Introduction
Circuit breakers are some of the most important components in modern electric power systems. The circuit breaker has to operate within extremely tight tolerances when a disturbance is detected in the network to protect sensitive and costly components such as transformers. They have to operate after months or in some cases years of inactivity. To ensure proper function and optimize network reliability, reliable and efficient test instruments and methods are needed.
Why test circuit breakers
Some of the most important of the many reasons for testing circuit breakers are to ensure they:
■ Provide protection for expensive equipment
■ Prevent outages that lead to loss of income
■ Ensure reliability of the electricity supply
■ Prevent downtime and darkness
■ Verify breaker performance
Substation breaker testing is an important task for any power utility
The breakers are there to facilitate the flow of current during normal operation and to interrupt current flow in the event of a fault. However, all electrically operated devices are, sooner or later, likely to experience some kind of failure. That failure can be caused by many factors, including ageing and external faults. The power utility operator has to be prepared and have a plan in place to handle every situation.
Breakers are mechanically sophisticated devices requiring periodic adjustments. The need for some of these adjustments can be determined visually and they can be given the attention needed without testing. However, in most cases, it will be necessary to carry out electrical testing to find out what is the cause of out-of-tolerance conditions. This guide primarily deals with electrical testing.
HV Circuit Breakers in a transmission scheme can be viewed as forming a tree starting with the generating station, fanning out to the transmission grid, to the distribution grid, and finally to the point of consumption.
The task for the utility is to generate power, transmit it and distribute it with maximum availability. While doing this, it is imperative that losses are kept to a minimum, and acceptable levels of power quality and safety are maintained. All of this must be done in an environmentally friendly manner. Breakers play an important part in making this happen. High voltage circuit breakers are extremely important for the function of modern electric power supply systems. The breaker is the active link that ultimately has the role of quickly opening the primary circuit when a fault occurs. Often, the breaker has to perform its duty within some tens of milliseconds, after months, perhaps years of idly being in circuit. Since RCM (reliability centred maintenance) and condition based maintenance have become the established strategies for most owners and operators of electric power delivery systems, the need for reliable and accurate test instruments for field use is clear.
Protection systems are put in place to detect all electrical faults or other abnormal operating conditions and they are coordinated to disconnect the smallest possible part of a power network in the event of a fault. With good system design, it should be possible to quickly restore normal operation. When a fault is detected by a protective relay and a trip impulse is sent to the breaker operating mechanism, the breaker has to function as specified and interrupt the current as soon as possible or severe damage may occur. The cost of damage caused by a malfunctioning circuit breaker can sometimes reach large sums.
Proper functioning of a breaker is reliant on a number of individual components that have to be calibrated and tested at regular intervals. The trigger for maintenance intervals differs greatly between power utilities but the intervals are often based on time since last test, number of operations, or severity of fault current operations. Environmental considerations such as humidity and temperature, whether the breaker is located in a desert or coastal region, also play into the maintenance scheme.
Mechanical wear and lubrication often affects the performance of breakers, so being able to trend mission critical parameters and compare these with factory thresholds helps to verify proper breaker function.
Standards
High voltage circuit breaker design and operation as well as type and routine tests are defined by international standards such as:
■ IEC 62271-SER ed1.0 - High-voltage switchgear and control gear.
■ ANSI/IEEE C37 - Guides and Standards for Circuit Breakers, Switchgear, Relays, Substations, and Fuses.
■ IEC/TR 62063 ed1.0 (1999-08) TC/SC 17A - High-voltage switchgear and control gear - The use of electronic and associated technologies in auxiliary equipment of switchgear and control gear.
Testing guide
Type of CB | Vacuum | Oil | Minimum Oil | SF6 | Air-blast | GIS | |||||
Voltage levels (kV) |
1 – 36 |
Any |
6 – 145 |
145 – 400 |
6 – 40 |
72 – 245 |
>245 |
6 – 40 |
40 – 130 |
>130 |
Any |
Timing | x | x | x | x | x | x | x | x | x | x | (x)1) |
Motion | (x)2) | x | x | x | x | x | x | (x)3) | (x)3) | (x)3) | x |
Coil current | x | x | x | x | x | x | x | (x)2) | (x)2) | (x)2) | x |
DRM | n/a | (x)2) | (x)2) | (x)2) | x | x | x | (x)2) | (x)2) | (x)2) | (x)6) |
SRM | x | x | x | x | x | x | x | x | x | x | (x)1) |
Vibration | (x)2) | (x)2) | (x)2) | x | (x)2) | x | x | (x)2) | (x)2) | (x)2) | x |
DCM | (x)2) | x | x | x | x | x | x | x | x | x | (x)1) |
Motor current | x | x | (x)4) | (x)4) | (x)4) | (x)4) | (x)4) | n/a | n/a | n/a | (x)4) |
Min. voltage to operate CB | x | x | x | x | x | x | x | x | x | x | x |
Minimum voltage | x | x | x | x | x | x | x | x | x | x | x |
Station voltage | x | x | x | x | x | x | x | x | x | x | x |
Gas density | n/a | n/a | n/a | x | x | x | n/a | n/a | n/a | n/a | |
Voltage integrity | x | n/a | n/a | n/a | n/a | n/a | n/a | n/a | n/a | n/a | n/a |
Air pressure/flow | n/a | n/a | n/a | n/a | n/a | n/a | n/a | x | x | x | n/a |
PIR contacts | n/a | x | (x)5) | (x)5) | (x)5) | (x)5) | (x)5) | (x)5) | (x)5) | (x)5) | (x)5) |
Grading capacitors | n/a | n/a | n/a | (x)5) | (x)5) | (x)5) | (x)5) | (x)5) | (x)5) | (x)5) | (x)5) |
Legend | |||||||||||
x | Applicable | ||||||||||
(x)1) | If accessible | ||||||||||
(x)2) | Possible | ||||||||||
(x)3) | Optional – if disconnector knife/contact is included in CB design. | ||||||||||
(x)4) | Motor current is only applicable on spring drives. | ||||||||||
(x)5) | Applicability and presence of grading capacitors as well as PIR contacts are depending on network design and not related to CB design. | ||||||||||
(x)6 | If accessible and if not vacuum CB | ||||||||||
n/a | Not applicable | ||||||||||
Comments | |||||||||||
Motor current | There are three different types of operating mechanisms; Spring, hydraulic and pneumatic. Motor current is only applicable on spring drives. | ||||||||||
PIR contacts Grading capacitors | Applicability and presence of grading capacitors as well as PIR contacts are depending on network design and not related to CB design. Usually not used in distribution networks. | ||||||||||
Oil CBs | Well defined travel trace and transducer attachment points. At higher voltages serial contacts per phase but those cannot be accessed, thus only timed as single contacts per phase. If PIR contact those are often sliding contacts in the main tank. No separation possible. Has to be timed as parallel contacts. PIR values can be down to 10 Ω. Coil current traces are essential. Always single operation mechanism if single tank design. | ||||||||||
Minimum Oil CBs 6 – 145 kV | 1 contact / phase, single operating mechanism. | Coil currents are essential, as well as operating mechanism damping dash-pots. Travel is essential and it is usually relatively easy to find documentation on transducer attachment points etc. | |||||||||
Minimum Oil CBs 145 – 400 kV | 2-6 contacts / phase. 400 kV always separate (3) operating mechanisms. | ||||||||||