Electric power conversion systems – Current conversion – Using semiconductor-type converter
Reissue Patent
1998-10-06
2001-04-03
Nguyen, Matthew (Department: 2838)
Electric power conversion systems
Current conversion
Using semiconductor-type converter
C363S058000
Reissue Patent
active
RE037126
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a multilevel voltage source inverter with separate DC sources, and more particularly to a multilevel voltage source inverter with separate DC sources including an apparatus and a method for use in flexible AC transmission system (FACTS) applications such as compensating reactive power and voltage balancing.
BACKGROUND
With long distance electrical power transmission and load growth, active control of reactive power (VAR) is indispensable with regard to stabilizing power systems and maintaining supply voltages. Static VAR generators (SVGs) using voltage-source inverters have been widely accepted as the next generation of reactive power controllers for power systems replacing conventional VAR compensators such as Thyristor Switched Capacitors (TSCs) and Thyristor Controlled Reactors (TCRs).
Delivering power from a power generating station to the ultimate power consumers over long transmission lines can be very costly for an electric utility. The electric utility passes on these costs to the ultimate consumers as higher electricity bills. Inductive and capacitive losses affect a reactive component of power which is measured in volt-ampere-reactive (VAR) units. These reactive power (VAR) losses may be compensated using a static VAR compensator to more economically transmit thereby reducing overall electricity bills as well as stabilizing the supplied voltage to the end user.
The state of the art VAR compensating approach uses transformer coupling voltage source inverters. A transformer coupling voltage source inverter comprising eight six-pulse converters connected in either a zig-zag, wye or delta configuration has a 48-pulse or a 48-step staircase inverter output voltage waveform which dramatically reduces harmonics. The major problem of using this transformer coupling approach resides in the transformer as a function of harmonic neutralizing magnetics. The transformer with the inherent harmonic neutralizing magnetics deficiency:
(a) is the most expensive equipment in the system;
(b) produces approximately 50% of the total system losses;
(c) occupies approximately 40% of the system layout; and
(d) causes difficulties in system control due to DC magnetizing and surge overvoltage problems resulting from saturation of the transformers on the transient state.
In recent years, a relatively new type of inverter, a multilevel voltage source inverter, has attracted the attention of many researchers. The transformerless multilevel inverter can reach high voltage and minimize induced harmonics as a function of inverter structure.
A multilevel, referred to as M-level, diode clamped inverter can reach high performance without the benefit of transformers. This inverter does, however, require the implementation of additional clamping diodes. The number of diodes required is equal to (M−1)*(M−2)*3 for an M-level inverter. For example, if M=51, for direct connection to a 69 kV power system, then the number of required clamping diodes will be 7350. These clamping diodes not only increase the cost of the system but also cause packaging/layout problems and introduce parasitic inductances into the system. Thus, for practicality, the number of levels of a conventional multilevel diode clamped inverter is typically limited to seven or nine levels.
A relatively new inverter structure, the multilevel flying capacitor inverter has the capability to solve the voltage balance problems and aforementioned problems associated with the multilevel diode clamped inverters. The required number of flying capacitors for an M-level inverter, provided that the voltage rating of each capacitor used is the same as the main power switches is determined by the formula, (M−1)*(M−2)*3/2+(M−1). Using the assumption of having capacitors with the same voltage rating, an M-level diode clamped inverter requires only (M−1) capacitors. Therefore, the flying capacitor inverter requires capacitors of substantial size compared with the conventional inverter. In addition, control is very complicated and higher switching frequency is required to balance the voltages between each capacitor in the inverter.
A multilevel cascade inverter with separate DC sources for reactive power compensation in AC power systems which is directed toward overcoming and is not susceptible to the above limitations and disadvantages is described herein. The multilevel voltage source inverter having separate DC sources eliminates the excessively large number of transformers required by conventional multipulse inverters, clamping diodes required by multilevel diode-clamped inverters and flying capacitors required by multilevel flying-capacitor inverters. The multilevel voltage source inverter having separate DC sources also has the following features:
(a) the multilevel voltage source inverter having separate DC sources is more suitable to high voltage, high power applications than conventional inverters;
(b) the multilevel voltage source inverter having separate DC sources generates a multistep staircase voltage waveform with the switching of each device only once per line cycle, thus reaching a nearly sinusoidal output voltage approximation by increasing the number of voltage levels;
(c) since the multilevel voltage source inverter having separate DC sources consists of cascade connections of a plurality of single-phase full bridge inverters fed with a separate DC source, neither voltage balancing nor voltage matching of switching devices is required; and
(d) system packaging and layout is streamlined due to the simplicity and symmetry of structure as well as the minimization of component count.
Thus, a need for a multilevel cascade voltage source inverter with separate DC sources for reactive power compensation in AC power systems is clearly evident.
OBJECTS OF THE INVENTION
Accordingly, it is an object of the present invention to provide a new and improved multilevel cascade voltage source inverter and more specifically a multilevel cascade voltage source inverter for connecting to an AC high voltage, high power system.
It is another object to provide a wye configured multilevel voltage source inverter for FACTS applications such as VAR compensation and voltage balancing of AC power systems.
It is another object to provide a delta configured multilevel voltage source inverter for FACTS applications such as VAR compensation and voltage balancing of AC power systems.
It is another object to provide a multilevel voltage source inverter for connecting to an AC high voltage, high power system for a variety of applications such as fuel cells, photovoltaic utility interface systems.
It is another object to provide a method for controlling the multilevel voltage source inverter to supply a sinusoidal approximation power waveform to an AC high voltage, high power system for a variety of applications from a plurality of DC voltage sources.
Further and other objects of the present invention will become apparent from the description contained herein.
SUMMARY OF THE INVENTION
In accordance with one aspect of the present invention, a multiple voltage source inverter for connecting to an AC power system comprising a plurality of full bridge inverters having a primary node and a secondary node, each of the full bridge inverters having a positive node and a negative node, each of the full bridge inverters having a voltage supporting device electrically connected in a parallel relationship between the positive node and the negative node; at least one cascade inverter phase, each of the cascade inverter phases having a plurality of the full bridge inverters, each of the cascade inverter phases having a consistent number of the full bridge inverters with respect to each phase, each of the full bridge inverters in each cascade inverter phase interconnected in a series relationship with the secondary node of one of the full bridge inverters connected to the primary node of another full bridge inverter, the series interconnection defining a first full bridge inverter and a
Lai Jih-Sheng
Peng Fang Zheng
Jones & Askew LLP
Lockheed Martin Energy Systems, Inc.
Nguyen Matthew
LandOfFree
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