Using Inverters to Back Up SCADA Systems in Power Substations

In today’s increasingly automated world, system controls must be highly reliable; and the more critical the resource is, the more foolproof the control system must be. For power utility substations, the need for continuous facility control under any circumstance demands that a failsafe electric supply be always available.

Utilities automate substations for obvious reasons, such as lower operating costs and enhanced reliability. This is why utilities have been upgrading their substations for years with ever-more sophisticated industrial control systems. Digital protective relays, remote terminal units, programmable controllers and high-speed computer systems utilizing SCADA (supervisory control and data acquisition) have all been part of this upgrade cycle.

In an automated substation, the reliability of these devices depends upon the reliability of the power being supplied to them. Substations use banks of batteries as their reliable source of power because the substation must operate even when AC power is lost. The battery voltages are 48, 125 or 250 VDC. The batteries are kept charged whenever AC power is available. In order to power the equipment for the automation process, the battery voltage must be converted to AC. The device that does this conversion is called an inverter. A battery and inverter system is similar to an uninterruptible power supply (UPS); but an UPS is not preferred because the substation supplies the batteries and charger and, therefore, the inverter is the device of choice. The power requirement for today’s substation electronic devices is approximately 860 watts. Allowing for contingencies and growth, an inverter should be able to supply between 1000 to 1200 watts. (Note: This is not VA but watts. Most units are rated in VA, with watts equal to about 0.65 VA. For all Behlman’s inverters, VA equals watts.)

There are three types of inverters. The first type is the on-line inverter that has an AC input (normally 120VAC at 60 Hz) and a DC battery input. The output always runs from an internal inverter that produces the AC output. If there is a loss of AC input, the batteries take over seamlessly. The second type is the standby (or off-line) inverter that has the same inputs as the on-line unit except that the AC input goes directly to the output via a bypass. Upon loss of the AC input, the output is switched to the internal inverter output supplied by the batteries. There is a short switchover time when there is no output. Usually the electronic devices have internal hold up so that this loss of input has no effect. This cannot be guaranteed and must be verified for each type of electronic device.

The third type is the DC battery input only. This unit supplies the AC power from an internal inverter that is always powered by the substation battery bank. The Behlman ACDC-1200 inverter is the on-line type. The Behlman INV-1200 is a battery-only type that can be configured as a standby unit. Both inverters supply a pure sine wave output. Behlman’s INV-1210 is identical to the INV-1200 except that the INV-1210 supplies a modified sine wave with peak and root mean square (RMS) equal to a sine wave.

Behlman’s low-cost inverters offer: rugged construction for use in a substation environment; proven reliability for long mean time between failure (MTBF); and small size for use in equipment racks.

Contact us for more information on how we can help you in this critical area.

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