
Specific details about the terminals are provided on the outline drawing. For bushings rated above 69 kV, the bottom terminal is typically included in a kit that includes the shield and mounting hardware. If a different configuration is required please contact ABB.
Figure 7 - Draw-lead stud
115 kV bushings and above are provided with a Type A (normally grounded) voltage tap as described in figure 1 of IEEE C57.19.01-2000 and shown here in figure 6. This tap is connected to one of the inner foil electrodes of the condenser, commonly referred to as the C1 foil. With the tap cover installed, the tap is grounded under normal operation. If the voltage tap is used in conjunction with a potential or monitoring device, the voltage between the tap and ground should be limited to 8 kV continuous.
Voltage tap
4 O Plus Dry technical guide
Routine testingAs part of the manufacturing process, the bushings are subjected to a number of routine tests. Routine testing ensures that each bushing has been manufactured using ABB quality standards and in accordance with proven design rules. Many of these tests have been incorporated into the manufacturing process including poka-yoke work stations as well as go/no-go checks throughout the manufacturing line. As a final assurance, each bushing is subjected to electrical tests at ambient temperature with the lower end of the bushing submerged in transformer oil. O Plus Dry bushings are tested to the higher of the two levels defined in the applicable IEEE or IEC standards. Overviews of some of these tests are included below:
− Pressure testing ensures there are no paths available for oil to leak out from a transformer through the bushing
− Power factor and capacitance tests ensure the insulation system was manufactured properly
− Withstand testing ensures a bushing can adequately transfer power into the transformer at voltage without suffering from flashover or puncture
− Partial discharge testing conducted at 2 times the line-to-ground voltage rating ensures practically void-free casting and that the insulation system will not erode during the life of the bushing
Certified test reports (CTRs) detail the final electrical testing and are available for each bushing ABB ships.
Usual service conditionsO Plus Dry bushings meet or exceed the usual service conditions as defined in IEEE C57.19.00-2004.
− O Plus Dry bushings can be operated in ambient temperatures ranging from -30 °C to +40 °C.
− All O Plus Dry bushings can be used at an altitude up to 1000 meters (3300 feet), and most can be used at much higher altitudes. Please see the outline drawing for individual bushing ratings.
− O Plus Dry bushings can be mounted at any angle from vertical to horizontal (0-90° from vertical).
− O Plus Dry bushings are designed for use in mineral oil.Please note that O Plus Dry bushings can often be applied outside the usual service conditions defined by IEEE. To ensure the suitability of O Plus Dry bushings for a particular environment please contact ABB.
The bushing’s maximum continuous line-to-ground voltage is indicated on the outline drawing and should serve as the limit for voltage application. Any [continuous service] voltage applied above these levels may cause damage to, and /or failure of, the bushing. For temporary overvoltage conditions, contact ABB.
Maximum voltage
OverloadingThe selection of properly rated bushings is critical to trouble-free operation. The current rating of a bushing is based on an assumed set of operating conditions (given in clause 4.1 of IEEE C57.19.00-2004), and as long as a bushing is operated within these conditions, one can be assured the bushing will not exceed its thermal rating. However, when a bushing is operated outside of these conditions extra care must be taken. Due to the thermal characteristics of resin, the overloading capability of resin impregnated bushings, including O Plus Dry bushings, is different than oil-filled porcelain bushings. Since the continuous operating temperature limit of 120 °C for O Plus Dry bushings is closer to the short-term thermal limit for the resin (130 °C), there is less inherent thermal margin in O Plus Dry bushings, as well as all resin-impregnated bushings.
Nonetheless, with regard to thermal performance, every bushing will have some amount of inherent margin allowing for some level of overloading. Bushings must meet target current ratings, as specified in the standard, but due to the use of common conductor sizes, varying insulation thicknesses, etc., some bushings may have more margin than others.
Furthermore, the actual operating conditions seldom match the assumed conditions of the standard, and bushings are often not operated at their full nameplate current. These differences can provide additional margin. Varying situations make it impractical to give standardized overload ratings or simple de-rating factors. So when a bushing is operated outside the assumed conditions, the operating temperature must be calculated to ensure it does not exceed the thermal limits of the bushing.
Design/Type testingO Plus Dry bushings have been thoroughly tested to ensure a lifetime of performance. The goal of design testing is to prove bushings manufactured using a defined set of rules actually perform as expected. Some of the design tests performed include cantilever tests to ensure bushings can physically support both transformer and line connections, over-voltage tests to simulate higher altitude applications, and wet electrical tests to simulate precipitous conditions. In addition to these tests, many of the same tests described in the routine testing section are performed, though sometimes with more severe test levels or limits. For full details regarding design tests please contact ABB for a design test report.
The hottest-spot temperature a bushing reaches in operation depends upon: bushing design (geometry and materials), load current and frequency, transformer top-oil temperature rise, ambient temperature, and the duration of operation under those conditions. IEEE C57.19.100-2012 provides for thermal constants, which may be used to estimate the hottest-spot temperature rise of a bushing given variations in these parameters. These constants are unique to a specific style of bushing, are derived from test data, and are provided by the bushing manufacturer. For O Plus Dry bushings, they are given on the outline drawing. The four constants (not truly constants, but relatively consistent over a reasonable range), and a description of their significance, are:
K1 This accounts for self-heating of the bushing, due primarily to I2R losses, but affected by cooling, etc.
K2 This accounts for the effect of the hot oil in which the bushing is immersed.
n This exponent relates bushing self-heating at overload conditions to that at rated conditions. Because the largest component of self-heating is I2R losses, this will be close to 2, but things such as cooling efficiency, variation of conductor resistance, and changes in thermal radiation with temperature are rolled into this exponent.
τ This is a time constant related to the timing of thermal transitions occurring after operating conditions are changed.
Under steady-state conditions, the hottest-spot temperature is given by:
where:θSS This is the steady state temperature of the hottest spot
of the bushing (°C)
∆θHS This is the steady-state hottest-spot temperature rise of the bushing over ambient (°C)
θA This is the temperature of the ambient air (°C)
∆θO This is the steady-state transformer top-oil temperature rise over ambient (°C)
Ia This is the actual load current the bushing is being operated at (A)
Ir This is the rated current of the bushing (A)
Please note the application described herein is slightly different than that described in IEEE C57.19.100-2012. This is because the IEEE standard seeks to estimate only the temperature rise above a fixed ambient temperature. As many bushings are operated at an ambient temperature differing from the assumed ambient temperature of the standard, the calculations illustrated herein consider the actual operating and ambient temperatures.
Another minor difference, which is just in terminology, is that the ratio (Ia ⁄ Ir) used here is referred to in the IEEE standard simply as “I”. However, the term “I” is universally used to represent current. This “I” (per IEEE) is not current, but rather the per unit current, referenced to the bushing rating. Additionally, listing the terms “Ia” and “Ir” separately, combines the math all in one equation.
Short circuit current ratingThere are no existing short circuit ratings for condenser bushings defined by the IEEE standards. There are, however, requirements for transformers in the IEEE standards which can serve as a guideline for bushing short circuit ratings. In IEEE C57.12.00-2006 § 7.13.1, the maximum short circuit duration for category II, III, and IV transformers is defined as two seconds. By solving the equation given in the same section, it can be determined that the symmetrical short circuit current rating is equal to 25 times the normal rated current.
In IEC 60137 § 4.3, it states that the short circuit current rating shall be 25 times the normal rated current for a two second duration with reference to IEC 60076-5. The rated short circuit current shall not exceed 100 kA symmetrical.
O Plus Dry bushings have been evaluated against the verification method of IEC 60137, as given in section 8.8, and are suitable for 25 times the rated current, for a duration of two seconds. The bushings should survive the short-circuit event, but it does not necessarily mean they are completely undamaged. As such, it is recommended that bushings experiencing a short-circuit event of the full magnitude described be replaced before reenergizing the transformer.
Additionally, this evaluation method relates to the thermal performance of the bushings. Mechanical loading is also of concern, but the mechanical loads developed depend upon how conductors are run, spaced, and supported. This must be done in such a manner that the forces generated at the bushings’ terminals do not exceed the short-term cantilever ratings of the bushings.
For short-duration overloads, the bushing does not have time to fully transition its temperature. This means overloads which otherwise would not be permissible may be okay for short intervals. The transition from the temperature at one set of conditions to that at another set of conditions is approximately exponential, and the time constant, “τ”, can be used to predict the temperature at a given time during this transition. The specific calculations are not shown here, but are explained in IEEE C57.19.100-2012, §4.2.2. However, even without doing complete calculations, the time constant can give an idea of what to expect. After five time constants, the temperature will be stable at the new operating conditions, so any overload duration exceeding five time constants is actually steady-state operation. After one time constant, the temperature will achieve 63 percent of the difference in temperatures at the “before” and “after” conditions. After three time constants, the temperature will be at 95 percent of the differential.
Please note that since the transient calculations given in the IEEE standard predict only how the temperature transitions from a starting value to an ending value, these values could be actual temperatures or they could be temperature rises. In the IEEE standard, those calculations start with temperature rises, and therefore yield a temperature rise. However, for the reasons discussed above, the limits for the bushing are given as actual temperatures, including the ambient temperature. Therefore, after calculating the temperature rise, the ambient temperature must be added to it, before comparing the temperature to the thermal limits of the bushing. However, if one were to calculate using actual temperatures instead - ie,the method previously described - then the ambient temperature is already included, and should not be added.
Once the temperature at the given operating conditions and duration is determined, it must be compared against the thermal limits of the bushing. For O Plus Dry bushings, the limit for continuous operating temperature (actual, not just rise above ambient), is 120 °C, and for short-duration excursions above this, the limit is 130 °C. These limits must not be exceeded.
As a final note, O Plus Dry bushings were designed for and tested at 60 Hz, and the thermal constants are based on 60 Hz. Due to increased conductor skin depth at 50 Hz, bushings operated at 50 Hz will run as cool as, or cooler than those operated at 60 Hz. Thus, operation at 50 Hz is acceptable without further consideration.
Given a bushing with: K1 = 17.9, K2 = 0.96 and n = 1.99, and rated for 1200 Amps, but operating at 1500 Amps, in a transformer with a top-oil rise of 59 °C, and at an outside ambient temperature of 34 °C, what will its hottest spot temperature be once the temperatures have become stable? Is this acceptable?
Therefore:
Since the temperature attained is less than the limit of 120 °C, this set of operating conditions is acceptable for continuous operation.
Sample overload calculation
O Plus Dry technical guide 7
069 N 3000 AA
Voltage class Type designation Amperage rating Specific identifier
025 = 25 kV N = (gray) O Plus Dry bushing 0412 = 400 A - draw-lead, 1200 A bottom-connected AA = no defined meaning
034 = 34 kV 0812 = 800 A - draw-lead, 1200 A bottom-connected BA = no defined meaning
069 = 69 kV 2000 = 2000 A - bottom connected only
115 = 115 kV 3000 = 3000 A - bottom-connected only
138 = 138 kV
Figure 8 - Style number explanation
Style number Information
025N0412AA 25 kV 400 A, draw-lead; 1200 A, bottom-connected
034N2000AA 34 kV 2000 A, bottom-connected
069N3000AA 69 kV 3000 A, bottom-connected
138N0812BA 138 kV 800 A draw-lead; 1200 A, bottom-connected
Figure 9 - Style number example
Style numberThe style number for the bushings will be written as “025N412AA”, where “025” is the bushing voltage class, “N” signifies the type of bushing, “0412” signifies the current rating in Amperes, and the final two characters eg, “AA”, are arbitrarily assigned by ABB to describe other characteristics of the bushing. See figure 8 for a graphical depiction of a style number and figure 9 for style number examples.
Draw-lead applicationProper sizing of the draw-lead cable is the responsibility of the transformer designer. The maximum rated current for draw-lead connected bushings rated 69 kV and below is 400 amperes. The maximum rated current for draw-lead connected bushings rated 115 kV and above is 800 amperes.
The transformer designer must know the inside diameter of the bushing conductor tube because this will limit the choice of the cable size. The draw-lead cable must be adequately insulated (ie, insulation thickness of 1 mm or more) to isolate it from the inner diameter of the bushing’s tube and avoid the formation of arcs between the draw-lead cable and bushing tube. The minimum diameter is established in the IEEE standard, but the outline drawing should be referenced to determine a specific bushing’s inside diameter.
If a draw-lead bushing is ordered, it will be shipped with a crimp-type draw-lead connector as shown in figure 7. If the standard draw-lead connector is not suitable for the application, contact ABB for additional options.
Ordering detailsWhen ordering please specify the following:
− Bushing type and style number − Current rating − Voltage class and BIL − Any non-standard requirement such as overload, impulse tests, or high temperature application must be specified at the time of quotation.
Bottom-connected applicationSpecification of the bottom-connected configuration is the responsibility of the transformer designer. In order to avoid overheating, proper sizing of the bottom terminal is imperative. Additionally, it is recommended that locking washers are used to ensure stable connections and prevent any hardware from vibrating loose due to transformer harmonics, resulting in damage to the transformer equipment.
8 O Plus Dry technical guide
Top terminal stud
Weather sheds
25 to 69 kV O Plus Dry electrical specificationsThe electrical ratings summarized in this guide are for bushings operated at 60 hertz. For specific details please refer to the outline drawing.
The information contained in this document is for general information purposes only. While ABB strives to keep the information up to date and correct, it makes no representations or warranties of any kind, express or implied, about the completeness, accuracy, reliability, suitability or availability with respect to the information, products, services, or related graphics contained in the document for any purpose. Any reliance placed on such information is therefore strictly at your own risk. ABB reserves the right to discontinue any product or service at any time.
