Slobodan N. Vukosavi ć
, Željko V. Despotovi ć
, Nikola Popov
, Nikola Lepojevi ć
School of Electrical Engineering, University of Belgrade
, boban@etf.rs
Institute “Mihajlo Pupin”, University of Belgrade
, zeljko.despotovic@pupin.rs
Abstract: The modern power supplies of electrostatic precipitators (ESP) are based on high voltage high frequency (HVHF) power converters. Developed HVFF ESP power, under commercial name
AR70 / 1000, is based on a distributed multi-resonant topology. This power it had to be subjected to a series of pure electrical tests (short circuit test, open circuit test, full load test), but also of thermal characterization and electro thermal testing, before delivery and exploitation in real conditions on the thermal power plant blocks. This paper describes a methodology of electrical thermal testing of the proposed ESP power, as well as experimental results obtained during tests and thermal heating in laboratory conditions at nominal output voltage 70kVdc and the nominal output power of 70kW.
The performed experimental results and procedure showed acceptable values and limitations, which was carried out the verification a given ESP power prior to delivery and installation of the ESP units on the real conditions on thermal power plants blocks.
Key words:
Power converters, ESP power, thermal characterization, thermal load, ESP testing
1.
INTRODUCTION
The conventional 50 Hz SCR design had been predominant solution for controlling the particulate emission from large electrostatic precipitators (ESP’s). Although capable to reach removal efficiencies up to 99, 8%, this design suffers a number of drawbacks, leading to poor energy efficiency, very large size of electrode plates, and it cannot compete with the high frequency power source.
High voltage high frequency (HVHF) ESP’s power supply and corresponding control require a lower size and weight of electrodes, offers significant energy savings, prevents back corona, brings up a very fast reaction to flashover, results in a much higher power factor, and has a transformer/rectifier set several times smaller and lighter compared to conventional 50Hz design
[1-8].
In many papers is presented a new HVHF distributed multi resonant topology (DMRT) for
ESP’s power [9-12]. DMRT in the secondary circuit enables reducing of commutation losses and the insulation stress, hence suppressing the catalytic effects of the electric field high speed changes and preventing chemical reactions leading to accelerated dielectric aging [12-13]. DMRT power converter enables ZCS commutation of IGBT power switches, significantly lowering the overall converter losses [14-15]. The basic electrical scheme of HVHF power with incorporated of DMT is shown in Fig.1. The functional blocks are arranged in accordance to disposition on the realized ESP power, named under commercial trade mark AR70/1000, i.e. output DC voltage 70kV and output
DC of 1000mA. This ESP power was developed on the School of Electrical Engineering, University of Belgrade (DDC Laboratory). Basic functionality and features of this ESP power are given in the internet address http://www.esp.etf.rs/ .

Fig.1. Basic electrical scheme HVHF distributed multi resonant topology AR70/1000 for ESP power
Since the focus of this paper thermal characterization of power AR70 / 1000, emphasis will be given to the basic thermal units or subsystems, which are clearly shown in Fig.1. There are three main thermal parts of ESP power AR 70/1000: cubicle of HF IGBT power converter, oil tank with corresponding equipment and HV insulator in which is connect ESP load.
The main heat sources in a HF cubicle are: input three phase diode rectifier (with or without three phase input reactor), capacitor in dc link circuit, IGBT H-bridge and bus bar system (U,V, +AT and -AT). Much stronger heat sources located within the oil tank: dc reactor
L dc
, ac reactor
L ac in primary circuit of HVHF transformer, HVHF transformer and HV rectifier (the chains of highvoltage diode and associated RC voltage spike suppressors). These last two components largely determine the thermal behaviour of ESP power AR 70/1000.
Thermal characterization and testing of AR70/1000 were performed in laboratory conditions as on the part of many other tests (short-circuit, no-load, full load and overload). These tests should be carried prior to final delivery and installation of HVHF units in real operating conditions of the ESP installation on thermal power plants.
2.
ELECTRO-THERMAL INSTALLATION FOR TESTING ESP POWER AR70/1000
In order to perform a correct thermal characterization and thermal power AR70-1000 tests it was necessary to provide a high-voltage RC load (parallel connection of resistor R and capacitor C), nominal voltage of 70kV, nominal current of 1000mA and accordingly nominal output power of
70kW. Like this established load approximately correspond to the real ESP load according Oglesby as stated in [16]. Since this load is one of the key parts of electro-thermal installations that will be described in detail.
The basic block diagram of the HV load, which actually emulates real ESP (emission and collecting electrodes system) and the way of its connection with power AR70/1000, are shown in
Fig.2. The basic elements of this load are: oil tank with accessories, HV resistors, cooling system,
HV insulator and extern HV capacitor. In the oil tank are three high-voltage resistor 23k Ω / 23kW
(connected in series) i.e.
R
L
= 68k Ω /70kW, one shunt resistor
R sh
= 220 Ω on which is measured voltage drop i.e. load current and corresponding measuring equipment. In fact, is constantly a measure and displays oil temperature in the tank through the thermal probe with indicator
(measuring range 0-100Cº), but it is in the tank installed thermo-switch that has a no-voltage contact which gives the signal that the oil temperature reached above a maximum threshold of 70°C. When the oil temperature in tank exceeds this threshold, unconditionally during testing, come to turn-off of power AR70/1000.

Fig.2. Block scheme test installation and RC load of ESP power AR70/1000; disposition of AR70/1000 is given in two projections (top and side views)
The case of the oil tank has dimensions 600x600x1200mm, with a separately earthing connection. On it, at the one lateral side are performed two holes for the inlet and outlet of the circulating oil, while at the opposite side is carried out the one hatch in which is mounted HV silicon insulator.
The inlet an outlet of oil tank had two manual valves with associated taps. The oil circulation is provided by pump, whose motor is electrical supplying from the frequency converter (inverter).
This converter is powered from single-phase voltage 230V, 50Hz, and at its outputs gives the threephase voltage variable frequency. With this converter is adjusted the flow of circulating oil, which provides cooling of resistive part of load. In heat exchanger “oil-water” is provided reliably dissipate heat from load.
HV insulator is reliably mounted on the oil tank, very well done sealing and at its end was attached to high voltage capacitor
C
L
= 25nF/100kV. The parallel connection of resistor
R
L
and capacitor
C
L
, approximately correspond to the real ESP load.
The measurement of the high voltage at the RC load is achieved from high voltage resistors
R
1
and R
2
(in series connection) and the micro-ammeter. Besides this, it provided an additional measurement of high-voltage by electrostatic voltmeter (ESV) with bright spot at the linear division scale 0-100% (i.e. 0-100kV). In this way, it is possible to remotely and visually monitoring the value of high voltage at the RC load.
HV output of AR70/1000 is connected to insulator of load through HV cable 200kV/2.5mm².
It is important to note that it is extremely reliably derived connections all of earthing (PE connections: the oil tank, HV capacitor, power AR70/1000 and high voltage measuring equipment).

As an additional measure of protection against direct touch with HV installation was provided
Faraday cage made of metal mesh grid, which was also grounded in more places and connected to other terminals for earthing.
Fig.3 shows the detailed installation which was carried out by electro thermal testing of ESP power AR70/1000. Fig.3 (a) shows the single-pole electric scheme of test installation and topologies of key functional blocks of HF power converter (named DS200™) and HV output parts
(HVHF transformer, HV rectifier, HV insulator,…etc.) . Figure 3 (b) shows the three-dimensional appearance of ESP power AR70/1000 with disposition of all the key components that largely affect of the thermal characterization. The positions of the components 1-6 in the electrical scheme correspond to the positions at the three-dimensional view of ESP power AR70/1000.
Fig.3. Electrical installation of ESP power AR70/1000; (a) Single pole electrical scheme of test installation,
(b) three-dimensional view of AR70/1000 power with the positions of associated equipment’s

In anticipated tests established three groups of electrical measurements: at the input terminal of AR70/1000 (connection to power supply network), electrical measurement within HF cabinet
(cubicle) and measurements on HV side.
On the input terminal is provided three-phases measurements of input current
U , also measurement of active and reactive power P and Q
I and voltage
, respectively, as well as power factor
.
Inside of the HF cabinet are realized measurement of instantaneous values of current I dc
and voltage
U dc
in the DC link circuit of DS2000 converter, respectively, and is actual values of current
I ac and voltage
U ac
at the output terminals DS 2000 converter i.e. IGBT H-bridge (primary winding of HVHF transformer). The third group of electrical measurement is performed on HV side. Thereby were measured output high voltage (output voltage of HV rectifier)
U
DC
and output current
I
DC of
AR70/1000 unit.
Based on the above-mentioned measured values of current and voltage are obtained power, and based on them are obtained the values of dissipation losses in certain parts of interest (as shown in
Fig.4): dissipation losses at the input of three-phase rectifier
P γ 1
, dissipation in DC link circuit
(capacitor
C dc
and inductance
L dc
)
P γ 2
, dissipation losses in IGBT H-bridge
P γ 3
and finally dissipation losses on the HVHF transformer and HV rectifier, a total of P γ 4
.
Fig.4. Distribution of power losses in ESP power AR70/1000
Each of these losses on their way affects on the thermal characterization and rising temperatures in certain parts of the ESP power AR70/1000. The largest impact they have losses
P γ 4
, considering that their largest amount, however, should not ignore the impact of losses in the dc inductance, or the impact of losses in the IGBT H-bridge, the fact that his heat sinks with its back side mounted in oil tank (in which HVHF is located), such as showing in Figs.1,3.
Thermal characterization was achieved for the various operating modes of ESP power
AR70/100. Change the operating modes of AR70/1000 power are achieved through the two reference values: switching frequency f [ kHz ] of IGBT H-bridge converter (in ZCS resonant mode) and the pulse width of their output voltage i.e. “duty-cycle”
[%] . In references [x-y] is explained in detail how the two reference values affect on the operating modes of DMRT integrated in assembly of IGBT H-bridge, HVHF transformer and HV rectifier.
3.
THERMAL CHARACTERIZATION OF ESP POWER AR70/1000
In this section will be presented positions (i.e. measuring point) at which they are placed temperature sensors in certain areas of interest within the ESP power AR70/1000. Fig.5 presents the disposition of equipment within the considered power AR70/1000 as well as the exact arrangement of measurement points

The temperature measurements were carried out in 14 measurement points (T1-T14) at the transformer oil tank and within cubicle of HF power. Also certain group of measurements is made on the HF device DS2000 and at the HVHF transformer. Measurements at all points are achieved
Pt1000 sensors (except for measurement points T3-T8 and T9, T10) and suitable measuring transmitters. The obtained measuring signals are introduced in the simple data acquisition system and after that was performed computer processing of the results.
Detailed specifications of all measurement points, according to the dispatch planning drawing in Fig.5, is given in Table I. This table shows the three groups of temperature measurements that fully correspond to the thermal characterization:
(i)-measuring points in HVHF transformer oil tank
(ii)-measuring points at HVHF transformer (windings, magnetic circuit and oil temperature)
(iii)-measuring points in HF converter DS2000 (H-bridge, input 3ph- rectifier, DSP controller)
Fig.5. Disposition of measuring points of characteristic temperatures in ESP power AR70/1000, (a) view on the right lateral side, (b) view on the top side, (c) view on the front side
Based on this above setting measurement points was carried out a series of experiments and thermal tests , after which the unit AR70/1000 was ready for delivery to ESP station on thermal power plant.

T1
(i) MEASURNIG POINT IN OIL TANK
(T1….T14)
Heat sink DS2000 Θ
Cu1
(ii) TEMPERATURE IN HVHF
TRANSFORMER ( end measured values)
Primary side winding (Cu1)
T2 Oil temperature in tank
T3-T8 Heat sink ribs (lateral side
0%-20%-40%-60%-80%-100%)
T9
T10
T11
T12
Air temperature (fans of DS2000)
Top side of oil tank
Indoor of cubicle-position 1
Indoor of cubicle-position 2
Θ
Θ
Cu2
Fe
Secondary side winding (Cu2)
Magnetic circuit (ferrite-Fe)
Θ transf.oil
Temperature of transformer oil
(iii) MEASURED TEMPERATURE IN DS2000
(end measured values)
Θ 12
Θ 45
V-bus bar (AT+) *see Fig.1
Block capacitor in DS2000, 0.47uF/850V,
MKP-CS13 ( <70ºC)
T13 Temperature of electrolytic capacitors in DC link circuit
Θ
78
Hexagonal pad bollard , U bus bar
(AT+)
Θ
10-11
Temperature of electrolytic capacitor in DC link circuit
Table I
- Characteristic measuring points in the thermal characterization of ESP power AR70/1000
4.
EXPERIMENTAL RESULTS
This section presents the experimental results obtained in the framework of the thermal characterization of ESP power AR70/1000. A series of thermal tests is achieved a view to finding the most acceptable solutions of the cooling system of proposed ESP power at different operating regimes (at different operating frequency and output power). The aim of the tests was to achieve maximum output power of 70kW (70kV/1A), and in order not to overheat the power unit
AR70/1000.
Fig.6. Test No1: Thermal characterization of ESP power AR70/1000 at switching frequency f=6.9 kHz, and output power 61.35kW; power losses P γ 4
=4kW
Fig.6 shows the experimental results for Test No1 . In this case, is adjusted operating frequency resonant inverter (IGBT H-bridge in ZCS regime) at 6.9 kHz. Output current AR70 it

amounted 207V / 220 Ω = 0.94A, the output voltage of 65kV or output power of 61.35kW. Power losses (in most are localized in oil tank of AR70) in these conditions amounted to 4kW. The temperature in the oil tank of load it amounted in a steady state 48 °C, variable frequency drive of circulating pump was set to 30Hz, and the flow of circulation oil in tank of load was about 2.1 l/s.
The ambient temperature (average daily) was about 12.5°C (min 10°C, max 15°C). Time duration of test is amounted about 13h.
Under these conditions, the highest increase in temperature had measuring points T1, T2, T8 and T13. In these most critical points T1 (i.e. heat sink of DS2000) and T13 (electrolytic capacitor in DC link circuit) temperatures are amounted about 69°C and 71°C, or temperature increases of
54°C and 56°C, respectively. The other temperature increases are not critical.
Fig.7. Test No2: Thermal characterization of ESP power AR70/1000 at switching frequency f=6.4kHz, and output power 57.25kW; power losses P γ 4
=3kW
Reducing the operating frequency at 6.4kHz and output power at 57.25kW (and thus power loss at 3kW) there was a reduction in the temperature increase (<50°C) of the above mentioned temperatures (T1- heat sink of DS2000 and T13-electrolytic capacitor in DC link circuit) as shown in the diagrams of
Test No2
, in Fig.7. However, the output power is significantly reduced, which was not the final and desired goal.
In
Test No. 3
, our goal was to raise the operating frequency to 7.4 kHz and output power to
67.5kW (69kV/0.98A), in the other almost the same conditions that applied to a
Test No1
and
Test
No2
. In this case, there are a certain difficulties and problems. After six hours of operation, there was a overheating of the DC electrolyte and the excessive temperature rise in the heat-sink of
DS2000, as shown in Fig.8. The result is a AR70/1000 power failure since there has been a response to the thermal protection device DS2000
("FAULT 128 -HEAT SINK"
). After an hour of pause, the unit AR70/1000 is turned on again, and after operation for about two hours there was a power failure again. Temperature of the DC electrolytes and heat-sink of DS2000 have reached critical temperature thresholds of 75°C and 80°C, respectively. This test showed that under these conditions the ESP power cannot give the full value of the output power, i.e.70kW.

Fig.8. Test No3: Thermal characterization of ESP power AR70/1000 at switching frequency f=7.4 kHz, and output power 67.45kW; power losses P γ 4
=3.65kW
Fig.9. Test No4: Thermal characterization of ESP power AR70/1000 at switching frequency f=7.4 kHz, and output power 65.6kW; power losses P γ 4
=3.67kW
In Fig.9 shows the temperature responses obtained in the
Test No4
, for the case when the ambient temperature was below 5°C, compared to the previous Test No3. This resulted in a slightly

lower temperature increase (T1- heat sink of DS2000 and T13-electrolytic capacitor in DC link circuit) than in the previous case, as can be seen in Fig.9.
But regardless of these results, it is generally a problem of critical temperature rise had solved the forced cooling AR70/1000 unit. The following tests just give the results that they have achieved a positive effect.
Fig.10. Test No5: Thermal characterization of ESP power AR70/1000 at switching frequency f=7.4 kHz, and output power 68.08kW; power losses P γ 4
=3.8kW, air flow speed (side fans) 2.1 m/s
Fig.11. Test No6: Thermal characterization of ESP power AR70/1000 at switching frequency f=7.4 kHz, and output power 68.18kW; power losses P γ 4
=4.5kW, air flow speed (side fans) 3.5 m/s

The following are presented the results of three tests in which was examines the influence of forced cooling. Forced cooling was provided by fans, which have been set to all the three ribbed side of transformer oil tank, as shown in Figs 5 (a) and 5(c). On each of the ribbed sides of transformer oil tank, from the bottom side is set of five fans with the ability to adjust their speed (by voltage adjusting) or airflow velocity. Thermal characterization of this case was carried out for three air velocity 2m/s, 3.5m/s and 5.2m/s at approximately the same other conditions: output power about 68kW, operating frequency of HF resonant IGBT converter of 7.4kHz and time duration of test about 10h. At these tests average value of the ambient temperature was about 15°C.
Fig.12. Test No7: Thermal characterization of ESP power AR70/1000 at switching frequency f=7.4 kHz, and output power 68.10kW; power losses P γ 4
=4.5kW, air flow speed (side fans) 5.2 m/s
On Fig.10 shows that air velocity from 2.1m/ s significant affect on reducing the oil temperature-T2 in transformer tank. In the case of natural cooling (tests No1-4) the temperature in the steady state it amounted 60-65°C. After the application of forced cooling (airflow velocity
2.1m/s), this value of temperature was decrease of about 10°C, and it was about 52°C. Similar also applies to the temperature rise.
On Fig.11 shows that air velocity from 3.5m /s additionally reduces the temperature of the oil in the transformer tank, for Δ T=8°C, or on the value about 44°C. The best effect is achieved at the airflow velocity of 5.2m / s (Fig.12), in which case the oil temperature is reduced to the value less than 40°C.
Forced cooling has shown positive effects on the decrease of the heat sink temperature T2 of converter DS2000, who with his ribbed side immersed in transformer oil tank. This value was reduced from 70°C (the case with natural cooling) to an acceptable the value of 55°C (test No7 -
Fig.12).
A more modest effect of forced cooling was applied on decrease the temperature of the electrolytes in DC link circuit, or on the temperature T13. In the case of forced cooling with a velocity from 2.1m/s temperature T13 was still above the threshold value from 70 ° C (Fig.10). At velocity from 3.5m/s the temperature was also above the threshold of 70°C (Fig.11). At the flow velocity from 5.2m/s, this temperature was reduced at the value of about 69ºC. For this reason, it is performed additionally forced cooling of HVHF cubicle of AR70/1000 power, with fan, which is placed on the lower right side of the cubicle, as shown in Figs.3 and 5 (in/out air flow). Applying this cooling system the temperature of DC electrolytes was significantly reduced (on values approximately 40-50ºC).

5.
CONCLUSIONS
The paper describes electro thermal installation that was used for testing and thermal characterization of ESP power AR70 / 1000 (nominal output voltage of 70kV and output current of
1000mA). In addition to the many standard electrical tests was performed detailed thermal testing of this power. These tests have been mandatory before delivery and after that, installing these devices on ESP station, on thermal power plants. Thermal tests showed some disadvantages of the natural cooling initially anticipated. This primarily related to the components within the HF resonant converters DS2000 (DC electrolytes and IGBT modules mounted on its own heat-sink).
Also, the temperature of the oil in the transformer tank is in these conditions, was at the upper limit.
Applying forced cooling of the transformer oil tank and cubicle of HF resonant IGBT converter, are solved all these problems and achieved the acceptable values increase in temperature in relation to ambient temperature under all operating regimes and in steady state. After this ESP power
AR70/1000, were ready for final delivery. The results of testing and thermal characterization of
ESP power AR70/1000 are presented by numerous experimental tests that are shown in this paper.
ACKNOWLEDGEMENT
The proposed multiresonant topology represents the basis of power supply AR70/1000 developed in the framework of scientific cooperation between the
School of Electrical Engineering
, the University of Belgrade and Mihailo Pupin Institute , University of Belgrade. This investigation has been carried out with the financial support of the Serbian Ministry of Science- Project No: TR33022-“
Integrated system for flue gas cleansing and development of technologies for zero pollution power plants ”
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