Determination of Parameters of Doubly-Fed Induction Generators H. Dehnavifard, Student Member, IEEE, A. C. Wozniak, M. A. Khan, Senior Member, IEEE, and P. S. Barendse, Member, IEEE Abstract -- In this paper, the identification of equivalent circuit parameters of a doubly-fed induction generator (DFIG) is investigated. To date, several methods have been presented to identify or estimate induction machine (IM) parameters by using IEEE Std. 112. However, for accurate results using this standard, adjustments according to the machine’s design are required. This paper presents adjustments for IEEE standard tests for DFIGs; tests are conducted on a 5kW DFIG prototype and a 2.2kW IM. The results are accurate and will assist in the reliable simulation of grid networks with connected DFIGs. Index Terms-- Wind generator, doubly-fed induction generator, induction machine, IEEE Standards, induction machine parameters.
I
I.
INTRODUCTION
NDUCTION machines (IMs) account for more than 60% of all electrical machines in the power system network, due to them being less expensive, their simplistic design and robust structure [1], [2]. These machines’ impact on power system behavior is notable either as wind generators or motors, therefore, accurate machine parameters estimation or measurement is essential for power system stability. Also, accurate parameter determination would result in efficient control using ac drives. IEEE Std. 112 presents the procedure testing IMs and for determining the equivalent circuit parameters [3]. Thus, a few methods have been presented in literature in order to estimate these parameters for IMs based on steady-state, frequency and transient measurements [4]– [6]. A method using a transient test with a Kalman filter is presented in [7], [8]. It describes the dynamic equations in the rotor reference frame or stator reference frame by a leastsquare method [6], [4]. This transformation causes a smoother variation of variables which keeps them almost constant in steady-state. Doubly-fed induction generators (DFIGs) currently dominate the wind energy market [9]. Although they are also tested using IEEE Std. 112, these machines have a slightly different design structure due to the presence of rotor windings [10]. In the testing, the number of coil turns is assumed to be equal for both the stator and rotor but are in fact not equal [11]–[13]. This assumption would decrease the accuracy of the results. This paper justifies IEEE Std. 112 tests configuration. A prototyped DFIG is tested to validate the methodology. In
H. Dehnavifard is with the Department of Electrical Engineering, University of Cape Town, Cape Town, South Africa (e-mail:
[email protected]). A. Wozniak is with the Department of Electrical Engineering, University of Cape Town, Cape Town, South Africa (e-mail:
[email protected]). A. Khan is with the Department of Electrical Engineering, University of Cape Town, Cape Town, South Africa (e-mail:
[email protected]). P. Barendse is with the Department of Electrical Engineering, University of Cape Town, Cape Town, South Africa (e-mail:
[email protected] ).
978-1-5090-2537-4/16/$31.00 ©2016 IEEE
section II, DFIG operational modes and IEEE Std. 112 are presented. In section III, the improved IEEE Std. 112 test procedure and its justified configuration using finite element analysis (FEA) are presented. The experimental results are shown in section IV and finally, conclusions are made in section V. II.
DFIGS AND IEEE STANDARDS
A.
DFIGs The classic induction generators are commonly employed by fixed-speed or limited-speed wind turbines. However, DFIGs are variable speed wind turbines with a partial scale frequency. Therefore, frequency converters perform the reactive power compensation and smoother grid connection [14], [15]. The DFIG schematic is presented in Fig. 1. The DFIG can handle large variations in wind speeds while providing a constant frequency and voltage on the grid coupled side. The smaller frequency converter makes this concept more acceptable economically [16], [17].
Fig. 1. Variable speed wind turbine configuration with DFIG
1) IMs Operation Mode As known, IMs are usually applied in three modes: motoring, generating and plugging. The motoring mode is when the stator terminals are connected to a supplier and the rotor rotates in the direction of the stator rotating magnetic field. Generating mode is when the rotor turns faster than synchronous speed in the same direction as the stator rotating field. If the rotor rotates in the opposite direction of the stator rotating field, this is plugging mode [18], [19]. However, it is slightly different for DFIGs, owing to the connection through either the stator or the rotor. 2) DFIGs Operation mode The grid-connected generators must produce power at constant voltage and frequency. DFIGs can do that by adjusting the rotor current and frequency for varying wind speeds. Also, the voltage amplitude will be steady as long as the specific flux value remains constant [20]. The induced stator voltage frequency depends on the rotor voltage frequency and its rotational speed:
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=
+
(1)
Where is the stator voltage frequency, is the rotor voltage is the rotor mechanical frequency, which frequency and =
×
(2)
The DFIG has two modes of operation depending on the rotational speed: sub-synchronous and super-synchronous. The positive direction of power flow is expressed as: =
−
(3)
is input mechanical power, and are the stator Where and the rotor power, respectively. The rotor power may be calculated: =
−
(4)
Where, is mechanical torque, and are the mechanical and synchronous speed, respectively. As known, the angular velocity slip expression is: =
−
(5)
By substituting the slip expression into rotor power expression, it will be: =−
(6)
The stator power as a function of the slip is given by: =
(7)
Hence, the slip and the rotational speed of rotor have an impact on the direction of power flow. Also, the slip polarity indicates that the power is drawn or delivered at the stator and the rotor terminals [20]. B.
IEEE standards 112 The equivalent circuit parameters of either motors or generators may be measured by this standard procedure. The IEEE Std. 112 has been improved since 1964 due to improvement in the instrumentation technology, increasing the accuracy of measurements by making corrections in test’s technique and variation in the user requirements [3]. The power source must supply 3-phase balance voltage, smoothly sinusoidal waveform, and the frequency within 0.5% of the rated frequency in IEEE Std. 112 procedure. The ambient temperature of the test location must be at room temperature (25°C). Typically, IMs have routine tests as follows: 1Preliminary tests: The first test is usually the measurement of the winding resistance when the machine is cold. 2Idle running tests: The core loss, windage and friction losses are determined by running the machine without a load. 3Tests under load: This test is used to determine the machine efficiency, power factor, speed, current and temperature rise. 4Tests with Locked-rotor: The mechanical stress and temperature in the machine will increase under locked-rotor test. Therefore, it is essential to • Start the test when the machine is at the ambient
temperature. The locking instruments have enough strength and must be installed with adequate attention to first keep the rotor in the right direction. This protects the operator from possible injury. In this test, the input voltage is applied to the machine until the current is at the rated value. The machine temperature also may not rise above its rated temperature plus 40°C. In addition, the readings must be taken within 5 seconds. 5Choice of tests The alternative tests can be considered as per the customer’s request. The schedule of factory and field tests by the manufacturer would result in the use alternative methods or new equipment being used to perform the test, in some cases. The manufacturer’s choice of method will govern in lieu of prior agreement or contract specification.
•
III.
IEEE STANDARD ADJUSTMENT
As considered, the no-load test, a blocked rotor test and dc measurement of the winding resistance are used to find the equivalent circuit parameters of IMs [3], [4], [21]. The machine is typically in the motoring mode and assumed the rotor winding is short-circuited. The stator is connected to the grid and induces magnetic fields on the rotor winding. The magnetic fields are strong enough to produce electromagnetic torque. As a classic wound rotor machine, the machine will have similar behavior if the stator get short-circuited while the magnetic fields are produced by the rotor coils. However, the stator coils have fewer turns than the rotor coils in DFIGs, which causes the weaker magnetic fields and eventually weaker magnetic torque. In this section, finite element analysis (FEA) is used to investigate a DFIG while it is fed through the stator or the rotor coils. The specifications of a 3phase 5kW DFIG prototype seen in Fig. 2 are used for analysis using Maxwell 2D (Table 1). In Fig. 3, the flux lines in the machine are seen. Fig. 3a shows the flux lines when the DFIG is fed through the stator windings and the rotor windings are short-circuited, while Fig. 3b shows when the DFIG is fed through the rotor with the stator windings short-circuited.
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Fig. 2. A 5kW prototyped DFIG
TABLE I THE SPECIFICATION OF 5KW PROTOTYPED DFIG Parameters kVA rating [kW] Stator Rated Current [A] Stator Rated Voltage [V] Frequency[Hz] Speed[rpm] Number of turn Stator Coil Number of turn Rotor Coil Outer diameter [mm] Internal Stator Diameter [mm] Length [mm]
As indicated by m1 and m2, the flux lines are stronger when the machine is fed through the rotor. This is justified by the fact that the number of coil turns on the stator is less than the rotor in order to produce enough electromagnetic fields in generator mode. As shown, IEEE Std. 112 procedure has not categorized the tests based on IM’s design aspects. The FEA proves IEEE Std. 112 tests results would be more accurate for DFIGs if they were fed through the rotor. It must be noted that the equivalent circuit will change when the machine is fed through the rotor as shown in Fig. 4.
DFIG 5 72.2 30 50 1750 4 20 238 130 130
(a)
(b) Fig. 3. The flux lines is generated by a) the stator b) the rotor coils.
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IV.
EXPERIMENTAL TESTS AND RESULTS
TABLE II THE SPECIFICATION OF A WOUND ROTOR 2.2KW IM Parameters IM kVA rating [kW] 2.2 Rated Current [A] 4 Rated Voltage [V] 380 Frequency[Hz] 50 Speed[rpm] 1375
The IEEE standards are usually used to identify equivalent circuit parameters of an IM. Table 2 presents a 3-phase wound rotor-2.2kW IM and the details of which are seen in Fig. 5. The motor resistances are measured by DC tests. For each feeding method (through the stator or the rotor), the no-load and the lock-rotor tests are illustrated in tables 3 and 4, respectively. The input voltages and the lines currents are similar under both no-load tests. This means the number of turns on the stator and the rotor coils are equal. In addition, it justifies the equivalence between the generated flux by both the rotor and the stator. Also, the active and the reactive powers are close enough to assume the core losses are about the same for the stator and the rotor feedings. In this paper, the calculated parameter values are shifted to the stator side in order to be able to compare them with each other. Table 5 indicates the equivalent circuit parameters of a 2.2kW IM. The value of the parameters are close enough to prove that the different feeding policy for IM has not changed the test results. The rotor core is usually smaller than the stator yoke. Therefore, the core loss has decreased while the IM is fed through the rotor windings. In addition, the wires of rotor winding are narrow due to passing through semi-open slots on the rotor. Thus, it increases the rotor coil resistance which will result in more copper loss. In general, the machine structure and casing are designed to feed through the stator. It is experienced that the vibration and noise are increased when the classic IM fed through the rotor.
Test Noload Lockrotor
Test Noload Lockrotor
TABLE III THE STATOR COILS FEEDING OF 2.2KW IM Active Reactive Input Speed Line Power Power voltage [rpm] Current [kW] [kvar] [V]
Power Factor
0.34
0.88
1490
380
1.42
0.355
0.312
0.423
0
102.6
3
0.593
TABLE IV THE ROTOR COILS FEEDING OF 2.2KW IM Active Reactive Input Speed Line Power Power voltage [rpm] Current [kW] [kvar] [V]
Power Factor
0.4
0.85
1490
380
1.4
0.42
0.327
0.458
0
108.5
3
0.582
Fig. 4. IM’s equivalent circuit while feeding through the rotor Fig. 5. IM test’s rig
Feeding Stator Rotor
Stator Resistance[Ω] 3.35 3.9
TABLE V THE 2.2KW IM EQUIVALENT CIRCUIT PARAMETERS Referred-to-Stator Referred-to-Stator Stator Rotor Rotor Reactance[Ω] Resistance[Ω] Reactance[Ω] 12.45 30.4 12.45 12.85 31.5 12.85
The 3-phase 5kW DFIG has been tested in a similar way. It is connected to the grid through the stator in motoring mode when the rotor was short-circuited same as the 2.2kW test. The test measurements are presented in tables 6 and 7 for each test, respectively. The measurement results illustrate the difference between the tests. The input voltage is 27 percent of rated
Magnetization Reactance[Ω]
Core Loss Resistance[Ω]
151.65 157.32
420.9 330.75
voltage for locked-rotor test in the IM. However, it is 75 percent for the stator feeding and 59 percent for the rotor feeding tests in the DFIG. The active power increases almost 6.5 times under no-load test and 10 times under locked-rotor test when DFIG is fed through the rotor. The power factor decreases from 25 to 11 percent under no-load tests and it
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changes from 83 to 75 percent under locked-rotor tests. Table 8 illustrates the 5kW DFIG parameter values for the stator and the rotor feedings. The stator and rotor resistances are measured 0.145Ω and 3.13Ω by simple DC test (the rotor referred-resistance is 0.125Ω). These values are close to the rotor feeding results in table 8. In addition, the number of the rotor windings is 5 times more than the stator windings and the prototyped machine’s airgap is 1.6mm (Table 1). These values justify the calculated-values for the stator, the rotor and the magnetization reactances while the machine is fed through the rotor. As seen, the core loss resistances are dramatically different. Table 1 shows that the stator yoke is bigger than the rotor core therefore it confirms the fact that the core losses on the stator side increase when the machine feeds through the stator. Figures 6 and 7 present DFIG steady-state behavior by adapting equivalent circuit parameters from table VIII. They illustrate that the results of rotor feeding tests are more accurate for the 5kW prototyped machine.
Feeding Stator Rotor
Stator Resistance[Ω] 0.145 0.157
Test Noload Lockrotor
Test Noload Lockrotor
TABLE VI THE STATOR COILS FEEDING OF 5KW DFIG Active Reactive Input Speed Line Power Power voltage [rpm] Current [kW] [kvar] [V]
Power Factor
0.035
0.135
1490
36.2
2.22
0.25
0.195
0.113
0
26.63
50
0.83
TABLE VII THE ROTOR COILS FEEDING OF 5KW DFIG Active Reactive Input Speed Line Power Power voltage [rpm] Current [kW] [kvar] [V]
Power Factor
0.232
2.02
1490
182.5
6.46
0.114
1.981
1.747
0
107.91
14.13
0.75
TABLE VIII THE 5KW DFIG EQUIVALENT CIRCUIT PARAMETERS Referred-to-Stator Referred-to-Stator Stator Rotor Rotor Reactance[Ω] Resistance[Ω] Reactance[Ω] 3.27 3.49 3.27 0.133 0.125 0.133
Mag. Reactance[Ω]
Core Loss Resistance[Ω]
6.43 0.53
37.3 5.61
For instance, the output power is about 5kW and the electromagnetic torque is 32N.m at rated speed for the rotor feeding graph. Also during experimental testing, the output torque could not pass 9N.m when the machine was fed through the stator while 32.5N.m is achieved when it is fed though the rotor. V.
Fig. 6. Output power versus speed for the stator and the rotor feedings
DFIG and IM operational modes are presented to show the magnetic fields source in each mode. Also, IEEE standard process to measure IM’s parameters is described. An IM and a DFIG are tested through IEEE standards firstly, while the stator was connected to the grid and secondly when they were fed through the rotor. The results indicate that the parameters are similar for IMs, however, they are different for DFIGs due to their specific winding design. The steady-state characteristics of the DFIG show that the parameters calculated by the rotor test are more realistic. This research shows that the feeding method must be considered in the determination of the DFIG’s parameters in IEEE standards. VI. [1] [2]
[3] [4] Fig. 7. Electromagnetic torque versus speed for the stator and the rotor feedings
CONCLUSION
[5]
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REFERENCES
I. Boldea and S. A. Nasar, The Induction Machines Design Handbook. CRC Press/Taylor & Francis, 2010. A. Pye, “Getting More from the Motor,” E & T MAGAZINE, 2015. [Online]. Available: http://eandt.theiet.org/magazine/2015/02/efficient-motordrives.cfm. [Accessed: 16-Feb-2016]. Standards Coordinating Committee 21, “IEEE Standard Test Procedure for Polyphase Induction Motors and Generators,” 2004. Keun Lee, S. Frank, P. K. Sen, L. G. Polese, M. Alahmad, and C. Waters, “Estimation of induction motor equivalent circuit parameters from nameplate data,” in 2012 North American Power Symposium (NAPS), 2012, pp. 1–6. P. Kumar, A. Dalal, and A. K. Singh, “Identification of three phase induction machines equivalent circuits parameters using multi-objective
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VII.
H. Dehnavifard was born in Tehran, Iran in 1985. He completed his Bachelor’s and Master degrees in Electrical Engineering at Amirkabir University of Technology, Tehran in 2008 and 2011 respectively. He came to South Africa to pursue his PhD in Electrical Engineering at the University of Cape Town in 2013. He is currently a 4th year PhD candidate and intends to submit his thesis in June 2016. His research interests are electrical power systems, electrical machine design, renewable energy, semiconductor gas sensor, and RFID. A. C. Wozniak received a B.Sc. (Eng.) degree in electrical engineering from Wits SA. He is currently the Principles Technical Officer for the machines and power laboratories of the electrical engineering department at UCT. His research interests include electrical machines, drives and renewables. M. A. Khan received the B.Sc. (Eng.), M.Sc., and Ph.D. degrees in electrical engineering from the University of Cape Town in South Africa. He is currently an Associate Professor at the University of Cape Town, where his research interests include electrical machines, drives and renewable energy systems. P. S. Barendse received the B.Sc. (Eng.), M.Sc., and Ph.D. degrees in electrical engineering from the University of Cape Town in South Africa. He is currently an Associate Professor at the University of Cape Town. His research interests include condition monitoring, drives, fuel cells, and energy efficiency.
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BIOGRAPHY
