2. Disconnect the secondary phase conductors and secure them in the clear. Caution: Secondary phases are still energized. 3. Open the disconnects. Remove any risers. It is now safe to work on the transformer bank.
58 Distribution Transformer Handbook PHASING AND PARALLELING THREE-PHASE INSTALLATIONS Voltage measurements are taken to match phases, prior to connecting transformers in parallel. Phasing and Paralleling Circuits With a Field Neutral This illustration shows typical delta and wye circuits with field neutrals. The circuits could be secondaries or primaries. Either circuit at the left can be paralleled with either circuit at the right. The neutral is dashed to indicate the connection could be through the earth or a conductor. Typical circuits with a field neutral. Equipment required: Voltmeter (if paralleling secondaries) or phasing stick (if paralleling primaries) rated for twice the phase-to-phase voltage, or higher. Follow these steps when phasing and paralleling installations with a field neutral. Step Action 1. Measure each circuit for normal phase-to-phase voltages and phase-to-neutral voltages. 2. If the secondary is being paralleled to maintain temporary customer service while another transformer bank is being rebuilt, take load checks to be sure the bank remaining in service will not be overloaded excessively. 3. If there is not a continuous system neutral, measure for voltage between the neutrals of the two circuits. 4. If no voltage exists, or if a small voltage exists (5% or less), you may connect the two neutrals. C 1 N 1 B 1 A 1 C 2 N 2 B 2 A 2 C 2 N 2 B 2 A 2 C 1 N 1 B 1 A 1
Installing Transformers 59 Note: Make a sketch of the circuits, showing the proper connections. Then proceed. 5. Measure the voltage from A 1 to a phase on circuit #2 that gives a near-zero voltage reading (5% or less). This is A 2 . Check the voltage from A 1 to B 2 and from A 1 to C 2 . These should read normal phase-to-phase voltages. Note: On delta-connected, combination lighting and power banks, if you do not get the above readings, you have a transformer mid-tapped that is connected to different phases in each bank. An outage will be required on one bank to correct this condition. The primary or secondary connections may be altered to provide uniformity between the two banks. Note: When paralleling wye or delta banks, the phase angle must be the same. Phase angle differences between banks will be indicated by higher than required voltages during tests. Caution: When replacing a bank, the original phase sequence must be maintained to avoid damage to customer equipment. 6. Measure the voltage from B 1 to a phase in circuit #2 that gives a near-zero reading. This is B 2 . Check voltages from B 1 to A 2 and from B 1 to C 2 . These should read normal phase-to-phase voltages.
- Fall '19
- Transformer Concepts