Electrical power conditioner

Details Electrical power conditioner Electrical power conditioner

2000-08-04

2003-09-23

Toatley, Jr., Gregory J. (Department: 2836)

Electricity: electrical systems and devices

Safety and protection of systems and devices

Transient responsive

C307S105000

Reexamination Certificate

active

06624997

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a transformer based power conditioner to provide high purity electrical power, preferably having capacitor based circuitry on the transformer secondary to provide noise filtering, and preferably having semiconductors on the transformer primary functioning as suppressors for transient electrical surges.
2. Description of the Related Art
High purity electrical power generally means that the power is substantially free from voltage spikes and sags with no significant neutral-to-ground voltage. A number of electronic devices require such high purity power. Among them are medical imaging systems such as X-rays, computer tomography, magnetic resonance imaging, and radiation treatment systems. All of these devices require a large amount of current but only for a short duration, that is, when the X-ray or magnetic generator is operational. The power during this exposure must be clean for good image quality. Additionally, the stand-by power between exposures must also be clean for the reliable operation of the computerized control and imaging processing subsystems, which operate between exposures. An example of a power conditioner to provide high purity electrical power is shown in U.S. Pat. No. 5,012,382.
For these types of systems, the voltage drop during the exposure period should be minimal, typically less than about 8%. This voltage drop is a result of the impedance of all upstream wiring, connections and transformers in the circuit. The reason for this limitation on voltage drop is that the exposure duration in the medical imaging systems is often calculated based on the magnitude of the line voltage present immediately before exposure. Significant changes in this voltage during exposure can result in unpredictable dosages. It is also important that operation of the generator does not produce voltage sags or spikes on the power lines which interfere with the reliable operation of other system components.
Other systems requiring high purity power include automated test equipment and telecommunications equipment. Automated test equipment (ATE) is used in a number of applications, one of which is the manufacture of semiconductors and printed circuit boards. For example, during the testing of semiconductors (like computer microprocessors) the ATE provides an array of inputs to the microprocessor and detects the response and response speed to those inputs. Impure power can cause the ATE to interpret the responses incorrectly, and as a result, eliminate good parts or retain bad parts. Printed circuit board (PCB) ATE provides an array of inputs to PCBs and determines if the traces have continuity to the desired areas of the PCB. Impure power can cause the ATE to give erroneous indications about PCB continuity. Manufacturers of such equipment continue to reduce the semiconductor and PCB test voltages to reduce the power requirements, allowing the ATE to be smaller and more cost effective for the end user. However, with lower operating test voltages, the susceptibility of the ATE to impure power becomes greater and the need for cleaner power increases.
Telecommunications equipment also requires clean power for similar reasons. Clean power allows the communications equipment to transmit with higher quality. This results in better sound and data quality and fewer dropped connections. Another advantage is that clean power ensures that telecommunications equipment will continue to operate without interruption for longer periods.
Surge suppression devices or L-C filters, or both, on the building wiring near the load provide another way to achieve the desired power quality. These devices shunt impulses above certain voltage or frequency levels from one wire to another. They typically are comprised of metal oxide varistors (MOVs), silicon avalanche diodes (SADs), gas discharge tubes, capacitors and inductors, and often incorporate resistors. An example of a transient voltage surge suppressor using MOVs and silicon surge suppression diodes is discussed in U.S. Pat. No. 4,802,055. Another example of MOVs used for electrical transient suppression is described in U.S. Pat. No. 5,038,245. Still another example of a way to suppress transients is discussed in U.S. Pat. No. 4,156,838, which does not employ magnetic coupling.
There are several limitations with many of these types of devices and filters. They shunt away voltage spikes or dips (“normal mode noise”), but in doing so increase the current in the neutral conductor, creating neutral-ground potentials (“common mode noise”) which can be even more damaging or disruptive than normal mode noise. Their effects are limited since they can only protect to a certain voltage or frequency level. For example, MOV's and avalanche diodes “wear out” with time and lose their effectiveness.
For this protection scheme to be as effective as possible, the inductance in series with the shunt elements (surge suppressors and capacitors in L-C filters) must be minimized. The wire length connecting the suppressors to the conductors makes a measurable difference in their effectiveness. Often, these devices are connected to the power lines by wires which are five to 50 feet in length due to physical placement constraints in the field or limited knowledge on the part of the installers, or both. The length of the wires between the power lines and the suppressors also limits the effectiveness of scaling the product for optimal performance. Surge suppressors are also frequently sized inappropriately to simplify installation at the expense of performance.
Also limiting the performance of shunt elements is the lack of ability to improve performance by cascading the filtering elements. Many of these filters employ single stage filtering that limits the effectiveness of the elements within the suppressors.
In an attempt to minimize lead length, maximize performance, and make site performance consistent, series power line filters have been used in these applications. Many of these devices have a series element (typically an inductor) that adds impedance to the line and increases cost significantly, but improves high frequency filtering performance greatly. The added series impedance has negative effects on power quality, particularly voltage regulation, during high current demand, as described earlier. It also limits the cost effectiveness of scaling these types of products.
Another alternative solution is to use a conventional shielded isolation transformer. The shield in the transformer increases the isolation of the output from the conducted ground (common mode) noise. A neutral-ground bond converts common mode noise to normal mode noise and allows a more effective use of filters and surge suppressors on the transformer as described above.
The extra impedance of the series power line filters (with inductors) or shielded isolation transformer methods results in lower power quality when the filters or transformers interact with computer loads. Modern computers have “switch mode” power supplies which draw their current in short bursts where the change in current with respect to time (di/dT) is fast, being the equivalent to 120 Hz or greater, instead of the usual 60 Hz for most conventional loads. Even at low load factors, these rapidly varying loads cause conventional transformers to produce outputs with a flat-topped voltage waveform and voltage spikes. Other switching transients within the system only add to the problem.
Often shielded isolation transformers and suppression/filtering devices are combined in the field in an attempt to provide the quality power desired. The limitations mentioned above also apply to this combination.
In contrast to the use of passive shunt element based solutions, other attempts to provide high purity power for these systems have utilized conventional voltage regulation schemes. Those attempts have generally not been successful. Electronic-controlled tap-switching voltage regulators are undesirable because the step voltage changes produced by tap changes du

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