Electricity: power supply or regulation systems – Including a transformer or an inductor – With compensation
Reexamination Certificate
2001-06-28
2002-11-12
Sterrett, Jeffrey (Department: 2838)
Electricity: power supply or regulation systems
Including a transformer or an inductor
With compensation
C361S143000
Reexamination Certificate
active
06479976
ABSTRACT:
BACKGROUND OF THE INVENTION
Current transformers are widely used in the measurement of alternating electric current. Current transformers provide an isolated secondary current that is proportional to, and smaller than, the primary current that is being measured. The primary winding of a current transformer is connected in series with the primary current that is to be measured. A secondary winding is magnetically coupled to the primary winding by a suitable magnetic core. The secondary winding is normally connected to a load that has low impedance so that secondary current can flow freely. A secondary current is induced in the secondary winding by magnetic coupling of the primary winding to the secondary winding, the magnetic coupling being strengthened by the magnetic core that is common to both windings. The secondary current is proportionally smaller than the primary current by the turns ratio of the primary and secondary windings (not taking current transformer errors into account). The primary winding frequently consists of only one turn, which is often just a current-carrying conductor installed through an opening in the middle of the current transformer magnetic core. The secondary winding usually consists of multiple turns wrapped around the magnetic core.
In order for the secondary current generated by a current transformer to be an accurate representation of the primary current, the impedance of the secondary circuit must be kept low so that current can flow freely. The impedance of the secondary circuit is often called the “burden.” The burden generally includes all impedances in the loop through which the secondary current flows, including stray winding impedances, stray impedances of connecting conductors, and the impedances of any other components connected in the loop (such as current-sensing resistors and relay operating coils). In order for a current transformer to drive a secondary current through a non-zero burden, a voltage must be induced in the secondary winding. The induced voltage is proportional to secondary current and is proportional to the burden, in accordance with Ohm's law (voltage equals current times impedance). The induced voltage is induced in the secondary winding by a fluctuating induction level in the magnetic core (the induced voltage is proportional to the rate of change of magnetic flux in accordance with Faraday's Law). The fluctuating induction level is associated with a magnetizing current in accordance with well-known electromagnetic principles. The magnetizing current accounts for most of the error in the secondary current. Generally speaking, the accuracy of a current transformer is inversely related to the burden of the secondary circuit. A higher burden causes the secondary current to be a less accurate representation of the primary current.
FIG. 1
illustrates one prior-art configuration that is commonly used to sense alternating current. A primary current J
1
flows in a primary conductor
4
which has an insulating covering
3
, both of which pass through a magnetic core
1
. Primary conductor
4
functions as a primary winding with only one turn. Though shown with one end disconnected, primary conductor
4
is normally connected as part of a larger electrical or electronic system. Magnetic core
1
has a secondary winding
2
wrapped around the core to form a current transformer. Winding
2
is shown with ten turns around magnetic core
1
, though the actual number of turns may vary widely depending on the application. Magnetic core
1
is shown as a toroid, though wide variation in current transformer configurations is possible. Secondary winding
2
is connected to a current-sensing resistor RI having relatively low resistance to allow a secondary current J
2
to flow freely. Secondary current J
2
is normally smaller than primary current J
1
by the turns ratio of the current transformer. Current J
2
flows through resistor R
1
, thereby generating a voltage signal V
1
, which is instantaneously proportional to current J
2
. Voltage signal V
1
is usually connected as an input to a larger current monitoring or control system (not shown). If an ideal system were possible (not having current transformer errors and other component imperfections), voltage signal V
1
would be precisely instantaneously proportional to primary current J
1
.
The primary weakness of ordinary current transformers is their inability to measure d-c current when simply connected to a passive linear burden. While the configuration shown in
FIG. 1
functions well with balanced a-c primary currents (with no d-c components), it does not function well with unbalanced currents. For example,
FIG. 4A
shows primary current J
1
as a rectified current waveform (this is one form of pulsed d-c current).
FIG. 4B
shows the secondary current waveform that is typically produced by the configuration shown in FIG.
1
. Times T
1
, T
2
, and T
3
are included for ease of reference and comparison. The waveform is distorted and has a large “zero offset error” (the d-c offset error, which is most obvious whenever primary current is zero amps and secondary current is not zero amps). While the amount of distortion varies greatly between different kinds of current transformers, the large zero offset error generally does not vary much.
FIG. 2
shows a prior-art configuration that may be used to eliminate the zero offset error when measuring pulsed d-c currents. A diode D
1
is added in series with resistor R
1
to prevent zero offset error in the secondary current. This diode adds additional burden to the secondary circuit, thereby contributing somewhat to distortion of the secondary current waveform, as illustrated by
FIG. 4C
(with the waveform of
FIG. 4A
representing the primary current). The amount of distortion present may vary widely, depending on the particular components utilized and the magnitude of the primary current.
FIG. 4D
shows a typical waveform for voltage V
2
(the secondary winding voltage of the configuration shown in FIG.
2
). The negative part of the waveform is the sum of the diode forward voltage drop and voltage signal V
1
. The positive part of the waveform is the reverse voltage across the diode as the diode prevents negative current from flowing. The peak magnitude of this positive pulse increases significantly as the accuracy of the current transformer is improved (by selecting current transformers better suited to this measurement configuration). This large voltage pulse is magnetically coupled to the primary circuit, and may cause problems in applications that are sensitive to noise.
In order to measure d-c current accurately and in an isolated manner, it is common practice to use devices generally known as “Hall-effect current sensors.” These sensors generally combine a Hall-effect sensor and a magnetic core in various ways to enable the measurement of d-c currents and a-c currents (symmetrical or unsymmetrical, with or without d-c components). Though Hall-effect current sensors are widely used, their cost is sometimes prohibitive, and many Hall-effect current sensors lack stability over time, and may therefore require frequent recalibration.
One alternative to Hall-effect current sensors is disclosed in U.S. Pat. No. 6,160,697 to Edel, issued Dec. 12, 2000. That patent describes how a voltage source (or, more generally, a “voltage device”) may be connected in the secondary circuit of an ordinary current transformer and controlled in such a way so as to control the induction level of the current transformer core. In one embodiment, the voltage source is controlled to implement a demagnetizing sequence that demagnetizes the current transformer. After such a demagnetizing sequence, the current transformer is able to measure d-c current for a limited time period (after this time period another demagnetizing sequence is required). However, secondary current is corrupted during the brief demagnetizing sequence, so d-c current cannot be measured continuously utilizing this method.
U.S. Pat. No. 6,160,697 also describes how the vo
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