The present invention pertains to fiber optical current and voltage sensors and, more particularly, to applications involving filters for use in such sensors.
One important aspect of electrical power systems deployed by the power generation industry is the ability to measure power carried over high voltage transmission lines. Power measurement has typically been performed on the high voltage side of the systems before the voltage is stepped down for distribution. However, the need is growing for more frequent and more accurate power measurements (e.g., voltage and current measurements) throughout the power distribution systems. Some recent innovations associated with making such power measurements involve the use of fiber optic sensors as described in more detail in, for example, the above-incorporated by reference U.S. Patent Publication 2011/0204875.
There are a number of different electrical power systems which require such monitoring. For example, in nuclear power applications, standby power transformers are employed to run critical equipment in the case of a failure. Normally, these standby power transformers are unloaded, but the high side is connected. By keeping their high sides connected, standby power transformers can be quickly switched on when needed. However, there have been several incidents where these generators were not fully connected at the high side (i.e., they had an open-phase condition), and it was not known until the emergency power was needed. In response to this problem, the Nuclear Regulatory Commission in the United States is implementing a requirement that all such standby generators have monitoring equipment installed to detect the open phase condition.
If unloaded, the current flowing into the high side of such standby power transformers is substantially the current attributable to the magnetization current of the power transformer's core, e.g., typically on the order of less than one ampere such as 50 to 800 mA. Alternatively, if the standby power transformer is slightly loaded, e.g., due to the presence of a monitoring load, then the current flowing into the high side of the standby power transformer might be a bit higher, e.g., on the order of an ampere. Regardless, when in the undesirable open-phase condition, the current flowing into the high side of the standby power transformer will drop to approximately zero (there may still be some very small capacitive current).
Thus, in order to tell if a standby power transformer is properly connected or not, a monitoring system can be used to determine when the high side current flow drops below some predetermined threshold, e.g., 25 to 500 mA. In order to reliably measure such small currents, the current sensor must be able to resolve currents several times lower than the low-level threshold, or typically about 10 mA. This value may vary according to the specific transformer under consideration. One way to design an optical current sensor to be able to detect current levels as low as 10 mA is by using a combination of a large number of fiber sensing turns in combination with a number of wire wraps around the sensing fiber loops. For example, Alstom has fielded a product which employs 60 wraps of sensing fiber together with 10 wraps of the primary conductor, yielding a multiplication factor of the current of 600.
However, in some applications it is not practical to employ multiple wraps of the primary conductor around the fiber optic current sensor. Also, it is quite expensive to install a large number of sensing fiber turns (e.g., 600 fiber turns) in a single optical current sensor, which would be needed absent the multiplicative effect provided by the multiple wraps of the primary conductor around the fiber optic current sensor.
Another approach to providing an optical current sensor which can detect such low levels of current involves passing the output of the optical current sensor through a comb filter as, for example, described in U.S. patent application Ser. No. 12/598,651, entitled “Adaptive Filters for Fiber Optic Sensors, also referred to herein as U.S. Patent Publication 2011/0204875, the disclosure of which is incorporated here by reference. In U.S. Patent Publication No. 2011/0204875, the comb filter is triggered off the current waveform that itself is being filtered. The raw current passes through a narrow band pass filter around the largest frequency component to be measured (normally the fundamental frequency). The zero crossings of the waveform at the output of this filter are used to track the frequency of the signal, and thus to provide the reset function required in the comb signal processing.
The system described in U.S. Patent Publication No. 2011/0204875 is therefore limited in its ability to accurately track the frequency (and thus properly time the resets of the comb filter) when the signal-to-noise ratio becomes too low. At some low level of primary signal, the noise coming through the band pass filter substantially interferes with the zero crossings of the resultant signal, distorting their location, or even adding extraneous zero crossings. Thus the system described in U.S. Patent Publication No. 2011/0204875 suffers a limitation in its ability to recover a small signal from a large noise.
Accordingly, it would be desirable to provide optical current sensors, transformers, systems and sensing methods which address the foregoing issues.