Current Affairs: A Better Meter Made
MORE THAN JUST A METERA major development in building energy management was replacement of electro-mechanical W/hr meters with inexpensive electronic me-ters permitting cost-effective submetering. Today, microprocessor-based devices provide so much sophistication that the term commonly em-ployed is power monitoring, rather than metering.
Modern power monitors or circuit monitors measure basic parameters such as voltage, current, and kW, but also have additional capa-bilities including alarm logging, power quality measurement, data storage, oscillography, and waveform capture. Provided with input/output mod-ules and various communication options, these devices can be networked together into a dedicated supervisory control and data acquisition (SCADA) system.
Human machine interface (HMI) software running on a PC can present the information available on graphical screens as well as provide the same alarm reporting, trending, and data historian functions that are common to BMS.
INTEGRATIONAlternatively, devices or groups of devices can communicate directly with the BMS, eliminating the separate network and allowing the in-formation to be used in energy management algorithms such as demand limiting or chiller optimization routines. While most devices were de-veloped for use on proprietary SCADA networks, open protocols are common today with both Modbus and Ethernet options available from most manufacturers.
As power monitoring technology is being pushed downward into devices such as circuit breaker trip units and motor protection relays, protective functions and tripping capability are being incorporated into circuit monitors, and the line between metering and protection is becoming blurred. In the power system of the future, meters, protective relays, circuit breakers, motor starters, and drives may all be interconnected to allow a detailed picture of system performance and energy usage.
ADDING LOADSAn advantage of incorporating power monitoring in circuit breaker trip units is the ability to track load on individual circuits. The National Electrical Code (NEC) permits adding load to an existing feeder that has spare capacity on the basis of actual loading only if either one year of demand history or a continuous 30-day recording during the maximum usage period is available. If data is not available, the available capacity must be calculated on a connected load basis, which is conservative and may result in higher cost for new feeder capacity.
The cost of adding power monitoring to feeder circuit breakers may be saved many times over where frequent modifications and addi-tions would otherwise require the installation and subsequent removal of temporary meters.
POWER QUALITYPower quality (PQ) refers to the fact that electronic loads, are sensitive to, and can cause, power system disturbances more complex than simply the absence of voltage. Under the PQ umbrella, we include over- and under-voltage (sustained), voltage surges and sags (transient), frequency variation, noise, and harmonic distortion. Power monitors measure, log, and alarm PQ parameters, and record disturbance data that can be helpful in locating the source. Many devices sample and store AC voltage and current waveforms just prior to, during, and after the event. These often contain clues to the nature and origin of the disturbance.
With external disturbances, time stamping and waveform data can help the utility identify the source if it resulted from action on their sys-tem. Time correlation of events to the operation of other equipment through the BMS can identify the source of internal disturbances, such as voltage dips on starting large motors. We use the harmonic monitoring capability of power monitors to verify that buildings with large numbers of drives or uninterruptible power systems (UPS) are within acceptable limits of harmonic distortion.