Circuit Breakers - ASHRAE-D-21513-20100324

The circuit breakers in this study consist of both low and medium voltage AC devices and DC breakers. The actuator of the breaker does not contribute a significant portion of the overall device rate of heat loss. The power loss of the circuit breaker comes from I²R ohmic heating.

The ranges of devices under consideration consist of DC breakers from 100 to 1500 amp, low voltage breakers up to 4000 amp, and medium voltage breakers from 1200 to 3000 amp.

Review of Environmental Heat Gains

Standards

There is only one standard which comes close to treating breaker power loss and this standard is IEEE Std. C37.09, Test Procedure for AC High-Voltage Circuit Breakers Rated on a

Symmetrical Current Basis. According to IEEE Std. C37.09, the DC resistance is measured by passing 100 amps through the breaker and then determining the voltage drop. The motivation for testing the DC resistance is simply to determine the continuity of the conducting path. These are over a hundred standards in the IEEE/ANSI C37 series and C37.09 is the only standard which provides a test procedure for the breaker DC resistance. No breaker standard reviewed in this work was found to address circuit breaker heat loss.

Equipment Heat Losses

The circuit breaker dissipates heat through I²R losses. The DC resistance provides only part of the picture for AC devices since skin effect tends to increase the resistance value. Also, conductor temperature plays a roll in determining resistance. The influence of a conducting enclosure around the breaker can increase the losses of AC breakers through stray loss created by eddy-currents in the enclosure material. Proximity effect of the individual breaker poles,

mounted close to one another, can also increase the heat loss of the breaker compared to a single pole. The environmental temperature could also play a roll in influencing the conductor

temperature and the breaker resistance. All of the items mentioned here have some influence on the total breaker heat loss.

The thermal performance of the breaker is of concern to the electrical design engineer.

Overheating, especially in those breakers that are thermally activated is to be avoided. This overheating, caused by excessive current, alters the shape (through thermal expansion) of the latch holding the breaker in the “on” or energized position. Once the latch changes shape, the spring pressing against the breaker switch all the while it is energized is free to move the breaker to the “off” position, thus, tripping the breaker. Should overheating, caused by other factors such as the environment, take place in the absence of excessive current then nuisance tripping of the breaker occurs. This information provides an upper limit for the breaker conductor temperature and, as a result, the power loss.

No information has been found in this work that accounts for all of the various influencing conditions cited above. A discussion on how the breaker heat losses can be modeled is now presented. Increases in conductor resistance above the DC value caused by skin effect might

possibly be estimated through analytical or empirical corrections. Likewise, the DC resistance could be measured at room temperature using a current much smaller than the device rating which could be several thousand amps. In this case, at the rated current, the breaker conductor temperature approaches the maximum temperature rise specified by the standards. If the DC resistance was measured at reduced current and at room temperature, then the temperature rise caused by the rated AC current increases the electrical resistance above the measured DC value corrected for skin effect. By using a resistance corrected for both skin effect and temperature, the I²R calculation at rated current might provide a useful estimate of the overall device losses.

The study of the increase in breaker heat losses with an enclosure might provide another empirical means to estimate the enclosure influence. Likewise, the determination of the sensitivity of the heat losses to ambient temperature, with and without an enclosure, might provide a way to model the power loss should this be a significant factor. This brief discussion highlights a strategy that might provide a means of predicting the breaker heat loss.

Manufacturers

A list of circuit breaker manufacturers was assembled and the manufacturers were contacted by e-mail concerning breaker losses and loss test methods. This information was used in

completing Table 3. From one manufacturer, a catalog containing the DC power loss values was obtained while from another a spreadsheet providing loss as a function of load current for

breakers of several frame sizes was obtained. The purpose of the DC power loss values, provided by one manufacturer, is to provide information useful to field maintenance and testing of breakers. The DC power loss of an AC breaker is measured by passing a DC current through the breaker equal in value to the rated RMS current. No information on the measurement uncertainty of the manufacturer supplied data was found.

Measurement Uncertainty

The standard C37.09 does not address instrument uncertainty in the measurement of the DC resistance. If the DC resistance is to provide a possible means to model the heat losses, the quality of the resistance value must be known. As stated earlier, no breaker manufacturer measurement uncertainty information has been found.

Information Deficiencies

Starting with the DC power loss value, the DC resistance can be found. The uncertainty of the DC resistance must be determined. If the heat loss is to be based upon the DC ohms to which empirical factors are applied, then the quality of the base resistance value must be known. The influence of AC skin effect, load created conductor temperature increase, enclosures, and ambient temperature has to be determined in order to adequately account for all the contributing factors of breaker heat loss. None of this information on influences is currently available.

Test Plan

The goal of the test plan is to provide a means of predicting the heat loss of a circuit breaker which accounts for the various factors mentioned. To achieve this end, the plan has as a first

sub-goal to determine the uncertainty of the power loss information that is publicly available.

The second sub-goal is to quantify the influence of all of the other factors. Based upon the measured data, a decision on whether the breaker power losses can be approached through empirical factors or not has to be made. If empirical factors can be used then the treatment of breaker losses would be a fast and simple calculation since once the breaker DC resistance is corrected for skin effect, temperature, enclosure, and ambient temperature then the heat loss of the breaker can be estimated given the load current. If the empirical approach is not practical, then tables of loss information must be developed. The initial approach is to study the loss influence factors for several breakers in order to build a body of information for determining the empirical factors or to construct tables to satisfy the needs of this study. Several breakers will have to be tested to assemble this information and to provide a basis of deciding how to model the breaker losses. This activity would be conducted during Phase II of this work.

Before the steps of the test plan are enumerated, a discussion of the measurement of breaker power loss will be presented. This discussion will lead to the construction of an additional test apparatus.

The philosophy of breaker usage is that if the breaker performs its desired task, and if the dc resistance is below some specified value, then whatever the losses happen to be are just accepted as part of the cost of doing business. One reason for this situation is that circuit breaker (CB) power losses are difficult to measure. One obvious way to measure the losses would be to place wattmeters on both the line side and the load side of the breaker and simply subtract the two readings. Since the readings would be so close, subtractive cancellation would render the result useless. To illustrate this point, consider the example of the three-pole, 300 A, 480 V CB operated at the rated voltage and current supplying a unity power factor load. The power is

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3 = W. Based on the tests of a CB removed from service, the power loss is probably no more than 100 W. This is one part in 2500, virtually impossible to measure.

If it is assumed that dielectric losses are independent of ohmic losses, then it becomes possible to test a CB on the bench with a modest amount of test equipment. A wattmeter could be used to measure the loss at rated voltage and no current to determine the dielectric losses. To determine the load losses, rated current could be passed through the breaker at low voltage and these power losses could be measured by a wattmeter. The sum of the two wattmeter readings would be the CB power loss.

The dielectric loss can usually be neglected without significant error (a reasonable assumption for medium voltage levels and below). As a confirmation of small dielectric losses, a modern wattmeter could not measure a loss (less than 0.1 W) when the middle pole of a 300 A CB was energized at rated voltage and the outer poles were grounded. This brief test demonstrates that only the load losses need to be measured.

In order to test CB load losses, a source is needed that will deliver hundreds of amperes at a fraction of a volt. Unfortunately, such sources are not readily available from equipment manufacturers. After a source is located, there is still a problem with test leads. Even heavy cables will have significant voltage drops at these current levels. Connections need to be bolted.

Even the best plug and socket will probably dissipate a similar amount of power as each pole of

the CB would dissipate. The desired qualities of the breaker test apparatus include that it is inexpensive, easy to use, constructed of readily available components, low uncertainty

(say±5%), and portable. One method of driving rated current through a CB and measuring the power is shown in Figure 2. The power source is a single-phase autotransformer, rated at 120VAC and 8 A. (larger ratings would obviously work but are not essential). Next is a modern single-phase wattmeter. A wide variety is commercially available. The recommendation is to use a wattmeter that provides a visual and a digital readout (data acquisition system compatible) of watts, power factor or VA, volts, and amps. The wattmeter should have a convenient means of excitation and must be compatible with the autotranformer. The capacitor limits the current to the very low impedance to be tested and allows the use of the full range of the variable

autotransformer without damage to the instruments. A capacitance of 150 µF will allow 5A to flow at a voltage of about 90 VAC. An inductive reactor could also be used in series, but a motor run capacitor is more efficient. The power loss in the capacitor and leads is measured by moving the lead from one terminal of the load (high current) side of the current transformer to the opposite terminal (thus forming a short) and applying power. On the prototype of this apparatus, a figure of 7.5 W at 5 A was measured in a preliminary test.

120 VAC Variable

Autotransformer

Current Transformer

5:400 Turns Ratio C₁

150 µF

Wattmeter Breaker

Load

Figure 2: Apparatus for Testing Losses in Circuit Breakers

High currents are obtained with a current transformer (CT) operated backwards from the normal application. Usually a conductor carrying a large current is passed through the CT opening and the low current winding are used to measure a proportional output current. For example, a 400:5 CT would produce an output of 5 A when 400 A passed through the opening. It is unusual to operate a CT in this reverse mode, so a word of explanation is in order. Any 60 Hz, iron core transformer will act like an ideal transformer under no load conditions, provided the voltage is not excessive. In the no load configuration, the unexcited winding is left as an open circuit and the current in this winding is zero while the voltage is proportional to the excitation voltage through the turns ratio. If the unexcited winding is short - circuited, the winding voltage is zero

ratio. If the excitation voltage is too high, the corresponding magnetic flux causes the iron to saturate. There will be a large magnetizing current and the output tends to decrease from the expected value without saturation. A CT is just like any other transformer in that it can be operated in either direction (5 A in or 5A out) as long as the iron does not saturate. As the load resistance increases from zero, the output (and input) voltages rises. When the voltage gets to the voltage rating of the transformer, there will be saturation of the magnetic path. Therefore a CT can supply rated current to a resistive load (such as the resistance of one pole of a circuit breaker) if the resistance is not too large.

Neither the voltage rating nor the VA rating of a CT is published. The only number published is something called the burden, specifying the maximum load for which the CT maintains its rated accuracy. The CT is still useful for loads beyond the specified burden. The actual rating would have to be determined experimentally. For example, a GE JCW – O 400:5 CT goes into

saturation at about 22 V on the 5A winding. It would be called a 100 VA transformer. This is generally consistent with its weight of about five pounds. The rated voltage on the high current side would be 100/400 = 0.25 V. The maximum impedance would be 0.25/400 = 625 µΩ. By way of reference, a 30-inch length of 2 gauge welding cable with heavy copper lugs soldered at each end has a resistance of about 500 µΩ. Two such cables in parallel would have a resistance of about 250 µΩ, so a resistance of up to 625 – 250 = 375 µΩ could be measured with this circuit. A larger resistance could be measured by putting two or three CTs in parallel, or by using lower resistance test leads.

Actual measurement of the power loss in a CB at room temperature requires a two step process.

First, the test leads are inserted through the opening of the CT, and the terminations are securely bolted together creating a short circuit. There is now a shorted turn through the CT. Power measurements are made at several convenient current levels. The wattmeter would measure the losses in the capacitor, the CT, the low current leads, and the high current leads. Second, the test lead termination is unbolted and the CB now takes the place of the short circuit. Power

measurements are repeated at the same current levels. The difference in the wattmeter readings, with and without the breaker, is the power loss in one pole or phase of the CB. For example, circuit losses for a 400:5 CT with two test leads were measured at 48 W for rated current. When a 400 A CB was inserted, the measured losses increased to about 78 W. Each pole of this

particular CB is dissipating78 – 48 = 30 W when rated current is flowing. The total loss (maximum) in three-phase operation would be 3(30) = 90 W. Normally, a CB is operated at no more than about 80% of its rated current, so the actual loss would be less than the maximum.

Likewise a used CB was tested where pitting, wear, and oxidation of the breaker contacts could have taken place. New breakers may have lower losses than the particular device that was used to test the prototype apparatus.

It is not difficult to measure power and the true rms voltage and current on the low current side of the CT to within 1%. The current in the high current side can be measured to within 1% with a second CT operated in the conventional fashion. It is probably not realistic to expect the actual power losses in a given CB to be known to this level of uncertainty, for a number of reasons.

Two are obvious since they apply to most types of electrical equipment. The first is that the ohmic loss varies as the square of the current, and the actual current is usually not known. The second is that ohmic loss increases with temperature and the actual conductor temperature is

usually not known. But reasonable estimates can be made for load diversity and for ambient temperature. A good engineer might get within 5% or 10% with these estimates. If this is true, there is little need to measure power to an uncertainty of less than 1%. Metering class CTs (0.1% uncertainty) are not needed for this work. A less expensive relay class CT will be adequate.

Another, hard to control factor is the variability of the contact resistance. There are three contact surfaces involved in this testing, two at the terminals of the CT, and one at the pole inside the breaker. The history of the surfaces (dust, grease, corrosion, pitting) can make a substantial difference in measured power loss. Values may change when a CB is opened and reclosed. For example, the 400 A CB mentioned above and used with the prototype apparatus tests had

measured resistances of 140, 140, and 200 µΩ of its three phases, using 20 A DC. The measured power loss of the same phases was 27.1 and 31.4 W, respectively. Both methods indicate that the third pole has higher losses, presumably due to surface conditions. If the CB could be disassembled and all surfaces polished, the readings would probably be closer. Calculated loses at the 400 amp AC current using the DC resistance measured at 20 A DC were 22.4, 22.4, and 31.8 W. The AC resistance is always higher than the DC resistance and the percentage

difference between AC and DC resistance increases as the circuit breaker rating increases. The measured and calculated powers look plausible for the first two poles. For the third pole it appears that 20 A was not enough to thoroughly wet the surfaces. A higher DC current would yield a lower resistance, giving a calculated loss closer to 25 W than 31.8 W.

A demonstration of the influence of an enclosure where hysteresis and eddy current losses in the metals located near the current carrying conductors was performed. Laying a CB on a steel sheet increased the measured power losses by 2% above the value obtained by mounting the CB on wood or some other non-conducting surface.

The influence of the enclosure created stray loss on the overall breaker power loss needs to be examined for AC devices. Since the breaker transfers heat to the surroundings via free

convection, it is unclear at this time if the enclosure alters or hinders the free convective air flow (especially when the enclosure is ventilated) and, thus, the breaker electrical resistance and associated power losses.

There are several steps in the test plan. This idea will have to be examined. The steps of the test plan are:

1) The apparatus of Figure 2 needs to be constructed. The intended limit on the test currents is 2000 amp. Required for this testing will be two CTs of the 400:5 turns ratio size and two CTs of the 1000/2000:5 rating. The second two CTs provide 2000 amp test capability. Two CTs are required for either test setup where one CT supplies the load and the other CT is used to measure the load current. The wattmeter needs to

In document ASHRAE-D-21513-20100324 (Page 54-67)