Circuit breaker and transducers - part 1
There are numerous types of equipment in the electrical transmission and distribution network that perform specific and necessary operations. The circuit breaker is no exception as it serves to protect the valuable assets in the substation. The circuit breaker is, however, unique because at one time it must function as a nearly perfect conductor and at another it must function as a nearly perfect insulator. It must transition from one state to the other in milliseconds, sometimes dispersing enormous amounts of energy during the transition.
A circuit breaker is defined as “a mechanical switching device” and the way to determine if the mechanical switching function is working correctly is to hook up a time and travel analyser to evaluate the operating characteristics. When bulk oil circuit breakers were the prevalent technology in substations, travel measurements were performed routinely. However, for reasons such as lack of outage time, lack of maintenance personnel and the complexity of hooking up a transducer to the circuit breaker, travel measurements are now performed much less frequently. Often, they are being replaced with contact timing only or reduced to first trip testing.
A recent study by CIGRE working group A3.06, released in 2012, found that 50% of the major failures in a circuit breaker were due to the operating mechanism and 30% were due to the electrical control and auxiliary circuits. While contact timing and first trip are important tests and should definitely be performed, they do not fully test the operating characteristics of the mechanism. Travel measurements should therefore not be neglected if the true health of the circuit breaker is to be determined.
Basic measurements made with the transducer
Although a motion transducer can be attached to the circuit breaker to determine the travel of different parts of its components such as the dashpot, its primary application to measure the motion of the main/arcing contacts in the circuit breaker, and this article focuses on that application.
The transducer can be linked to many different parts of the circuit breaker: directly to the pull rod of the main contacts, directly to the mechanism, somewhere on the linkage in between, or even to an auxiliary switch.
Many parameters are determined from the transducer but the most important is the stroke of the circuit breaker, as all other parameters are derived from this. The stroke is defined as the total travel distance of the contacts from resting position in the closed state to the resting position in the open state, or vice versa. It is essential to connect the transducer to the manufacturer’s recommended attachment point on the circuit breaker and to apply the appropriate correction factors if a direct connection to the contacts cannot be made. At the very least, the measurement of the stroke during periodic maintenance must be consistent so that it is possible to trend the results.
If the circuit breaker is gang operated – that is, it has one mechanism operating all three phases – only one transducer is needed. If the circuit breaker has separate operating mechanisms for each phase, an individual transducer should be used for each mechanism.
Once the stroke of the circuit breaker is determined, it is possible to derive the velocity of the contacts in different regions of the travel. The most common region to measure velocity is the arcing zone of the circuit breaker, where it interrupts or clears the fault. Occasionally damping is also measured on the travel curve by calculating the velocity in the damping zone or the time between two predefined points on the travel curve in the damping zone.
By observing the closing time along with the travel measurement, the penetration or contact wipe can be determined – this is how far the contacts are engaged. Penetration is the length measured from initial contact touch to the final resting position after the operation. Overtravel is measured directly from the travel curve and is the maximum displacement beyond the resting position that the contacts reach during the operation.
Similarly, rebound is measured from minimum displacement, after the maximum displacement (overtravel), to the final resting position of the contacts. Figure 1 shows examples of the parameters that can be measured with a travel transducer. Other parameters can be determined but they are all derivatives of the actual stroke measurement of the contacts, which is why it is important to connect the transducer correctly and to measure the stroke accurately.
Figure 1 – Typical motion trace
Types of transducer and parameters needed for proper measurements
Two types of transducers are used to measure the contact travel of circuit breakers: linear and rotary. A linear transducer will measure a length in either inches or millimetres whereas a rotary transducer will measure an angle, typically in degrees, which must be converted to a length measurement. There are various types of transducer, including resistive, optical and magnetic, and they give either analogue or digital outputs.
Linear transducers are available in lengths ranging from 25 mm or less, commonly used for vacuum circuit breakers, all the way up to 1000 mm or more. Typical lengths are 225 to 300 mm for SF6 dead tank circuit breakers and 500 to 600 mm for bulk oil circuit breakers. When preparing to make motion measurements, one must first decide whether to use a rotary or a linear transducer.. Although a lot of breakers allow the use of either type, the manufacturer will often specify a preference and it is always recommended to use the manufacturer’s specified attachment point, transducer type and, if needed, conversion factor.
CAUTION! Before connecting the transducer, always ensure that the circuit breaker is in the open position. Make sure no energy is stored in the mechanism, or if it is impractical to discharge all the energy, as is the case with some pneumatic and hydraulic mechanisms, make sure that the maintenance pin that blocks operation is set. Finally remove the power to the control circuitry. No matter where the transducer is placed - no part of the transducer, mounting bracket, or travel rod (if used) - must be in the direct path of any moving parts of the circuit breaker that could cause damage to the transducer or its accessories.
If a linear transducer is used, it must be of suitable length to cover the total travel it will encounter, including overtravel, on both close and open operations. When it is uncertain that the transducer is of suitable length, a common practice is to attach the transducer in, for example, either the open or the closed position, then detach the transducer and operate the circuit breaker so it changes state. It is then possible to attempt to reattach the transducer to confirm that it is of suitable length. Once a transducer of adequate length is selected, the next consideration is to make sure the transducer will fit in the space available.
There are two types of measurements made with linear transducers, the first is direct measurement, as shown in Figure 2, where the transducer or linkage rod is connected directly to the moving contacts. Direct measurements are common in bulk oil, minimum oil, most vacuum and some dead tank SF6 circuit breakers. Although finding a suitable mounting bracket and correctly mounting the transducer can be difficult at times, direct measurement is beneficial because the actual stroke of the transducer is equal to the actual stroke of the contacts. No conversion factor is needed and all parameters measured are directly representative of the motion of the contacts in the circuit breaker; the motion isn’t distorted by gears, linkages or mechanical play in the interconnections.
Figure 2 – Bulk Oil circuit breaker with direct linear connection
The second type of measurement with a linear transducer is an indirect measurement as shown in Figure 3. The transducer is not connected directly to the moving contacts but to a part of the circuit breaker that is connected to the moving contacts, such as the mechanism or interconnecting linkage. When this type of measurement is used, the travel of the transducer may or may not be equal to the travel of the main contacts. If the travel of the transducer is different, a conversion factor must be used to obtain the correct stroke length and travel parameters. For example, if the conversion factor is 1.5, 80 mm of transducer stroke would be equivalent to 120 mm of contact stroke.
Figure 3 – SF6 dead tank circuit breaker with indirect linear connection
When a rotary transducer is used, the measurement is in degrees or occasionally radians, and must be converted to a unit of length. There are two types of conversion. The first is a constant conversion where one degree is equal to a certain length throughout the entire travel of the contacts. This is common where the mechanical linkage is simple with few interconnecting parts. When the linkage is more complicated, the ratio of the angle to length may not be constant throughout the total travel of the contacts. For example, one degree might correspond to 1.5 mm for the first ten degrees but for the next ten degrees, one degree might correspond to 2.5 mm. In these cases, a conversion table must be used.
Rotary transducers have the advantage of being relatively small and, with a few accessories, one kit can be used with many different styles and types of circuit breaker. The disadvantage of rotary transducers is that a conversion factor or table is always needed to calculate the correct parameters for the circuit breaker under. If the conversion factor or table is not included in the manual, the breaker manufacturer must be contacted. Rotary transducers are most commonly used on live tank SF6 circuit breakers but are also used on certain types of dead tank SF6, bulk oil and generator breakers. See Figures 4 and 5 for examples of rotary transducer connections.
Figure 4 – Rotary transducer on SF6 live tank circuit breaker – image goes here
Figure 5 – Rotary transducer on SF6 dead tank circuit breaker – image goes here
Once the correct transducer is selected and the conversion factor (if needed) is determined, most parameters such as stroke, overtravel, rebound and penetration will come out of the measurements automatically. An exception is velocity; to calculate velocity, the analyser must be told where on the travel curve the velocity should be measured. Two points on the curve are selected and the average velocity between these points is calculated. The points chosen can reference many different points on the curve such as distance below close, distance above open, percentage of stroke, distance below upper point etc.
Another common reference point is an event during the timing – contact touch or contact separation, for example. The two most important factors that influence the selection of the speed calculation points are that the velocity is measured on a linear portion of the travel curve and that it is measured during the arcing zone. The calculation points are therefore generally near contact touch for the closing operation and contact separation for the opening operation.
A common mistake when calculating velocity is to take the total travel distance (stroke) and divide it by the total time it takes for the contacts to reach the fully closed position. This will determine the average velocity for total travel, NOT the velocity during the arcing zone. The acceleration at the beginning of the travel and deceleration at the end will mask the instantaneous velocity around the arcing zone.
Speed calculation points can vary by manufacturer, type of circuit breaker, type of mechanism, and so on, but the manufacturer’s advice should be followed to select the correct speed calculation points. This information is generally available in the manual or in the original test report provided with the circuit breaker. If no information is given, it is recommended to use contact touch and 10 ms before this for the close calculation points, and contact separation and 10 ms after this for the open calculation points.
See Table 1 for a list of common transducer types and speed calculation points used by different manufacturers, but note that the manufacturer or the manual should always be consulted to determine the correct transducer and speed calculation points.
Table 1 – Typical transducer types and speed calculation points
To be continued
The second part of this article, which will appear in a future issue of Electrical Tester, will include a case study of investigations carried out on a Siemens SPS2-38-40-2 circuit breaker, and will also look at what to do when little or no information to support travel measurements is available from the circuit breaker manufacturer.