Mains electricity is the term used to describe the common household electric power supply in the United States, being 120 Volts (V) at 60 Hertz (Hz). Electric utilities are required to provide this supply base within certain tolerances per tariffs imposed by state utility commissions, which verify compliance with state regulations. For example, the Pennsylvania Utility Commission (PUC) verifies compliance with the Pennsylvania Code for Electric Service. Since voltages constantly fluctuate with varying electric loads on the electric system, utilities employ devices at the substation and on distribution lines, such as load tap changers, voltage regulators and capacitors, to help regulate the voltage that is provided to the customer. When determining proper secondary voltage of distribution transformers in substations that feed these lines, it is important to understand the substation application of 120V.Based on the PA code, the allowable variation in voltage measured at the service terminals of a residential customer may not exceed 5% above or below 120V. Therefore, the allowable voltage range for a customer is 114-126V. Since loading causes voltage drops along a distribution circuit, it would be ideal to set the secondary voltage of a substation distribution transformer as high as possible but still be in tariff, in that customers at the beginning of the circuit would receive 126V and customers at the end of the circuit would receive 114V. This variation is smoothed out thru the use of voltage regulators and capacitors throughout the circuit so that all customers receive as close as possible to its nominal voltage supply.
To show what the secondary voltage of the transformer is, an auxiliary potential transformer (PT) steps the secondary voltage down to 120V. In the example, the PT has a turns ratio of 8,400:120. If a substation inspector observed 120V on the voltmeter, the secondary voltage of the transformer would be at 8,400V. A reading of 115V would indicate that the voltage would be lower than 8,400V and a reading of 125V would indicate that the voltage would be higher than 8,400V. Therefore, the use of the 120V voltmeter is a substation application that indicates nominal voltage and should not be confused with the 120V that a customer would receive.
Rated voltage is stated on the transformer nameplate. It provides rated voltages for each setting of the de-energized tap changer (DETC), which changes the turns ratio of the transformer by either adding or subtracting turns of the primary winding. Set while the transformer is out of service, the DETC typically has five settings and shorts out more turns as the setting number increases. Therefore, the higher the DETC setting, the less stepping down of primary voltage to secondary voltage. The nameplate voltage of the transformer is taken from DETC position 3. In the example, a delta-wye distribution transformer has a nameplate voltage of 69,000V primary - 13,200Y/7,620V secondary. Its DETC was set for 5. Therefore, in effect, the transformer is rated for 65,600V primary - 13,200Y/7,620V secondary. By moving from 3 to 5, less turns are left in the primary winding, resulting in less stepping down of voltage.
The next thing to consider when determining proper secondary voltage at a substation is the utility's standard for the turns ratio in all pole-top, pad-mount and underground transformers, on a particular distribution circuit, that transform the distribution line voltage down to 120V for customer use. In the example, the distribution circuit's nominal voltage is 13,200Y/7,620V. Customers are connected phase to ground with transformers that are rated 7,620V primary - 240/120V secondary. Therefore, all the line transformers have a turns ratio of 7,620:120, which is different than the 8,400:120 for the PT at the substation. Since customers close to the substation can go up to 126V, an ideal secondary voltage at the substation transformer would be 8,001V, which would indicate 114V on the PT's voltmeter.
To help regulate the secondary voltage at the substation under varying loads, load tap changers (LTC) are employed on the secondary side of the distribution transformer at the substation. It is important that the LTC pass thru the neutral tap when loads change from peak to off-peak and from off-peak to peak, to allow for proper wiping of the contacts of the reversing switch. Therefore, protection engineers calculate the proper secondary voltage of a transformer at what would be needed at the neutral tap. The LTC uses the secondary of the PT to sense when it would need to make an adjustment.
The picture shows how to determine what DETC position should be used based on how a nominal 69kV subtransmission system is normally operated at and what secondary voltage would be needed for proper voltage regulation by the LTC.
Article by D Scrobe III
