M-0329B LTC Backup Control
The M-0329B LTC Backup Control prevents a defective LTC tapchanger control from running the voltage outside the upper and lower voltage limits and, in addition, will prevent the line drop compensator from raising the voltage too high under full load or overload conditions. Selectable bandcenter and deadband settings allow the M-0329B to match any transformer control. The M-0329B responds to a voltage excursion within 0.2 seconds and operates within ±1% voltage accuracy over a range of -40° to +80°C.
Prevents a defective LTC tapchanger control from running the voltage outside the upper and lower limits.
- Prevents the line-drop compensator from raising the voltage too high under full or overload conditions.
- Selectable bandcenter and deadband ranges allow the M-0329B to match any transformer control.
- Responds to a voltage excursion within 0.2 seconds.
- Operates within ±1% voltage accuracy
The M‑0329B Backup Control offers reliable voltage protection from both improperly set and malfunctioning Tapchanger controls. The most common voltage error in setting tapchanger controls occurs when values of Line Drop Compensation are set in the tapchanger control that result in unexpectedly high voltage at the transformer due to higher than anticipated load currents.
Modern digital tapchanger controls typically offer Upper and Lower Voltage Limits in the form of Block Raise and Block Lower Setpoints, and can provide runback, as in the M‑0329B. However, the M‑0329B is a stand‑alone, line voltage operated, analog device that keeps operating regardless of the condition of the main control’s processor and/ or internal power supply.
The M‑0329B has bandcenter and bandwidth settings, similar to the primary tapchanger control. In the majority of applications, the bandcenter should be set to the same numerical voltage value as the primary control. The bandwidth of the M‑0329B should be set to at least twice the numerical value of the bandwidth setting of the primary control. The band edges of the M‑0329B’s bandwidth are the Upper and Lower Voltage Limits, (also known as Block Raise and Block Lower), beyond which, the M‑0329B prohibits Raise and Lower commands from the primary control from energizing the tapchanger motor.
The M‑0329B also has a “Deadband” setting. This is a voltage band of 1, 2, 3, or 4 Volts, and this value is selected by setting dip switches on the side of the M‑0329B. The lower edge of the Deadband setting begins at the Upper Voltage Limit of the M‑0329B. The upper edge of the Deadband is referred to as the “Voltage Runback Threshold”. When the measured voltage exceeds this threshold due to load shedding or some other external event (the tapchanger is already blocked at this point), the M‑0329B can issue its own Lower command. How quickly this occurs is determined by the time delay setting on the M‑0329B, which is settable from 1 to 30 seconds. Once this Force Lower command is issued, the M‑0329B will not cancel the command until the measured voltage is below the upper edge of the Deadband (Voltage Runback Threshold).
If, for some reason, the voltage remains above the Voltage Runback Threshold (an inoperative tapchanger), the M‑0329B will time out for 180 seconds (3 minutes) and then close an Over‑Voltage Alarm contact. It may be helpful in setting the M‑0329B to adjust the bandwidth setting and Deadband such that the Voltage Runback Threshold matches the maximum allowed system voltage, so that the over‑voltage alarm corresponds with the actual maximum value.
It is also helpful to consider setting the M‑0329B bandwidth to accommodate the maximum amount of anticipated Voltage Reduction (if used). To accommodate the maximum voltage reduction and the maximum allowable voltage, it is sometimes necessary to skew the bandcenter setting of the M‑0329B to accommodate both. Consult the factory if you are unsure how to determine the value for a skewed bandcenter.