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.
Fig 1: IMAGE coming soon
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: IMAGE coming soon
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.
LINK: Tap Changer Geometry design choices. Coming soon.
Definition of the Problem
The transformer needs to operate on a range of voltages from 120-133 Volts (Line-Neutral) and have a resolution of one degree of phase shift per tap with an overall operating phase shift range of +/- 10 degrees. This will mean 10 taps per lef 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 in a range of 50 to 80 milliseconds. The tap changer needs to be able to transition directly from +1 to 0 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 whos action is at the exact midpoint of the system (where the change from -1 to 0 to +1 takes place).
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
This project will strive to fully understand various on-load tap changer design advantages, disadvantages, and use cases to come up with the simplest, most economical design that is still capable of performing to the specifications defined by the client. The two major topics covered will be the design of the transformer and the design of the tap changer. The transformer design will encompass the type and configuration of transformers, the location of taps, the number of taps, and the connection to the tap changer. The tap changer design will cover the different design options, as well as justification for why one design component is better than another. Finally, the wiring diagram and drawings of the components assembled are attached.
PROBLEM 1: Make Before Break
First and foremost in a transformer tap changer, you have to change taps. It sounds obvious, but there is more to it than meets the eye. 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 create this phase shift without having to shut off the power.
This leads us to what is called an on-load tap changer. When changing from one contact to another on-load, you do see the possibility of an arc as well as an electrical transient in the system. Tp 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 or reactive bridge. The bridge would connect the two tap positions on either side of a resistive or reactive load via two pre-selector arms.
Using a reactive or resistive bridge is necessary when working with transformers. A bridge with an impedance component in it ensures both that current in the transformer windings would not exceed the winding ratings and the longer life of the tap changer components.
PROBLEM 2: Resistive or Reactive Bridge
Cognizant of our need for a resistive or reactive bridge, we needed to weigh the benegits and the costs to decide which route to take. The resistive bridge is easier to fabricate and simpler to design. The reactive or inductive bridge involves the creation of an inductor and the consideration of an electrical time constant due to resistance within the tap changer and complex impedance (inductive) in the transformer windings. An inductive bridge is more efficient as the power doesn't get wasted as dissipated thermal energy during the transitions. The decision came down to time, money, and difficulty, all of which lend themselves to the resistive bridge.
PROBLEM 3: Synchronize the Polarity Switch
In order to create the needed negative phase shift we would need to reqire the connections to the transformer. Once again it was the desire of our client for this to 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 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 that must be considered. 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. If this occurs, a potentially dangerous and economically unviable voltage change and transient 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 - coming soon.
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.
PROBLEM 4: Tap Changes in 50-80 milliseconds
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 needs 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.
In much of the used documentation dealing with industrial tap changer designs we found that a device called a Geneva Mechanism combined with a spring was used. 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.
We decided to use the Geneva gear for our tap changing action, and it was determined that 11 slots and a small driver would be used. Using the relations found in a book of mechanisms we sized both the Geneva gear and driving gear. Finally, we chose to use a ball bearing ring to gold 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. This allows a spring attached to a flywheel or thin diameter disk to move without obstruction and power the tap change.
PROBLEM 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, we came to a realization of how the spring system would work. The spring should reach its maximum position just before the driving gear's peg enters the Geneva gear slow 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, a button of manual switch, seemed the most logical, however it also presented a complex extra step for the operator. That invalidated the options, as the operator should be able to turn the handle one direction or another and have the device do the rest for him.
We finally devised a noth 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 correct amount of time.
Conclusion
All of the problems listed, along with the best solution to each, have been formed into one proposed design for a Phase Shifting Transformer and associated Load Tap Changer. All transformers will be ordered from Control Transformer.
FIGURE 4 - Proposed Tap Changer Design
The tap changer design will consist of two shafts, a driver and a driven shaft. A handle will be the input with an option of attaching a motor later. The shaft will be vertically supported by a bearing mounted to the inside wall of the vear box. The driving shaft will have the driving gear of the Geneva device attached to it as well as the spring to force a faster rotation after it has been wound half way. The driven shaft will have a switch that works as a stop and can follow a 720 degree helix on the bottom of the shaft. The polarity switch will be completed by placing three conductive strips into the helix groove and placing contacts on the stop arm.
On top of the gear box will be the tap changer shell. The shell will hold all of the 33 contacts necessary for the tap changer to change taps. The driven shaft will have three arms attached to it, sitting inside the tap changer shell. The arms will sit inside of the tap changer shell on level with each corresponding ring of eleven contacts needed for that specific phase.
With all of these items accounted for, the tap changer design will meet all of the needs specified by our client, as well as the necessary operating specifications. These have been combined in the simplest and most easily operated design.