GB2400172A

GB2400172A - Optical AC current sensor

Info

Publication number: GB2400172A
Application number: GB0307979A
Authority: GB
Inventors: Xuekang Shan; Haiyu Li
Original assignee: Individual
Current assignee: Individual
Priority date: 2003-04-05
Filing date: 2003-04-05
Publication date: 2004-10-06
Anticipated expiration: 2023-04-05
Also published as: GB2400172B; GB0307979D0

Abstract

A sensor for measuring the AC current flowing in a conductor 1 comprises a coil 2, electro-optic modulator 4, light source 5, photo-detector 6, DC bias circuit 7 and signal processor 8. The coil 2 converts the current to an AC voltage, and the optical signal from the light source 5 is then modulated according to the depth of the driving voltage by the electro-optic modulator 4. The modulated signal is sensed by the photo-detector 6 and manipulated by the signal processor 8 to provide an estimate of the AC current as a function of the ratio of the amplitude of the voltage signal to the amplitude of the time-averaged signal. A DC bias circuit 7 is required to correctly sense positive and negative voltages. The current sensor is theoretically independent of optical power and polarisation effects and depends only on the modulation depth.

Description

A Diffractive MEMS Based Optical AC Current Sensor The present invention relates to measuring AC current by optical means and, in particular, to the combination of a conventional AC current measurement method with optical signal modulation, transmission, and detection.

AC current measurement in electric power industry is traditionally carried out using Current Transformers (CT). In high voltage environments, for example, when the line voltage is 400 kV, electrical insulation is difficult, and thus CTs for high voltage applications are very bulky and expensive.

As is well known, optical fibres are capable of transmitting optical signals with wide bandwidth and low loss. Optical fibres have also found wide applications in sensor technology, and many types of optical fibre sensor have been commercialized. One of the advantages of optical fibre sensors is that they can be used in a high voltage environment, because optical fibres are intrinsically good insulators.

At present, there are commercial optical current sensors, also called optical CTs sometimes. These products are based on the Faraday Rotation effect in optical fibres or in a bulk optical material. The polarisation states of the optical signal in the optical fibre or in the bulk optical material are affected by the magnetic field generated by electric current carrying conductor, and the changes in the polarisation states are detected by an optical receiver. But they suffer from the disadvantages that the Faraday Rotation Effect in optical fibres is very weak, and that because polarisation states in optical fibres are susceptible to environmental disturbances, detection of polarisation changes due to the current to be measured is generally difficult.

The present invention describes a new concept of an Optical AC current sensor.

The new Optical AC Current Sensor is based on an electro-optic amplitude modulator, whose modulation depth is in a fixed relationship with the driving voltage. This driving voltage is derived from the AC current or voltage under measurement, in one of the methods to be described later. The CW optical power from the optical source is thus modulated by the driving voltage, the modulation depth is in a fixed relationship to the driving voltage, and the modulated optical signal is then detected by the optical receiver.

In the optical receiver, the optical power is converted into electrical signal via a photo-diode and the associated electronic circuit. The AC component, Vac, of the electrical signal, is separated from the total signal, and its amplitude is then measured as Vet. At the same time, the total signal is time averaged to get a quasi DC voltage Vavg, which is proportional to the total time averaged optical power received by the optical receiver. The line current being measured by the Optical AC Current sensor then is Iiine = f ( V) arg where f is a mathematical function determined by data fitting. In general, f can take the form of f(x)=a+bx+cx2 +dx3 +ex4 + where a, b, c, dare constants determined by data-curve fitting.

When the optical source power drifts, the ratio V1/Vavg is theoretically independent of optical power and only dependent on the modulation depth.

For the application of this invention as an Optical CT, in one form, it uses a conventional coil, which can either be air cored or ferrite cored and is installed in the vicinity of an AC current carrying conductor, to convert the alternating magnetic field around the conductor into an AC voltage, and then this voltage is used to drive the electro-optic amplitude modulator. In another form, a shunt resistive element is connected in series with the current carrying conductor, and the voltage drop across this element is used to drive the electro-optic amplitude modulator.

In some forms of electro-optic amplitude modulators, the modulation depth is optical polarisation dependent, and thus it is not a stable representation of the current under measurement. Preferably the electrooptic amplitude modulator used in this current sensor is polarisation insensitive, such as certain types of Variable Optical Attenuators (VOA). A preferred electro-optic amplitude modulator is a Diffractive MEMS (MEMS stands for MicroElectricalMechanical Systems) based VOA, which is compact, low cost, highly reliable (Trillions attenuation cycles can be expected, equivalent to more than one thousand years of life for 50 Hz line frequency), purely voltage driven ( high impedance, thus not current consuming), and very importantly, insensitive to polarisation and vibration.

Most types of electro-optic amplitude modulators or VOAs, including the said Diffractive MEMS based VOA, need a DC bias voltage in order to be able to change optical attenuations in both positive and negative directions. In this invention, this bias voltage is obtained from the line current via a coil and voltage regulation circuitry (Fig. 1), or by optical remote power supply (Fig. 2). Because the Diffractive MEMS based VOA used in this sensor does not consume current and only requires a bias voltage of Revolts, the bias supply circuitry is fairly simple (Fig. 4).

Electro-optic amplitude modulators and VOAs may suffer from temperature changes, because the IEC has defined the CT operational temperature range to be from -40 to +85 C. The temperature change may result in change of modulation depth, which will lead to measurement error. This drift can be compensated by use of temperature sensitive resistor(s) in the modulator driving circuitry, and an example is shown in Fig 4.

Although the Diffractive MEMS based VOA is insensitive to polarisation, to further reduce possible residual polarisation sensitivity, a broad line width (1 0-30nm) optical source (LED) is preferred. After transmission in the optical fibre which always has some degree of birefringence, the light emitted by the LED is depolarised, and any residual polarisation dependent effects in the electro-optic amplitude modulator are reduced to negligible level.

A preferred embodiment of the invention will be described with reference to the accompanying drawing in which: FIGURE 1 shows a block diagram of an AC current sensor using a ferrite cored coil for electro-optic amplitude modulator bias supply; FIGURE 2 shows a block diagram of an AC current sensor using a photovoltaic cell for electro-optic amplitude modulator bias supply; FIGURE 3 is a block diagram of the optical receiver showing how the Vac and Vave are obtained from the received optical signal; FIGURE 4 shows temperature compensation of modulation depth of the electro-optic amplitude modulator.

As shown in Figure 1, the AC current under measurement is converted into an AC voltage by the measuring coil (1), to drive the electro-optic amplitude modulator (4). The CW optical signal from the light source (5) is then modulated and detected by the photo detector (6). And then the detected signal is processed and displayed by the signal processing and display unit (8). The bias coil (3) which is ferrite cored and the DC bias circuit(7) generate the bias voltage for the electro- optic amplitude modulator (4).

Figure 2 show another way of generating the bias voltage: using another light source (3) an optical fibre, a photovoltaic cell (9), and the DC bias circuit (7), to supply a DC bias to the electro-optic amplitude modulator (4).

Figure 3 shows the block diagram of the optical receiver. The optical signal is converted into electrical signal by the photo detector ( 1) and the O/E converter (2). This converted signal comprises an AC component Vac superimposed on top of a time average DC voltage Vavg, as shown in the inset of Figure 3.

The converted electrical signal is then split into three paths: the first goes to one of the inputs of an adder (6), the second to a polarity inverter (3) via a DC blocking capacitor, and the third to a rectifier via a DC blocking capacitor. The inverter (3) inverts the polarity of the input voltage Vac, and then at the output of the adder (6) Vavg is obtained. Vavg is then converted into a digital signal by A to D converter (7). Rectifier (5) rectifies Vac into a single polarity voltage, which then passes through a low pass filter (11), and is converted into a quasi DC voltage Vet. V, is then converted by A to D converter 8 into a digital signal. The micro-processor (9) processes these two digital signals and drives the display unit (10).

Figure 4 shows how temperature drift in the modulation depth of the electro-optic amplitude modulator (5) is compensated by a thermistor (2). If the modulation depth increases with temperature increase, a negative temperature coefficient thermistor (2) can be used, and the effective driving voltage to the modulator will then decrease with temperature; if the modulation depth decreases with temperature, a positive temperature coefficient thermistor (2) should be used, and then the effective driving voltage will increase with temperature.

Figure 5 shows using a shunt resistor ( 15) to obtain the voltage to drive the electro-optic amplitude modulator (4), instead of using a wire coil.

Claims

Claims: 1. An optical AC voltage sensor having an electro-optic amplitude

modulation means having optical fibre input and output, and having a modulation depth versus driving voltage;
2. A wire coil, air cored or ferrite cored, installed in the vicinity of an alternating current carrying conductor, to convert the current induced alternating magnetic flux into an alternating voltage, to drive the said electro-optic amplitude modulator as in Claim 1;
3. An optical source having optical fibre output and stable output power and desired optical wavelength and wide spectral width (30nm), to provide CW optical signal to the said electro-optic amplitude modulator as in Claim 1;
4. An optical receiver which converts optical signal into electrical signal, to receive the modulated optical signal from the said eiectro- optic amplitude modulator as in Claim 1;
5. An electrical circuit that may have analogue circuit and a digital signal processor, that separates the AC component of the electrical signal from the time average signal that comes from the said optical receiver as in Claim 4, and calculates the ratio of the AC component to the time average signal; and calculates and displays the current under measurement;
6. A wire coil, ferrite cored, installed in the vicinity of an AC current carrying conductor, to convert alternating magnetic flux into alternating voltage. This alternating voltage is then rectified and regulated to supply a DC bias voltage to said electro-optic amplitude modulator as in Claim 1; The ferrite core saturates at high magnetic flux to limit the DC bias voltage at very high line current, for example, when there is a short circuit;
7. A photovoltaic cell, shined by an optical power source via an optical fibre, to generate a DC voltage to bias the said electro- optic amplitude modulator as in Claim 1;
8. A modulator driving circuitry that comprises temperature sensitive resistor(s), to compensate the temperature drift of modulation depth of the said electro-optic amplitude modulator as in Claim 1;
9. A short length conductor, whose resistance has been calibrated, connected in the current carrying conductor, to generate an alternating voltage to drive the said electro-optic amplitude modulator as in Claim 1; Amended claims have been filed as follows Claims: 1. An optical AC voltage sensor having an optical source, an optical receiver and an electro-optic amplitude modulation means having optical fibre input and output; as an example, this electro-optic amplitude modulation means is based on Diffractive Micro-Electro- Mechanical Systems (MEMS) variable optical attenuator; 2. An optical source having optical fibre output and stable output power and desired optical wavelength and wide spectral width (30nm), to provide CW optical signal to the said electro-optic amplitude modulator as in Claim 1; 3. An electrical circuit that may have analogue circuit and a digital signal processor, that separates the AC component of the electrical signal from the time average signal that comes from the said optical receiver as in Claim 4, and calculates the ratio of the AC component to the time average signal; and calculates and displays the current under measurement; 4. A photovoltaic cell, shined by an optical power source via an optical fibre, to generate a DC voltage to bias the said electro-optic amplitude modulator as in Claim 1; 5. A modulator driving circuitry that comprises temperature sensitive resistor(s), to compensate the temperature drift of modulation depth of the said electro-optic amplitude modulator as in Claim 1;