Understanding Power Measurements...
The threatened limitations of conventional electrical power sources have focused a great deal of attention on power, its application, monitoring and correction. Power economics now plays a critical role in industry as never before. With the high cost of power generation, transmission and distribution, it is of paramount concern to effectively monitor and control the use of energy.
The electric utilities primary goal is to meet the power demand of its customers at all times and under all conditions. But, as the electrical demand grows in size and complexity, additions and modifications to existing electric power networks have become increasingly expensive. Electric power measuring and monitoring have become even more critical because of down time associated with equipment breakdown and material failures.
For economic reasons, electric power is generated by utility companies at relatively high voltages (4160, 6900 and 13,800 volts are typical). These high voltages are then reduced at the consumption site by step-down transformers to lower values which may be safely and more easily used in commercial, industrial and residential applications.
Personal and property safety are the most important factors in the operation of electrical system operation. Reliability is the first consideration in providing safety. The reliability of any electrical system depends upon knowledge, preventive maintenance and subsequently the test equipment used to monitor that system.
Typical Voltage ConfigurationsSingle Phase Systems
Single-phase residential loads are almost universally supplied through 120/240 V, 3-wire, single-phase services. Large
appliances such as ranges, water heaters and clothes dryers are supplied at 240 V. Lighting, small appliances and outlet receptacles are supplied at 120 V. In this system the two "hot" or current carrying conductors are 180 degrees out-of-phase with respect to neutral.Three-Phase, 3-Wire Systems
In this type of system, commonly known as the "DELTA" configuration, the voltage between each pair of line wires is the actual transformer voltage. This system is frequently used for power loads in commercial and industrial buildings. In such cases, service to the premises is made at 208 V, three-phase. Feeders carry the power to panel boards supplying branch circuits for motor loads. Lighting loads are usually handled by a separate single-phase service. The 480 V distribution is often used in industrial buildings with substantial motor loads.
Three-Phase, 4-Wire Systems
Known as the "WYE" type connection, this is the system most commonly used in commercial and industrial buildings. In office or other commercial buildings, the 480 V three-phase, 4-wire feeders are carried to each floor, where 480 V three-phase is tapped to a power panel or motors. General area fluorescent lighting that uses 277 V ballasts is connected between each leg and neutral; 208Y/120 three-phase, 4-wire circuits are derived from step-down transformers for local lighting and receptacle outlets.
phase-to-phase = 208/480 V
A balanced load is an AC power system using more than two wires, where the current flow is equal in each of the current carrying conductors. Many systems today represent an unbalanced condition due to uneven loading on a particular phase. This often occurs when electrical expansion is effected with little regard to even distribution of loads between phases or several nonlinear loads on the same system.RMS vs. Average Sensing
The term rms (root-mean-square) is used in relation to alternating current waveforms and simply means "equivalent" or "effective", referring to the amount of work done by the equivalent value of direct current (DC). The term rms is necessary to describe the value of alternating current, which is constantly changing in amplitude and polarity at regular intervals. Rms measurements provide a more accurate representation of actual current or voltage values. This is very important for nonlinear (distorted) waveforms.
Until recently, most loads were "linear"; that is, the load impedance remained essentially constant regardless of the applied voltage. With expanding markets of computers, uninterruptible power supplies and variable speed motor drives, resulting nonlinear waveforms are drastically different.
Measuring non-sinusoidal voltage and current waveforms requires a true rms meter. Conventional meters usually measure the average value of the amplitudes of a waveform. Some meters are calibrated to read the equivalent rms value (.707 x peak); this type calibration is a true representation only when the waveform with a pure sinewave (i.e., no distortion). When distortion occurs, the relationship between average readings and true rms values changes drastically.
Only a meter which measures true rms values gives accurate readings for a non-sinusoidal waveform. Rms measuring circuits sample the input signal at a high rate of speed. The meters internal circuitry digitizes and squares each sample, adds it to the previous samples squared, and take the square root of the total. This is the true rms value.
The amount of electrical energy consumed over time is known as the demand. Demand is the average load placed on the utility to provide power (KILOWATTS) to a customer over a utility-specified time interval (typically 15 or 30 minutes). If demand requirements are irregular, the utility must have more capability available than would be required if the customer load requirements remained constant. To provide for this time varying demand, the utility must invest in the proper size equipment to provide for these power peaks. Brief high peaks such as those present when large equipment initially comes on line are not critical in the overall equation because the duration is short with respect to the demand averaging interval.
Watts and vars are instantaneous measurements representing what is happening in a circuit at any given moment. Since these parameters vary so greatly within any time period, it is necessary to integrate (sum) electrical usage over time. The fundamental unit for measuring usage is the watthour (Wh), or more typically the kilowatthour (kWh). This value represents usage of 1000 watts for one hour.
Content for this article was supplied by AEMC Instruments.