Heat generation estimation for historic switchgear Type D (to 4.76kV) and Type F (to 15kV) with air magnetic circuit breakers

Author: Ted Olsen

09/01/2007 - Volume V - Issue I

We are often asked for estimated heat generation data for our equipment. These requests are most common for our modern products, but we also have requests for data on the types D and F switchgear designs which used air magnetic circuit breakers. This issue of Tech Topics provides information on these historic designs to allow calculation of approximate heat generated by the switchgear under assumed loading conditions.

 

The heat generation data in tables (pages 2-3) is based on full rated continuous current. Actual heat generation estimates must consider the true loading of the equipment. The amount of heat generated is related to the square of the current, so a circuit breaker operating at one-half rated current will have heat generation only one-quarter of that at full rated continuous current. Because the effect of the square relationship is very significant, it is overly conservative to estimate heat generation based on the assumption that all sections and all circuit breakers each carry their rated continuous current at all times. Air conditioning systems sized based upon such estimates will be larger than the real operating conditions require.

 

To estimate the heat generated under actual loading conditions, determine the component heat generation for each of the components indicated in the table. Estimated heat generation for circuit breakers should be adjusted for actual loading based on the ratio of the squares of the actual current loading for each individual vertical section, but this is frequently ignored in the interests of simplification. Instead, the main circuit breaker loading is usually assumed to be equal to the main bus loading in all vertical sections.

 

Example: Assume a lineup of type F switchgear, with five vertical sections, one 2000A main breaker (loaded to 1400A), three 1200A feeder breakers (loading 250A, 600A, and 550A), with 2000A main bus and including space heaters. The lineup includes one VT rollout, one 10kVA CPT, and non-complex electromechanical relaying (e.g., three induction disk relays) and instrumentation (e.g., an ammeter with selector switch). The calculation would be as follows:

 
Category
Heat generation
A
2000A breaker at 1400A = 1290 x (1400/2000)^2 = 
632 W
 
A
1200A breaker at 250A = 910 x (250/1200)^2 = 
39 W
 
A
1200A breaker at 600A = 910 x (600/1200)^2 = 
228 W
 
A
1200A breakers at 550A = 910 x (550/1200)^2 = 
191 W
 
A
Total heat generation, circuit breaker cells
 
1090 W
B
Vertical sections w/2000A bus at 1400A = 5 x 210 x (1400/2000)^2 =
 
515 W
C
Space heaters for 3 vertical sections = 5 x 500 =
 
2500 W
D
VT rollout = 1 x 50 =
 
50 W
E
CPT = 1 x 4% x 10kVA = 
 
400 W
F
Relaying and instrumentation = 4 x 100 = 
 
400W
 
Total estimated heat generation under assumed loading conditions
 
4955 W

If true loading were not considered, (i.e., all calculations performed on the basis of full rated current), the calculations would yield a heat generation of 8420 watts, or about 170% of the "real" heat generation.

Type D switchgear (to 4.76kV) with type MA or type FA air magnetic circuit breakers

Approximate full load heat generation (in watts)
Category
 
 
 
 
 
 
Rated current
1200A
2000A
3000A
3750A
 
Actual current
1200A
2000A
3000A
3750A
Circuit breaker cell, with circuit breaker
 
750W
1050W
2390W
3520W
Vertical section, with main bus
 
100W
150W
310W
480W
Space heaters, per vertical section
 
500W
VT trunnion
 
50W
CPT (drawout or stationary)
 
4% of CPT kVA rating
Relaying and instrumentation, per circuit breaker cell:
 
 
Microprocessor type
50W
 
Electromechanical - non-complex
100W
 
Electromechanical - complex
200-300W

Notes on assumptions:

  1. Space heaters, when provided, are not normally controlled by a thermostat; hence, their load is represented as a continuous load. The purpose of space heaters is to prevent condensation, and this is not limited by the absolute temperature. Even when a thermostat is used to control the heaters, it is set to shut the heaters off at a temperature of approximately 110 degrees F. Therefore, in an air conditioned room, the heaters would be energized continuously.
  2. Heat generated by current transformers is ignored as it is usually insignificant and varies according to the CT ratio as well as the loading.
  3. The CPT heat generation estimate is very conservative and assumes that the CPT is operated at full rated capacity. If normal loading is at less than full rating, heat generation may be adjusted by the square of the percent loading.
  4. Relaying and instrumentation heat generation estimates are very approximate, and are normally estimated on the basis of the number of circuit breaker cells. Extensive relaying and instrumentation may warrant additional conservatism in the estimation of associated heat generation.
  5. Conversion factor: watts x 3.415179 = BTU / hour.

Type F switchgear (to 15kV) with type FB or type FC air magnetic circuit breakers

Approximate full load heat generation (in watts)
Category
 
 
 
 
 
 
Rated current
1200A
2000A
3000A
3750A
 
Actual current
1200A
2000A
3000A
3750A
Circuit breaker cell, with circuit breaker
 
910W
1290W
2390W
3520W
Vertical section, with main bus
 
140W
210W
310W
480W
Space heaters, per vertical section
 
500W
VT trunnion
 
50W
CPT (drawout or stationary)
 
4% of CPT kVA rating
Relaying and instrumentation, per circuit breaker cell:
 
 
Microprocessor type
50W
 
Electromechanical - non-complex
100W
 
Electromechanical - complex
200-300W

Notes on assumptions:

  1. Space heaters, when provided, are not normally controlled by a thermostat; hence, their load is represented as a continuous load. The purpose of space heaters is to prevent condensation, and this is not limited by the absolute temperature. Even when a thermostat is used to control the heaters, it is set to shut the heaters off at a temperature of approximately 110 degrees F. Therefore, in an air-conditioned room, the heaters would be energized continuously.
  2. Heat generated by current transformers is ignored as it is usually insignificant, and varies according to the CT ratio as well as the loading.
  3. The CPT heat generation estimate is very conservative and assumes that the CPT is operated at full rated capacity. If normal loading is at less than full rating, heat generation may be adjusted by the square of the percent loading.
  4. Relaying and instrumentation may warrant additional conservatism in the estimation of associated heat generation.
  5. Conversion factor: watts x 3.415179 = BTU / hour.