About the Project
The Phase Shifting Transformer is not a new concept. Electric Utilities must constantly consider how to most efficiently transport useful power from an area of generation to the consumer endpoint. A phase shifting transformer has the ability to adjust the difference in phase angle between voltages at the transmitting and receiving end of a transmission line by varying voltages internal to the transformer. Since the flow of real power (P) is directly related to this phase shift, being able to control phase shift allows the utility to control the ratio of power flowing through two parallel transmission lines. This increases the efficiency of the existing transmission lines when handling a large load by allowing the utility to keep power flows from exceeding the ratings of the transmission line. Figure 1 illustrates how power flows in a transmission line can be controlled via the introduction of a phase shift.
Figure 1.
As need for power change, a real-time change in phase angle difference can be made to allows for the varying demand in a region.
Because the requirements for this transformer allow us to fabricate on such a small scale, we can avoid the need to immerse the tap changer in oil. Many larger scale transformers use fluid to reduce arcing and to cool more efficiently. Oil not only helps keep temperatures within safety margins, but is also a better insulator than air, therefore being better suited to prevent excessive energy from jumping from one conductor to another insude the tap changer. With such a large amount of power coursing through the full scale tap changers, arcing can be catastrophic - destroying the transformer, surrounding components, and real estate.
Fig 2: Phase Shifting Transformer Schematic
The selected design allows for the free convection of air for cooling as well as allowing everything to be exposed. This provides greater access for measurements during tests and increased system clarity (this design will be placed in an academic atmosphere and will be for educational use). The open design also caters to the possibility of future improvements and expansions.
Our primary tasks will be to specify the size of the transformers needed to create the series and shunt units within the Phase Shifting Transformer. We ordered two 10kVA transformers last semester with the appropriate taps. By changing which tap is connected to the circuit, the tap changer can control how much voltage is placed across the primary windings of the shunt unit. That voltage is then induced in quadrature to the line voltage present in the secondary windings of the shunt unit, and a phase shift then takes place. The angle of shift produced is the difference in phase angle between the input line voltage and the output line voltage. The component we will design and build completely from scratch will be the tap changer. The tap changer allows us to change which tap on the excitation transformer winding is connected to the overall circuit, thereby selecting which voltage will be used. Changing tap selection on the excitation transformer allows us to get a higher or lower voltage. That voltage change produces a change in phase shift in the line voltage present in the secondary winding of the shunt unit. The tap changer will be designed based on the functionality and needs specified by our client. Many designs have been reviewed and evaluated while converging on the final design selected to be fabricated.
Definition of the Problem
The transformer should operate on a range of voltages up to 133 volts and have a resolution of one degree (0.01744 radians) of phase shift per tap with an overall operating phase shift change of +/- 10 degrees. This means 10 taps per phase of the shunt transformer, or 30 taps in all. Contact #1 would correspond to +/-10 degrees and contact #11 would correspond to a phase-shift of zero. Having ten taps means eleven contacts (one for a zero shift position). The change from one contact to another must take place within 50 to 80 milliseconds. The tap changer needs to be able to transition directly from +1 to zero to -1 degrees of phase. It also needs to stop movement at the points of maximum phase shift amplitude (+/- 10 degrees). In order to allow for dynamic change of phase shift direction there must also be a winding polarity switch that takes place at the midpoint of the system (where the change from -1 to 0 to +1 takes place). This switch would ideally be automatic and synchronous with the switching motion of the main contact arm.
Criteria
The design needs to be as simple as possible, yet still be able to meet requirements. Contact with the next point needs to be made before disconnecting from the previous contact. Each arm of the three phases needs to have the ability to be offset each contact from the other by +/- 1 contact to simulate an unbalanced situation (the net voltage of the three phases is no longer zero).
Scope
Through this project the team strove to gain a comprehensive understanding of the advantages and disadvantages of various load tap changer designs and use cases and come up with the simplest most economical design that was still capable of performing to the specifications defined by the client. The two major topics covered were: 1) The design of the transformer and 2) The design of the tap changer. The transformer design encompassed the type and configuration of transformers, the location of taps, the number of taps, and the connection to the tap changer. That design, once it arrived, needed to be characterized and wired properly. The tap changer design considered the different design options, as well as justification for why one design configuration is better than another. The wiring diagram and drawings of the components assembled are attached to this report, as are pictures of the final products.
Design Factor 1: Make Before Break
First and foremost, a transformer’s load tap changer needs to be capable of changing taps. Changing taps requires the disconnecting of one contact from the circuit while reconnecting the circuit at another point. This can be done one of two ways: with the power on, or with the power off. With the power off you do not have to worry about the possibility of an arc or the speed at which you change contacts. Living in today’s on demand world, however, it is much more desirable to be able to change taps without shutting off the power.
This leads us to what is called an on-load tap changer (or just load tap changer). When changing from one contact to another on-load, you do see possibility of an arc as well as an electrical transient in the system. To keep that from happening you have to bridge the connections during the change. “Make before break” refers to making contact with the new point before releasing from the last point of contact.
There are two solutions to this issue. One is a larger selector contact wide enough to bridge the gap between two tap contacts essentially shorting the two winding connections together, or the second, a resistive/reactive bridge. The bridge would connect the two tap positions on either side of a resistive or reactive load via two pre-selector arms.
In an interview with ABB Corporation, Dave Geibel, the engineering manager at the Tennessee branch told us about the need for a resistive or reactive bridge in order ensure both that current in the transformer windings would not exceed the winding ratings and longer life of the tap changer components
Design Factor 2: Resistive or Reactive Bridging
Cognizant of our need for a resistive or reactive bridge, we needed to weigh the benefits and the costs to decide which route to take. In Europe the majority of tap changers employ a resistive bridge. In the US it is the opposite. The resistive bridge is easier to fabricate and a simpler to design. The reactive or inductive bridge involves the creation of an inductor and the considerations of an electrical time constant due to resistance within the tap changer and complex impedance (inductive) in the transformer windings. An inductor is more efficient as the power doesn’t get wasted as dissipated heat during the transitions. The decision came down to time and money and difficulty, all of which lend themselves to the resistor. The resistor was also the recommendation of ABB’s engineers during the interview, given the academic purpose and design timeline.
Design Factor 3: Synchronize the Polarity Switch
In order to create the needed negative phase shifts we would need to rewire the connections to the transformer. Once again it was the desire of our client for this occur automatically with the rotation of our tap changer.
The polarity switch needs to take place when the tap changer is at its zero phase shift position. Depending on the direction of the tap changer it will either choose to tap positive or negative. There are multiple solutions to this problem. First, the motion of tapping in one direction or another could be used to physically make a switch. The second option would be to have a contact strip that would be followed with a split to make the polarity switch after each rotation. The third option would be to have a manual switch that would be moved by the operator or some external synchronized mechanism when the switch would need to take place.
There are two issues that present themselves during this scenario. One problem is the need to stop the motion of the tap changer from going around again after it has reached its minimum or maximum phase shift. This occurs as a result of the potentially dangerous and economically unviable voltage change and transient that would occur with a direct switch from +/- 10 degrees back to zero. The other problem is the need to make before break when changing polarity with the power on.
One solution was realized after the afore-mentioned problems came to light. Combine a helical path with three small conductive strips with a split in the middle (see Figure 3).
FIGURE 3: Helical Polarity Switch
Starting at maximum phase shift the arm would follow the helix around the shaft, jumping the conductive gap and making a phase shift at the zero-tap position, going to 0 to (-)0, and continuing on the other side of the strip as the shaft continues to rotate. In order to control how far the shaft can rotate in either direction, the helix will need to only be present for 720 degrees, or two rotations, of the shaft as to allow one full rotation for negative and another for positive phase shifting of the voltage in the connected transmission lines.
Design Factor 4: Tap Changes in 50-80 ms
In order for the phase shifting transformer to work correctly the tap changer must be able to move from one phase to another within a range of 50-80 milliseconds. The system must to have the ability to store mechanical energy and use it in the tap changing process when it is ready. There is a need for a constant motion to be stored as energy to be released in a single movement of the tap changer. It is theoretically possible to design a high torque precision stepper motor and associated controller to provide the rotational speed and exact angle needed, but it was determined that providing a mechanical solution would be more reliable and cost effective. All of the documentation of industrial tap-changer designs used for research there was a device called a Geneva Mechanism [See Appendix A.1for more information] combined with a spring. When the spring is stretched to its greatest point it has sufficient energy stored in tension to carry the change through in a specified time.
The solution arrived at was to use a Geneva gear with 11 slots and a small driver. Using the relations found in [1] we sized both the Geneva gear and driving gear. Finally we chose to use a ball bearing ring to hold the shaft vertically while still allowing it to rotate freely. The shaft will be tapered at the bottom to allow the bearing to hold the shaft vertically. This allows a spring attached to a flywheel or thin diameter disk to move without obstruction and power the tap change. While the Team was able to complete the Geneva gear, the spring component of the Mechanism was not completed.
Design Factor 5: Tap Change Both Directions
Upon creating this spring and gear system to create an exact output position of the tap changer based on an input rotation from the driver, we came to a realization about how the spring system would work. In order to work best the spring should reach its maximum position just before the driving gear’s peg enters the Geneva gear slot in the direction of rotation. This implies that we must be able to modify where the spring is located or pulls depending on the direction the Geneva gear needs to turn. Without this, the time to change taps would not be consistent in both directions.
In order to solve this we had to put a switch in place. This either means hitting a button to make something automatically switch places when you are ready to reverse, putting a mechanical control that can detect which direction the shaft is rotating in, or physically moving the spring before changing tap position in the opposite direction.
The first option of a button or manual switch seemed the most logical; however it also presented a complex extra step for the operator. That was not an option as the operator wants to be able to turn the handle one direction or another and have device do the rest for him. This also removes the option for any kind of manual switch.
Finally we devised a notch section and a stop on each end that allows the shaft to rotate freely for a set angle when changing directions. When it then hits the stop in the new direction the spring will be aligned again as is needed. If the driver handle is rotated in either direction the appropriately located spring will stretch and pull the Geneva gear through its motion in the allotted time period. As stated in the previous Design Factor segment, this spring system was not fully implemented before the project deadline was reached.
All of the problems listed and the best solution to each were formed into one proposed design for a phase-shifting transformer and associated tap changer.
FIGURE 4 - Proposed Tap Changer Design Design Evaluation
Our design specifications as they stood at the end of first semester are summarized as follows:
Firstly, the transformer was to be rated at 10kVA with a rated voltage of 230 V. It was to have voltage difference between successive taps of 1.8% and to have 11 taps. This part was met entirely to specification. The transformers were designed, the work order was sent out, and Control Transformer delivered them exactly to specification.
Next we have the cart. The cart needed to have a footprint no larger than 2 feet by 4 feet so that we could easily store it in the power lab Gauss Johnson Engineering Laboratory or room G10 of the Buchanan Engineering Laboratory. The cart was easily assembled and, though it may not roll or be as easy to maneuver as we might like, is rated to hold the weight of the entire phase shifting transformer system. The wheels could easily be changed out for larger diameter wheels with a better outdoor rolling capability in the future.
Finally we come to the tap changer, which was by far the largest and most important part of this project. The tap changer was designed and fabricated completely in-house.
Here is where our design has diverged from the initial specifications. First addressed are those items that worked as designed. The tap changer has 11 contacts per phase to match the needed switching positions. The contacts in the collars were more than large enough to meet the current requirements for the current that would be flowing through the tap changer. The contacts in the collars had plenty of force acting outward to create good electrical connection, and the wires running from the collars and tap changer shell were clearly labeled to easily obtain desired measurements and clarify what, on paper, is a complex wiring diagram.
The tap changer also correctly stopped motion after 720 degrees of rotation. As originally designed however, the stop mechanism did not work. It was supposed to move vertically as the walls around the large diameter section were supposed to restrain the movement in the lateral direction. However, the moment created about point O caused the part to rotate until it came in contact with the wall, and though it didn’t have much room to rotate, it was enough to cause the piece to catch and not slide, even when lubricated. However, upon realizing it was trying to rotate, we reversed its orientation and moved the supports closer to the helix to shorten the moment arm significantly. By doing this we were able to solve the problem in a short amount of time without requiring any additional machining.
Now for a summary of those items on the list that did not work correctly, why they didn’t work, and a list of things that could not be tested as a product of the former.
When the tap changer was assembled the first time, we realized that the sinusoid formed by the interior walls of the shell and the concave surface of the contacts in the shell was causing the pre-selector arms on the collars to catch on the walls and instead of deflecting into the shell, they deflected sideways causing the shaft to stop rotating. Originally we wanted the pre-selectors to be spaced at an angle such that a component of the force exerted on the contact by the act of rotation would cause the pre-selector spring to compress, allowing the entire assembly to move from one tap to the next. However, given the diameter of the selected springs, the collar holes to hold the spring and contact assembly would have interfered with each other if drilled at the design angle. We had to drill the holes in a parallel spacing, increasing the angle difference between the rotational force applied to the contact and the force needed to compress the spring. We used a nylon disk as opposed to the three armed aluminum housing seen in the original design to provide simpler electrical isolation. Now that they were oriented in parallel, the contacts no longer saw a component of the force from the wall in line with the direction of the spring compression. Since the contacts were only soldered on to the wires and placed against the springs, the pre-selectors had a tendency to bend sideways and rotate about the soldering point.
Removing a majority of the sinusoid allowed the tap changer to rotate from point to point without catching, but it still didn’t move smoothly. At this point we decided to test the quality of the electrical connections between the main contact on the collars and the contacts in shell. We realized that not all of the collars were lining up correctly with the contacts in the shell due to imprecise machining. Only 8 in 10 contacts made an electrical connection, and many of those were not getting solid contact on the faces and were only making point contact. As a product of these failures we were never able to test either the mechanical or electrical transient response of the system and the delay when changing taps because it was not safe to hook the tap changer up to a power source.
The tap changer was never bolted securely in place. Considering that it wasn’t working, the Team felt that it would be a poor decision to permanently secure something that needed to be partially redesigned. The system as a whole did not meet specifications because the connections could not be used safely with an electrical power source. The primary reason for this was precision of machining. Aside from the bearings, springs and fasteners, every single part in the tap changer was fabricated by the Team in the University of Idaho machine shop. The copper contacts both from the collars and the ones pressed in the shell were not all exactly the same dimension; the concave and convex surfaces did not have perfectly matching curvature, and the holes for the contacts didn’t get drilled in the exact spacing needed.
Other machining problems came to light with holes for fasting the collars to the main shaft: They were not in the correct position (or even at the same offset angles the others were), and this resulted in a few of the contacts lining up correctly while others in another phase made only point contact.
Lessons Learned
There were several key lessons we learned while working on this project:
- We are not professional machinists and if somethign needs to be very accurate or to a tight tolerance it is important to have it made professionally.
- It is important to analyze even the simplest part with at least a free body diagram and a quick motion study. Don't assume it will always move or act the way you think it will on a computer without friction, gravity or moments.
- Lastly, and probably most important is to spend more time really understand what the client wants from the project. We spent too much time in our design allowing a change from one tap to the next to happen quickly thinking everything have to move quickly when in reality the tap changer only needed to move one step at a time and stay in the new tap position until all testing is concluded. Only the transition needs to be fast, and doesn't need to be fluid to do three or four quickly on a row.
Concrete knowledge of exactly what you are making and of what your client wants can really help simplify a design. Anything not clarified early on compounds itself into large complications very late in the fabrication stage, and when you are only given time to fabricate once, one of these complications can make or break an entire project.
Recommendations for Future Work
There are two fundamental angles from which future work could be approached. We will outline each and recommend the approach we think will be most productive.
The first option would be to correct the current problems with the existing hardware. To make the current design work as specified, we would first recommend remaking the tap changer shell so that it is a smooth circle on the inside (rather than the “ripples” that it has in the original design). The contacts would still remain the same with the holes in the shell being drilled so that the rim of the bowl of the contact is flush with the inside of the shell. Furthermore we recommend that the shell diameter be significantly increased, in order to provide more space for moving parts. However, the height of the shell should not be increased if at possible because the current height is very close to the maximum tool depth on the CNC mill that was used to machine the shell. Recommended machining procedure for this assembly would be to drill holes in the appropriate places around the outside perimeter of solid cylindrical stock, then insert the copper contacts as rods machined to the designed diameter but somewhat longer than designed. If possible, permanently fix the contacts in place. Finally, program the CNC mill to cut out the center to the specified diameter, thus ensuring that the contact faces are flush with the shell as possible (since they will be machined at the same time).
The contact heads should all be made the same size as the center one in the current design (instead of making the pre-selector heads smaller) and the contact collars should be changed so that the outside perimeter is not a perfect circle. Instead, the region where the contact heads sit should be flattened so that the mouths of all the holes are more or less flat (this avoids the tendency of the pre-selector heads to twist). The collars should also be expanded in proportion to the shell. This will provide enough space to allow the pre-selector holes to be drilled at an angle relative to the center hole (radial to the collar such that the force applied by the spring is exactly normal to the surface of the inside of the shell). It should be noted that it is very important that the size ratio between the width of the contact heads, the width of the contacts and the distance between adjacent contacts be adjusted such that electrical contact is never broken. If the collars are significantly enlarged, it may be worth considering removing material in non-critical areas in order to reduce the inertia of the collar. In the same case, a stress analysis should be performed on the main shaft to ensure that the angle of twist due to the torque load (induced from the spring force on the contacts) is not sufficient to introduce error in the switching.
Finally, the driving spring mechanism needs to be reconsidered. As currently designed, it does not work the same when the handle is rotated both directions as originally intended. The 90 degree slot in the Geneva drive gear was designed to compensate for this issue, however we neglected to consider that our design called for the spring to be attached at the peg of the drive wheel and thus this slot is useless. It may be necessary to add a separate collar attached to the same shaft to hold the spring, or a whole new design might be necessary.
The advantage of this approach is that it is potentially a more manageable work load and would be less costly if managed correctly. The drawback to this approach is that the design that we chose may not be the most effective one to accomplish the task.
The second option for future work is to start over and completely redesign the tap changer (the electrical portion does not need to be redesigned). For this approach, we would recommend further investigation into the flag type tap changer design, an idea which we did not pursue in the initial stages because of insufficient understanding of the problem at that time. The flag type design is simpler, which means it is likely to be easier to build and design. Finally, it may even turn out to be a less expensive option due to the simplicity of the design. We would recommend this approach to future work on this project.
In either case, further work should be done on the on-load polarity switch. The client eliminated it from the project early in the second semester of work due to the complexity of the design, so we only got as far as some detailed design work on it. The current design requires high precision machining which we are incapable of doing here in the University of Idaho machine shop, so we recommend a significant overhaul of this design.
Finally, we would recommend from our own experience that future teams avoid designs that require any sort of precision machining or precise fits between parts. The University of Idaho machine shop does not have the equipment and students do not have the expertise to do that sort of work. It may be possible to get some machining done professionally, but in that case it would be important to watch the budget and be sure that the product received is worth the cost. In summary, avoid excessive moving parts, limit the amount of machine shop work required and investigate simpler alternatives.