On-Load Tap Changer - GE 845 Instruction Manual

SYSTEM

On-load Tap Changer

4–104
This section contains the settings to configure the tap position input. The 845 relay offers
tap position detection by monitoring either a resistive input from the tap changer control
circuitry, a dcmA analog input, or by Binary Coded Decimal (BCD) inputs. Based on the
detected tap, the 845 relay dynamically corrects for CT ratio mismatch resulting from the
changes of the voltage ratio of the transformer. Thus, the percent differential function of
the device can be set for greater sensitivity. See Section: Transformer Setup > Windings >
Magnitude Compensation > item 2. LTC compensation set to "Enabled", for more details on
the tap position input.
Tap detection using Resistive Input
A typical tap position input layout is shown in the figure: Tap position input. Minimum tap
position and minimum tap voltage correspond to the "TAP 1" label in the figure.
Figure 4-29: Tap position input
In this method of detection, the resistive output from the tap changer is connected to the
Ohms input on the relay. The maximum resistance supported by the Ohms input is 5.1kΩ.
The user defines the minimum and maximum tap, voltage increment per tap, and the
Ohms per tap.
Tap detection using dcmA analog input
In this method, the user selects Analog Input 1 dedicated for detection of DC milliamp
value corresponding to a tap position. To detect the tap, the relay measures the dcmA
current at the terminal of the Analog Input 1 and compares it to the corresponding dcmA
current computed for each tap.
Tap detection using BCD input
The Binary Coded Decimal (BCD) is another way of detecting tap position by using the
combination of six bits each driven by contact input. The user wires and selects the
contact input per bit number. Each combination of six bits corresponds to a certain tap
position. Refer to the following table: All BCD combinations supported by the relay:
Path:
Setpoints > Monitoring > Transformer > Tap Changer
TAP POSITION DETECTION
Range: Disabled, Enabled
Default: Disabled
845 TRANSFORMER PROTECTION SYSTEM – INSTRUCTION MANUAL
CHAPTER 4: SETPOINTS