Load Tap Changing Control

The control used for the tapchanger of an LTC transformer or step-voltage regulator is a prime illustration of electronic technology advances applied to the power industry. Electro-mechanical (balance-beam and induction disc), discrete static, integrated circuit static and now digital designs have evolved over the past forty years. This increased product sophistication has made possible many features beyond the primary objective of simple raise-lower commands in response to the measured secondary voltage. BASIC CONTROL This very basic LTC control, as routinely provided and as required by IEEE/ANSI standards, requires five basic setpoints. Three of these are used with every installation. Voltage Level (bandcenter) This is the voltage, spoken in terms of the 120 V basis of the control, which is desired to be held at the load. The load will usually be removed from the transformer, in which case the electrical distance to the load is defined by the line drop compensation settings. Simplistically, if LDC (see below) is set to zero, the voltage level is the output voltage of the LTC transformer. It is commonly required that the voltage at the load be held in the range of 114 V to 126 V although particular utilities may impose far more stringent criteria. Voltage Bandwidth Due to the voltage step change nature of the LTC output, there must be some range of voltage about the voltage level setting which is acceptable to the control and will be recognized by the control as being “in-band.” Thus, the bandwidth, also expressed in volts on the 120 V base is the total voltage range, one-half of which is allowed above, and one-half below the voltage level setting. There is always a minimum acceptable bandwidth setting, usually considered to be twice the voltage change per LTC step change, or 1.5 volts in the common system. In fact, 2.0 and 2.5 volts are the most common settings with 3.0 V and higher values being used where tight regulation is not required. Time Delay An intentional time delay is always included so as to avoid tapchanger operations when the voltage excursion outside of the bandwidth is of short duration. A good example case is that of a large motor starting on the system. The voltage level may be pulled low, but will be expected to recover in perhaps 15 seconds. To have made a raise tapchange for the short period would not significantly help in motor starting, and would require consecutive lowering operations after the motor came to speed, with attendant accelerated wear of the tapchanger. Consequently, the intentional delay is set, most often in the range of 30 to 60 seconds. Figure 1 depicts the voltage on the substation bus, VB, as a function of time without regard to line drop compensation. The tolerable voltage band is that within VBANDCENTER ±0.5 VBANDWIDTH. In time, the bus voltage will drop below the bandedge, at which time the time delay is initiated. At the completion of the established time delay period, the control delivers a command to the tapchanger drive motor. Tap changer action causes a step change of the voltage, bringing the voltage level “in-band.” Line Drop Compensation There are usually two settings associated with line drop compensation which provide the means to individually program the control to compensate for one—the resistive, and two—the reactive voltage drop on the line between the transformer and the load location. The difficulty in the use of LDC is that there is seldom a realworld situation applicable to the classical illustration of LDC application, i.e. a case where there is one distribution feeder of appreciable length, which is terminated in the only load for that feeder. In spite of this, LDC is often used with the recognition that the system may not be ideally suited for it.