GE Industrial Solutions
MV Airinsulated Equipment Derating for High Altitude Installations  Dany Huamán, Peru´s Specification Engineer
Digital Specification Program Latin America
Overview This paper demonstrates and compares the correction factors required for electrical ratings of IEC and ANSI airinsulated, medium voltage switchgear when installed and operated at altitudes above their standard designs. Application cases addressed in this paper indicate that discharge voltage (BIL) is the rating most impacted by altitude. This white paper is intended as a reference for the proper selection of the type of medium voltage switchgear to be used according to its altitude of service. An incorrect selection can lead to problems such as dielectric failures, oversizing of the electrical equipment (higher cost), or the application of other solutions such as gas insulated switchgear (also higher cost).
1. Introduction South America can be characterized by geographical conditions requiring the need for electrical equipment installations at high altitude. The Andes between Argentina, Chile, Bolivia, Peru, Ecuador, Colombia and part of Venezuela, are a mountain range that in which you can find cities, industries and mining operations at altitudes higher than 1,000 meters above sea level. At these heights, the dielectric properties of the electrical operating ranges of the equipment are affected due to the reduction in air density. In this paper we will present two application examples. Both of which show the main electrical factors of switchgear operation affected by high altitude installations. They include: voltage and basic impulse level (BIL) ratings, with the latter being of greater importance when the altitude is greater than 1000 m.a.s.l. since the correction factor is directly related to the value of an exponential function. 2. Standardization of electrical switchgear ratings 2.1 Conditions of normal operation Normal operating conditions as defined by the construction standards of IEC and ANSI to function normally under the following variables and ranges: environmental temperature, operating altitude, humidity and solar radiation. For unusual operating conditions the standards recommend the use of correction factors.
Table No. 1: Normal Operation Standard Temperature Instant environment 0 ° C Minimum Maximum Maximum daily average Altitude Maximum altitude (meters) Solar radiation Solar radiation Moisture Average relative humidity over a period 24 hours 01 month
IEC IEC 62271200
ANSI IEEE C37.20.2
5 °C 40 °C 35 °C
30 °C +40 °C Not specified
≤ 1000
≤ 1000
Not specified
Not significant ANSI Std C37.241986
95% 90%
(*) Not specified Not specified
Table referenced IEC standards 62271200 and ANSI C37.20.2
Note: (*) The ANSI C37 19992000 recommended in clause 8.1.4.3 resistors using appropriate quantities and of sufficient strength to minimize condensation in all compartments.  General Electric draws on maximum noncondensing humidity of 90% for its medium voltage equipment (CCM). Manual implementation GET6840C.
2.2 Electrical Ratings Electric ratings are defined by IEC and ANSI standards as ratings of voltage, current and insulation levels. 2.2.1 Voltage Range The IEC standard defines nominal voltage as: Un (kV) and the ANSI standard defines the maximum voltage as: Umax (kV). Table No. 2: Voltage Ranges IEC Serie I Un (kV) 3.6 7.2 12 17.5 24 36 40.5 IEEE Umax (kV) 4.76 8.25 15 27 38 Table referenced standards IEC 62271100: 2006 and ANSI C37.06.12000 I referred to IEC Series indoor equipment
2.2.2 Current Range Defined values of current (Ir) according to each standard. Table No.3: Current Range IEC ANSI
(A) (A)
630
1250
1600
2500 3150 1200 2000 3000 Table referenced standards IEC 62271100: 2006 and ANSI C37.06.12000 (*) There is no forced ventilation
2.2.3. Insulation Levels The standards classify insulation levels for switchgear as:  Power Frequency Voltage (RMS)  Discharge Voltage Table No.4: Insulation Levels Voltaje (kV) Voltaje Frecuencia Voltaje decarga Grado Umax Rango Un Industrial (kV rms) Atmosférica BIL (kVp) ANSI IEC ANSI IEC ANSI IEC 4.16 19 60 7.2 7.2 36 20 95 60 12 28 75 13.8 36 95 17.5 38 95 24 50 125 27 60 125 36 80 170 38 80 150 40.5 95 185 Table referenced standards IEC 62271100: 2006 and ANSI C37.06.12000
3. Altitude correction factor 3.1 According to IEC standard The standard clauses of IEC 62271200, recommend the use of correction factors for installations in unusual conditions such as altitude. Critical variables affected: 3.1.1 Voltage Correction Factor The voltage correction is directly related to the effect of atmospheric pressure at high altitudes, which is referenced in the standard IEC 72123. The following formula is used for the calculation of the correction factor: Item Voltage
Factor m=1
Table No.5: Voltage correction factors for altitude ACF
Ka = e m (H1000)/8150
Table referenced standards IEC 600712: 1996 Note: The exponent m is dependent on several parameters including the minimum discharge path which is generally unknown at the design stage. Determining the exponent m is based on the IEC standard 601. H= Altitude above sea level in meters give m= 1.0 for coordination atmospheric pulse voltage it can handle. m = 1.0 for industrial frequency voltages can be supported by air spaces and clean insulators.
3.1.2. Current Correction Factor The current correction is directly related to the effect of atmospheric pressure at high altitudes and is referenced to IEC 72123. The correction factor can be calculated from the following formula. Item Corrientes
Table No.6: Correction factors for current ACF ACF = 1 –0.02 * (H1000)/1000 Table referring to IEC 600712 Note: H = Altitude above sea level given in meters
3.2 According to ANSI standard As stated by ANSI C37.20.21986, clause 8.1.3, for unusual conditions such as altitude, it recommends the use of correction factors for the voltage and current, given in the following table: Table No.7: Correction for altitude Altitud (m)
Altitud (ft)
1000 1200 1500 1800 2000 2100 2400 2700 3000 3600 4000 4300 4900 5500 6000
3300 4000 5000 6000 6600 7000 8000 9000 10000 12000 13000 14000 16000 18000 20000
From standard ANSI C37.201999.
ACF for dielectric ACF for continuous withstand voltage current 1.00 1.00 0.98 1.00 0.95 0.99 0.92 0.99 0.91 0.99 0.89 0.98 0.86 0.97 0.83 0.97 0.80 0.96 0.75 0.95 0.72 0.94 0.70 0.94 0.65 0.925 0.61 0.91 0.56 0.9
For calculation of correction factors for voltage and current for altitudes above 1,000 meters General Electric uses the following equations: 3.2.1
Correction factor for voltage
The correction factor can be calculated with the following formulas: Table No. 8: Altitude correction factors Ítem
Altura (feet)
ACF
A<3300
ACF= 1.0
3300≤A<5000
ACF = 1.0 – [0.0294 x (A – 3.3 Mft)]
5000≤A<10000
ACF = 0.95 – [0.03 x (A – 5.0 Mft)]
A≥10000
ACF= 1.0 – [0.02 x A]
Voltaje
The table refers to catalog GET6840C, General Electric, reflecting the adoption of ANSI C37.14
3.2.2
Current Correction Factor
The correction factor can be calculated with the following formulas: Table 9: altitude correction factors Ítem
Altura (feet)
ACF
A<3300
ACF= 1.0
3300≤A<5000
ACF = 1.0  0.00588 x (A – 3.3 Mft)
5000≤A<10000
ACF = 0.99 – 0.006 x (A – 5.0 Mft)
A≥10000
ACF = 1.0 – 0.004 x A
Corriente
The table refers to catalog GET6840C, General Electric, reflecting the adoption of ANSI C37.14
4. Examples of application 4.1 Case #1 ABC Mining Company has a project at an altitude of 3,000m and must select the medium voltage switchgear for their primary substation. The operating voltage is 13.8 kV and the current is 1500A with a main bus short circuit current of 23 kA. According to IEC standard: The switchgear required at an altitude of 3,000m should be derated in electric ratings. 1. Calculating the voltage correction factor: Ka = e (3,0001,000)/8,150 = 1.278 2. Calculation of current correction factor: ACF = 1 – (3,0001,000)/1,000 * 0.02 = 0.96 3. Selection of switchgear: Table No. 10: altitude correction factors IEC Voltaje de operación a 3,000 m.s.n.m
Factor Ka
Nuevo rango a 3,000 m.s.n.m
Rango Switchgear (ver Tabla 2,3,4)
13,8 kV
1.278
17.63 kV
24 kV
Voltaje de descarga atmosférica
95 kV
1.278
121.41 kV
125 kV
Corriente Continua
1,500 A
0.96
1440 A
1600 A
Rango Voltaje Operación
Prepared by author
According to the above table, the equipment to be used will be GE SecoGear switchgear 24 kV, 1600A current and a BIL of 125 kV, 25 kA. According to ANSI standard: The switchgear required at an altitude of 3,000m should be derated in electric ratings. 1. Calculating the voltage correction factor: Si A ≥ 10 Mft; USAR: ACF= 1.0 – [0.02 x A] ACF = 1.0[0.02 x (10)] = 0.8
2. Calculation of current correction: factor Si A ≥ 10 Mft; USAR: ACF = 1.0 – 0.004 x A ACF = 1.0 – 0.004 x (10) = 0.96 3. Selection of switchgear: Table 11: Correction factors for altitude ANSI / NEMA Voltaje de operación a 3,000 m.s.n.m
Factor
Nuevo rango a 3,000 m.s.n.m
Rango Switchgear (ver Tabla 2,3,4)
13,8 kV
0.8
17.25 kV
27 kV
Voltaje de descarga atmosférica
95 kV
0.8
118.75 kV
125 kV
Corriente Continua
1,500 A
0.96
1440 A
2000 A
Rango Voltaje Operación
Prepared by author
According to the above table, the equipment to be used will be GE PowellVac switchgear 27 kV, 2000A current and a BIL of 125 kV, 25 kA. 4.2 Case 2: DEF Mining Company has a project at an altitude of 4,500m and must select the medium voltage switchgear for its primary substation. The operating voltage is 22.9 kV and the current is 1500A with a main busbar short circuit current of 25 kA. According to IEC standard: 1. Calculating voltage correction factor: Ka = e (4,8001,000)/8,150 = 1.594 2. Calculation of current correction factor: ACF = 1 – (45001000)/1000 * 0.02 = 0.93
3. Selection of switchgear: Table No.12: Correction factors for altitude IEC Factor Ka
Nuevo rango a 4,800 m.s.n.m
Rango Switchgear (ver Tabla 2,3,4)
22,9 kV
1.56
35.724 kV
40.5 kV
Voltaje de frecuencia industrial
50 kV
1.56
78kV
95 Kv
Voltaje de descarga atmosférica
125 kV
1.56
195 kV
185 kV
Corriente Continua
2,500 A
0.93
2,325 A
2,500 A
Rango
Voltaje de operación a 4,500 m.s.n.m
Voltaje Operación
Prepared by author
According to the above table, the equipment to be used will be GE SecoGear switchgear rated 40.5 kV, 2500A of current and with a BIL of 185 kV, 25 kA. (You would need to have a conversation with your customer to assure that the 185 kV would be sufficient.)
According to ANSI standard: 1. Calculating voltage correction factor: Si A ≥ 10 Mft; USAR: ACF= 1.0 – [0.02 x A] ACF = 1.0[0.02 x (14,753/1,000)] = 0.70 2. Calculation of current correction: factor Si A ≥ 10 Mft; USAR: ACF = 1.0 – 0.004 x A ACF = 1.0 – 0.004 x (14,753/1,000) = 0.94 3. Selection of switchgear: Table No. 13: Correction for altitude ANSI / NEMA Voltaje de operación a 4,500 m.s.n.m
Factor
Nuevo rango a 4,500 m.s.n.m
Rango Switchgear (ver Tabla 2,3,4)
22,9 kV
0.7
31,42 kV
38 kV
Voltaje de frecuencia industrial
50 kV
0.7
71.42 kV
80 Kv
Voltaje de descarga atmosférica
125 kV
0.7
178.6 kV
150 kV
Corriente Continua
1,500 A
0.94
1410 A
2000 A
Rango Voltaje Operación
Prepared by author
According to the above table, the equipment recommended will be a model PowellVac GE switchgear 38 kV, 2000A current and BIL of 150 kV, 25 kA. (You would need to have a conversation with your customer to assure that the 150 kV would be sufficient.)
5. Conclusion From the above cases we can conclude that altitude is an important factor in the derating of electrical characteristics (voltage and current) for switchgear (Table No. 2, 3 and 4). It is shown that nominal voltage, power frequency voltage and discharge voltage (BIL) are most affected because their derating factors are related exponentially with the altitude (Table 6). For Case No. 2, it can be observed that to an altitude of 4500 masl and a service voltage of 22.9 kV, the derating of electric characteristics would require a switchgear rating of 38 kV NEMA or 40.5 kV IEC. In both cases the standard electrical ratings of the equipment did not meet the necessary requirements. It can be noted that the altitude correction factors cause the discharge voltage (BIL) levels to exceed any standard equipment ratings as listed by NEMA and IEC. An alternative solution is to follow the recommendations of IEEE C37.20.2 where in clause 8.1.3 an arrester is recommended for altitudes above 1,000m to maintain transient voltages below the permissible limit of the equipment or you should examine the possibility of using SF6 gas insulated switchgear. This type of equipment is not affected by altitude as its active parts are enclosed in SF6. A thorough technical and economic analysis should be used to determine which is the best type of equipment to be applied in each specific installation.
6. Bibliography:

PCIC Paper No. 2012  015, "IEC & ANSI Standards Medium Voltage Distribution Equipment, Review & Analysis". Marcelo Valdez, Xuhui Ren, Shridhaval Sapre, Marty Trivette, Steven Meiners.

ED2.0 IEC 62271200 (201110)  Highvoltage switchgear and control gear Part 200: AC metalenclosed switchgear and control gear for rated voltages above 1 kV and up to 52kV. Geneva, Switzerland: IEC.

IEEE C37.20.21999  ANSI Standard for MetalClad Switchgear, New York, NY: ANSI.

IEEE C37.041999  IEEE Standard Rating Structure for AC HighVoltage Circuit Breaker.

ANSI C37.06.12000, Guide for HighVoltage Circuit Breakers Rated on a Symmetrical Current Basis Designated.

GET6840CSelection Guide Medium Voltage Starter NEMA, Limitamp General Electric.

GET 6600FGuia Medium Voltage Switchgear Application NEMA  Power / Vac.

Selection Guide Air Insulated Switchgear Medium Voltage IEC – GE SecoGear.

Selection Guide for Gas Insulated Switchgear Medium Voltage IEC – GE SecoCube.