This is a US National Phase application claiming priority to International Application No. PCT/EP2011/060136 having an International Filing Date of Jun. 17, 2011, incorporated herein in its entirety by reference.
The present invention relates to an electrostatic precipitator unit with at least two individual power supplies for pulsed operation of electrostatic precipitators, wherein the power supplies are powered by a common feeding. The invention further relates to methods of operation of such an electrostatic precipitator unit.
With the increasing concern for environmental pollution, the reduction of particle emissions by using Electrostatic Precipitators (ESPs) is a highly important issue for coal fired power plants. ESPs are highly suitable dust collectors. Their design is robust and they are very reliable. Moreover, they are most efficient. Degrees of separation above 99.9% are not unusual. When compared with fabric filters, their operating costs are low and their risk of damage and stoppage owing to functional disorders is considerably smaller. They are a natural choice in many cases. In an ESP, polluted gas is conducted between electrodes connected to an ESP power supply. Usually, this is a high-voltage transformer with thyristor control on the primary side and a rectifier bridge on the secondary side. This arrangement is connected to the ordinary AC mains and thus is supplied at a frequency, which is 50 or 60 Hz. Power control is effected by varying the firing delays of the thyristors. The smaller the firing angle, i.e., the longer the conducting period, the more current supplied to the ESP and the higher the voltage between the electrodes of the ESP. Modern ESPs are divided into several bus sections to increase collection efficiency. Each of these bus sections has its own power supply (PS), which is controlled individually and has a typical output power range of 10-200 kW and an output voltage range of 30-150 kVDC.
Modern ESP's power supplies are often based on resonant converters because of the transformer's shortcomings and to have soft switching for a wide operation range. One exemplary power supply for ESP's is known from US 2009/0129124.
Modern ESP's are often operated in pulsed mode. Pulsed operation of an electrostatic precipitator considerably influences the mains power quality, since it can result in high line current distortion (total harmonic distortion) and unbalanced mains phase loading. Thus, interruption and malfunction of equipment fed by a common feeding, audible noise, heating in transformers, generators and power lines, electric resonance in the mains, and mechanical oscillations in generators, engines, etc., can be caused. These problems can become worse if, for example, a group of power supplies with pulsed operation are fed by a common feeding, since the pulses in different supplies can occur at the same instant. On the other hand, if the pulses in each power supply are scheduled in an optimal way it should be possible to reduce the undesirable effects in this type of operation, so that the power consumption becomes more continuous in time. At the moment, the power supply systems for ESP application do not use a strategy for pulse scheduling and don't allow for such. Therefore, arbitrary current waveform at the input occurs.
This is where the present invention has its origin, proposing a new and improved electrostatic precipitator setup for the operation of at least two power supplies connected to a common feeding. The power supplies provide pulsed power output for pulsed operation of one or several ESPs operated with said power supplies.
So, this invention deals with mains' energy quality optimization for a group of power supplies feeding an Electrostatic Precipitator (ESP) with pulsed operation.
The modified electrostatic precipitator device and the corresponding optimization strategy that will be presented herein, can be applied to any group of power supplies operating in pulsed mode. Thereby, a considerable improvement of the line current by just controlling the starting time of the different pulses can be achieved without any additional means.
The “best case” for an ESP system occurs when all supplies at full load operate feeding the ESP with continuous power, when the mains' phase currents are balanced, and when the relation between the average value of the power consumption and the harmonic components are at the lowest. Considering this, the main idea of this optimization is to allow arrangement of the pulses in an optimal sequence, so that the group of pulsed power supplies has similar line behaviour to that of an equivalent single power supply operating in continuous mode.
So, one of the cores of the invention can be summarized as provided below.
The proposed modified electrostatic precipitator unit allows arrangement of the pulses of the individual power supplies in an optimal sequence by shifting the initial pulses of each power supply by a delay time with respect to one reference. The aim is essentially to provide a structure, which enables filling of the gaps between the reference pulses with the pulses of the other power supplies.
Best behavior is observed when the pulses are essentially uniformly distributed within the reference pulse period and by shifting all pulses of one field by the same delay with respect to the other fields.
More specifically, the present invention relates to an electrostatic precipitator unit with at least two individual power supplies for pulsed operation of electrostatic precipitators, wherein the power supplies are powered by a common feeding (mains).
In accordance with the present invention, each individual power supply comprises a control unit, and these individual control units are at least indirectly connected among each other by communication lines allowing for a controlled relative scheduling of the pulsed operation of the individual power supplies.
The power supplies of the unit, which are powered by a common feeding, can be powering at least two individual electrostatic precipitators, e.g., each being part of a different exhaust duct. In each of these ducts, there may be several bus sections powered by individual independent power supplies also powered by a common feeding.
According to another embodiment, the power supplies are part of one electrostatic precipitator, typically powering different bus sections thereof or powering different modules within one bus section.
The control may be realised in the unit either in a manner such that there is communication lines between the individual control units and one control unit takes the lead and controls the relative scheduling, or this control is shared between the control units forming part of individual power supplies. On the other hand, it is also possible and in accordance with another preferred embodiment of the present invention, that the unit further comprises a control computer (which can be dedicated computer, or which can be a computer also dealing with other tasks in the precipitator or in the power plant) connected to the communication lines and controlling the scheduling of the power supplies.
The relative scheduling of the pulsed operation of the individual power supplies can be effected in that one power supply is defined to be the reference power supply, and the initial pulses of each further power supply are shifted by controlled delays with respect to the pulses of the reference power supply, so as to fill the gaps between the reference pulses with the pulses of the further power supplies. In this case, the controlled delays can be determined so as to essentially uniformly distribute the pulses of the further power supplies in the pulse period of the reference power supply, wherein preferably, if the accumulated pulse width of all power supplies is smaller than the largest pulse period, the controlled delays are determined such that the gaps between all pulses are essentially identical, if the accumulated pulse width of all power supplies is equal to the largest pulse period the controlled delays are determined such that there are no gaps between pulses, and if the accumulated pulse width of all power supplies is larger than the largest pulse period, the overlap length of all pulses is equal.
Typically the power supplies used in this context are high voltage transformer based, preferably IGBT (integrated gate bipolar transistor) based converters, preferably series loaded resonant converters allowing for high power and high voltage, preferably said high power being in a range of 10-200 kW and/or said high voltage being in a range of 50-150 kV DC.
According to yet another preferred embodiment, the system is adapted to operate with DC pulses provided to the electrostatic precipitators having pulse widths in the range of 0.1-20 ms, and/or having pulse periods in the range of 0.5 ms-2 s, wherein preferably the pulse ratio defined as the pulse width divided by the pulse period is in the range of 1- 1/2000.
The electrostatic precipitator may comprise at least one bus section for pulsed operation and at least one further bus section for continuous operation.
Furthermore the unit may, in accordance with another preferred embodiment, comprise at least three power supplies, preferably at least four power supplies, most preferably at least six power supplies, preferably all of them connected and powered by a common feeding and at least indirectly connected by communication lines. In case of large precipitator units, there may be up to 24 or even 36 power supplies or more, which are individually controlled and scheduled, with all powered by a common feeding.
Furthermore, the present invention relates to an industrial application comprising an electrostatic precipitator unit as described above, e.g., a power plant, preferably a fossil fuel operated power plant, most preferably a coal operated power plant, the exhaust gases from which are cleaned by the electrostatic precipitator unit. The electrostatic precipitator unit can also be used for another dust producing process, such as a sinter band sieving system, a cement manufacturing process, or the like.
In addition to the above, the present invention relates to a method for the operation of a unit as outlined above, wherein preferably one power supply is defined to be the reference power supply, and wherein the initial pulses of each further power supply are shifted by controlled delays with respect to the pulses of the reference power supply, so as to fill the gaps between the pulses of the reference power supply with the pulses of the further power supplies, and wherein preferentially the reference power supply is the power supply of the system which has the largest pulse period.
According to a preferred embodiment of this method, the controlled delays are determined so as to essentially uniformly distribute the pulses of the further power supplies in the pulse period of the reference power supply, wherein preferably, if the accumulated pulse width of all power supplies is smaller than the largest pulse period, the controlled delays are determined such that the gaps between all pulses are essentially identical, if the accumulated pulse width of all power supplies is equal to the largest pulse period the controlled delays are determined such that there are no gaps between pulses, and if the accumulated pulse width of all power supplies is larger than the largest pulse period, the overlap length of all pulses is equal.
Further embodiments of the invention are laid down in the dependent claims.
Preferred embodiments of the invention are described in the following with reference to the drawings, which are for the purpose of illustrating the present preferred embodiments of the invention and not for the purpose of limiting the same. In the drawings,
Usually, an ESP system is divided into several bus sections to improve particulate collection efficiency. In small systems, only 2 or 3 bus sections are connected in series, and in large ones, several bus sections are connected in parallel and in series. Different power supplies with different power ratings often energize the bus sections in order to optimize the collection efficiency of a single bus section.
Each of the fields 6-8 has two rows of individually powered precipitator systems (four cells and six fields), leading to 24 bus sections, and to this end, 24 power supplies (PS) are provided to energize the precipitators. The general topology of such a power supply will be discussed further below. The power supplies are energized via the common feeding 1, which via a low or medium voltage line 2 and distribution transformers 3 connects to the individual power supplies. In other words, the totality of the power supplies is connected to a common feeding system 1, and if these power supplies or at least a fraction thereof are operated in pulsed mode, the load on the main can be heavily unbalanced.
A high frequency, three-phase mains power supply 11 for powering one of the individual precipitators in a setup according to
For pulsed operation of such a power supply, the full bridge inverter is operated in pulsed mode via the control unit 23 and the drivers 22. In order to control the whole system, there is provided a current and voltage sensor 16, the output of which is used for controlling the unit 23.
The present invention is not limited to high frequency, three-phase power supplies as illustrated in
The charging method for each ESP power supply 11 can be either a continuous mode 25 or a pulsed mode 26 of current 27; see
Problems occur when a group of ESP power supplies 11 are operating in pulsed charging mode and fed by the same mains 1, as illustrated in
The current pulse from each ESP power supply has variable pulse width PW and variable pulse period time TP as defined in
The example in
The second example illustrated in
The proposed solution is that the different ESP power supplies or groups of ESP power supplies communicate as illustrated in
A different approach for the communication interface can be to use a dedicated host computer, for managing the time slots for the controller unit in each local ESP power supply.
The variation in the power flow can be minimized by using a line distortion optimization algorithm in each local controller. The purpose is to limit the number of pulses from different ESP power supplies that occurs at the same instant; see
To summarise what distinguishes the invention from existing technologies is that:
This allows to solve at least the following problems:
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10166579 | Jun 2010 | EP | regional |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/EP2011/060136 | 6/17/2011 | WO | 00 | 3/25/2013 |
Publishing Document | Publishing Date | Country | Kind |
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WO2011/157831 | 12/22/2011 | WO | A |
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