efectos de la mala calidad de suministro electrico

2008 Dr. Luis Morán T. 1

EFECTOS DE LA MALA CALIDAD DE SUMINISTRO

ELECTRICO

CAPÍTULO 5


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.

General Comments


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.


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.


Voltage concern for electronic type of loads.(The CBEMA Curve).


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.


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.


Power Field Data.

ANSI C84.1 - 1989 Steady State Voltage Range106 %

87 %

110 %

70 %

> 700Events per year

> 240Events per year

0 - 200 Events per year

20 - 140Events per year

0 - 16 Events per year

0 - 10/yr

Duration

0.01 s 0.02 s 1 s 3 s

19 per year

Per

cent

of N

omin

al V

olta

ge


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.


200

250

150

100

50

0

0.0001 0.001 0.01 0.1 1 10

Time (s)

Overvoltage Conditions

-100

-50Undervoltage Conditions

RatedVoltage

100 1000

AceptablePower

Cha

nge

in B

us V

olta

ge (%

)

8.33

ms

0.5

Cyc

les

Modified CBEMA curve; Actual ITIC/IEEE 1100


The following values have been picked from this new curve.

Voltage spike

500% V 200% V 120% V 110% V0.01 cycle 1 us 0.53s continous

Voltage sag

70% V 80% V 90% V 0.5 s 10 s continous

Momentary interruption0 V for 20 ms


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.


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.


Typical Voltage Tolerance Curve for Computers


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.


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:

i) Current harmonics cause an increase in copper losses and stray flux losses.

ii) Voltage harmonics cause an increase in iron losses.

The overall effect is an increase in transformer heating, as compared to purely sinusoidal operation.


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.

IEEE C57.12.00-1987 proposes a limit on the harmonics in transformer current.


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.


Designed features that minimize harmonic current losses.

Neutral and terminal connection sized at 200 % of normal.

Allow operation up to nameplate capacity without derating.

K-factor transformer have additional thermal capacity of known limits:


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.


Typical Tranformer Derating Factor (for nonlinear loads)


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.

h

h hI 22)(


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.


K-Factor Calculation for a Typical Nonlinear Loadh (harmonic

number)Ih (nonlinear load

current)(Ih)2 ih = (Ih)/(h)2)1/2 (ih)2 (ih)2h2

1 100,00% 1,000 0,792 0,626 0,6263 65,7 0,432 0,52 0,27 2,4345 37,7 0,142 0,298 0,089 2,2267 12,7 0,016 0,101 0,01 0,4959 4,4 0,002 0,035 0,001 0,098

11 5,3 0,003 0,042 0,002 0,21313 2,5 0,001 0,02 0,000 0,0615 1,9 0,000 0,015 0,000 0,05117 1,8 0,000 0,014 0,000 0,05919 1,1 0,000 0,009 0,000 0,02721 0,6 0,000 0,005 0,000 0,0123 0,8 0,000 0,006 0,000 0,02125 0,4 0,000 0,003 0,000 0,006

Total - 1,596 - 1,00 6,33


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.


Overcurrent protection Limits.


TX Inrush

TRF-1A952A-D01

A8 PropuestoD6AM

TRF-1A9

52A-D01

A8 Propuesto

D6AM

100 1K 10K 100K0.01

0.10

1

10

100

1000

CURRENT IN AMPERES

2.tcc Ref. Voltage: 13200 Current Scale X 10 0̂

TIME

IN S

EC

ON

DS

TRF-111/14.63/18.37 MVA

220/6 kVZ=11%

220 kV

512000:5

IRI1-I5E5HD

A9

A1

50/51

51G

1000:5

50:5

Siemens7SK88

IRI1-E5HD

A8

34.6800A

50/51150:5

KCGG-140

52A-D01


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.


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.


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.


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.


Temp. Rise at normal Flux Level 2-kHz PWM Carrier Frecuency

60708090

100110120130140

0 10 20 30 40 50 60 70 80 90

Frecuency in Hz

Tem

pera

ture

Ris

e (º

C)

Stator Winding (PWM) Rotor (PWM)

Stator Winding (sine) Rotor (sine)

Temperature rise variation with speed (stator frequency)


Temp. Rise at normal Flux Level 2-kHz PWM Carrier Frecuency

40

60

80

100

120

140

0 10 20 30 40 50 60 70 80 90

Frecuency in Hz

Tem

pera

ture

Ris

e (º

C)

Stator Rise @ 75% Load Rotor Rise @ 75% LoadStator Rise @ 100% Load Rotor Rise @ 100% Load

Temperature-rise variation with speed and load.


Rotor / Stator Temperature Ratio

100

110

120

130

140

150

160

170

0 10 20 30 40 50 60 70 80 90

Frecuency in Hz

Rot

or R

ise

Div

ided

by

Sta

tor

Ris

e in

%

100% Load 75% Load

Rotor rise relative to stator rise.


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)] yearsT = Ta + TThe hot spot temperature of the stator insulation, Ta is the ambient temperature in ºC, T 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.


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%.


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.


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.


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.



122 ºC)Case 2:

Voltage harmonic distortion of 22% with 5th, 7th, 11th, 13th.

Final stator winding tempe-rature 126 ºC.



122 ºC)Case 3:

Voltage harmonic distortion of 30% with 5th, 7th, 11th, 13th.

3% voltage unbalance.

Final stator winding tempe-rature 132 ºC


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.

Unbalance (%) Harmonics (%)

2 5 10 15 5 10 15 20 25

Stator Temp (ºC) 122,7 127,2 141,03 161,2 123,

37123,4

7123,

7 125,9 126,8

Motor life reduction (%) 4 32,9 80,14 97 9,15 9,89 11,8 26,9 31,6


Voltage fluctuation tolerance in static frequency

changers.Protections Eurotherm Drives

Serie 690+.ABB

ACS 600.ABB ACS 500

SAMI GS.

Overcurrent Protection Yes 3.5xIn

3.75xIn (Instantaneous),

2.65xIn (rms)Overload protection n.e. No 1.5xIn (rms)Dc overvoltage Protection Si 1.3xUn 1.35xUn

Dc undervoltage protection Si 0.65xUn 0.65xUn

Maximum Temperature Yes 125 ºC 70 ºC

Auxiliary Voltage < 17 V n.e. Protected against short circuit.

Ground Fault Protection n.e. Yes Yes

Protection against locked rotor Yes Yes Yes

Overtemperature in the motor Yes Yes Yes

Open phase Yes 13% ripple in dc bus n.e.


• 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:

Ejemplos industriales, Planta Inforsa.

TR 1

Scc = 1315 MVA Coci 3Ф

20 MVA13.2/6.6 KV

Z= 8.7 %

60 MVA220/13.2 KV

Z= 10 %

60 MVA220/13.2 KV

Z= 10 %

60 MVA220/13.2 KV

Z= 10 %TR 2TR 3

TR 4

OsciloscopioBarra B2 6.6 kV

Barra A1 13.2 kVBarra A4 13.2 kV

RPM²³RPM²

RPM¹³

Osciloscopio

1.5 MVA6.6/0.460/0.266

KVZ= 6.5 %

TR

RPM¹

Osciloscopio

K24 K01

K22Leyenda para I nstrumentos.

El superíndice ¹ ² ³ : indica el período en donde se efectuó la medición.

- período 1: entre 13-18 Julio.- período 2: entre 18-20 Julio.- período 3: entre 20-27 Julio.

OsciloscopioBarra 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).

• Registros en 220 kV.

Fase Amplitud (valor peak)

Frecuencia Duración Fecha registro

a 141.66 kV 798 Hz 20 ms 15 Julio17:52:27,73

b 160.97 kV 794 Hz 20 ms 15 Julio17:52:27,73

c 138.00 kV 791 Hz 20 ms 15 Julio17:52:27,73



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.

• Registros baja tension Barra 480 V, máquina 1



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.

Efecto NOTCH provocado por la conmutación.

• Registros baja tension Barra 480 V, máquina 2



Barra 23 kVS/E Móvil 2

23/7.2 kV

PV-02

Desde Tesoro 23 kV

Circuito Mina

7.2 kV 6.9 kV

1800 mts900 mts 940 mts

600 V

Pala 1

7.2 kV

600 V

Pala 2

10 MVA

600 mts – 350 MCM300 mts – 2/0 AWG

640 mts – 350 MCM300 mts – 2/0 AWG

600 mts – 350 MCM1200 mts – 2/0 AWG

•Cargas en Operación: 2 palas P&H y 1 perforadora (Subestación Móvil de 10 MVA).

Ejemplos industriales, Palas P&H


Perfil de Tensión en Puntos de mediciónPerfil de Tensión en Puntos de medición

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

(a) (b)

(c) (d)

23,6 kV

7,04 kV 581 V

6,99 V



Ciclo de Trabajo de la PalaCiclo de Trabajo de la Pala

Ciclo de Trabajo de la Pala

Etapa Movimiento1 Se carga el balde con mineral2 Giro de la tornamesa hacia el camión3 Frenado de la tornamesa y descarga del balde4 Giro de la tornamesa hacia el lado del mineral5 Frenado de la tornamesa y bajada del balde6 Se baja el balde

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).

Las fluctuaciones de voltajeasociadas a las fuertes variaciones de potencia activa y reactiva asociados al ciclo de trabajo de las palas.


efectos de la mala calidad de suministro electrico

Documents

luis morn t

power equipment

voltage sags

electric power loads

voltage concern

power quality

voltage spike500

voltage distortion