High current measurement system incorporating an air-core transducer

[0002] 1. Field of the Invention

[0003] The present invention relates to devices used to measure current flowing through high voltage electrical power transmission lines.

[0004] 2. Related Art

[0005] The measurement of high currents present in power transmission lines has previously been accomplished with devices which use iron-core transformers. The winding ratio is chosen so that the large primary current is converted to a proportional smaller secondary current. The use of iron-core transformers has a number of disadvantages including the weight of such transformers and the difficulty involved, and time required, in installation. A further important disadvantage is that iron-core transformers have an inherent saturation point. At the saturation point, the magnetic flux density in the iron core does not maintain a linear relationship with the primary current, and, as a result, the secondary current is not proportional to the primary current. An accurate measurement of the primary current cannot be accomplished when the secondary current is not proportional to the primary current.

[0006] In order to avoid the limitations associated with iron-core transformers, some devices have used air-core transducers, such as Rogowski coils. One particular advantage of air-core transducers is the elimination of saturation problems. Saturation can not occur in air-core transducers. The physics of air-core transducers are such that the output of the coil is a voltage which is proportional to the time derivative of the primary current. As a result, one limitation of conventional devices which incorporate air-core transducers is that an integration operation must be performed before the magnitude and phase of the primary current can be determined. The integration operation has been performed in two locations.

[0007] Any electronics mounted electrically close to the power transmission line (the source end) requires a power source. When the integration operation is performed at the source end, additional power is required to power the integration circuits. Another approach has been to perform the integration operation at a location where the metering equipment is located (the remote end) to eliminate the additional power required for the integration operation at the source end. However, when the integration operation is performed at the remote end, the link connecting the source end and the remote end must have a very wide frequency bandwidth and a large voltage dynamic range. One skilled in the art will appreciate that equipment having a wide bandwidth and large dynamic range is inherently more susceptible to electrical noise effects than other equipment.

[0008] In accordance with the invention, an air-core current transducer measurement system is provided which overcomes the problems associated with both iron-core current transformers and conventional air-core current transducers and which combines the advantages associated with conventional air-core current transducers (e.g., ease of installation, no saturation under overload, and small size and weight) with the permanent installation advantages of iron-core current transformers. The measurement system of the invention is particularly useful, inter alia, in providing alternating current measurements at specific locations in a power system and, more generally, in connection with metering and protective systems for electrical power equipment.

[0009] According to the invention, an air-core current transducer measurement system is is provided for measuring current flow in a high voltage electrical conductor such as transmission line, the system comprising: a high voltage end including an electrically conductive case for shielding the electrical circuitry at that end and an isolated low voltage end coupled to the high voltage end and providing a scaled output representing the current flow in the high voltage electrical conductor. A current transducer located at the high voltage end includes a coil for sensing current flow in the conductor or transmission line and produces a time derivative output signal. An electrostatic shield connected to the conductive case provides electrostatic shielding of the coil. A low pass filter, connected to the coil and to the input of electrically conductive case, filters out electrical noise. An integrator, located in the conductive case, converts the time derivative signal into a scaled signal corresponding to the scaled output signal provided at the low voltage end.

[0010] Preferably, the high voltage end and low voltage end are connected together by a fiber optic cable. Advantageously, the conductive case further includes a signal converter for converting the signal produced by the integrator into a light signal for transmission over the fiber optic cable. The air-core transducer measurement system preferably further comprises a light signal converter at said low-voltage end for converting the light signal into the scaled output signal.

[0011] In an advantageous implementation, a laser is located at the low voltage end, a photovoltaic power supply is located at the high voltage end for supplying electrical energy to the integrator and the signal converter, and a fiber optic cable couples the laser to the photovoltaic power supply. A laser power safety interlock circuit is preferably provided for removing power from the laser when a predetermined fault condition occurs. Advantageously, the interlock circuit receives as an input the light signal produced by the signal converter.

[0012] The current transducer preferably comprises a Rogowski coil, a torroidal coil with a core material with a relative permeability of approximately unity. The integrator typically comprises an analog integrator, although the integrator can also be implemented digitally.

[0013] Other features and advantages of the invention will be set forth in, or apparent from, the following detailed description of the preferred embodiments of the invention.

[0014] FIG. 1 is a schematic block diagram of a device in accordance with a preferred embodiment of the invention for measuring current flowing through a high voltage electrical power transmission line or other high voltage conductor; and

[0015] FIG. 2 is a schematic circuit diagram of a portion of the device of FIG. 1.

[0016] Referring to FIG. 1, there is shown a device which is denoted 10 and which is constructed in accordance with a first preferred embodiment of the invention for measuring current flowing through a current carrying conductor represented by a high voltage electrical power transmission line L. A Rogowski coil 12 is placed around the power transmission line L and acts as a transducer, with the voltage V_ininduced therein being proportional to the rate of change of the current in the transmission line L. In particular,

[0017] V_in=k*di/dt

[0018] where k is a constant and di/dt is the first derivative with respect to time of the magnitude of the current in the transmission line L. As indicated in FIG. 1, the Rogowski coil 12 is electrostatically shielded. The electrostatic shield is shown at 12ain FIG. 2 (with the Rogowski coil being shown at 12b) and is designed to provide complete electrostatic shielding without becoming an electrically shorted turn of the coil 12. As is also indicated in FIG. 2, the shield 12ais connected to an electrically conductive case 16aat the high-potential end of the link, which case is connected to transmission line L.

[0019] The output of the Rogowski coil 12 passes through a low pass filter 14 which filters out high frequency electrical noise. This connection made through a shielded cable, such as a triax 13, as shown in FIG. 2. In FIG. 2, low pass filter 14 is formed by a series resistor R1 and a shunt capacitor C1 having the values indicated although as will be apparent to those skilled in the art, low pass filter 14 can be of any suitable design and construction of a suitable filter would be well within the skill of those of ordinary skill in the art.

[0020] The output of the low pass filter 14 is connected to a fiber optic transmitter package 16. The fiber optic transmitter package 16 is located close to the transmission line L and includes an integrating and scaling circuit 18 at the input end to which filter 14 is connected. Transmitter package 16 includes the electrically conductive case 16ato which electrostatic shield 12ais connected, as described above. The voltage V_ininduced in the Rogowski coil 12 must be integrated in order to produce a voltage proportional to the current in transmission line L. V_inis integrated and scaled by integrating and scaling circuit 18 to produce a signal waveform which is a scaled representation of the current in transmission line L. The remaining units of the fiber optic transmitter package 20 are discussed below. The integrating and scaling circuit 18 can employ an analog integrator or can be implemented digitally.

[0021] A fiber optic receiver package 20, which is sited at a location remote from the transmission line L, includes a laser 22 that produces the energy that is used to power integration and scaling circuit 18. An electrical power supply 23 is used to provide power to laser 22, through a laser power safety interlock circuit 25. Laser power interlock circuit 25 is described in more detail below. Laser 22 produces a laser beam which travels through a fiber optic cable 24 to a photovoltaic power supply 26. The photovoltaic power supply 26 is a device used to convert laser energy into electrical energy. The electrical energy output of photovoltaic power supply 26 powers the integration and scaling circuit 18.

[0022] The output of integration and scaling circuit 18 is connected to a light conversion circuit 28, which converts the electrical signal from circuit 18 into a light signal, such as a laser or other light beam. Power for light conversion circuit 28 is also supplied by photovoltaic power supply 26. The light signal produced by light conversion circuit 28 travels through a fiber optic cable 30 to an electrical conversion circuit or converter 32 in the fiber optic receiver package 20. The electrical conversion circuit 32 transforms the light signal input into an electrical signal. The electrical signal leaves the fiber optic receiver package 20 at an output port 34. The electrical signal at the output port 34 is a signal (analog or digital) proportional to the current in transmission line L and may be connected to a meter, a protection device or another electrical device or devices.

[0023] The light signal which travels through fiber optic cable 30 is also an input to the laser power safety interlock circuit 25. Laser power interlock circuit 25 monitors the light signal and removes power from laser 22 when a malfunction in the system occurs.

[0024] Fiber optic transmitter package 16 and fiber optic receiver package 20 can be separated by great distances and are only connected to each other by relatively small and lightweight fiber optic cables 24 and 30. Thus, a fiber optic cable electrical isolation barrier is created.

[0025] Although the invention has been described in detail with respect to preferred embodiments thereof, it will be apparent to those skilled in the art that variations and modifications can be effected in these embodiments without departing from the spirit and scope of the invention.