|Title of Invention||
A NEW FOUR-STAGE POWER CONTROLLER FOR IMPROVED ENERGY EFFICIENCY AND SAVING IN REACTIVE POWER FOR THREE-PHASE INDUCTION MOTORS AND WIND-DRIVEN GRID-CONNECTED INDUCTION GENERATORS
|Abstract||The Invention relates to a New Four-stage Power Controller for the operation of three-phase Induction Machines, worked either as motors or Wind-driven Grid-connected Induction Generators. This controller is meant for changing the stator connection from parallel delta to parallel star or series delta or series star, when the power output of the machine varies from full-load to no-load. With this operation the supply current waveforms of the machine remain sinusoidal, without harmonics in all the stator connections. Compared to operating the machine with a fixed stator connection, the operation with the proposed controller would result in (i) reduced kWh and kVARh taken by the induction motor from the supply, on a daily basis, (ii) increased kWh supplied to the grid and reduced kVARh drawn from the grid by the wind-driven induction generator annually, (iii) reduced kVA rating for power factor improvement capacitors, for both motors and generators, (iv) reduced starting current for motor or generator, when started in series delta or star connection. Fig. No. 2 24|
|Full Text||FORM 2
THE PATENTS ACT 1970 (39 OF 1970)
THE PATENTS RULES, 2003
1. TITLE OF THE INVENTION
A New Four-Stage Power Controller for Improved Energy Efficiency and
Saving in Reactive Power for Three-phase Induction Motors and
Wind-Driven Grid-Connected Induction Generators
3. PREAMBLE TO THE DESCRIPTION
The following specification particularly describes the invention and the manner in
which it is to be performed
In any three-phase induction machine designed for a particular pole number, there will be at least two identical sections per phase which could be connected in parallel. The induction machine may be operated either as a (i) motor taking electrical input and delivering mechanical output or (ii) generator taking mechanical input such as from wind turbine and delivering electrical output to the ac grid. So, for the rated power and rated voltage, the machine can be designed with delta connection with the two identical sections of each phase being connected in parallel. Thus, the stator connection can be called parallel delta (AA). For example, the two sections of a-phase could be (Aj - A2) and (A3 - A4) can be connected in parallel as shown in Fig. 1. Thus, a total of 12 terminals for three-phase winding (four terminals for each phase i.e., two terminals for each section) could be brought out. The machine is operated with parallel delta connection at or near rated load. As the load on the machine reduces, the stator winding could be switched to parallel star, series delta or series star at the appropriate reduced loads.
In applications requiring varying loads, such a four-stage operation would result in improved overall efficiency and power factor compared to a fixed stator connection or even compared to two-stage switching, namely star and delta. However, since the four-stage operation requires twelve terminals to be taken out to the terminal box, this operation is recommended for medium and large size machines.
Based on the load variation on the machine, the stator winding has to be switched from one setting to the other. Such a controller can be built as a solid state configuration, using 14 pairs of anti-parallel thyristor units as shown in Fig. 2, suitable for the proposed four-stage switching. Appropriate thyristor units should be given gate pulse and switched on to obtain the required stator connections. The thyristor units to be switched on and the corresponding stator connections are given in Table 1. For the sake of immediate reference, all the four stator connections along with the thyristor units switched on are shown in Fig. 3.
A prototype proposed four-stage power controller was also fabricated to make these switchings from one connection to the other and tested with the experimental machine.
Specifications for thvristors
Each thyristor units consists of two anti-parallel thyristors as shown in Fig. 4.
Fourteen such thyristor units are required in the power circuit as mentioned in the caption of
The voltage rating of the thyristors = Line voltage rating of the motor (VR) The current rating of the thyristors = rated phase current of the motor
Induction motors are the widely used electrical drives in industries in ratings varying from few kW to few thousand kW and they consume more than 50% of generated electrical power. These motors are designed with high power factor and efficiency at full load conditions. For example, a 415 V, 50 Hz, 100 kW, 750 rpm, three-phase squirrel-cage induction motor may have an efficiency of 93% and a power factor of 0.78 at full load condition. However, there are many applications wherein, over a major duration of the daily duty cycle, the motor works at much less than full load rating. At these reduced loads, both efficiency and power factor of operation become lower. To improve these performance values at reduced load conditions, the motor could be operated at a suitable lower voltage, thereby decreasing the core loss and magnetizing current and hence increasing the efficiency and power factor.
Even though induction machines are largely used for motoring operation, in recent times, with increased emphasis on renewable energy sources, induction generators are being increasingly used in wind farms in Wind Energy Electric Conversion Systems, for delivering power to the grid. Due to seasonal variation in wind speed, these generators deliver less than 25 % of the rated power, for more than 70 % of time in a year. So, for these generators also, reducing the stator applied voltage with decreasing wind speeds, would result in increased kWh supplied to the grid and reduced kVARh drawn from the grid [1-3].
To obtain such suitable reduced voltages for application to induction motors or generators, use of three-phase phase control circuits employing anti-parallel thyristor units has been suggested [4-6]. But with these circuits, a non-sinusoidal voltage is applied to the stator of the induction machine and this results in a non-sinusoidal current, leading to undesirable current harmonics in the supply lines. So, a method of reducing the applied phase voltage to the motor, without affecting its sinusoidal waveform would be very much desirable.
2. Delta-Star Switching
2.1 Applied to Induction motors
It has been suggested earlier that a three-phase induction motor, designed with a delta-connected stator winding could be switched to star-connection, when the load on the motor reduces below about 40% of full load. This switching gives higher values of efficiency and
power factor at reduced loads . Considering a given load cycle in a day, this switching results in reduced kWh and kVARh taken by the motor thereby leading to reduced tariff to be paid by the consumer to the power supply authority. Such a two-stage delta/star operation requires, two end terminals of each phase winding to be taken out from the stator winding i.e., totally six terminals, for change-over operation from delta to star and vice-versa as shown in Fig. 5.
2.2 Applied to Induction Generator
Similarly, it has also been suggested earlier that, in wind farms , three-phase induction generators driven by wind turbines, can be designed with delta connected stator winding. This delta connection could be used when the wind speed is normal. At times of decreasing wind speed, the stator winding can be switched to star connection using the same controller shown in Fig. 5. This results in improved power factor at lower ranges of power output and hence decreases the reactive power drawn from the grid.
3. Present Proposal for Four-Stage Switching 3.1. Applied to three-phase Induction Motors
In this context, further saving in kWh and kVARh could be obtained if two more stages are added i.e., totally giving four settings for the stator windings. As per this present proposal (for which the patent being sought) these four settings are:
(i) parallel delta (AA) : two sections in parallel per phase
(ii) parallel star (YY) : two sections in parallel per phase
(iii) series delta (A) : two sections in series per phase
(iv) series star (Y) : two sections in series per phase
Starting with parallel delta at full load, subsequent settings are used during suitable reduced load ranges. Thus, with the same sinusoidal applied line voltage (VR) to the motor terminals, the voltage per section for each of the settings and the corresponding stator wire current are as follows:
Thus, with subsequent settings the magnetising current and iron loss reduce, thereby improving the power factor and efficiency leading to kWh and kVARh.
The details of switching controller, for which the patent is being sought is explained along with the specification.
3,1.1 Case study
The usefulness of this four-stage proposed controller is illustrated with a case study on a three-phase, 400 V, 50 Hz, 250 kW, 4-pole squirrel-cage induction machine designed with parallel delta stator connection, as follows:
Typical resistance and reactance parameters (all in terms of stator) for this machine are R, = 0.033 a R2 = 0.025 Q, X, = 0.150 Q, X2 = 0.150 Q, Rm = 40.0 Q and Xm = 5.250 Q.
Using the exact equivalent circuit of the motor, the following performance quantities of the motor were calculated for various loads (i.e., various slips) starting from the full load: (i) mechanical power output (Pout), kW (ii) electrical power input (Pin), kW (iii) reactive power input (Qin), kVAR (iv) output torque (Tout) > Nm (v) input power factor (PF) and (vi) efficiency (n).
These results are given in Table L It may be noted from the first row of the Table corresponding to 250 kW output power rating,
the rated wire current of the stator winding = phase current / number of parallel paths
= 251.18/2-126 A.
Similar calculations of performance can be made for the other stator connections using the respective resistance and reactance parameters. For parallel star connection, parameters are same as that of the parallel delta connection. For series delta and series star connections, the parameters are four times as that of parallel connection. The calculations thus made are shown in Tables 2, 3 and 4. It should be noted that the stator line voltage is same for all the connections. Further, it is seen that the maximum power output obtainable for each setting, corresponds to the rated wire current of the stator winding namely 126 A. So, starting from this operating point only, the performance values are given in Tables 2, 3 and 4.
Performance of a three-phase, 400 V, 50 Hz, 250 kW, 4-pole squirrel-cage induction motor
with parallel star-connection
Note : Stator wire current = Iphase /2
3.1,2 At what value of reduced load, the stator winding should be switched from one connection to the other ?
This question should be first answered before operating the controller, for overall improvement in power factor and efficiency. As seen from Table 1, in the case study motor of 250 kW, when operated with parallel delta, if the load reduces to 140 kW, the power factor and efficiency values reduce to 0.836 and 90.61%, respectively. It is seen from Table 2 that at this load of 140 kW, if the stator winding is changed to parallel star connection, the power factor and efficiency values would increase to 0.894 and 91.25% (the wire current being within the permissible limit of 126 A). So, it is advantageous to operate the motor with parallel star connection from 140 kW downwards.
As seen from Table 2, when operated with parallel star, if the load reduces to 75 kW, the power factor and efficiency values reduce to 0.892 and 92.23%, respectively. It is seen from Table 3 that at this load of 75 kW, if the stator winding is switched to series delta connection, the power factor and efficiency values would increase to 0.904 and 92.31% (the wire current being within the permissible limit of 126 A). So, it is advantageous to operate the motor with series delta connection from 75 kW downwards.
As seen from Table 3, when operated with series delta, if the load reduces to 35 kW, the power factor and efficiency values reduce to 0.836 and 90.61%, respectively. It is seen from Table 4 that at this load of 35 kW, if the stator winding is brought to series star connection, the power factor and efficiency values would increase to 0.894 and 91.25%, respectively (the wire current being within the permissible limit of 126 A). So, it is advantageous to operate the motor with series star connection from 35 kW downwards to no-load.
It should be ascertained that at each setting, (i) the slip is not large and (ii) that the ratio of maximum torque to full load torque at any setting is not less than 1.5.
All these factors are again summarized for the sake of clarity in Table 5.
3.1.3 Benefits of using the proposed four-stage switching
For the calculation of kWh and kVARh taken by the motor, the loading pattern of the motor over the day is to be known and it depends on the application of the motor. For the present study, four loading patterns given in Table 6 are considered.
3.1.4 kWh and kVARh calculations
To prove the benefit of using the four-stage switching, with each of the loading pattern given in Table 6, the kWh and kVARh taken by the motor in a day, are calculated for the following cases:
(i) motor operated with the proposed four-stage switching
i.e. load ranges for each switching being as per the Table 5 (ii) motor operated with the two-stage delta-star switching
i.e. with parallel delta connection in the range of 250 kW to 140 kW
and in the parallel star connection in the range of 140 kW to no-load (iii) No switching (i.e., motor operated only in parallel delta connection at all loads)
(i) With the proposed four-stage switching
For any given load on the motor, the real power (kW) and reactive power (kVAR) input to the motor can be calculated, similar to the calculations shown in Tables 1 to 4. Then, kWh and kVARh can be calculated taking into account, the time duration at each load setting. As an example, such calculations made with loading pattern 1, are given in Table 7.
Table 7 Daily kWh and kVARh taken by the 250 kW case study motor for loading pattern 1
for four-stage stator switching (stator winding connection for each load is also indicated)
(ii) With the two-stage delta-star switching (for the sake of comparison)
If only two settings namely parallel delta and parallel star are used, the motor should be operated from 250 kW to 140 kW in parallel delta and from 140 kW to no-load in parallel star. For this two-stage operation, the kWh and kVARh calculations made with loading pattern 1 are given in Table 8.
Table 8 Daily kWh and kVARh taken by the 250 kW case study motor for loading pattern 1
for two-stage stator switching (stator winding connection for each load is also indicated)
(iii) With fixed stator connection (for the sake of comparison)
Let the motor be operated with a fixed stator connection from full load to no-load namely, parallel delta connection itself. The kWh and kVARh calculations made with loading pattern 1 are given in Table 9.
For each of the above three cases, similar calculations were made for the other loading patterns listed earlier in Table 5. The summary of kWh and kVARh values is given in Table 10. Taking the single setting as the reference, the percentage saving in kWh and kVARh in the two-stage switching and in the proposed four-stage switching, are also shown in the Table 10- It can be concluded that, compared to the fixed stator connection or the two-stage operation, the proposed four-stage operation gives an increased saving in kWh and kVARh. Consequently, in industries employing a number of medium or large size three-phase induction motors, there will be a reduction in the Energy bill and overall kVA demand and hence in the kVA tariff. This increase in saving becomes more and more in the case of motors working at light loads for a greater time duration in a day.
In other words, if the proposed four switching is adopted, the input power factor of the motor is above 0.84 at any load above about 10% of the rated load. So, if the terminal capacitor bank is to be connected across the motor terminals for further improvement in power factor, a capacitor bank of much smaller kVA rating will be sufficient.
Let us consider the reduction in kWh tariff. Consider for example, the loading pattern 1 and that the motor operates for 28 days in a month. Taking the energy cost as Rs.4.00 per kWh, the saving in energy cost per month, if four-stage switching controller is used is (refer Tables 7,8 and 9) Rs.14,952 (2695.44 - 2561.94 = 133.5 kWh x Rs.4 x 28 days) compared to single setting
Rs. 2,050 (2580.24 - 2561.94=18.3 kWh x Rs.4 x 28 days) compared to two-stage setting
Like wise the kWh and KVARh consumption of motor per day are calculated for all the loading pattern (considered in Table 6) are compiled in Table 10 for the single (AA), two and four settings.
Comparison of kWh and kVARh for four-stage, two-stage and single setting stator connections for the different loading patterns for the 250 kW case study motor
Note : Percentage saving is with respect to single setting. 3.1.5 Experimental Results
To confirm the working and usefulness of the four-stage switching, a small induction motor in the laboratory was wound with a 3-phase, 4-pole, double layer winding with the following details:
Number of stator slots = 48, coil pitch = 8 slots, coils made of 18 SWG copper wire (area of cross-section of the wire = 1.1675 mm2).
With such a winding, an operating voltage with parallel delta (delta connection with two parallel paths per phase) worked out to be 170V in the knee of the magnetization curve. With rated wire current of 5.8 A (current density is assumed to be 5 A/mm2), the full load output of the motor was obtained as 4.2 kW. Considering the same factors explained in section 3.1.2, the load ranges at which each stator connections are to be set were decided and they are given Table 11.
Load ranges and the corresponding stator connections for the experimental motor
Load tests were conducted on the motor from no-load to full load with appropriate stator connections at the various load ranges as shown in Table 11. The variation of efficiency and power factor with output are shown in Figs. 6 and 7, respectively.
For the sake of comparison, a load test was also conducted from no-load to full-load keeping the stator connection with parallel delta only. The variation of efficiency and power factor for this case is also given in Figs. 6 and 7. It is seen that switching the stator connection to AA, YY, A and Y at appropriate load ranges results in improved efficiency and power factor. This will obviously lead to saving in kWh and kVARh.
It may be noted that the motor can be conveniently started in the series star connection mode of the stator winding with reduced starting current compared to direct online starting with parallel delta. For the experimental motor, the starting current transient along with the line voltage recorded is shown in Fig. 8
3.2. Applied to three-phase Grid-connected Induction Generators
For the generator operation also, the four-stage switching can be adopted as that explained in section 3.1 for the case of motor operation.
3.2.1 Case study
The usefulness of this four-stage proposed controller is illustrated, with a case study on the same three-phase, 400 V, 50 Hz, 250 kW, 4-pole squirrel-cage induction motor considered in section 3.1.1, now used for generator operation, when the rotor is driven by a wind turbine above synchronous speed.
Using the exact equivalent circuit of the machine, the following performance quantities were calculated for generator operation for various values of negative slips (which varies with varying wind speed) for all the four stator winding connections, namely, AA, YY, A and Y:
(i) mechanical power input (Pjn), kW
(ii) electrical power output of the generator supplied to the grid (Pout), kW
(iii) reactive power input to the generator drawn from the grid (Qin), kVAR
(iv) input power factor (PF) and
(v) efficiency (TI).
Then, using the same considerations on the variation of power factor, efficiency, wire current and slip (as was done for the motor), the electrical power output ranges and the corresponding stator connections are ascertained for the generator operation. These details are given in Table 12.
3.2.2 Benefits of using the proposed four-stage switching
As explained in earlier sections, in the case of induction motor, the mechanical load on the motor varies as the application demands. In the case of induction generator, the mechanical input to the generator from the wind turbine, varies as per the annual seasonal variations in the wind velocity in a given location. Hence the electrical power output of the generator fed to the grid varies with wind speed.
The data regarding the wind velocity variation over one year period and the corresponding mechanical input for a 250 kW wind turbine is given in Table 13 ,
Mechanical power input to a wind driven generator versus the time duration
over one year period
3.2.3 kWh and kVARh calculations
To prove the benefit of using the four-stage switching, the kWh supplied to the grid and kVARh taken by the generator are calculated on an annual basis (since wind speed varies seasonally over a one year period). Such calculations were made for the following cases:
(i) generator operated with the proposed four-stage switching (ii) generator operated with the two-stage delta-star switching
i.e. with parallel delta connection in the 266 kW to 150 kW range
and in the parallel star connection in the 150 kW to no-load range (iii) generator operated only in parallel delta connection throughout the power range of
operation i.e., at all wind speeds
(i) With the proposed four-stage switching
For any given input to the generator, the real power output (kW) of the generator and reactive power (kVAR) input to the generator can be calculated. Then, kWh and kVARh can be calculated taking into account the time duration for each power output of the generator. So, with four-stage switching, such calculations made are given in Table 14.
Table 14 kWh and kVARh obtained with four-stage stator switching
for the 250 kW case study generator (stator winding connection for each power output are also indicated)
(ii) With the two-stage delta-star switching (for the sake of comparison)
If only two settings namely parallel delta and parallel star are used, the generator should be operated from 266 kW to 150 kW in parallel delta and from 150 kW to no-load in parallel star. For this two-stage operation, the kWh and kVARh calculations made are given in Table 15.
(Hi) With fixed stator connection (for the sake of comparison)
Let the generator be operated with a fixed stator connection from full load to no-load with parallel delta connection itself. The kWh and kVARh calculation made are given in Table 16.
Also it should be noted that, the power factor on the grid side of the generator is above 0.83 at any operating point above about 10% of the rated power. So, if the terminal capacitor bank is to be connected across the generator terminals for further improvement in power factor, a capacitor bank of much smaller kVA rating will be sufficient.
3.2.4 Experimental Results
The induction machine considered in section 3.1.5 as a motor is now operated as a generator using a dc motor as a prime mover in the laboratory. Considering the same factors explained in section 3.1.2, the power ranges (i.e., real power input to the grid) at which each stator connections are to be set were decided and they are given in Table 18.
Power ranges and the corresponding stator connections for the experimental generator
Load tests were conducted on the generator from no-load to full load with appropriate stator connections at the various power ranges as shown in Table 18. The variation of efficiency and power factor with output are shown in Figs. 9 and 10, respectively.
For the sake of comparison, a load test was also conducted from no-load to full-load keeping the stator connection with parallel delta only. The variation of efficiency and power factor for this case are also given in Figs. 9 and 10. It is seen that switching the stator connection from AA to YY or to A or to Y, at appropriate power ranges, results in improved efficiency and power factor. This will obviously leads to increased kWh fed to the grid and reduced kVARh drawn from the grid.
 N. Kumaresan and M. Subbiah, "Innovative reactive power saving in wind-driven grid-connected induction generators using a delta-star stator winding: part /, Performance analysis of the delta-star generator and test results", Wind Engineering, Vol.27, No.2, 2003, pp.107-120.
 N. Kumaresan and M. Subbiah, "Innovative reactive power saving in wind-driven grid-connected induction generators using a delta-star stator winding: part II, Estimation of annual Wh and VARh of the delta-star generator and comparison with alternative schemes'", Wind Engineering, Vol.27, No.3, 2003, pp.195-204.
 S. Sudha, N. Ammasaigounden and M. Subbiah "A thyristor controller for power factor improvement of wind-farm induction generators", pp. session V-19-26, Proceedings of the Fourth International Seminar on Power Electronics & Automation ELECRAMA-99, IEEMA, Mumbai, India, 1999.
 S A Hamed and B J Chalmers, "Analysis of variable-voltage thyristor controlled induction motors", IEE Proc. Vol.137, pp.184-193, May 1990
 M.A. Abdel-Halim, "Solid-state control of a grid connected induction generator", Electric Power Components and systems, Vol.29, 2001, pp. 163-178.
 M.A. Abdel-halim, A.F. Almarshoud, and A.I. Alolah , "Performance of grid-connected induction generator under naturally commutated ac voltage controller", Electric Power Components and Systems, Vol.32, No.7, July 2004, pp.691-700.
 M.G. Say, "Alternating current machines", The English Language Book Society and Pitman Publishing, Edinburgh, UK, 1976.
Taking Rs. 4.00 per kWh as revenue for feeding real power to the grid and Rs. 0.50 per kVARh as penalty for consuming reactive power from the grid, the net revenue for the various schemes of stator connections for induction generator operation are given in Table 17. It is seen from this Table that the proposed four-stage switching offers maximum revenue.
|Indian Patent Application Number||537/CHE/2006|
|PG Journal Number||24/2009|
|Date of Filing||24-Mar-2006|
|Name of Patentee||DR. N. KUMARESAN|
|Applicant Address||LECTURER, DEPARTMENT OF ELECTRICAL AND ELECTRONICS ENGG., NATIONAL INSTITUTE OF TECHNOLOGY, TIRUCHIRAPPALLI - 620 015,|
|PCT International Classification Number||H02P 25/18|
|PCT International Application Number||N/A|
|PCT International Filing date|