1. Field of the Invention
This invention relates to apparatus incorporating solid state inverters for providing compensation on a three-phase electric power line. More particularly, it relates to apparatus which injects a compensating voltage into the power line in series, such as for example, a dynamic voltage restorer or an active power line conditioner. The inverters are connected to the primary windings of the single transformer and are line powered through additional secondary windings on the transformer.
2. Background Information
There are various types of apparatus for providing compensation on electric power lines which incorporate power inverters. One such system is the Dynamic Voltage Restorer (DVR) an example of which is described in U.S. Pat. No. 5,329,222. The DVR injects a voltage in series with a utility source for the purpose of making up missing voltage that occurs when upstream faults cause short term voltage sags. The DVR described in the above mentioned patent uses three single-phase inverter modules with a common dc link which are connected to the utility line through injection transformers. The common dc link approach allows the use of one common source of energy to supply three separate injection systems, one for each phase, in a three-phase system. This common source of energy can be for instance, a capacitor bank or an interface to draw the required power from the line, even when the voltage is sagged. The injection transformers isolate the power electronics from the power line allowing the maximum injection capacity to be less than the source voltage. It has been shown that the vast majority of sags in utility systems are less than 50% of the phase-to-neutral voltage. The DVR can correct these sags with an inverter rating of less than the connected downstream load which is protected by the DVR.
In practice, the inverter modules of the DVR can be implemented as individual power devices, if the ratings are high enough; series strings of power devices as is the present practice and described in U.S. Pat. No. 5,347,166; or single-phase inverter modules as described in U.S. Pat. No. 4,674,024. Larger units, using magnetics for phase-staggered waveform mixing are also possible.
As previously stated, the topologies which have an injection transformer allow a common dc link and energy storage interface since the injection transformer provides the required isolation. This technique requires three single-phase or one three-phase transformer with a five leg core since such a transformer must support zero sequence voltage components.
In many applications, it is possible, and even desirable to derive the power required to make up a sag from the source. No significant energy storage is provided. For instance, a line powered DVR is known in which a three-phase transformer having a rectifier bridge connected to the secondary draws power directly from the power line. This arrangement not only requires the injection transformers but also the three-phase transformer for drawing power from the line. When the input voltage sags, the line powered DVR responds by drawings more current from the source to provide the power required by the load. The product of the positive sequence source voltage and current remains constant.
patent application Ser. No. 09/017,034, filed Feb. 2, 1998, proposes a neutral point connected DVR in which a three-phase inverter wye connects the secondary windings of a three-phase load transformer to the load end of the power line. This eliminates the need for injection transformers and in fact does not require any additional transformers where the load transformer is already present in the system. In a line powered version of the neutral point connected DVR, the inverter is powered from the load side. If the inverter is to optimized for fifty percent injection, a line power transformer is required. This means that a total of two transformers are required.
Commonly owned Patent Application entitled Power Inverter Apparatus Using The Load Transformer Of An AC Power Line To Insert Series Compensation (Attorney Docket No. RDM 97-037), concurrently filed in the name of C. G. Hochgraf, discloses several methods of connecting an inverter to the primary or source side of a transformer in a DVR. This system has some advantages in common with the neutral point connected DVR. It keeps the transformer voltage constant during a sag condition, whereas the neutral point connected DVR keeps only the load voltage constant. In this version, only one transformer is required in most cases. In versions of this system where the primary winding is delta connected, and a three-phase inverter with a common dc link is used to provide the voltage compensation, one single-phase injection transformer is needed to provide the required isolation.
Another type of compensation equipment which uses series injection with transformer coupling is the active power line conditioner (APLC). The APLC includes a series inverter and a parallel inverter connected by a common dc link. The parallel inverter transfers real power to and from the series inverter and also supplies harmonic currents and the reactive component of the load current. The series path in the APLC suffers from the same limitations as the DVR due to the series injection transformer discussed above patent application Ser. No. 09/017,034, filed Feb. 2, 1998, discussed above, discloses connection of the parallel inverter to the load side of the power line. While this APLC eliminates the need for injection transformers by using the neutral point connection for the series inverter, it requires an additional transformer for the connection of the parallel inverter to the load side of the power line, if the inverter system is partially rated.
While these improved DVRs and APLCs have reduced the cost of an installation by eliminating or reducing the number of transformers needed for series injection of voltage compensation into a power line, the APLC, and the DVR if it is to be line powered and take advantage of the partial rating, still require a separate transformer connected to the power line.
There is a need therefore for improved DVRs and APLCs which do not require a separate or additional transformer to draw power from the line.