12592 Closed Transition Transfer

The issue of confidence in the control strategy of any power system that parallels with a utility-derived power source is a universal concern. The electric utility supplier in any such application must provide safety to its maintenance staff as well as continuity and reliability in electric service to its customers. For these reasons, utility companies are quite demanding of the synchronizing and paralleling control strategies that they will allow at the point of common coupling.

On the other side of this issue are deregulation and the increasing demand that automation of customer facilities places on continuity of electric service. Deregulation and increased automation, coupled with reductions in operations staffing, impose less intrusive load transfer strategies on electrical loads. Having suffered an interruption in the process upon loss of the utility power source, it is often costly to suffer a second disruption upon restoration to the utility service. Consequently, upon restoration of the critical load to the normal power source and for test transfer in either direction, closed transition transfer has become the preferred transfer control strategy. This transfer strategy is equally suitable for the DG scenario. The equipment is comprised of a power switching module and a control strategy.

The power switching module of the closed transition transfer switch is a dual operator type. The single operator type is the traditional open transition, double throw switch. When the operator is energized, both sets of contacts move simultaneously. In this operation, the closed contacts open before the open contacts can close. This is a binary device; in the steady-state condition, only one set of main contacts can be closed. In the dual operator switch, each set of main contacts has its own operator. This switch provides both closed and open transition transfer. Typical operation, when both power sources are available and synchronism exists, is for the open set of main contacts to close first, followed quickly by the opening of the (initially) closed main contacts. Because this transfer switch must provide for transfer from an inadequate source to an adequate source, it must also be capable of open transition transfer. In open transition transfer, the closed main contacts are opened before the open set is closed.

Regardless of the operating strategy, both switch designs should be compliant with the type test regimens of the CSA, IEC, and UL standards. The control module should be qualified to these same standards. Typically, control strategies must meet specified repetitive accuracies and stability over a temperature range of -5 to 40°C. The control strategies would include voltage and frequency sensing, time delays, and various other control functions; of interest to this discussion is only that strategy committed to closed transition transfer. At a minimum, the repetitive accuracy of the control strategy should be ± 1% of nominal at ambient temperature, and the stability should be ± 0.5% of setting across the temperature range.

In addition to measuring acceptability of the two power sources, the control strategy should also measure the difference between the sources in voltage, frequency, and phase angle. A typical operation sequence for closed transition transfer is to first determine acceptability of both power sources. Only when both sources are determined acceptable and have remained acceptable for a preset time delay should the strategy determine if the differentials are met. Only when the voltage difference between the two sources is less than 5%, the frequency difference is less than 0.2 Hz, and the phase angle crosses 5° (electrical) will the strategy initiate a closed transition transfer. Upon initiation of transfer, the open contacts are closed. After closure, the initially closed mains are opened. Timing for this type of closed transition transfer should be arranged to limit parallel operation of the sources to less than 100 ms.

Figure 12.7 shows a simplified logic flow chart of a suitable control strategy. Because the closed transition transfer strategy must be capable of both open and closed transition, both logic paths are illustrated in the figure. The path on the left is for closed transition transfer, the other is for open transition. Because the strategy is required to determine when either strategy is to be implemented, it must be capable of both open and closed transition. As an overview, closed transition transfer is initiated by restoration of the preferred source of power to acceptable values for the time set in the respective delay functions or by initiation of either the unit-mounted test switch or remote initiating contact. Open transition transfer is automatically initiated when the source to which the load is connected becomes unacceptable and the other source is determined to be acceptable.

Looking at Figure 12.7 and following the left logic path, assume that a closed transition transfer is called for and initiated. The strategy continuously checks the two sources of power to determine their acceptability. As long as they remain acceptable, the strategy will make the differential determination of voltage, frequency, and phase angle. Only when the sources are acceptable, the voltage difference is less than 5%, the frequency difference is less than 0.2 Hz, and the relative phase angle difference between the two sources is less than 5° (electrical) will the strategy initiate the transfer operation. When transferring the load to the normal (preferred) source, the strategy will close the CN contacts and initiate a timing function to permit an overlap time of no more than 100 ms. At the end of this time, the strategy will initiate opening of the CE contacts. (CN are the operator and main contacts for the preferred source, and CE are the operator and main contacts for the alternate source.)

The control strategy discussed herein assures the power system operators that paralleling will only occur when both sources are adequate and within a very narrow window of synchronism. Additionally, the control strategy includes automatic recovery should a malfunction occur during the closed transition transfer operation. The stability and repetitive accuracies cited are

Closed Transition Transfer

Opened Transition Transfer

Transfer Vu

EYes

Closed Transition Transfer

Lockout Controls

Opened Transition Transfer

Transfer to Emer.

Transfer to Norm.

Open CN

Open CN

Transfer to Norm.

Open CE

Close CN

Logic flow diagram for closed transition transfer of electric load; (left) closed transfer, (right) open transfer.

Close CE

recommended because these values meet both ANSI protective relay and the commercial standards mentioned.

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