Downstream events are of utmost importance to users of high-speed transfer systems.
Transfer systems — electrical equipment with the ability to transfer a load from a primary to a secondary source — come in a variety of voltage levels, speeds, and configurations. Solid-state transfer systems, for example, offer sub-cycle transfers and occupy a definite place in medium-voltage distribution, while low-speed conventional systems provide an adequate and economical solution for loads less sensitive to momentary interruptions and voltage sags. But a significant need exists for another solution in the performance range between these alternatives. In this article, I'll introduce a 2-cycle system that uses fast mechanical circuit breakers as the means of transfer.
A 1999 survey of utility industry professionals, conducted by the Electric Systems Technology Institute, indicated that the most commercially acceptable transfer times should be in the range of 2 cycles, but always less than 7 cycles. The same survey indicated that the preferred method of transferring loads from a primary to a secondary source was a dead-bus transfer using conventional mechanical circuit breakers, but that approach did not achieve performance in the 2-cycle range. The 2-cycle mechanical transfer system described here fits well into this framework as a balance between performance (speed) and cost (see Fig. 1).
Mechanical transfer system controls are critical because they contribute to speed performance, as well as circuit protection. The medium-voltage power circuit breaker normally comes with some type of overcurrent relay to protect the load and downstream circuit. The control system of the electromechanical transfer scheme should start with this functionality.
Downstream events are of utmost importance to users of high-speed transfer systems. The control system must ensure that a downstream fault will not be transferred to the secondary source. To do this, use a fast directional indication (FDI) of fault data in the decision process to calculate the current direction. If the fault current is downstream, the system should lock out the transfer and issue a trip command to the circuit breaker connected to that source. Likewise, if the FDI shows the fault is upstream, the control system should command the circuit breaker to open, disconnect the load from its primary feeder, and connect it to the secondary feeder by closing the secondary feeder breaker.
The system voltage of primary and secondary sources also plays an important role in this scenario. In the case of upstream faults, the distance to the fault and the system impedance dictates voltage drop on the system. Therefore, you should use an under-voltage variable along with current direction in making the transfer decision.
If the fault is nearby, voltage will drop below the undervoltage setting quickly, and the transfer command will be issued. However, if the fault is farther away and the system impedance is high enough, the voltage may stay above the undervoltage setting long enough for the circuit breaker nearer the fault to have time to clear that fault. In this case, no transfer occurs.
You must carefully analyze the load's characteristics and ability to ride through a voltage drop on the system. You also should consider voltage swell as a power quality transfer variable, as well as the voltage on the secondary feeder. Obviously, there is no need to transfer to a bad secondary source or a dead bus.
Finally, make sure you compare the phase angle and frequency of the load bus and the secondary feeder. Phase-angle measurement becomes important when large motor loads are present. On loss of primary voltage, the motor becomes a generator and places a residual voltage on the load bus. Depending on the load size, the phase angle of the bus residual voltage can change quickly, compared to the phase angle of the secondary feeder. Transferring these types of loads with different phase angles will increase the motor current and can cause damage to the motors. By keeping the phase angles within limits, the transfer can take place with as little “bumping” of the motor as possible.
Fig. 2, on page 14, shows how the ratio of the maximum-to-rated motor current (I max/I rated) compares at a fast transfer of 38 ms vs. a residual voltage transfer of 80 ms. A short circuit was induced at the medium-voltage side of the transformer, simulating the maximum current using both fast- and residual-voltage transfers. In all cases, using a fast transfer scheme significantly reduced the motor current.
Typically, there are two ways to configure transfer schemes. Fig. 3 depicts the Main-Main configuration with a common load bus, while Fig. 4 depicts the Main-Tie-Main configuration with distributed loads.
In the Main-Main configuration, you must measure 3-phase current and voltage for primary and secondary sources. System studies of utility and industrial installations indicate that single-phase faults cause more than 60% of system faults. For this reason, all phases (A, B, and C) must be monitored for voltage and current.
Use of the voltage on the load bus compares the phase angle and frequency, so in this case, only one phase is needed. The Main-Tie-Main arrangement needs one additional voltage sensor to provide measurements from each side of the tie breaker.
Application of high-speed transfer systems also requires that you make sure the load functions properly during transfer to an alternate source. In the typical transfer scheme, a “dead time” (see Fig. 5, on page 17) exists as one source disconnects from the primary source and connects to the secondary source. The length of this dead time directly correlates to the ability of the load to operate properly before and after the transfer occurs.
The design of a high-speed mechanical transfer system contains several key elements: power circuit breakers that operate extremely fast, transfer switch control technology, and a switchgear platform that allows integration of the component technologies into a safe, compact enclosure that can be readily connected to a load bus.
Circuit Breakers. The speed at which the transfer takes place is largely dependent on the circuit breakers used in the system. While conventional transfer schemes include standard mechanical circuit breakers, the opening and closing speeds are too slow (usually 40 ms to 60 ms, with a total transfer time of about 100 ms) to achieve 2-cycle transfer performance.
You can achieve much faster operation by using a circuit breaker with a unique contactor operation method, (see photo). Large permanent magnets provide closing and opening forces, replacing springs and operating linkages. Opening and closing coils shift the magnetic flux and allow the permanent magnets to change the state of the primary contacts quickly, and the primary contacts then remain latched. This technology results in closing and opening speeds of 17 ms and 10 ms respectively, and total transfer times within 2 cycles. Furthermore, the endurance and reliability of the circuit breaker increases because there are no mechanical linkages.
Control Technology. Protection and control relays are used to measure load and fault currents, system voltage, frequency, and phase. The relay system function includes the additional transfer logic. Fig. 7, on page 18, shows a transfer switch relay that handles all normal protection functions as well as the transfer scheme functions and measurements. Front-panel operator interface is a convenient addition to the relay function for status monitoring. Multiple high-speed binary outputs allow noncritical load shedding during a transfer function. Communication with SCADA systems is possible over fiber optic links or RS-485 connections.
Metal-Clad Switchgear. Metal-clad switchgear houses the circuit breakers, control technology, and all other standard wiring and instrumentation. This platform (see Fig. 8, on page 20) offers such features as a wide range of ratings, insulated bus, primary compartment segregation, and drawout circuit breakers for visible circuit isolation and easier maintenance. Contemporary metal-clad equipment includes stacked circuit breakers (two-high construction), which allows configuration of the mechanical transfer switch with both circuit breakers in a single, compact transfer switch section connected to a load bus.
Using a power circuit breaker transfer system has several advantages. Most transfer systems currently available are limited to the Main-Main transfer configuration. This scenario requires transfer of the entire load from one feeder to another. Using power circuit breakers allows you to take advantage of the Main-Tie-Main configuration, where the load can be distributed between the two sources while transferring only the load of the troubled feeder — not the entire load on the troubled feeder. If noncritical loads exist, load shedding can reduce inrush and continuous load currents.
In addition, alternative solutions provide limited continuous and interrupting current levels. These systems typically rate at 600A continuous current and up to 25 kA interrupting current — inadequate levels for heavy industrial applications.
With power circuit breakers, you can increase the continuous and interrupting current levels to the full ratings of metal-clad switchgear. In addition, you will achieve all protection relaying without the use of separate upstream breakers. Relays monitor the fault current direction. In the case of a downstream fault, the system will interrupt the fault current without initiating a transfer of that fault to the alternate source. Best of all, you can switch to a fast-transfer functioning with only a marginal increase in the cost of mechanical switchgear.
New technology can be a major contributor to power quality problems. Electronic motor drives on critical processes, for example, have now made those processes susceptible to fluctuations in power. In many industrial applications and utility distribution substations, engineers can improve power reliability beyond the means of conventional switchgear and circuit breakers and without the high costs of solid-state transfer switches. They can do this by combining advances in control technology with extremely fast circuit breaker operation, and by integrating protection and transfer functions into metal-clad switchgear.
Richard Tyner is developing new distributed generation technologies as a senior research and development engineer with ABB Inc. He has been awarded two patents for circuit breaker control and system monitoring. Tyner has a BS degree from Francis Marion University.
A. E. Turner and E. R. Collins Jr., “The Impact of Power System Disturbances on AC-Coil Contactors, “Proceedings of the 1997 IEEE Textile, Film, and Fiber Industry Conference, Greenville S.C., May 1997.
E. R. Collins Jr. and A. Mansoor, “Effects of Voltage Sags on AC Motor Drives,” Proceedings of the 1997 IEEE Textile, Film, and Fiber Industry Conference, Greenville S.C., May 1997.
Ralf Krumm, “Network study of Motors connected to Transfer Systems,” ABB Power Distribution, Mannheim, Germany, 1999.