1. 16th INTERNATIONAL SYMPOSIUM on POWER ELECTRONICS –
Ee 2011 NOVI SAD, REPUBLIC OF SERBIA, October 26th - 28th, 2011
RETROFIT POWER SUPPLY OF ELECTROSTATIC PRECIPITATORS ON THERMAL POWER PLANT “MORAVA”
Slobodan Vukosavić*, Željko Despotović**, Nikola Popov*
*University of Belgrade, School of Electrical Engineering, Belgrade, Serbia
**Institute "Mihajlo Pupin",
Belgrade, Serbia

2. INTRODUCTION
Power supply, DC voltage controlled, 100 kV and kW rating, are key component in electrostatic precipitation systems in power plants and industry
Conventional power supplies are based on T/R unit with thyristor control
Electrostatic precipitator (ESP) is controlled by thyristors, using 50Hz high voltage transformer (HVT) and high voltage rectifier (HVR).
This technology was very effective in recent years, but it was not made great progress in ESP systems
Although the mentioned thyristor topology is very robust, simple and flexible, it has a lot of serious disadvantages!!!!!

3. DISADVANTAGE of 50Hz THYRISTOR TOPOLOGY
poor quality input current,
low power factor,
slow response,
low efficiency
significant greater size and weight of HV equipment

4. HVHF Power Supply
Today it is working intensively on development of high voltage high frequency (HVHF) switching power supply on ESP market.
They are gradually introduced in the ESP supply systems with affordable price and they become very competitive in relation to thyristor control combined with T/R set, which has been used in industry for a long time
Applied in ESP applications, HF converters have a significant impact on ESP systems and with aspect of construction, operation and maintenance

5. HYBRID (RETROFIT) Power Supply
In some applications hybrid solution is very convenient, which combine a good performance of HF converters and existing 50/60Hz transformers
Reasons for development of hybrid solutions is that most of ESP in thermal power plants (TPPs) already has a 50Hz T/R unit, so there can be easily achieved fast control voltage with low ripple, with all the positive effects of HF supply
Hybrid solutions provide several significant improvements over conventional 50Hz-thyristor systems

6. HYBRID (RETROFIT) Power Supply-IMPROVEMENTS
much faster and precise control of ESP parameters
it is possible to achieve a rapid increase of voltage and very fast response to load changes
fast voltage control provides better control of electrical charge hence better dust removal also
modulation of the output voltage in synchronism with phases of emission and collecting electrodes rapping

7. HYBRID ESP POWER SUPPLY SYSTEM DESCRIPTION

8. ESP VOLTAGE CONTROL
The Basic idea of hybrid power supply is money saving while retaining existing T/R unit
Converter output voltage has 50Hz component and HF component also
Such operating mode may lead to additional stress of HVT and HVR, which are designed for relatively low frequency.
Since ESP is representing capacitive load, supplying of its electrodes with DC current can be achieved by control of output DC voltage
Hence, for analysis of system, equivalent circuit is made and it’s representing basic model of system

9. MODEL of RETROFIT ESP POWER SUPPLY
The HF model of HVT is represented
with leakage inductance, magnetizing
inductance and ideal transformer with
1:N transfer ratio.
Active power losses in winding are
modelled with RT.
In real case this resistance is very small
( ≈ 20 mW).
ESP model is represented with
equivalent circuit that is proposed by
Oglesby
Dynamic resistance of corona discharge is represented with RP while capacitance of ESP is represented with CP
Dynamic impedance of powder layer is represented with Rd and Cd parameters.
The model of hybrid power supply of ESP;
(a)-model T/R set, (b)- Oglesby model of ESP

10. DEVELOPPED PROTOTYPE of HYBRID POWER SUPPLY
Developed hybrid power supply is applied at thermal power plant (TPP) „Morava“ as integral part of ESP system.
IGBT converter has 300kW maximum power rating, and 120kW permanent power rating. It is fully developed and designed in School of Electrical Engineering, University of Belgrade, at the Laboratory for digital control of power converters and power drives.
Reliability of developed IGBT converters technology has been proven in practice and gives a MTBF (mean time expected between failure) of 18 years.
The importance of the reliability of IGBT converters in ESP is clearer if we look at the fact that key global manufacturers of ESP withdraw their products from market due to the IGBT converter failure.

11. The disposition of hybrid power supply at ESP system on TPP "Morava"
1- HF resonant converter 3x0.4kV/70kVDC/1A
2-HF resonant converter AR70/1000
3x0.4kV/70kVDC/1A
3-HVHF step up transformer 0.4kV/70kV,
4- Hybrid power supply ETF DBS 180/340, 3x0.4kV/70kVDC adjusted to HVT 50Hz
5-Exsiting HVT 0.4kV/55kV, 50Hz, 55kVA

12. EXPERIMENTAL RESULTS
The following will be presented he experimental results obtained during testing of hybrid power supply in TPP „Morava“

13. Oscilloscopic records of HVT primary voltage and current in case of reference current frequency of 20Hz; CH1-voltage (1V/mV), CH2-current (50A/V) time base : 20ms/c
Oscilloscopic records of HVT primary voltage and current in case of reference current frequency of 50Hz;
CH1-voltage (1V/mV), CH2-current (1A/mV)
time base: 5ms/c

14. Oscilloscopic records of HVT primary voltage and output voltage of ESP in the moment of spark; CH1-current (yellow trace) 100A/V,
CH2- negative value of ESP voltage (blue trace) 10kV/V,
time base : 40ms/c
Oscilloscopic records of HVT primary voltage and current at effect overcurrent protection; CH1-voltage (1V/mV), CH2-current (30A/V) time base : 40μs/c

15. EXPERIMENTAL RESULTS- PARTICLE EMISSION
Comparison of dust particle emission in recirculation line of ESP system at TPP “Morava”; (a)- hybrid power supply, (b)-50Hz power supply
Signal of particle emission was logged from sensor which was installed in recirculation line of ESP system on TPP „Morava”.
While data collecting, block was in regime of constant power, about 90MW and with constant quality of coal.
Measurement was performed for three characteristic cases for right side of ESP system: thyristor system combined with hybrid system, without supplying of right side of ESP and two thyristor systems.
While this experiment left side of ESP system was connected to two thyristor supply unit. It is noticeable that in the first case output concentration of 300mg/m3, while in case of double thyristor supply concentration is 500mg/m3. In state without supply of right side concentration was over the limit of 2000mg/m3.

16. ACTUAL EMISSION > 50mg/m³ ??????
It should be noted that the result obtained from previous figure is higher than allowable limit of emission of 50mg/m3.
As the first cause of this result is the existing electrode system of ESP which was designed and installed in 1970, for rated purification of 150mg/m3.
With described hybrid system it is possible to archive voltage of 40kV-45kV which is absolute maximum for designed electrode distance of 300mm.
Although current of 1000 mA was noted.
The greater voltage values and efficient separation is possible to achieve if electrode distance could be increased, which would present a major reconstruction of mechanical construction of whole ESP.
However, significant improvement of separation is achieved with coordination of rapping and voltage supplying of electrode.

17. It should be noted that this value is above the legal limit.
Comparison of dust particle emission in recirculation line of ESP system at TPP “Morava”; (a)- hybrid power supply + coordinated rapping system , (b)-50Hz power supply
The obtained result is very optimistic and the most lower concentration ever in this facility.
It should be noted that this value is above the legal limit.
This result is represented in order to demonstrate the relative impact of hybrid system in relation to conventional system

18. CONCLUSION
This paper presents solution for a hybrid power ESP supply, which keeps T/R unit of conventional thyristor system.
Instead of thyristor based voltage converter is built H-bridge IGBT converter for controlling of primary winding voltage.
The basis of control system is implemented algorithm which has to adjust 50 Hz designed transformer to 10 kHz IGBT power supply.
As a result of applied hardware and control, high voltage waveform is obtained from HF supply, which leads better efficiency of ESP.
The developed algorithm is flexible and enables user any modification and improvement of ESP working.
The technical solution of presented hybrid supply has commercial name ESP ETF_DBS_180 and is under patent.

19. ACKNOWLEDGMENT
The experimental confirmation of the proposed controlling and measuring the efficiency of ESP were made on a real pilot plant at the ESP system of TPP “Morava”.
Implementation of this project is funded by the “P.D.TENT”– Obrenovac and by the Serbian Ministry of Science, under project of technological development –No: TR33022.

20. THANK YOU IN ATTENTION!!!!!