Fast motor bus transfer: the three methods

Short answer

Motor bus transfer moves a bus with running motors from one source to another without stopping the process. IEEE C37.96 classifies open-transition transfers as fast, in-phase and residual voltage. All three aim at the same thing: closing the new source when the difference between it and the voltage generated by the motors will not produce torques or currents that damage them.

What motor bus transfer is

A medium-voltage bus feeding large motors can lose its normal source because of an upstream fault or a switching operation. If the load is essential, the bus must move to a backup source without the process stopping. The problem is that the motors keep spinning and, for a few moments, act as generators: the bus holds a residual voltage that decays in magnitude and frequency and drifts in phase relative to the new source.

Closing the new source onto that voltage is the same as connecting two systems out of synchronism. The vector difference between the two voltages appears across the motor impedances and produces currents and transient torques that can exceed rated torque several times, with cumulative damage to shafts, couplings and windings.

IEEE C37.96-2012, in clause 6.4, covers motor bus transfer and groups the open-transition methods into three: fast, in-phase and residual voltage.

Why the residual voltage rotates and decays

When the source opens, each motor keeps supplying voltage while the rotor flux dies away. The bus behaves like a group of machines slowing down together:

The magnitude falls with the open-circuit time constant of the motors.
The frequency falls according to the inertia of the motors and their loads.
The angle relative to the new source grows continuously because of the frequency difference.

On the phasor plane, the bus voltage traces a spiral towards the origin while the new source stays fixed. The difference vector between the two decides the stress of the reconnection. ANSI/NEMA C50.41 limits the resultant volts per hertz at the instant of reconnection to 1.33 pu, and that is the reference criterion against which each method is evaluated.

The three methods

Fast transfer. The new source closes in the first cycles after the loss, when the residual voltage is still high but the angle is still small. It requires the new source to be available and in phase with the old one before the event, fast breakers and logic that checks in real time that the angle and magnitude difference are still within the window. If the window closes before the breaker acts, the logic moves to the next method.

In-phase transfer. If a fast transfer was not possible, the logic measures the slip between the bus and the new source and predicts when the angle will next pass through zero. It sends the close command ahead by the breaker closing time, so the contacts touch with both voltages in phase. It is the same principle as the synchronized closing used to connect generators.

Residual voltage transfer. As a last resort, the logic waits for the bus voltage to fall below a low threshold, where the angle no longer produces significant stress. It is the slowest: the motors decelerate further, the subsequent reacceleration draws more current from the new source and some processes cannot tolerate it.

Which method completes the transfer

Is the angle and voltage difference inside the fast window when the command is given?
- YesFast transfer
- NoDoes the slip allow the next in-phase crossing to be predicted?
  - YesIn-phase transfer, with the close command advanced by the breaker time
  - NoHas the bus voltage fallen below the residual threshold?
    - YesResidual voltage transfer
    - NoWait, or block and shed load according to the project logic

The sequence fast, then in-phase, then residual, with automatic fallback from one to the next, is how most motor bus transfer relays work.

Simultaneous and sequential mode

Sequential: the old source opens and only when its contact confirms the opening is the close command given to the new one. There is never a parallel, at the cost of a few more cycles of interruption.
Simultaneous: both commands go out at once. The closing time of one breaker is usually longer than the opening time of the other, and the interruption is reduced almost to zero. If the old source does not open, two sources are left in parallel; the logic must detect it and open one of the two breakers within a short time.

There is also the closed-transition transfer, planned and with synchronism check, to move from one healthy source to another without interruption. It is covered in open versus closed transition transfer.

Where it is applied

Auxiliary systems of thermal power plants, where fans, feedwater pumps and mills cannot stop without tripping the unit.
Refineries, petrochemical and chemical plants, with compressors and pumps whose shutdown trips the process safety systems.
Mining, cement and paper, with mill and large fan buses.
Medium-voltage main-tie-main switchgear with motors, where the automatic transfer must wait for a safe condition before closing the tie.

What is studied before setting it up

Fast transfer is not set without a study of the bus. In general, the residual voltage decay is modelled with the actual motors and their load, the real breaker times are compared with the available window, the C50.41 criterion is checked in the worst cases and the reacceleration of the motors on the new source is reviewed, since it can cause a voltage drop similar to starting large motors. It is also coordinated with each motor’s 27, which must not trip during the transfer.

What goes wrong in practice

The transfer always ends in residual because the breaker times are longer than the fast window, or because the backup source was already out of phase before the event.
Late in-phase closures because the breaker closing time differs from the set value; it changes with temperature and ageing.
Motors tripped by undervoltage before the bus transfers, which turns the fast transfer into a partial shutdown.
Uncontrolled parallel in simultaneous mode when the old source did not open and the supervision logic did not act.
Transfers started by faults on the bus itself, which the logic must block as in any automatic transfer.

How it is tested

In general terms, a test set with two independent voltage systems simulates the bus collapse and its slip, and each window, the fallback from one method to another, the blocking and the total times with the real breakers are checked. At commissioning, real transfers are recorded with oscillography to compare the closing angle and voltage with the study. Synchronism check 25 shares much of the test logic.

We can help with your plant’s transfer

At Tiza Group we do the pre-commissioning study of the motor bus, checking the transfer windows, and the remote relay configuration of the logic in the transfer relay, for substations and industry. If a transfer ended in a shutdown or in tripped motors, we can quote the event analysis.

References

IEEE C37.96-2012, Guide for AC Motor Protection, clause 6.4 (motor bus transfer).
ANSI/NEMA C50.41, Polyphase Induction Motors for Power Generating Stations.
IEEE PSRC, Motor Bus Transfer Applications Issues and Considerations, working group report.
IEEE C37.2, Standard for Electrical Power System Device Function Numbers, Acronyms, and Contact Designations.

Frequently asked questions

Why isn't it enough to wait a fixed time before closing the new source?

Because the voltage generated by the motors decays and rotates differently under each load condition. A fixed time can land exactly when the angle difference is at its maximum. The fast and in-phase methods measure the actual voltage and angle; the residual method waits until the voltage is low enough for the angle to stop mattering.

What is the difference between simultaneous and sequential mode?

In sequential mode the close command to the new source is given after the opening of the old source is confirmed. In simultaneous mode both commands go out at once, relying on the new breaker taking longer to close than the old one takes to open; it is faster, but if the old breaker does not open the two sources are paralleled and the logic must detect and correct it.

Is fast bus transfer suitable for any plant?

It is suitable where there are large motors whose shutdown is costly: thermal power plants, refineries, petrochemicals, mining, cement and paper. In plants with mostly passive load or small motors a slow transfer is usually enough. The decision comes from a study of the bus with its actual motors.