The present invention is related to a method for measuring the instantaneous
value of a mono-phase crest voltage.

State of the art

To solve problems regarding the regulation of power systems operating
in a mono-phase network, the knowledge of the instantaneous voltage of the fundamental
is imperative. In a classical approach the measuring method consists in rectifying
the mono-phase voltage and subsequently filtering the resulting signal. This method
has two important drawbacks :

If the rectified voltage filtering is very weak, the voltage amplitude fluctuates
over a small value as compared to the crest value.

Some filtering is recommended to obtain a stable readout of the average value.
However, if the filtering is too strong, the response time is too long to measure
fast changes of the network voltage.

Aims of the invention

The present invention aims to provide a method for measuring the instantaneous
value of a mono-phase crest voltage overcoming the problems of the prior art solutions.

Summary of the invention

The invention relates to a method for measuring the instantaneous
value of a monophase crest voltage signal V_{e}, comprising the steps of

applying the voltage signal V_{e} to a first input of a phase comparing
circuit and a reference signal to a second input of said phase comparing circuit,

in the phase comparing circuit deriving from the phase difference between the
reference signal and the voltage signal V_{e} a correction signal and outputting
the correction signal,

filtering the correction signal,

using the filtered correction signal to adjust the reference signal,

when the phase difference between the reference signal and the voltage signal
V_{e} is cancelled, determining the instantaneous value of the voltage signal
V_{e}

In a preferred embodiment the reference signal has a frequency of
approximately 50 Hz.

Advantageously the step of filtering comprises a low-pass filtering
followed by a PI regulation.

Preferably the correction signal is a linear combination of the product
of the voltage signal V_{e} and the cosine of the instantaneous phase of
the reference signal and the product of the voltage signal V_{e} and the
sine of the instantaneous phase of the reference signal.

The invention also relates to a device for measuring the instantaneous
value of a mono-phase crest voltage, carrying out the method as described above.

Short description of the drawings

Fig. 1 represents the prior art approach.

Fig. 2 represents the instantaneous phase of the voltage signal.

Fig. 3 represents the measuring method according to the invention.

Fig. 4 shows how the crest value is obtained applying the method of
the invention.

Detailed description of the invention

Figure 1 illustrates the prior art solution, wherein the mono-phase
voltage signal is first rectified and subsequently filtered by a pass band filter.

In the method of the invention the instantaneous value of the mono-phase
voltage is determined without the need for strong filtering.

The setting essentially comprises a Phase Locked Loop, needed to synchronise
on the system. This avoids the need for searching the instant of zero pass of the
AC voltage signal and the uncertainties that come along with it. In this way one
obtains a sinusoidal phase reference inside the microprocessor reflecting the AC
voltage phase present on the system. The voltage signal has a very good frequency
stability. Therefore the Phase Locked Loop can be relatively slow, while still offering
a very high operating stability.

The method is based on a digital reference based on which all projections
will be undertaken. The digital reference of the system voltage preferably relates
to the AC voltage V_{e} itself for the following reasons:

it does not contain the harmonic of the system, not even the harmonic generated
by the bridge itself.

it consists of two sine waves, one in phase and one in quadrature with the input
voltage, which facilitates the calculation of the reactive and the active component
of the current for example.

In order to obtain a reference vector having a relation to the input
AC voltage, a phase locked loop is used. The fundamental equations of this loop
are the following (see Fig.2):

with :
&thetas;= ∫(ωs + ε) dt = ∫ωr dt
whereby &thetas; denotes the instantaneous phase of the rotating vector, such that
Ed and Eq denote the instantaneous in-phase and quadrature components of the rotating
(unit) vector, respectively. Further, ω_{s} denotes the approximate
(and constant) reference pulsation of the 50Hz system. ε represents the correction
signal to the value of this theoretical pulsation ω_{s} and ω_{r}
the resulting pulsation of the digital reference of the rotating vector.

The phase locked loop cancels the phase difference between the signal
at pulsation (ω_{s} + ε) and the external input signal on which
we want to synchronise (V_{e} in this case). The signal at the reference
pulsation ω_{s} is adjusted by adding or subtracting a value ε
that depends on the phase difference between the reference signal and the input
voltage (cfr. infra). The instantaneous phase of the resulting signal (at pulsation
ω_{r}) is determined and so one obtains (see Fig.3) :
A = E_{d}· V_{e}B = E_{q}· V_{e}
Value A depends on the sine of the phase difference between the voltage V_{e}
and the internal reference sine wave. It is only used in case of large phase differences.
B depends on the cosine of the phase difference. As the parameter A is only used
for a big phase shift, only the negative value of A is used. In this way there is
a privileged sense of rotation for the PI regulator in the PLL.

A value C is defined such that
C = 0 if A > 0C = A if A < 0
An input signal e_{k} to the regulator is then calculated :
e_{k} = -B + C if B > 0e_{k} = -B - C if B < 0
So the error to the regulator input e_{k} takes different values according
to the sign of A. This assures a continuous functioning of the system and prevents
a synchronisation with the system to a phase difference of 180°.

A low-pass filter eliminates the double frequency component contained
in e_{k} resulting from the multiplication of the network sine wave and
the reference pulsation. Most preferably next a PI regulator is used in the phase
locked loop. At the PI regulator's output an error signal ε is obtained, given
by
ε = K_{p} · e_{k} + ∫K_{i} · e_{k}
K_{p} and K_{i} hereby denote the proportional and integral parameters
of the regulator, respectively.

When, for example, the power is switched on and the input voltage
undergoes a transient effect, the phase locked loop is in a non-synchronised transitory
mode. To accomplish the PLL's synchronisation it is necessary to bring it into steady
state regime (i.e. to render ε constant). ε is then kept constant by adding
a high-pass filter (not shown in Fig.3) and monitoring there is no AC signal at
the high-pass filter's output.

Similarly, for security reasons associated with the PLL, the excursion
s should not exceed about thirty rad/s (tolerance of the 50 Hz system).

The voltage crest value can easily be obtained by an instantaneous
voltage measurement (see also Fig.4):
V_{e} = V_{e} / (E_{d})

Anspruch[en]

Method for measuring the instantaneous value of a monophase crest voltage signal
V_{e}, comprising the steps of

applying said voltage signal V_{e} to a first input of a phase comparing
circuit and a reference signal to a second input of said phase comparing circuit,

deriving from the phase difference between said reference signal and said voltage
signal V_{e} in said phase comparing circuit a correction signal and outputting
said correction signal,

filtering said correction signal,

using said filtered correction signal to adjust said reference signal,

determining the instantaneous value of said voltage signal V_{e}, when
the phase difference between said reference signal and said voltage signal V_{e}
is cancelled.

Method as in claim 1, wherein the step of filtering comprises a low-pass filtering
followed by a PI regulation.

Method as in claim 1 or 2, wherein said correction signal is a linear combination
of the product of said voltage signal V_{e} and the cosine of the instantaneous
phase of said reference signal and the product of said voltage signal V_{e}
and the sine of the instantaneous phase of said reference signal.

Method as in any of the previous claims, wherein said reference signal has a
frequency of approximately 50 Hz.

Device for measuring the instantaneous value of a mono-phase crest voltage,
carrying out the method as in any of the previous claims.