Introduction

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.