(1) Power distribution within the station shall be through a double ended motor control center. Pumps and other loads shall be divided as equally as possible between the two sections of the motor control center. Vital loads like lighting and control power shall be capable of being connected automatically through an auto-transfer switch to either half of the motor control center. Bus shall be copper.

For one-line diagram with one feed from the power company and standby emergency generator see Diagram 1 on Drawing DG-02-3. The tie breaker in this arrangement will be normally kept open.

For one-line diagram with two feeds from the power company, see Diagram 2 on Drawing DG-02-3. The tie breaker, in this arrangement, will be normally closed and one of the two incoming line main circuit breakers (selected as the normal or preferred source) will be closed. The two main circuit breakers shall be electrically operated and an automatic throw-over control shall be provided for automatic transfer to the second source, if the preferred source fails and for automatic return to the preferred source on restoration of the preferred source.

Generally, it is difficult to get two services at 480 volts which can be paralleled from power companies, especially for small loads, and hence the above arrangement is suggested.

It shall be possible by means of a selector switch to select either of the two lines as the preferred source.

The compartment for power company metering shall comply with the power company requirements.

A preferred motor control center layout is shown in Diagram 3 on Drawing DG-02-3. This layout will generally be the same for one feed from the power company and a standby generator or for two feeds from the power company, except for differences in the width and depth of the incoming sections because of the location of the power company metering CTs and PTs at the motor control center. (The meters are to be located outside the pumping station in both cases.)

The main incoming line circuit breakers shall be of the insulated case type with current monitors and solid state tripping devices with adjustable settings for continuous current, long time delay, short time delay, ground fault trip, and fixed instantaneous trip. All trip devices shall have trip indicators.

All circuit breakers shall have external handles which clearly indicate when breaker is “ON,” “OFF,” or “TRIPPED” and lockable in the “OFF” position.

The interrupting rating of all circuit breakers and bus bracing shall be adequate for the available fault currents.

For the pumps, one vertical section per pump, housing the branch circuit breaker, starter, individual pump controls, indicating lights, etc., is preferred. All indicating lights shall be of the push-to-test type.

The pump motor branch circuit breakers shall have auxiliary contacts for use in the control circuitry (see pump control schematics).

The lock-out relay (device 86) shall be of the hand reset type.

Size of the control power transformer in starter shall be coordinated with control power requirements, heaters, etc.

(2) Spare Parts and Special Tools. The following spare parts and tools shall be specified to be furnished with the motor control center:

(3) Acceptable manufacturers: Square D, Autocon, Allen Bradley.

(1) If the horsepower of the motors, total load and future possible growth indicate the need for 2,400V or 4,160V as the utilization voltage, a substation to step down the power company’s distribution voltage available in the area to the utilization voltage would be needed.

A power distribution one-line diagram of a 13.8kV to 2,400V or 4,160V substation and motor control center is shown in Drawing DG-02-4. One-line diagram of a substation with a primary voltage of 13.8kV is included here, as a distribution voltage of 13.8kV appears to be more common especially in areas where booster pumping stations are likely to be built.

The substation is to be located outside the pumping station at the most suitable location. Metalclad switchgear with drawout type power circuit breakers in outdoor walk-in enclosures is preferred as they are easy to install, are less susceptible to problems due to bad weather, and for aesthetic reasons.

The one-line diagram and the protective relaying scheme shown represents the preferred arrangement from a reliability point of view. This scheme assumes that two feeders can be paralleled on the secondary side of the transformers. Normal operation would be with both the primary breakers, both the secondary breakers and the secondary tie breaker closed and the automatic reclosing scheme in service. The tie switch on the primary side would be open.

In the one-line diagram, the transformer secondary main breakers and secondary tie breaker are shown to be located in the outdoor metalclad switchgear. This arrangement has two main advantages:

However, other physical arrangements like locating the secondary breakers at the motor control center have to be considered and the best arrangement depending on site conditions shall be chosen on a case-by-case basis.

The power company serving the area is to be consulted to ascertain if paralleling on the secondary side would be permitted. If paralleling will not be permitted, the scheme shown on Drawing DG-02-4 has to be suitably modified, incorporating an automatic transfer scheme satisfactory to the power company.

The power circuit breakers shall be DC close and DC trip. The primary breakers shall be provided with DC undervoltage release, so that in case DC voltage fails or falls below a safe value while the breaker is closed, the breakers would open. This coil should have as low a VA burden as possible.

An emergency trip pushbutton shall be provided at the motor control center inside the pumping station for remotely tripping both the incoming primary breakers. This trip pushbutton shall be protected from accidental operation, preferably by installing it in a recessed pocket.

The switchgear for each incoming line shall be in a separate enclosure and each line up shall be separated at least by six feet to minimize chances of common mode failure. The front aisle space in the walk-in enclosures shall provide sufficient space to permit the passage of two breakers side by side (not less than six feet).

The surface of the concrete pad for the switchgear and transformers shall be so sloped that water does not stagnate and cause deterioration of the bottom plates. Also, as the actual pad sizes and reinforcement (to some extent) may vary with the equipment actually furnished, coordination of pad sizes and design have to be by the contractor before pads are poured. These shall be clearly indicated on the drawings.

(2) Automatic Reclosing Scheme. An automatic reclosing scheme (if permitted by the power company) shall be designed to automatically reclose the incoming primary and the secondary breakers of either line up, on restoration of power in either line after the secondary and primary breakers trip on reverse power for a fault in the power company line. Reclosing shall take place only after a time delay (adjustable) after the power company successfully re-energizes the line. Reclosing shall be inhibited for any fault within the substation. Operation of any lock-out relay shall automatically change the control transfer device 43 to the manual position to prevent auto-reclosing when lock-out relays are reset. Auto reclosing shall also be inhibited if breakers are tripped manually.

(3) DC Battery System.

(4) Motor Control Center. The motor control center line up shall include incoming disconnect switches (quick make, quick break, load interruptor type) (or circuit breakers), bus tie switch (or circuit breaker), high voltage starters with draw-out type contactors and current limiting power fuses (or circuit breakers), relays, metering, etc., as indicated on the one-line diagram. Bus bars shall be copper.

The step-down transformer for station auxiliaries shall be capable of being fed from either side of the motor control center. The contactors shall be electrically interlocked so that only one contactor can be closed at one time. Controls shall provide for automatic transfer of the transformer feeder from one bus to the other, if voltage fails on the bus to which it is connected normally. Indicating lights for open (green) and closed (red) positions and open-close switches shall be provided for each contactor.

Motor protection shall include, as a minimum, thermal overcurrent, with instantaneous unit (49/50) ground overcurrent (50G) and phase sequence and undervoltage (47). Bearing temperature protection shall be included for motors 500 hp and above. Medium voltage motors rated 500 hp and above shall also be provided with lightning and surge protection. The surge arrestors and capacitors shall be located as close to the motor as possible, preferably in the terminal box, which has to be adequately sized for this purpose. Single phasing protection and mechanical interlocks to prevent the opening of the isolating switch when the contactor is closed and other required safety interlocks shall be incorporated in the contactor. Provision shall be made for plugging in an auxiliary test source for checking the operation of the contactor and the low voltage pilot circuits, when the contactor is in the test or drawn out position.

Sufficient space shall be provided in front of the motor control center for taking out the contactors.

One vertical section per pump housing the contactor, fuses, individual pump controls, indication lights, etc., is preferred. The common pump control and telemetering panels in free standing unitized sections shall form an integral part of the motor control center.