Power Conditioning System

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DEPT. OF ELECTRIC.
ENGINEERING

FACULTY OF
ENGINEERING

UNIVERSITY OF
BOLOGNA

Research field: Power Conditioning Systems

ACTIVE POWER FILTER

POWER CONDITIONIN
SYSTEM WITH ENERGY STORAGE

POWER CONDITIONING SYSTEM
FOR GENERATION SYSTEM

BIBLIOGRAPHY

"...many lacks of the electric energy distribution network may be solved by the use of state of the art power electronics and energy conversion systems...."

Active Power Filters

WHY ACTIVE FILTER?

High power non-linear loads, such as ac/dc rectifiers, cause undesirable phenomena in the operation of power systems like harmonic pollution and reactive power demand. Parallel active filters have been recognized as a viable solution to current harmonic and reactive power compensation. Various active power filter configurations and control strategies have been proposed and developed in the last decade in order to reduce these undesirable phenomena.

HOW DOES IT WORK?

The principle of operation is based on the use of a power electronic structure capable to inject towards the mains the reactive and harmonic current required by the loads. According to their principle of operation, the parallel active filters behave as harmonics current source, leading the source currents to be sinusoidal and in phase with the corresponding voltages, whatever it is the load current absorption.

In a three phase network, a three phase inverter can be used as active filter. The dc side of the inverter is connected to a capacitor in order to stabilize a proper voltage level for the correct operation of the the inverter as current source. The three output phases are connected to the corresponding source phases, through three ac link inductors.

By using a portion of the energy stored in the dc capacitor the active filter is also capable to compensate both the load current unbalance, and to smooth the effect of a source voltage unbalance.

Fig. 1 Simplified scheme of a shunt Active Power Filter

The compensation of harmonic current and reactive power can be obtained by using different solutions for the filtering algorithm. These algorithms generates the reference currents the filter has to inject, then the filtering performance as harmonic compensator are given by using high speed, high precision current regulators.

The filter inverter absorbs a small amount of active power from the grid, that corresponds to the losses in the static switches and in the passive components (dc-link capacitor, ac-link inductors).

WHAT DO WE DO?

Our team has developed several active power filter configurations. These systems are characterized by the use of very simple algorithms, that can work properly also by measuring only one current system (source or load). The current compensation algorithm is based on the tracking of sinusoidal and balanced waveforms reference for the source currents whatever it is the load currents. These techniques don't require the computation of the harmonic contents of the load current, so they are simpler than other techniques have been proposed and can be implemented on low powerful, low cost microprocessors.

In the proposed structure a regulator has the task to generate the reference value of the amplitude of the active current demanded by the mains. This is the value that yields to zero the error in the dc link voltage level. Then a high speed, high precision current regulation has been implemented for the control of the source current Space Vector Modulation (SVM). This current regulator is based on the use of the Space Vector Modulation (SVM), in order to minimize the ripple superimposed on the source current and voltage.

This algorithm employs a reduced number of voltage and current transducers, contributing to reduce the cost of the system. Numerical simulations, carried out on four different configurations of APF, have shown that the performance of the developed, simplified structure of APF are quite similar to that of standard configurations.

An experimental set up has been has been realized for the configuration that requires the measurement of only two source currents and of the dc-link voltage.


Fig. 2 - Reactive power compensation and current harmonic reduction operated by the APF

The proposed active filter works properly also in the case of voltage non idealities, like the voltage unbalance and the low order harmonic distortion. A reliable Three Phase Locked Loop (TPLL) recognizes the fundamental positive sequence of the source voltage also in the worst cases of voltage harmonic distortion. Under these conditions the APF absorbs sinusoidal and balanced currents from the source, that are in phase with the corresponding fundamental positive sequence component of the source voltages.

Fig. 3 - Reactive power compensation operated by the APF under voltage unbalance

The developed regulation structure for active power filter can be usefully employed in combination with other structures connected to the dc link bus. These structures are static converters capable to exchange active power between an energy storage device or an energy source and the dc-link. Throughout the inverter this active power can be easily transferred to the grid.

Power Conditioning Systems with Energy Storage Capability

A Power Conditioning System (PCS) with energy storage capability can be considered a viable solution for improving the quality and the reliability of the electric energy supply. Several tasks can be performed at the same time, such as reactive power compensation, current harmonic reduction, and smoothing of pulsating loads. Moreover, the PCS can operate as Uninterruptible Power Supply (UPS) during short time interruptions of the grid supply. The proposed PCS is a flexible structure that can be coupled to several energy storage device like battery, flywheel, supercapacitor, Superconductive magnetic energy storage (SMES).

In practical applications a high reliability power supply is required for critical loads. In general, this requirement is fulfilled by a standard UPS configuration based on the series connection of a Voltage Source Inverter (VSI) between the grid and the load. The energy is usually stored in a lead acid battery bank. It can be noted that, about 90% of grid faults are very short (less of 1s), and in the remaining cases the UPS supplies the load for the time interval required for starting up a diesel or turbine generator set. In these cases the UPS supplies the load for no more than 30 seconds. As a consequence, for these high power and low energy applications the lead acid battery bank is often oversized, and an extra cost is paid for an energy reserve that will never be used.

Fig. 4. Schematic drawing of the PCS structure

Using new types of energy storage devices, such as superconducting magnets, flywheels or supercapacitors, which are more suitable for high power and low energy applications, it is possible to better exploit the energy reserve.

The energy storage device is connected to a dc-dc converter allowing the bi-directional transfer of energy with the dc-link intermediate bus. The topology of the dc-dc chopper depends on the type of energy storage device.

These new storage devices allow additional tasks to be performed using the same hardware structure required for the UPS operation. The additional tasks consist of reactive power compensation, current harmonic reduction, load unbalance compensation, and smoothing of pulsating loads. In this way, the UPS behaves as a Power Conditioning System (PCS) when the grid supply is present, improving significantly the power quality in the grid section next to the Point of Common Coupling (PCC).

Fig. 5 Flicker compensation

In this research activity the principle of operation of several configuration of PCS have been discussed. The analysis has been focused on the control of the power flow among the system components. Numerical analysis and simulations have been performed to demonstrate the effectiveness of the PCS to reduce flicker phenomena and to compensate harmonic current components and reactive power and to operate as Uninterruptible Power Supply (UPS).

Then a laboratory scale prototype has been assembled in the LAME laboratory of the Department of Electrical Engineering of Bologna. In this prototype a flywheel, driven by an induction machine, has been used as storage device.

Power Conditioning Systems for Generation System

Power conditioning systems suitable to be used in conjunction with an energy source can be derived from the basic structure of an Active Power Filter and its control system.

Two configurations of PCS have been developed in order to allow the energy transfer from an advanced generation systems to the grid:

· New Cogeneration System
· Photovoltaic panels

NEW COGENERATION SYSTEM

In industrial plants and gas networks it is frequently required to reduce pressure of steam or gas to a lower value. The pressure reduction is carried out, in most cases, by using special pressure reducing valves. Theoretically the process of pressure reduction is associated with a loss of entropy that can be utilized, by a special turbine construction, to obtain mechanical energy that can be converted into electric energy by a generator.

A basic question determining the application of the system is the return time on the investment. Return times calculated from the investment cost and the cost of electric energy are relatively short particularly in developing countries having a high cost of electric energy.

Fig. 6 Block diagram of a co-generation plant and
its control system

The pressure reduction is associated with a waste of entropy which can be saved by the system described. Examples for applications where pressure reduction is frequently needed are chemical and food processing plants and hospitals where low pressure steam is used for sterilization besides central heating.

A special feature of a turbine using a relatively low pressure working medium is the need of high speed operation. Thus a high speed (12,000 rpm) permanent magnet, three-phase synchronous generator, is direct connected to the turbine shaft. The electric energy produced can be fed back into the existing utility mains by an ac/ac power converter. The converter system is composed by a three-phase diode rectifier supplying the dc link of a voltage source inverter. The parallel connection to the ac line is made trough a three-phase link reactor.

Fig. 7 Electrical scheme of the PCS used to interface a synchronous generator with the grid

The aim of this work is the analysis of a control algorithm for the electric power conditioner of the co-generation system, which could be implemented on a DSP based controller. The features of this controller are:

·         possibility to use the power conditioning system for additional tasks
·         simplified structure
·         low computational time
·         robustness and reliability.

The main feature of this structure of power conditioning system is its possibility to behave as an active filter, i.e. compensation of reactive power and current harmonics of non-linear loads connected to the same mains. This possibility comes out looking at the structure of the dc/ac conversion system, which is similar to that of an active filter.

In order to compensate reactive power and current harmonic in addition to the rated active power, the dc/ac converter has to be oversized. This additional investment cost is justified thanks to the good performances obtained by the active filter.

PHOTOVOLTAIC PANELS

Photovoltaic technology is the most promising candidate for the large scale spreading of renewable energy source. In order to contribute to lower the cost of new installation of photovoltaic panels, the activity research of our group has been targeted to the developed of very simple power electronic conversion structure for the transfer of the photovoltaic energy to the grid. We have developed two structures of power conditioning systems for PV panels

· A three phase, single-stage converter, suitable for large photovoltaic power plants.
· A single phase, single-stage converter, suitable for small photovoltaic generation systems

THREE PHASE, SINGLE-STAGE CONVERTER

The Photovoltaic sources are combined with the structure of a power active filters in order to regulate both the active and the reactive power injected into the mains. In this way, the system is able to compensate undesired load characteristics such as phase unbalances, power flickers, and low-order current harmonics, improving the power quality.

In order to avoid an intermediate dc-dc chopper, a direct coupling between the PV panels and the dc-link of the voltage source inverter is utilized. The dc-link voltage is continuously regulated between upper and lower limits for tracking the maximum power point of the PV panels. The proposed MPPT algorithm is based on real-time processing of current and voltage ripple of the PV source that is generated by the switching operation of the PWM inverter. The system performance is investigated emphasizing the transient behavior in presence of sudden variations of the solar irradiance.

Fig. 8 - Schematic diagram of the three - phase PV generation system

SINGLE-PHASE, SINGLE-STAGE CONVERTER

A new maximum power point tracking algorithm for single-stage converters connecting photovoltaic panels to a single-phase grid has been developed.

The algorithm is based on the analysis of the current and voltage low-frequency oscillations introduced in the PV panels by the single-phase utility grid. The proposed control technique allows the generation of sinusoidal grid currents with unity power factor. The algorithm has been developed to allow an array of PV modules to be connected to the grid by using a single-stage converter. This simple structure yields higher efficiency and reliability when compared with standard solutions based on double-stage converters configuration. The converter has been developed for the installation on very small PV generation systems like photovoltaic roofs that now give an important share of new installations of PV panels.


Fig. 9 - Schematic diagram of the single - phase PV generation system	Fig. 10 - P-V and I-V characteristic during the start up and a step change of the solar irradiance (experimental results)

For further information about power conditioning systems, please feel free to contact

Claudio Rossi (mail:claudio.rossi@mail.ing.unibo.it)

ACTIVE POWER FILTER

POWER CONDITIONIN
SYSTEM WITH ENERGY STORAGE

POWER CONDITIONING SYSTEM
FOR GENERATION SYSTEM

BIBLIOGRAPHY