Electricity: measuring and testing – Measuring – testing – or sensing electricity – per se
Reexamination Certificate
2000-05-03
2003-12-30
Cuneo, Kamand (Department: 2829)
Electricity: measuring and testing
Measuring, testing, or sensing electricity, per se
C324S096000, C324S127000
Reexamination Certificate
active
06670799
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a device for measuring current in high voltage (HV) power systems.
BACKGROUND OF THE PRESENT INVENTION
The concept of the hybrid current transformer with various means of sensing the current and various means of making the electrical to optical conversion is known. The motivation for using optics based measurement and/or signal transmission in HV power systems is that optical signals may be transmitted by inherently insulating means such as optical fibers. The hybrid current transformer combines traditional current sensing methods, including inductive or resistive current sensing, with optical signal transmission. The known current sensing means will be discussed first, followed by optical modulation methods.
CURRENT SENSING
The most direct method of sensing a current is to use a shunt resistor, having low resistance, in line with the sensed current. A voltage will be generated across the resistor proportional to the current through the resistor and the resistance. This method has the advantage over the Rogowski coil (discussed below) in that it does not require time integration, and it can measure DC current in addition to AC current. An example of the shunt resistor method is shown in U.S. Pat. No. 4,629,979. When used for HV applications, this example uses active electronics in the HV environment to transmit an analog or digital optical signal from the HV to low voltage (LV) environments. In the analog case, the optical signal is generated by applying a frequency modulated carrier signal to a light emitting diode.
U.S. Pat. No. 4,070,572 uses a shunt in combination with active circuitry to amplitude modulate an LED light source to transmit the measured current signal from HV to LV. This system uses active components in the HV environment and also places the light source in the HV environment.
U.S. Pat. No. 5,446,372 (also U.S. Pat. No. 5,420,504) primarily involves the physical design of a shunt, but does include the possibility of “electro-optically transmitting” the measured signal using an “electro-optical interface.” The electro-optical interface is not described in detail but the measured signal is digitized prior to transmission.
The most common current sensing means in AC power systems is the inductive current sensor comprising a coil, which is inductively coupled with the sensed current. Within this broad class, there are two sub-classes: 1) devices which produce a secondary current which is proportional to the sensed or primary current, 2) devices which produce a voltage which is proportional to the time derivative of the sensed current. In general, the burden or load resistance placed on a coil will determine which of the two sub-classes apply. Specifically, when the inductive reactance of the coil is larger than the combined resistance of the coil and the burden, then the device will behave according to sub-class 1.
The conventional current transformer used in the power utility industry belongs to sub-class 1. These devices have no intrinsic voltage output, which makes them somewhat less appropriate for driving optical voltage sensors. A coil with sufficiently large inductance can however generate a voltage signal by placing a small resistance across the coil terminal and still behave according to sub-class 1. In this case, the voltage will be proportional to the secondary current and hence also proportional to the primary current. This method of current to voltage conversion is utilized as described by C. McGarrity, et al., AA fiber-optic system for three-phase current sensing using a hybrid sensing technique, @
Review of Scientific Instruments
, Vol. 63, No. 3, pp 2035-2039, 1992, to drive an optical modulator. McGarrity's system is passive, using a current transformer and load resistor to generate a voltage signal which is applied to an interferometer.
A disadvantage of sub-class 1 devices is that they invariably use high permeability materials in the core of the coil. In addition to making the coil heavy, high-permeability materials are generally non-linear and can saturate when measuring large fault currents.
Devices, which fit sub-class 2, are generally referred to as Rogowski coils although other names are sometimes used such as linear coupler. Occasionally, Rogowski coils are classed as current transformers although, strictly speaking, they are a time-derivative of current to voltage transformer. They will operate in this manner even with an infinite load resistance, thus producing no current at all. A load resistor is usually used and can be sized to optimize the transient response of the coil, (see D. A. Ward, J. La T. Exon, “Using Rogowski coils for transient current measurement,”
Engineering Science and Education Journal
, June 1993, pp. 105-113) or to compensate for the thermal expansion of the core (see G. Carlson, F. Fisher, “Voltage and current sensors for a 1200 kV gas insulated bus,” 7
th
IEEE/PES Transmission and Distribution Conference and Exposition
, Apr. 1-6, 1979, pp. 200-207).
In order to measure current, sub-class 2 devices must be used in combination with an integrator to recover the sensed current signal from the time-derivative. Two analogue integrator types can be used: passive or active although the passive integrator is usually only used at higher frequencies (much higher than 50 or 60 Hz power frequencies) (see D. A. Ward.) The difficulty encountered in making a passive integrator that operates at low frequencies is that as the integrator time constant is made larger, the voltage output from the integrator decreases. This can be compensated for to some extent by increasing the Rogowski coil's output voltage (by increasing its mutual inductance) but at the expense of stressing the voltage withstand ability of the coil's winding insulation.
For example using a single pole passive integrator for high-accuracy metering applications having a phase accuracy at 60 Hz of 5 minutes of arc, the integrator pole location should be about 1000 times lower in frequency, or at 60 mHz. The voltage signal from the Rogowski coil will be 1000 times larger than the voltage signal from the integrator i.e. to obtain a 1 V integrator signal, the Rogowski voltage will be 1000 V at 60 Hz. If a bandwidth of 6 kHz is desired, an additional factor of 100 in Rogowski coil voltage must be tolerated, pushing its voltage level to 100 kV. This number will further increase by the over-current factor that is desired. A Rogowski coil and integrator capable of such high voltage levels is not cost justifiable.
A third type of integrator can also be used by digitally sampling the time-derivative signal and subsequently digitally integrating it.
The location of the integrator is also important. If the integrator is located in the LV environment, then the time-derivative signal must be transmitted from the HV to the LV environment. This places large demands on the transmitting means in terms of dynamic range. Either active integration or digital integration can be used in this case, but both pose problems due to amplification of low frequency signal corruption introduced by the optical system.
Low frequency signal corruption can be filtered out for revenue metering applications and as such, integration in the LV environment is appropriate for metering applications.
An alternative to locating the integrator in the LV environment is to place it in the HV environment. Powering a digital or active integrator in the HV environment is not a trivial task. Several powering methods which tap power from the HV line have been used including using an auxiliary current transformer, capacitive dividers, and resistive dividers (see R. Malewski, A High-voltage current transformers with optical signal transmission,@
Optical Engineering
, Vol. 20, No. 1, 1981, pp. 54-57.) All of these methods represent a finite turn-on time when energizing a line which can be a hazard when energizing a faulted line. Batteries may be used to get around this problem but with the added problem of maintaining them. Power
Bull Jeffery David
Jaeger Nicolas August Fleming
Rahmatian Farnosh
Cuneo Kamand
Hollington Jermele
NXT Phase Corporation
Rowley C. A
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