It is recommended that the I-Trap™ be installed as close as possible to the harmonic producing loads on the three phase system. This location would typically be on the line side of a distribution panel that feeds the harmonic producing equipment.
A measurement of current flowing in the neutral conductor from the load to the supply transformer will need to be taken with an RMS meter.
The vector addition of three phase fundamental currents will add to zero on a balanced system without harmonics. However, if triplen* harmonic currents are present, they will add in the neutral.
With high concentrations of non-linear loads, the neutral current can be as high as two times the line current.
* Harmonics which are odd multiples of three (3rd, 9th, 15th, 21st, etc.). Odd triplen harmonics add together in the neutral conductor
The I-Trap™ is a version of a Zig-Zag autotransformer that has been designed to provide low zero sequence impedance.
An example of an everyday application is always a good way to explain the intent of the “Code.”
Example: A 1 kVA transformer Catalog No. T111683 has a primary of 120 x 240V and a secondary of 12 x 24V. It is to be connected as an autotransformer at the time of installation to raise 208V to 230V single phase.
When this 1 kVA unit is connected as an autotransformer for this voltage combination, its kVA rating is increased to 9.58 kVA (may also be expressed as 9,580 VA). This is the rating to be used for determining the full load input amps and the sizing of the overcurrent protect device (fuse or breaker) on the input.
When the full load current is greater than 9 amps, the overcurrent protective device (usually a fuse or nonadjustable breaker) amp rating can be up to 125 percent of the full load rating of the autotransformer input amps.
The National Electrical Code, Article 450-4 (a) Exception, permits the use of the next higher standard ampere rating of the overcurrent device. This is shown in Article 240-6 of the N.E.C.
An autotransformer is a transformer in which the primary (input) and the secondary (output) are electrically connected to each other.
An isolation transformer, also known as an insulating transformer, has complete electrical separation between the primary (input) and the secondary (output).
This is illustrated in the drawing below:
An autotransformer changes or transforms only a portion of the electrical energy it transmits. The rest of the electrical energy flows directly through the electrical connections between the primary and secondary. An isolation transformer (insulating transformer) changes or transforms all of the electrical energy it transmits. Consequently, an autotransformer is smaller, lighter in weight, and less costly than a comparable kVA size insulating transformer.
Please refer to this question for additional information on autotransformers.
Buck-boost transformers are frequently field-connected as auto transformers.
The National Electrical Code Article 450-4 addresses overcurrent protection of autotransformers. A copy is reproduced below for easy reference.
450-4. Autotransformers 600 Volts, Nominal, or Less.
- (a) Overcurrent Protection. Each autotransformer 600 volts, nominal, or less shall be protected by an individual overcurrent device installed in series with each ungrounded input conductor. Such overcurrent device shall be rated or set at not more than 125 percent of the rated full-load input current of the autotransformer. An overcurrent device shall not be installed in series with the shunt winding (the winding common to both the input and the output circuits) of the autotransformer between Points A and B as shown in diagram 450-4.
- Exception: Where the rated input current of an autotransformer is 9 am peres or more and 125 percent of this current does not correspond to a standard rating of a fuse or non-adjustable circuit breaker, the next higher standard rating described in Section 240-6 shall be permitted. When the rated input current is less than 9 amperes, an overcurrent device rated or set at not more than 167 percent of the input current shall be permitted.
- (b) Transformer Field-Connected as an Autotransformer. A transformer field-connected as an autotransformer shall be identified for use at elevated voltage.