|Title of Invention||
"DC/AC ARC FURNACE DIGITAL MODEL FOR DIGITAL SIMULATION STUDY"
|Abstract||A DC / AC arc furnace for digital simulation study comprising a plurality of rectifiers (15) having a control (10). The rectifiers are connected to an incoming supply 33 kV BUS (12)/220 kV Bus (11) and a filter (13) is connected to said supply bus (12) and said rectifiers are connected to a load (14), the control block having an input of DC currents Idc (1) of rectifiers ( 15) is connected to a low pass filter 1 / (1 + K, S) (2) and from the output of the low pass filter a current reference Iref is subs traded to generate an error signal (3) and is connected to a control function (4,5 & 6) comprising of an integrator function (1/K2S) (5) and limiter (4) said control functions, is connected to the firing circuit (7) and said firing circuit is connected to receive the synchronization signal (8) generates triggering pulse and is connected to thyristor (9).|
|Full Text||The invention relates to a DC / AC are furnace digital model for digital simulation study to make possible to define correct specification of arc furnace related static voltage Ampere Reactive (VAR) compensation equipment by utilities and evaluate the design performance of compensator equipment by equipment manufacturer.
DC and AC arc furnace are being used by steel manufacturers in steel plants. Arc furnace load has adverse effects on power supply system like unbalance, low power factor, harmonics and flicker. Static voltage ampere reactive ( VAR ) compensators ( SVC's ) are used to reduce these disturbances on supply network.
At present no digital simulator module is available, instead analogue simulators are used to study the effect of static compensators. There are disadvantages associated with the present system. One of the main disadvantage is that the study can be done only after control system has been manufactured.
Another disadvantage associated with present system is that it requires very expensive and complex simulators, facilities of which are generally not available in every country.
Yet another disadvantage with the present system is that simulation of furnace characteristics is very difficult.
Therefore the main object of the present invention " DC / AC arc furnace digital model for digital simulation study" is to find out the effect of the complex characteristic of an arc furnace load on supply network.
Another object of present invention is that this invention makes it possible to test the design and fine tune the design parameters even before manufacturing compensator equipment.
Yet another object of present invention is , to make possible to define correct specifications of the arc furnace related compensator equipments by utilities.
Still another object of the present invention is that it is very simple to use and can be used for any possible system configuration.
According to the present invention there is provided a DC / AC Arc furnace digital modof for simulation study comprising a plurality of rectifiers having a control, said rectifiers are connected to an incoming supply 220 kV /33kV BUS and a filter is connected to said supply bus and said rectifiers are connected to a load characterized by a mathematical formula and that the control block having an input of DC currents Idc of rectifiers is connected to a low pass filter 1/'(1+'K1S ) and from the output
of the low pass filters a current reference Iref is subtracted to generate an error signal and is connected to a control function comprising of an integrator function (1/K 2S) and limiter, said control functions is connected to the firing circuit and said firing circuit is connected to receive the synchronisation signal generates a triggering pulse and is connected to thyristor.
The nature of the invention, it's objective and further advantages residing in the same will be apparent from the following description made with reference to non - limiting exemplary embodiments of the invention represented in the accompanying drawings :
Figure 1. Shows the rectifier construction with control
block and load of the present invention.
Figure 2. Shows the voltage and current of one phase at 220KV
under 6Hz load variation condition.
Figure 3. Shows RMS value of 220KV bus voltage.
Figure 4. Shows the reactive power flowing at 220kV bus incomer
Figure 5. Shows reactive power flowing at 220KV bus with
Figure 6. Shows the control block for rectifier bridges.
According to the present invention the model essentially comprises of 3 parts,
a) Rectifier (2 X 12 ) pulse ) ( 15 )
b) control (10)
c) load model ( 14 )
Brief description of the invention
Figure 1 shows the circuit of the digital model of the invention 220 kV bus (11 ) is a typical 220 kV transmission like bus which basically consists of an ideal generator ( 220 kV ) and typical transmission line parameters like resistor ( 0 . 6775 ohm ) inductance (10.61 mH ) and capacitance (312nF ). The 33 kV bus ( 12 ) is basically a 33 kV supply voltage generated from 220 kV by using typical step down 220 / 33 kV star/star transformer. The control block (10) for the control of the device is shown in details in figure 6. The bank of filters 140 MVAR (13 ) consists of filters (reactor and capacitor) and are tuned to 2nd, 3rd, 4th, 5th, 7th, 11th and 13th harmonics generated at 33 kV bus (12). The 160 MVA load (14) is represented by a mathematical formula as specified in the detail description of the invention. The rectifiers (15) are a typical 6 pulse thyristor based rectifier and used to convert AC voltage into DC voltage. These rectifiers (15) are controlled by control block.
Figure 6 shows the detail circuit and function of the control block of control for rectifier suitable for DC ARC furnace (K1 = 0.001 and K2 =0.006 ). All the four rectifiers have separate control with a current reference of 1/4th the full load value.
The load ( 14 ) is modeled as a continuously variable resistance with time and is defined as a function of desired current. Generally the resistance of the furnace varies rapidly and enormously with time. However, as every function can be defined as a summation of various frequency components, a simple mathematical model presented here contains either a single frequency component or combination of more than one frequency component to see the effect of various frequencies of load on the network . The fluctuating frequencies including those around 10 Hz to which human eye is most sensitive are covered. More than one frequency component in desired ratio can be simulated.
Mathematical formula invented for Arc furnace load model is given by
where R is the resistance of load
Vaf is the secondary voltage of AC furnace transformer Ido is the Arc furnace DC current
k(l) is the percentage variation required for desired frequency ω is 2f (f is the desired frequency)
Above formula is designed for 160 MVA ( mega volt ampere ) DC ARC furnace. Frequency ω ( 2 f ) can be selected to any value. Same concept can also be used for AC Furnace model. For AC arc furnace model, the rectifiers and its control are not required and the above formula is'to be used for all three phases of the load .
TEST RESULT :
Furnace model so invented is used with the configuration of power system and DC arc furnace selected to study the effect of furnace on network as illustrated in fig 1 . A 160 MVA DC arc furnace (14) is connected to 33kV bus (12). Filter capacitor bank of 140 MVAR (13) is connected to 33 kV bus (12).
The controller (10) for rectifiers (15) is designed to limit the DC current to 1.5 PU and DC current recovery within 20ms during large load variations to match arc furnace characteristics.
The following values are selected for testing the invented model:
to =2f (f= 6Hz)
Power system frequency = 50Hz
Fig. 2 shows the voltage and current of one phase at 220KV under 6 Hz load variation condition. The phase angle is continuously changing indicating considerable fluctuation in reactive power. This is due to control characteristic of rectifier (15) which is set for constant DC current ( 130 kA in this case ). Rectifier (15) output voltage is varied by varying triggering angle in order to keep the DC current constant. The load (14) is modeled such that DC current, varying at the rate of 6 Hz , will flow provided no attempt is made by the rectifier to control the current.
Due to change in reactive power flow at 220kV incomer (11), voltage will change. This is illustrated in fig. 3 which shows RMS value of 220kV bus (11) voltage. Voltage is changing around 4 % in less than 5 cycles ( 50 Hz).
Fig.4 shows the reactive power flowing at 220 kV bus (11) incomer.In this case capacitive reactive power is increasing at 300ms (leading current is seen in fig. 2 ) resulting in voltage rise.
The transmission line selected has a X/R ratio of 6 which is considered low. High value of R is found to be reducing voltage fluctuation when a step change in furnace resistance was studied for different X/ R ratios and the results are compared .
The invented arc furnace digital model was also tested to assess the dynamic performance of a SVC equipment. A 150 MVAR thyristor controlled reactor (TCR) with necessary SVC control in delta configuration
was connected at 33 kV bus (12) of the system shown in fig. 1 . Reactive power flowing at 220 kV bus (11) was recorded ( fig . 5 ) . The performance of SVC control can be easily observed by a comparison of fig.5 with fig.4. (reactive power without TCR ) .
The disturbance on power supply network due to load has been studied extensively for various load conditions. Simulation technique invented is found to be very effective for spectroanalysis study of the load disturbance.
The invention described here in above is in relation to non- limiting embodiments and as defined by the accompanying claims.
1. A DC /AC arc furnace for digital simulation study comprising a
plurality of rectifiers (15) having a control (10), said rectifiers are connected
to an incoming supply 33 Kv BUS (12)/220 Kv Bus (11) and a filter (13) is
connected to said supply bus (12) and said rectifiers are connected to a load
(14), the control block having an input of DC currents Idc (1) of rectifiers
(15) is connected to a low pass filter 1 / (1 + K S) (2) and from the output of
the low pass filter a current reference Iref is subtracted to generate an error
signal (3) and is connected to a control function (4,5 & 6) comprising of an
integrator function (1/K2S) (5) and limiter (4) said control functions, is
connected to the firing circuit (7) and said firing circuit is connected to
receive the synchronization signal (8) generates triggering pulse and is
connected to thyristor (9),
2. A DC / AC arc furnace as claimed in claim 1, wherein said
rectifiers are 6 pulse thyristor based to convert AC voltage into DC
3. A DC /AC arc furnace as claimed in claim 1, wherein said load is a 160 MVA load.
4. A DC /AC arc furnace as claimed in claim 1, wherein said filter comprises of filters 140 MVAR which are reactor and capacitor and are tuned to 2nd, 3rd, 4th, 5th, 7th, 11th and 13thharmonics generated at said 33 kV bus.
5. ADC/ AC arc furnace for digital simulation study as herein
described and illustrated in the accompanying drawings.
|Indian Patent Application Number||919/DEL/1999|
|PG Journal Number||12/2008|
|Date of Filing||29-Jun-1999|
|Name of Patentee||BHARAT HEAVY ELECTRICALS LIMITED|
|Applicant Address||BHEL HOUSE, SHIRI FORT, NEW DELHI 110 049, INDIA.|
|PCT International Classification Number||C03B 5/027|
|PCT International Application Number||N/A|
|PCT International Filing date|