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Significance of data acquisition to smart grid

Posted: 17 Mar 2015 ?? ?Print Version ?Bookmark and Share

Keywords:smart grid? Data-acquisition systems? electric power system? wattmeters? AC?

Today's image of a smart grid consists of an efficient, reliable, self-healing electricity distribution grid. To be successful, this grid must accommodate distributed resources while still generating energy needed by equipment such as massive fleets of electric vehicles. Because so much depends on this grid, it must function efficiently at all times. The typical, even normal, system faults and failures found with many complex systems simply can't be tolerated. Thus, the smart grid must automatically detect system faults and quickly isolate them for fast repair. Data-acquisition systems play an important role in keeping the electricity flowing.

Utilities worldwide are deploying smart grid devices that provide accurate, time-aligned information about loads which change constantly. To accurately collect electric-power data, voltage and current measurements must be simultaneously gathered for all power lines because utilities must understand the timing among phases and ensure maximum efficiency and uptime. The most demanding application is measurement of 3-phase power, which requires multiple, time-aligned analogue inputs for voltage and current measurements.

3-phase electric-power measurements
A 3-phase electric power system carries three phases of AC (alternating current) at the same frequency. Each phase is separated from the other by 120. Figure 1 shows the waveform of the 3-phase voltage.

Figure 1: A 3-phase power waveform. The three phases are alternating currents (AC) with the same frequency. Each phase is separated from the other by 120.

Figure 2 shows the three single phases configured in 4-wire wye or star connection. A 3-wire wye connection is exactly the same as a 4-wire connection, but without the neutral line. The neutral line (black in figure 2) that connects to the centre of the wye configuration system is used with an unblanaced load. If the loads are well balanced, meaning that the currents taken from each phase are equal, the phase currents cancel one another and the neutral line carries no current. Consequently, a 3-wire connection is commonly used for a balanced load. The advantage of losing the fourth wire is the cost of the copper wire.

Figure 2: In 4-wire wye configuration, the neutral line (black) is used when the load is not balanced.

Power is the product of voltage and current across a load. A wattmeter consists of a current meter and voltage meter used together to measure power. For a 3-phase, three-wire system, at least two wattmeters are required to measure the total power consumption, as shown in figure 3. Total power is the sum of the wattage on the two wattmeters.

Figure 3: A 3-wire wye system load. Total power is the sum of the wattage on the two wattmeters.

A short digression for the circuit analysis of figure 3 is worthwhile here.

Let the centre of the 3-phase load be the 0V reference. Then:

Therefore:


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