Technical Article

Buck-Boost Transformer Operations and Limitations

June 04, 2021 by Alex Roderick

A buck-boost transformer is a small transformer designed to buck (lower) or boost (raise) line voltage. The major advantages of buck-boost transformers are their low cost and small size in comparison to general-purpose transformers.

Buck-boost transformers are typically used to make relatively small changes to a source voltage. The source voltage may need to be boosted to raise it or may need to be bucked to lower it to the needed value. Buck-boost transformers are typically wired as autotransformers. Buck-boost transformers generally are not used to boost voltage where there is a fluctuating load because the voltage may become very high when the load is reduced.

 

Operating a Buck-Boost Transformer

A buck-boost transformer is a small transformer designed to buck (lower) or boost (raise) line voltage. The major advantages of buck-boost transformers are their low cost and small size in comparison to general-purpose transformers.

Typical buck-boost transformers have dual-voltage primaries and secondaries. Buck-boost transformers are wired as autotransformers with the primary and secondary wired in series (see figure 1). A buck-boost transformer is usually used to change the voltage by about 5% to 25%. Common primary voltages are 120V, 240V, or sometimes 480V. Buck-boost transformers are typically available in sizes up to about 10kVA.

 

Figure 1. Buck-boost transformers reduce or increase the source voltage because the primary and secondary are wired in series with each other.
Figure 1. Buck-boost transformers reduce or increase the source voltage because the primary and secondary are wired in series with each other.

 

Note

Buck-boost transformers can be connected in parallel to supply a larger load than the rating of the individual transformers. The terminals must be connected in phase for like polarity, and they all must have the same turns ratio. The individual transformers share the load in inverse proportion to their impedances. The connection should be checked for current circulating between the individual transformers. Any significant current indicates a mismatch between the transformers. Individual overcurrent protection is required.

A common type of buck-boost transformer has the transformer primary rated at 120V × 240V and the secondary rated at 12V × 24V. This type of rating means that each of the two individual coils in the primary is rated at 120V. When the primary coils are wired in series, a source of up to 240V may be applied. When the coils are wound in series, this allows only 120V to drop across each coil.

The voltage connected across the primary should never be allowed to exceed the rating of the coils. When the primary coils are wired in parallel, no more than 120 V may be applied across them.

 

Note

An autotransformer may also be known as a compensator or balancing coil.

The amount of voltage available at the secondary to buck or boost the source depends on the voltage dropped across the primary. The polarity of the voltage that causes current to flow from H1 to H2 in the primary results in current flow from X1 to X2 in the secondary. Buck-boost transformers can be wired with subtractive polarity to buck the source voltage or with additive polarity to boost the source voltage.

For example, if the source is 240V and the primary is wired in series, each primary coil drops 120V. Each secondary coil then has 12V induced in it. The secondary coils can be wired in subtractive polarity to buck a total of 24V to bring the voltage down to 216V (Figure 1(a)).

The secondary coils can also be wired in additive polarity. If the source is 210V and the primary coils are wired in series, each primary coil drops 105V. Each secondary coil then has 10.5V induced in it. The voltage across each secondary coil adds to the primary to boost a total of 21 V to bring the voltage up to 231V.

 

Fusing

According to NEC® Section 450.4, Autotransformers 600V, Nominal, or Less, each buck-boost autotransformer 600 volts, nominal, or less shall be protected by an individual overcurrent device installed in series with each ungrounded input conductor. The overcurrent protection device cannot be installed in series with the shared winding of the autotransformer (see Figure 2).

 

Figure 2. A fuse is not allowed between the H1 and H4 taps of the primary.
Figure 2. A fuse is not allowed between the H1 and H4 taps of the primary.

 

For autotransformers, the NEC® makes a distinction between circuits with a rated input current of less than 9A and circuits with a rated input current of 9A or more. When the current is less than 9A, an overcurrent protection device is allowed to be rated at not more than 167% of the input current. When the current is 9A or more, the overcurrent protection device is allowed to be rated at not more than 125% of the full-load current of the autotransformer. When the current is 9A or more, and the calculated fuse rating does not correspond to a standard fuse rating, the next higher standard fuse rating is permitted. Standard fuse ratings are described in NEC® Section 240.6 Standard Ampere Ratings.

 

Grounding

NEC® Section 450.5, Grounding Auto-transformers, covers the provisions for grounding autotransformers. Section 450.5(A) lists four general provisions for grounding autotransformers used to create 3-phase, 4-wire circuits from 3-phase, 3-wire distribution systems (see figure 3). This section covers the provisions for point of connections, overcurrent protection, transformer fault sensing, and rating. 

 

Figure 3. Autotransformers must have a direct connection to the ungrounded phase conductors.
Figure 3. Autotransformers must have a direct connection to the ungrounded phase conductors.

 

Connections

An autotransformer may not be switched or provided with overcurrent protection that is independent of the main switch and overcurrent protection for the 3-phase, 4-wire system. The autotransformer must be directly connected to the ungrounded phase conductors.

 

Overcurrent Protection

An overcurrent protection device must be provided to cause the main switch or common overcurrent protection device to open if the load reaches 125% of its rating. However, delayed tripping is permitted to allow time for the common overcurrent protection device to open on branch or feeder circuits.

 

Fault Sensing

A fault-sensing system is required to guard against single-phasing or internal faults in 3-phase, 4-wire systems. The fault-sensing system can consist of current transformers wired to sense when an unbalance of 50%, or more is present in the line current (see figure 4).

 

Figure 4. NEC ® Section 450.5 requires fault sensing in 3-phase, 4-wire systems to guard against single-phasing or internal faults.
Figure 4. NEC ® Section 450.5 requires fault sensing in 3-phase, 4-wire systems to guard against single-phasing or internal faults.
 

Rating

The autotransformer must be rated for the highest possible unbalanced load current on the neutral of the 4-wire system.

 

Note

A buck-boost transformer with two primary and two secondary windings can be connected in many different ways to provide a wide range of possible buck or boost voltages. This provides the flexibility required from a buck-boost transformer.

 

Limitations

There are several common limitations of buck-boost transformers. Buck-boost transformers should never be used to buck or boost a source supplying varying loads and should not be used to convert a 208Y/120V source to a 240V/120V single-phase service. NEC limits the types of circuits where buck-boost transformers can be used. There are also limitations caused by the required overcurrent protection.

 

Fluctuating Loads

Buck-boost transformers are often used to adjust the voltage at the end of long transmission lines. However, buck-boost transformers should not be used to correct the voltage drop when the load fluctuates. The voltage drop on the line varies with load. When buck-boost transformers are designed and installed for high load conditions, unexpected high voltages may result during lightly loaded conditions.

 

240V/120V Service

Buck-boost transformers should not be used to create a 240 V/120 V single-phase service fed by a 208Y/120 V 3-phase, 4-wire source. If this is done, two neutrals would exist on the same circuit. This wiring arrangement creates unbalanced line-to-neutral voltages where one line is at 120 V, and the other line is greater than 130 V. This connection is only permitted for creating a 240 V, 2-wire system. The impedance of the transformer is the only limit to the current of the unbalanced neutral.

 

NEC Section 210.9

Buck-boost transformers must not be used to buck or boost three units in a wye for making a 3-phase, 4-wire wye circuit from a 3-phase, 3-wire delta circuit. The neutral created by this connection does not give proper line-to-neutral voltages under load. This application uses three buck-boost transformers in a 3-phase wye connection. This connection violates NEC® Section 210.9, Circuits Derived from Autotransformers. This wye connection can be used for 3-wire to 3-wire, 4-wire to 3-wire, and 4-wire to 4-wire applications.

 

Overcurrent Protection

If the fuse in the transformer primary blows, the secondary is still in series with the load across the supply. The secondary becomes a reactor in series with the load and drops the voltage at the equipment to a dangerously low level. If the overcurrent device on the feeder does not open, the motor or the secondary could be damaged. The voltage drop across the secondary could rise to a high level, and the additional current could destroy the winding.