Diep Nguyen
Carollo Engineers
Walnut Creek, California, USA

In the 90s, power quality has become a major concern for both the utilities and the consumers because of the increasing use of non-linear loads such as variable frequency drives (VFDs). While there are many benefits associated with the use of the VFDs such as energy saving, they, due to the inherent design of the power electronic components, can cause voltage and current distortions better knows as harmonics which can affect the operations of motors, transformers, protective relays and other sensitive electronic equipment connected to the same bus as the VFDs.

Recognizing this concern, IEEE Std 519-1981 was revised and later updated to become IEEE Std 519-1992 which was issued in April 1993. This Standard, also an ANSI Standard, addresses and limits the harmonics (voltage and current) to acceptable values from both the utilities and the consumer's viewpoint. It specifically requires that the harmonic indices for both voltage and current at the point of common coupling (PCC) defined as the point between non-linear loads such as the VFDs and other loads within an industrial plant be as low as 5 percent in some cases.

In order to meet this stringent new limit, in general, most medium size (150 hp and larger) VFD system must be designed such that the reactors and external harmonic filters and/or multiple pulse (above 6-pulse) converters are used. The filters are designed to trap the unwanted harmonics which are predominantly 5th (300 Hz) and 7th (420 Hz) generated by 6-pulse VFDs. Unfortunately, external filter solutions generally require thorough harmonic studies, system computer modeling, automatic control of the filters' connection and periodic tuning of the system parameters in order to avoid system resonant conditions. In addition, the total filter capacitance must be properly sized to prevent the power factor of the system becoming leading which is highly undesirable.

In the past couple years, several VFD manufactures are offering so-called "Clean Power Drives." These drives are designed to meet the distortion limits for both voltage and current at the drives' input regardless of how weak or stiff the power system source is. This paper presents a case study of an industrial application using clean power drives for a water pumping station. This installation successfully meets all stringent requirements of the IEEE Std 519-1992 at the drives' input.

Nguyen Tranh Chieu, Ph.D
21 Dartmouth Road
Mountain Lakes, New Jersey 07046 USA
Tel: (201) 263-6254(W), (201) 299-0019(H)
Fax: (201)263-6473

Taking off from the power pooling concept whereby a group of electric utilities collaborate to coordinate their activities with regard to the generation and transmission of electricity, normative cost allocation structures for the deregulated electric power industry are proposed. Derivations of the cost allocation schemes are made for both the short-run energy exchange pool as well as for the long-run central planning pool using the duality theory of linear programming. Benefits of the proposed structures regarding the adaptability of power pools in the changing regulatory environment as well as policy implications on transmission access are discussed. Applicability of the structures to the electric industry in developing countries are also discussed.

Khai D. Le and Mai Nguyen
ABB Power T&D Inc.
Cary, North Carolina, USA