EFECTOS DE LA MALA CALIDAD DE SUMINISTRO ELECTRICO

1. CAPÍTULO 5 EFECTOS DE LA MALA CALIDAD DE SUMINISTRO ELECTRICO Dr. Luis Morán T.

2. General Comments Electric power loads are designed to operate with sinusoidal voltage (constant amplitude and frequency) within certain tolerance defined and accepted by different standards. Not all suppliers follow the same standards, specially with electronic type of loads. Dr. Luis Morán T.

3. Most of electrical and electronics loads are sensible to voltage fluctuations (sags, swells, outage) and voltage distortion. • The basic problem is to know how much each load can tolerate these fluctuations and distortions without being damage and without affecting their operation. Dr. Luis Morán T.

4. The reliability of electronic loads is much more closely tied to the quality of the power supply, as compared to older or more traditional equipment that may have had relay controls, or electrical contactor controls. Dr. Luis Morán T.

5. Voltage concern for electronic type of loads. (The CBEMA Curve). Dr. Luis Morán T.

6. IEEE Transactions of Power Delivery, July 1990, pp. 1501-1513 “Power Quality – Two Different Perspective” • None of these curves have been truly scientifically generated in the sense that they were created from the theory of power disturbances. • The question of validity of these curves, their use in power distribution assesment, and their appropriateness for different types of loads are largely unknow and uncorrelated to actual field evaluations. Dr. Luis Morán T.

7. The: • Electric Power Research Institute (EPRI). • The Canadian Electric Association (CEA). • National Power Laboratory (NPL). Combined and assembled their data on voltage sags, spikes and interruptions. Dr. Luis Morán T.

8. Power Field Data. Dr. Luis Morán T.

9. In 1996, based on this study, the Information Technology Industry Council (ITIC), formerly the Computer Business Equipment Manufacturers Association (CBEMA), modified the well-know CBEMA curve to the shape shown in next slide. Dr. Luis Morán T.

10. Modified CBEMA curve; Actual ITIC/IEEE 1100 Dr. Luis Morán T.

11. The following values have been picked from this new curve. Voltage spike 500% V 200% V 120% V 110% V 0.01 cycle 1 us 0.53s continous Voltage sag 70% V 80% V 90% V 0.5 s 10 s continous Momentary interruption 0 V for 20 ms Dr. Luis Morán T.

12. Applicability: The curve is applicable to 120 V nominal voltages obtained from 120 V, 208 Y/120 V, and 120/240 V 60 Hz systems. Other nominal voltages and frequencies are not specifically considered and it is the responsibility of the user to determine the applicability of these documents for such conditions. Dr. Luis Morán T.

13. For all conditions, the term “nominal voltage” implies an ideal condition of 120 VRMS , 60 Hz. • Seven types of events are described in this composite envelope. • Steady state tolerances. • Line voltage swell. • Low frequency decaying ringwave. • High frequency impulse and ringwave. • Voltage sags. • Drop out. • No damage region. • Prohibit region. Dr. Luis Morán T.

14. Typical Voltage Tolerance Curve for Computers Dr. Luis Morán T.

15. Tolerance for Power Equipment. • Most of the tolerance for power equipment, such as motors, cables, transformers are specified by different standard. • Most of these standards dealt with classical voltage and current limits. • News analysis and studies have shown more concern about the operation of power equipment with distorted voltages and currents. Dr. Luis Morán T.

16. TRANSFORMERS Harmonics applied to transformers may result in increased audible noise  the effects on these components usually are those arising from parasitic heating. The effects of harmonics on transformers are the following: • Current harmonics cause an increase in copper losses and stray flux losses. • Voltage harmonics cause an increase in iron losses. The overall effect is an increase in transformer heating, as compared to purely sinusoidal operation. Dr. Luis Morán T.

17. IEEE C57.12.00-1987 proposes a limit on the harmonics in transformer current. • The upper limit of the current distortion factor is 5% at rated current. • Maximum rms overvoltages that the transformer should be able to withstand in steady state 5% at rated load and 10% at no load. Dr. Luis Morán T.

18. K – Factor Transformers. To protect against transformer overheating caused by harmonics, designers can specify: • derated equipment, that is oversized transformer that will run at a fraction of this rated capacity, • or K-factor transformer specially designed to accomodate harmonics currents. Dr. Luis Morán T.

19. K-factor transformer have additional thermal capacity of known limits: • Designed features that minimize harmonic current losses. • Neutral and terminal connection sized at 200 % of normal. • Allow operation up to nameplate capacity without derating. Dr. Luis Morán T.

20. Underwriters Laboratiry (UL) recognized the potential safety hazards associated with using standards tranformers with non linear loads and developed a rating system to indicate the capability of a transformer to handle harmonic loads. • The ratings are described in UL 1561 and know as K-factors. • K-factors is a weighting of the harmonic load currents according to their effect on transformer heating, as derived from ANSI/IEEE C 57.110. • The K-factor indicates the multiple of the 60 Hz winding eddy current losses the tranformer can safety dissipate. Dr. Luis Morán T.

21. Typical Tranformer Derating Factor (for nonlinear loads) Dr. Luis Morán T.

22. The higher the K-factor, the greater the harmonic heating effects: K-Factor = Ih is the load current at harmonic h, in (º/1) bases such that the total RMS current equals to 1 p.u. Dr. Luis Morán T.

23. Some K-factors use up to 15th harmonic, others 25th harmonic, and still others include up to the 50th harmonic. • Based on the underlying assumptions of C57-110, it seems reasonable to limit the K-factor calculation to harmonic currents less than the 25th component. Dr. Luis Morán T.

24. K-Factor Calculation for a Typical Nonlinear Load Dr. Luis Morán T.

25. In establishing standards transformers K-factor ratings, UL chose ratings of 1, 4, 9, 13, 30, 40 and 50. • Office areas with non linear loads and large computers rooms normally have observed K-factors between 4 to 9. • Areas with high concentrations of single-phase computers and terminals have observed K-factors of 13 to 17. Dr. Luis Morán T.

26. Overcurrent protection Limits. Dr. Luis Morán T.

27. Dr. Luis Morán T.

28. Motors. • Motors can be significantly impacted by the harmonic voltage distortion. • Harmonic voltage distortion at the motor terminals is translated into harmonic fluxes within the motor. • Harmonic fluxes do not contribute significantly to motor torque, but rotate at a frequency different than the rotor synchronous frequency  inducing high-frequency currents in the rotor. Dr. Luis Morán T.

29. The effect on motors is similar to that of negative sequence currents at fundamental frequency: • The additional fluxes do little more than induce additional losses. • Decreased efficiency, along with heating, vibration, and high pitched noises. Dr. Luis Morán T.

30. There is usually no need to derate motors if the voltage distortion remains below 5% THD, and 3% for any individual harmonic. • Excessive heating problems begin when the voltage distortion reaches 8 to 10% and higher. • Such distortion should be corrected for long motor life. Dr. Luis Morán T.

31. Principal operation characteristics of a motor (TEFC) connected to a PWM inverter. • The highest internal surface temperature can generally occur on the surface of the rotor (including the end rings). • Rotor temperatures are generally increased when an induction motor is fed from a PWM inverter instead of a sinusoidal voltage source. • The difference between the rotor and stator temperature varies with inverter set up, operating point, and motor design. • Low flux and low carrier frequency are two conditions that increase rotor temperature. • While the highest temperature (for a constant torque load) may occur at the lowest speeds, the differential between the rotor and stator tends to be maximum at the highest speed. Dr. Luis Morán T.

32. Temperature rise variation with speed (stator frequency) Dr. Luis Morán T.

33. Temperature-rise variation with speed and load. Dr. Luis Morán T.

34. Rotor rise relative to stator rise. Dr. Luis Morán T.

35. Motor Life Calculation: Motor life computation is based on the experimental aging curves derived by E. Brancato [1] and listed in the IEEE Std. 117. The life of Class F insulation material can be expressed by the following equation: L = 6.0exp[0.0815(155 – T)] years T = Ta + DT The hot spot temperature of the stator insulation, Ta is the ambient temperature in ºC, DT is the temperature rise ºC, determined from the heat transfer model. [1] E. Brancato, “Estimation of Lifetime Expectancies of Motors,” in IEEE Trans. Electrical Insulation Magazine, vol. 8, Nº 3, May/June 1992, pp. 5-15. Dr. Luis Morán T.

36. 100 HP Motor: Percent Loss of Life vs. Percent Harmonic Voltage. • For a 6% of 5th voltage harmonic component motor loss of life is 18%. • For a 0.25% of interharmonic (h=0.1), the motor loss of life is 18%. Dr. Luis Morán T.

37. All motors : Percent Loss of Life vs Percent Voltage Imbalance (sinusoidal voltages). • The percentage of motor loss of life is not equal for all type of motors, since it depends on the motor rated power. Dr. Luis Morán T.

38. 100 HP motor with 2% voltage unbalance : Percent Loss of Life vs Percent Harmonic Voltage. • The motor loss of life increases if voltage harmonic and unbalance are combined. Dr. Luis Morán T.

39. Temperature at the stator winding. (Steady state temperature for normal operating conditions 122 ºC) Case 1: • 5% voltage unbalance in the supply voltage. • Final stator winding temperature 128 ºC. Dr. Luis Morán T.

40. Temperature at the stator winding. (Steady state temperature for normal operating conditions 122 ºC) Case 2: • Voltage harmonic distortion of 22% with 5th, 7th, 11th, 13th. • Final stator winding tempe-rature 126 ºC. Dr. Luis Morán T.

41. Temperature at the stator winding. (Steady state temperature for normal operating conditions 122 ºC) Case 3: • Voltage harmonic distortion of 30% with 5th, 7th, 11th, 13th. • 3% voltage unbalance. • Final stator winding tempe-rature 132 ºC Dr. Luis Morán T.

42. Stator temperature rise and percentage motor loss of life. • A larger unbalance in the supplied voltage increases the final temperature in the stator winding and therefore reduces the motor life. • Voltage harmonic components slightly increase the stator winding temperature. Dr. Luis Morán T.

43. Voltage fluctuation tolerance in static frequency changers. Dr. Luis Morán T.

44. Ejemplos industriales, Planta Inforsa. • Análisis de señales de voltaje y corriente en barras de alta, media y baja tensión del sistema de distribución de energía eléctrica de la Planta Inforsa de CMPC. • Los puntos de medición en las distintas barras fueron los siguientes: Dr. Luis Morán T.

45. Ejemplos industriales, Planta Inforsa. • Registros en 220 kV. Existen perturbaciones transitorias de alta frecuencia y de menos de un ciclo de duración que exceden los límites establecidos. (1.3 veces el valor máximo a 750 Hz). Dr. Luis Morán T.

46. Ejemplos industriales, Planta Inforsa. • Registros baja tension Barra 480 V, máquina 1 Efecto NOTCH provocado por la conmutación. Formas de onda de un ciclo del voltaje y de la corriente (2.5 ms/div) Forma de onda del voltaje y de la corriente en el instante del cruce por cero de la tensión (50 ms/div), canto de bajada del voltaje Forma de onda del voltaje y de la corriente en el instante del cruce por cero de la tensión (50 ms/div), canto de subida del voltaje. Dr. Luis Morán T.

47. Ejemplos industriales, Planta Inforsa. • Registros baja tension Barra 480 V, máquina 2 Efecto NOTCH provocado por la conmutación. Formas de onda de un ciclo del voltaje y de la corriente (2.5 ms/div) Forma de onda del voltaje y de la corriente en el instante del cruce por cero de la tensión (50 ms/div), canto de bajada del voltaje Forma de onda del voltaje y de la corriente en el instante del cruce por cero de la tensión (50 ms/div), canto de subida del voltaje. Dr. Luis Morán T.

48. Ejemplos industriales, Palas P&H • Cargas en Operación: 2 palas P&H y 1 perforadora (Subestación Móvil de 10 MVA). Dr. Luis Morán T.

49. Ejemplos industriales, Palas P&H Perfil de Tensión en Puntos de medición 6,99 V 23,6 kV (a) (b) 581 V 7,04 kV (c) (d) Registros de Voltaje en distintos puntos de medición (a) 23 kV Primario S/E (b) 7.2 kV Secundario S/E (c) Terminales pala primario (d) Terminales pala secundario Dr. Luis Morán T.

50. Ejemplos industriales, Palas P&H Ciclo de Trabajo de la Pala Las fluctuaciones de voltaje asociadas a las fuertes variaciones de potencia activa y reactiva asociados al ciclo de trabajo de las palas. Ciclo de Trabajo de la Pala Caídas de voltaje en terminales de las palas son atribuibles a la pérdida de voltaje en las impedancias equivalentes de los transformadores (S/E móvil y pala) y de la línea (23 kV). Dr. Luis Morán T.