Two transformers can be paralleled if they have the same phase displacement, equal turns ratios and similar short-circuit impedances. If they have on-load tap changers, their voltage regulators 90 must be coordinated: with different taps, a reactive current circulates between them that does not feed the load. The usual methods are master-follower, circulating current minimization and power factor, and the logic must know at all times which transformers are actually in parallel.
When two transformers can be paralleled
Connecting two transformers in parallel, with their primaries on one bus and their secondaries on another, increases capacity and allows one to be taken out without interrupting the load. For them to share the load without unwanted currents, the IEC 60076-8 application guide and practice agree on the conditions:
- Same phase displacement (same clock number of the vector group) and same phase sequence. With different displacements, the angle difference between secondaries produces a current comparable to a short circuit. Our vector group compensation tool helps read those clock numbers.
- Equal turns ratios at the operating tap. A ratio difference creates a voltage difference between secondaries and a circulating current.
- Similar short-circuit impedances. Load is shared in inverse proportion to impedance: the transformer with the lower impedance takes more load.
- Ratings not too different, so that the difference of impedances in ohms does not unbalance the sharing.
The condition that depends most on control is the second one: with an on-load tap changer, the ratio changes in operation.
Circulating current
With an on-load tap changer on each transformer, each voltage regulator (device 90) moves its tap changer to keep the bus voltage within its band. If each regulator acts on its own, both see the same bus voltage but there is no reason for them to end up on the same tap: one may raise and the other stay.
When the taps differ, the secondaries have different internal voltages and a current circulates between the two transformers, limited only by the sum of their impedances. Since those impedances are almost purely inductive, the circulating current is almost entirely reactive: one transformer delivers it and the other absorbs it. It does not feed the load, increases losses and takes up capacity. Worse, it can make the regulators run away: one raises and the other lowers to correct the same bus voltage, and the tap difference grows.
Paralleling control methods
IEEE C57.153, the guide dedicated to paralleling regulating transformers, describes several methods. The most used are:
| Method | Principle | When it suits |
|---|---|---|
| Master-follower | One regulator leads and the others copy its tap position | Identical transformers, same number of taps and same step |
| Circulating current minimization | Each regulator calculates the circulating current from the currents of all units and corrects its tap to reduce it | Different transformers, or with different taps |
| Power factor | Each regulator compares its power factor with that of the group and corrects the difference | Different transformers, with regulators that offer this method |
| Negative reactance | Line drop compensation with a reactance that reduces the tendency to run away | Simple schemes without communication between regulators |
In master-follower, the follower needs the master’s position and must detect when it fails to match it (a tap changer that does not respond), to block the group and raise an alarm. In the circulating current and power factor methods, each regulator needs the currents, or a derived measurement, of the others. Some manufacturers offer variants under their own names, such as power factor difference (ΔcosΦ); read the documentation of each regulator before mixing brands.
- Are the transformers identical, with the same number of taps and the same step?
- YesIs the tap position of each transformer available in the other's regulator?
- YesMaster-follower or circulating current
- NoCirculating current or power factor, depending on what the regulators exchange
- NoCirculating current or power factor; master-follower does not match the voltages
How the regulators communicate
There are two ways to carry the information between regulators:
- Hardwired: the tap position as a code or an analog signal, and the raise and lower commands from the master to the followers. For circulating current, a current loop or a manufacturer’s own bus.
- Over the substation network: with IEC 61850, IEC 61850-7-4 models the regulator with the ATCC node (automatic tap changer control) and the tap changer with YLTC. The regulators exchange over GOOSE the position, the currents or the measurements the method requires. Devices such as the SEL-2414 or the TAPCON support this coordination over Ethernet.
With GOOSE the same rule applies as in interlocking: if the message from the other regulator is lost or arrives with invalid quality, the regulator must not keep running in parallel on stale data. The usual response is to block automatic regulation of the group and raise an alarm.
What happens when the bus tie opens
The paralleling logic cannot assume the transformers are always together. If the secondary bus tie opens, or a transformer goes out of service, each transformer ends up on its own bus and there is no longer a circulating current between them. A follower that keeps copying a master that is no longer on its bus regulates one bus with the reference of another.
That is why the parallel group is determined from the positions of the breakers and disconnectors: the regulator must know which transformers share a bus at each moment and switch to independent regulation when the tie opens. It is the same information the interlocking uses, and it should come from the same source.
What goes wrong in practice
- Fixed parallel group that does not follow the real topology.
- Master-follower with different transformers, which leaves a permanent circulating current.
- Follower that does not detect its own failure and stays on a different tap without an alarm.
- Regulators of different brands with incompatible methods.
- Loss of communication without blocking, which leaves each regulator running on stale data.
How it is verified
Secondary injection checks the paralleling method for each combination of transformers, the detection of the bus tie opening, the blocking on tap discrepancy or loss of communication, and the correction of a simulated circulating current. On site, real load confirms that the circulating current stays within what is expected.
Do your parallel transformers show circulating current, or taps that drift apart? At Tiza Group we review and configure the paralleling logic of the regulators, hardwired or over IEC 61850, for substations and industry, remotely through our remote relay configuration service. Contact us with the brand and model of your regulators.
Related guides
- Substation interlocking: types, levels and logic
- Automatic switching sequences in substations
- Main-tie-main interlocking: secondary selective
- Automatic capacitor bank control at MV and HV
References
- IEC 60076-8, Power transformers – Application guide (parallel operation).
- IEEE C57.153-2015, Guide for Paralleling Regulating Transformers.
- IEC 61850-7-4, Compatible logical node classes and data object classes (ATCC, YLTC).
- IEEE C37.2, Standard for Electrical Power System Device Function Numbers, Acronyms, and Contact Designations (device 90).
Frequently asked questions
What happens if two parallel transformers end up on different taps?
The voltage difference between their secondaries drives a current between them, limited only by the sum of their impedances. It is almost entirely reactive, does not feed the load, heats both transformers and takes up part of their capacity.
Does master-follower work for different transformers?
It works well with identical transformers, with the same number of taps and the same step. With different transformers, matching the tap position does not match the voltages, and the methods that measure circulating current or power factor are preferred.
What should the regulator do when the bus tie opens?
It must stop treating the transformers as a group and regulate each one separately. To do that, the paralleling logic reads the position of the breakers and disconnectors; if it does not, a follower may copy a master that is no longer on its bus.