Measurement and technology II Electric power measurement current

Measurement and technology II.

Electric power measurement

current division voltage division

Instantaneous power: depending on time Active power: The time average of the intantaneous power. It gives amount of dissipated energy Effective values! Apparent power Reactive power P/P_A called power factor, cos(fi)

The above has presented the power definitions applied to ac circuits with the restrictions of sinusoidal quantities. In the most general case of distorted quantities, obviously symbolic representation can no longer be applied. In any case, active power is always defined as the mean power dissipated in one period. As far as methods and instruments for ac power measurements are concerned, some circuit classification is required. In fact, the problems are different, arising in circuits as the frequency of power supply increases. Therefore, in the following, ac circuits will be classified into (1) line-frequency circuits, (2) lowand medium-frequency circuits (up to a few megahertz), (3) and (3) high-frequency circuits (up to a few gigahertz). Line-frequency circuits: related specifically to the three-phase power supply

Three voltmeter method

Because of the load, a delta e voltage imbalance arises proportional with the active power of the load

Wattmeters Based on Multipliers The multiplication and averaging processes (Figure 3. 8) involved in power measurements can be undertaken by electronic means. Electronic wattmeters fall into two categories: analog (continous) or discrete In continuous methods, multiplications are mainly carried out by means of analog electronic multipliers. In discrete methods, sampling wattmeters take simultaneous samples of voltage and current waveforms, digitize these values, and provide multiplication and averaging using digital techniques. Errors: 1. resistances of the voltage and current circuits have to be taken into account 2. phase errors of both current and voltage circuits increase the relative uncertainty of power measurement, 3. sinusoidal conditions.

The main analog multipliers are based on a transistor-based popular circuit such as Four-quadrant multiplier [3], which processes voltage and current to give the instantaneous power, and an integrator to provide the mean power (Figure 3. 9) Time Division Multipliers (TDMs) Hall effect-based multipliers.

Discrete utilities Approx. periodic time synchronous sampling

Line applications

Result: sum of P 1, P 2, P 3

High frequency methods

Pulse power measurement