Electricity: measuring and testing – Measuring – testing – or sensing electricity – per se – Plural inputs
Reexamination Certificate
2000-06-05
2004-12-07
Deb, Anjan K. (Department: 2858)
Electricity: measuring and testing
Measuring, testing, or sensing electricity, per se
Plural inputs
C324S141000, C324S076120, C702S061000
Reexamination Certificate
active
06828771
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention concerns improvements relating to electrical power measurement and more particularly, though not exclusively, to a method of measuring an electrical power parameter, such as Apparent Power or Power Factor, which is accurate for single and multiple phases under all conditions namely those of balanced and unbalanced loads, and distorted and non-distorted waveforms.
The quantities that have conventionally been used to measure the quality and quantity of electric power are generally considered to be true and accurate when power is supplied to the load in a sinusoidal waveform and the load bears equally on each phase of the supply. The parameters that are widely used in power measurement are Active Power (P), Apparent Power (S), Reactive Power (Q) and Power Factor (PF).
Active power, when integrated in time, is used for billing purposes and reflects the energy consumed. Power Factor in AC circuits is a convenient figure of merit representing the utilisation of the supplying system by consumers and is used to determine the quality of the load. Reactive Power contributes to transmission losses and is used to define the oscillation of energy caused by reactive elements in the load. It is an important quantity in defining the current carrying capacity of electrical transmission systems, in the design of consumer plants and equipment and is often used in the calculation of Power Factor. Apparent Power is used to measure the maximum demand for industrial loads and it reflects the capital investment in the supplying systems. Apparent Power is also used in power engineering to define the maximum ratings of electrical apparatus, and in revenue metering, it is used as a meaningful quantity in the general power theory of electrical networks.
Of the above-mentioned quantities only the first, Active Power, is widely accepted as holding for unbalanced or non-sinusoidal situations. Reactive Power, Apparent Power and Power Factor become less valid when the supply is distorted or the load is unbalanced.
Non-linear and unbalanced loads are widespread and have caused concerns to electricity suppliers, users and regulators for many years. Non-linear loads have detrimental effects on components of the power system, they give rise to harmonic power flows to other users of the supply, and they contribute to a deterioration of the supply quality. Also, load imbalance in polyphase systems reduces the performance of the distribution system, causes voltage asymmetries which may be harmful for customers' loads, and also contributes to a deterioration of the supply quality. In the absence of a proper system for power measurement for non-linear loads, various approximations and work-arounds are made. These are unsatisfactory from many points of view: consumers are not charged accurately, and equipment designers have to cope with a margin of error when their prototypes are tested, for example.
The lack of a universally accepted definition of Apparent Power, for example, leads to inconsistencies and inaccuracies in VA demand meters used for billing purposes. Meters from different manufacturers (or even different models by the same manufacturer) use different definitions of Apparent Power. Different operating principles may also be used. Each meter type therefore responds differently to different types of load, and no conventional meter gives consistent results for all loads. Under distorted waveform conditions their readings can differ by several percent even for the same balanced but non-linear, load. In extreme circumstances, the differences may be as high as 30%. Standard engineering text books tend to skate over this problem, but International institutions such as the EEC, IEEE and IEE are well aware of it.
Due to uncertainties in defining Reactive Power in non-sinusoidal situations, the use of Apparent Power is preferred in some countries for determining Power Factor. However, using existing standard power theory in AC circuits, Apparent Power itself cannot be defined consistently in an unbalanced three-phase system.
There is a considerable body of literature which describes the shortcomings of the current approach to calculating Apparent Power, Reactive Power, and Power Factor. Examples notable for their clarity are:
(1) A. Emanuel, “On the Definition of Power Factor and Apparent Power in Unbalanced Polyphase Circuits with Sinusoidal Voltage and Currents,”, IEEE Trans. On Power Delivery, Vol. 8, No. 3, pp. 841-852, July 1993;
(2) P. S. Filipski, Polyphase “Apparent Power—the Misleading Quantity In Non-Sinusoidal Power Theory: Are all Non-Sinusoidal Power Theories Doomed to Fail?,” ETEP, Vol.3, No.1, pp. 21-26, January/February 1993; and
(3) R. West, “The Measurement of Apparent and Reactive Power with Unbalanced Loads and Non-Sinusoidal Waveforms”, Distribution 2000 Conference, Sydney, 1997.
These authors described three key aspects of the problem Firstly, that there appears to be a good working definition in single-phase systems, where convention gives rise to calculated quantities to which one can attribute a physical meaning. Secondly, that there is a problem in polyphase systems of attributing a real physical meaning to the calculated parameters in existing theories. Intuitive reasoning appears to fail here. Thirdly, that the analysis of polyphase systems gives rise to no less than five different definitions for Apparent Power in three-phase systems with or without distorted waveforms. These results are often different by quite significant margins; particularly in unbalanced systems with distortion.
A good review of previous works has been presented in the paper by West (1997). The problems have been reported in many technical papers in for example by West (1997), Filipski (1993), Emanuel (1993), and in IEEE Working Group Report, “Practical Definitions for a Powers in Systems with Non-Sinusoidal Waveforms and Unbalanced Loads: A Discussion”, IEEE PWRD, Vol. 11, pp. 79-101., but yet no acceptable solutions have been found. The emphasis for calculating Apparent Power is now on System (equivalent) Apparent Power, see the papers by West (1997), Emanuel (1993), and IEEE Working Group Report. Also, the quantity know as RMS Apparent Power has also been considered by some technical institutions.
One of the main disadvantages with all the proposed methods for calculating Apparent Power is the fact that their results all depend on the voltage of reference point at which measurement is made. In single-phase circuits, the RMS values of voltage across the load and current through the load are clearly defined. However, in three-phase systems the Apparent Power should not depend on the reference voltage see paper by L. S. Czarnecki, “Power Related Phenomena in Three-Phase Unbalanced Systems”, IEEE PWRD, Vol. 10, pp. 1168-1176. This disadvantage of the conventional power measurement methods is readily apparent by the following example of a three-phase system.
Considering
FIG. 1
, in a balanced situation, points
0
, N
1
and N
2
have zero voltage. The RMS values of the phase voltages measured with respect to three references are the same which in turn it implies that the Apparent Powers are equal. However, if the load is unbalanced then N
1
, N
2
and
0
have different voltages. Only
0
retains zero voltage. Thus, Apparent Power calculated using the voltages measured with respect to
0
, N
1
, and N
2
will be different.
Now consider a three-phase load that is supplied by an ideal source (Z
s
=0). Assume that one of the lines, say “c”, is broken (I
c
=0). The measured voltage for phase “c” would be different on the sides of the broken point. If the voltage transducer is on the source side then it is the source voltage and if the potential transformer is on the load side, the measured voltage is influenced by the current of other two lines and the load impedance. Thus, the system (equivalent) Apparent Power (see papers by West (1997) and IEEE Working Group Report) will be different for the load.
One of the reasons for calculating Apparent Po
Deb Anjan K.
Electric Power Research Limited
Stevens & Showalter LLP
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