Overview of FunctionThe transformer voltage controller functionality (ANSI 90V) is used to control power transformers (two-winding transformers, three-winding transformers, grid coupling transformers) and auto transformers using a motor-operated tap changer. The function provides automatic voltage control within a specified voltage range on the secondary side of the transformers or, as an alternative, at a remote load point (Z compensation or R/X compensation) in the network. In order to compensate for the voltage variations in the meshed system, use the LDC-Z procedure (Z compensation). For voltage drops on the line, use the LDC-XandR procedure (R/X compensation). The control principle is based on the fact that a higher or lower command to the tap changer, as a function of the voltage change (ΔV) per tap, causes a voltage increase or decrease. The voltage control operates on a tap-for-tap basis and compares the measured actual voltage (Vact) to the specified target voltage (Vtarget). If the voltage difference is greater than the set bandwidth (B), a higher or lower command is sent to the tap changer once the set time delay (T1) has elapsed. Specifying the time delay (T1) depends on the set controller response (inverse or linear), to avoid unnecessary adjusting commands during brief voltage deviations from the target value and for coordination with other automatic voltage controllers in the system. The voltage controller function also monitors the currents on the upper-voltage side and the low-voltage side of the transformer to block the controller during impermissible operating states (overcurrent/undercurrent/ undervoltage). The voltage controller function also has limiting values that, in special operating cases, suppress higher commands in the case of overvoltage and lower commands in the case of undervoltage. Structure of the FunctionThe Two-winding transformer voltage controller, Three-winding transformer voltage controller, and Grid coupling transformer voltage controller function groups consist of 5 function blocks. Depending on the application, the function groups are preconfigured in the relevant application template by the manufacturer or can be copied into the corresponding device project during engineering.The following figure shows, for example, the functional scope of the Two-winding transformer voltage controller function group.
Figure 1: Structure/Embedding of the Function GroupThe functions General (GAPC), Tap changer (YLTC), and Voltage controller (ATCC) are logical node points in IEC 61850-8-1. The tap changer (YLTC) is the interface between the voltage controller (ATCC) and the motor-operated tap changer of the transformer (OLTC). This means that the voltage controller (ATCC) sends higher and lower commands to the tap changer. This tap changer issues command pulses to the motor-operated tap changer of the transformer (OLTC). The tap changer (YLTC) measures the tap positions and monitors the action of the motor-operated tap changer (OLTC). The function group has interfaces to the following measuring points:
- Voltage, 3-phase
- Current, 3-phase (optional)
- 2 x voltage, 3-phase
- 2 x current, 3-phase (optional)
Grid coupling transformer:
- 2 x 3-phase voltage
- 2 x 3-phase current
Optionally, you can assign the tap changer to a Transformer side function group. In this way, the current of the upper-voltage side is also monitored and the differential protection pickup blocks the voltage controller automatically.
Figure 2 shows these interfaces as a block structure.
Figure 2: Structure of the Voltage Control Function GroupFunction DescriptionIf the load is increased in an electricity-supply system, the voltage is reduced and vice versa. The power transformers are usually equipped with transformer tap changers (OLTC) so as to keep the power-system voltage at a constant level. As a result, the transformer ratio is changed in predefined steps. Changing the transformer taps causes the voltage to change. The Voltage control function is intended to control transformers with motor-operated transformer tap changers. This function is designed to control the following:
For two-winding transformers (2W): the voltage on the secondary circuit of the power transformer
For three-winding transformers (3W): the voltage of the secondary winding 1 or winding 2
For grid coupling transformers (GC): voltage of winding 1 or winding 2, selectively depending on the power direction
The control operation is based on a step-by-step principle. To move the tap changer one position higher or lower, a single control pulse is issued to the motor-drive mechanism of the tap changer. The length of the control pulse can be set over a large range so as to handle the very different types of tap-changer drives. The control pulse is issued if the measured voltage deviates from the set reference value by more than the preset voltage range for more than a given time period. The voltage can be controlled at the voltage measuring point or at the load point in the electrical power system. In this case, the load-point voltage is calculated on the basis of the measured load current and the known impedance between the voltage measuring point and the load point.The following figures show possible configurations of the voltage controller for two-winding transformers with and without current measurement.
Figure 3: Voltage-Controller Constellation for Two-Winding Transformers with Current Measurement for Load Compensation at the End of the Line(1) Only if a transformer side is present
Figure 4: Voltage-Controller Constellation for Two-Winding Transformers without Current MeasurementThree-Winding TransformersThree-winding transformers are special power transformers that have 2 separate windings on the secondary circuit and typically supply 2 different busbars. The voltage levels on the secondary circuit of the power transformers can either be the same or different. In addition to designs with 2 tap changers on the secondary circuits, in most cases, three-winding transformers are equipped with only one tap changer or on-load tap changer on the primary side. It is therefore necessary to feed both voltages of the secondary windings to the voltage measuring inputs (V1, V2) and to specify one of them, depending on the busbar situation, to the voltage controller as the control variable.The voltages of side 1 and 2 of the three-winding transformer are simultaneously monitored. In the process, the voltage to be monitored can be automatically selected via the load current of both sides or by using one parameter. This parameter is the settings group switching via binary input, protocol, or function keys. The uncontrolled voltage can be monitored in parallel to ensure that it remains within the defined voltage limits.If a current measuring point is assigned to a function group, the voltage to be controlled can be automatically selected dependent on the load. For automatic load change, the voltage of the transformer side into which the larger load current flows is controlled. The respective uncontrolled voltage is monitored for undervoltage and overvoltage. In contrast to overvoltage on the controlled side, for overvoltage on the uncontrolled side, the higher impulse is blocked and no fast step down to a lower tap occurs. In case of undervoltage on the uncontrolled side, the lower commands are blocked if this behavior is activated for undervoltage supervision.
Figure 5: Voltage-Controller Constellation for Three-Winding Transformers with Current Measurement for Load Compensation at the End of the Line
Figure 6: Voltage-Controller Constellation for Three-Winding Transformers without Current MeasurementGrid Coupling Transformers Grid coupling transformers are special power transformers that connect 2 electrical power systems to one another. The load-side voltage is controlled. The power flow can change during operation. That is why both voltages and currents, winding 1 and winding 2, voltage measuring inputs (V1, V2) and current measuring inputs (I1 and I2) must be fed. A voltage must be specified as a function of the load situation to the voltage controller as the control variable. The voltages of the windings 1 and 2 of the grid coupling transformers are simultaneously monitored. In the process, you can select the voltage to be controlled using one parameter. This parameter can be changed using the settings group switching via the binary input, protocol, or function keys.
Figure 7: Voltage-Controller Constellation for Grid Coupling TransformersOn-Load Tap Changer On-load tap changers are used to set the desired tap of a stepped winding of the transformer while under load. During switchover, the desired tap of the stepped winding is first selected by means of the tap selector. Next, the on-load tap changer switches over from the current-carrying tap to the selected tap. During this switchover, one tap of the stepped winding is briefly bridged by an ohmic resistor so that the load switchover takes place without power interruption. Physically, the tap changer is installed in the transformer tank or in a separate tank. Tap ChangerThe Tap changer function issues the adjusting commands to the on-load tap changer and receives the corresponding feedback. The function of the tap changer inside the voltage controller corresponds to that of the separate tap changer. The higher and lower tap commands are generated by the voltage controller. Voltage ControllerThe Voltage controller function controls the voltage within the bandwidth and within the set voltage limits. Using a parameter, you can turn the voltage controller on or off or set it for test mode. Via parameter, you can specify up to 4 target voltages. You can only activate one of these target voltages at a time using function key, communication, or a binary input. With a parameter, you can specify a valid target voltage via a communication network.If the voltage controller is switched off, adjusting commands cannot be given to the tap changer in automatic operation or in manual operation. The set on-site or remote switching authority is independent of this. You can set the controller operating mode via parameter or command to automatic operation or manual operation. Three-Winding Transformer Voltage ControllerFor the two-winding transformer voltage controller, the measurands that are used to manage the control are permanently defined. With the three-winding transformer voltage controller, these measurands can be specified using one parameter or they can be automatically selected.The winding is automatically selected by evaluating the load current in the windings. If the load current in one of the two windings is greater than the load current in the other winding by 5 % of the rated current for 10 s, then the winding voltage is controlled using the larger load current.Grid Coupling Transformer Voltage ControllerFor the two-winding transformer voltage controller, the measurands that are used to manage the control are permanently defined. With the grid coupling transformer voltage controller, these measurands can be selected using a parameter You can change the winding selection during operation using the settings group switching.You can also monitor the power-flow direction. If the difference in the power of Winding 1 and Winding 2 is greater than 10 % of the rated power on the control side, the Power-flow supervision indication appears and the function switches to the alarm state.