GB2217027A

GB2217027A - Active power measurement

Info

Publication number: GB2217027A
Application number: GB8807949A
Authority: GB
Inventors: A J Wilks
Original assignee: OMEGA ELECTRIC Ltd
Current assignee: OMEGA ELECTRIC Ltd
Priority date: 1988-04-05
Filing date: 1988-04-05
Publication date: 1989-10-18
Also published as: GB8807949D0

Abstract

In an arrangement to determine active power, current and voltage signals (i, v) are rectified (1, 2) in devices having long time constants, and the output signals are respectively used to produce a frequency signal (3) and a pulse width modulation signal (4); a phase detecting/phase converting arrangement (5, 6, 7) produces a phase difference signal which is fed to a cosine law generator (9) which gives a pulse width modulation output representing the power factor; an AND gate (10) then produces an output representing the active power (volts x amps x powerfactors). <IMAGE>

Description

DESCRIPTION ACTIVE POWER CALCULATOR FOR ENERGY MEASUREMENT IN A.C. SUPPLY SYSTEMS This invention relates to a means of calculating the amount of active power delivered to a load from an a.c. supply as in the case of a conventional electricity meter.

Solid state electricity meters require a means of calculating the power delivered to a load. This may be achieved by a four quadrant multiplier or by using a hall effect sensor as a multiplying element. Both these means have drift problems associated with temperature which affects offset, stability and linearity.

According to the present invention the active power delivered to a load may be calculated by a means which does not suffer from the problems detailed above.

A specific example will now be described with reference to the accompanying drawings.

Figure 1 shows an active power calculator using two full wave rectifiers, a voltage to frequency converter, a pulse width modulator, a phase to voltage converter, a cosine law generator and a further pulse width modulator.

Figure 2 shows in greater detail a means of achieving a phase to voltage converter and cosine law generator.

Referring to Figure 1 a representation of the instantaneous a.c.

current (i) and voltage (v) are applied to two full wave rectifiers (1 & 2). The full wave rectifiers (1 & 2) have a long time constant hence their outputs are effectively d.c. levels.

The output of (1) is applied to a voltage to frequency converter (3). The output of (2) is applied to a pulse width modulator (4) which produces a varying mark space ratio which operates at a frequency suitable to sample the output of (2).

The output of (1) and (2) are applied to polarity detectors (5 & BR< 6) which generate square wave outputs.

The square wave outputs are applied to a phase to voltage converter (7) whose output is a positive maximum for in phase signals (zero degrees out of phase), zero for signals in quadrature (90 degrees out of phase) and negative thereafter up to a maximum negative value of 180 degrees out of phase.

In the case of a conventional ' import' meter we are only concerned with the range 0 degrees to 90 degrees.

The output of (7) is applied to a cosine law generator (8) which generates a non linear output which follows a cosine law.

The output of (8) is applied to another pulse width modulator (9) running at a suitable frequency to sample the outputs of (3) and (4).

The AND function (10) at its output will provide a pulse train which is on an integral basis the product of voltage multiplied by current and by the power factor over the range 0 to 1.

With reference to Figure 2 the input comparators (5) and (6) convert the instantaneous signals into square waves which are applied to an exclusive NOR gate (11). The output of the gate is hence mostly at a '1' for in phase voltage and current, and at 90 degrees phase it will 'l' and '0' for equal times.

The pulse train output from (11) is applied to a smoothing filter (12) which has a balancing current injected by a resistor such that its output at 90 degrees phase is zero volts. Hence it can be seen that for in phase conditions its voltage output will be a maximum.

The output of (12) is applied to a non linear function generator (13) which may typically be constructed as shown whose output follows a curve for a linear output, the curve being the cosine law as shown in Figure 3.

Claims

1. An active power calculator for use in electrical active power meters based upon the multiplying action of a variable frequency gated with a varying mark space ratio at R fixed frequency, and gated also with a varying mark space ratio at a fixed frequency representing a power factor between zero and unity.

2. An active power calculator as in Claim 1 based upon the use of two full wave rectifiers, a voltage to frequency converter, a pulse width modulator, a phase to voltage converter, a cosine lav generator, a further pulse width modulator and a gating element.