PROTECTION AND CONTROL

Circuit Breaker Timing Tests -- Part 1
Timing Tests Are a Powerful Tool for Troubleshooting Faulty Breakers

Main circuit interrupters are:

• Disconnect-switch: used mainly to isolate equipment or portion of a circuit for repair or maintenance. The disconnect switch has little or no current interrupting abilities. Its operation has to be done without any current flow in the circuit.
  It is a safety device with, usually, a visible breaking contact and it can be locked in the open position.
• Interrupter: this switch is designed to close or open circuits and make or interrupt nominal currents. It is faster than a disconnect-switch and has current breaking capability.
• Circuit-breaker: The circuit breaker fills the same function as an interrupter but has the ability to interrupt short circuit currents as well. It is the ultimate protection device on the power network.

These tests are necessary to determine the real condition of the circuit breaker before starting service, to be able to establish a starting point to trace its evolution. One of the most important tests among these is the timing test, and it includes:

• measuring the exact instant that the contacts change states.
• verify the contacts' discrepancy
• verify the contacts' travel and speed

THE CIRCUIT BREAKER
The circuit breaker is the most important and complicated of all types of power circuit interruption equipment. This is due to its highly important capability of interrupting the powerful short circuit current, over and above its normal role of conducting, isolating and interrupting nominal load currents.

The power circuit component of the circuit breaker is where the main current flows or is interrupted, and it includes:

Arcing Chamber
The arcing chamber is a closed volume containing a fixed contact, a moving contact and the interrupting medium. The current is established when the moving contact touches the fixed contact and interrupted when they part.

An arc is created when the contacts part. The interrupting medium is responsible for quenching the arc and establishing the nominal level of isolation between the open contacts.

Several chambers may be connected in series to serve higher voltage levels. In this case a grading capacitor is installed in parallel with each chamber to balance the voltage across the contacts when parting.

Insertion resistor
The sudden modification of circuit characteristics, when circuit breakers operate, produces peak voltage impulses where the level is determined by the circuit characteristics. These impulses may reach very high levels and must be reduced. A well-known method is closing or opening in two or three steps on resistors.

• On trip: the voltage impulse levels are generally acceptable when interrupting nominal or short circuit currents, but they can be dangerously high when interrupting small capacitive or inductive currents.
• On close: sudden energizing of a circuit always generates voltage impulses with moderate levels, with the exception of closing or reclosing on long unloaded lines where the impulses, function of the line length, instant of closing or reclosing and discrepancy of the three poles, can reach extremely high levels.

A resistor with a predetermined value is placed in series with an auxiliary contact. Both are installed in parallel with the main arcing chamber.

The auxiliary switch is programmed to close a few milliseconds before the main contacts on closing and to open a few milliseconds after the opening of the main contacts on tripping. The programmed delay is called the insertion time.

Operating Mechanism
The operating mechanism is where the needed energy to part the contacts and to extinguish the arc is developed. It includes devices, called energy accumulators, to store the needed energy. Examples of accumulators include springs, and nitrogen-charged cylinders.

The most common operating mechanisms in circuit breakers are spring operated, hydraulically operated and pneumatically operated.

Control
The order to operate the breaker is launched in the control part of the circuit breaker, as an electric impulse of a fraction of a second duration. The order is then amplified in the operating mechanism to a complete circuit breaker operation capable of interrupting short circuit currents. The control includes:

• Closing and tripping coils
• Control relaying system
• Pressure switches and gauges
• Surveillance and alarm system
• Re-inflating system to restore the energy spent on the operation.

Correct function
The circuit breaker control must ensure correct closing action, whatever the closing current value, and ensure breaking (opening) at the required moment by releasing, by mechanical action or via a relay, the energy stored in the accumulators.

This energy has to face opposing forces when closing or breaking circuits under load or not, and even stronger forces with short circuit currents.

This means an excess of energy, when operated without load, has to be damped with a proper damping system.

Operation cycles
The circuit breaker has to be capable of executing different operation cycles and achieve fast breaking of short circuit currents -- the faster, the better for the network. Recent progress has reduced the response time from 5 to 3 cycles, and down to 2 cycles. It is already planned to have response times of 1 cycle. Operation has to be reliable, robust and easy to maintain.

CIRCUIT BREAKER TYPES
A circuit breaker has to interrupt weak capacitive or inductive currents, up to high short circuit currents and, as a result, to extinguish powerful electric arcs. The main problem is then, essentially, an arcing problem. Another problem is overvoltage impulses; this is related to the nature of the circuit where it is installed.

One of the major factors influencing the capacity of circuit breakers is the interrupting medium. It affects a circuit breaker's concept and design. By this principle, circuit breakers are classified in families according to the type of interrupting medium used.

Interrupting Medium
A good number of substances have acceptable qualities to be interrupting mediums.

Three of them are widely preferred by circuit breaker designers around the world. This is due to their excellent breaking and insulating properties that lead to high performance and economic designs. They are:

Mineral oil
Mineral oil was, until recently, the interrupting medium of choice.

It has excellent breaking and insulating quality, especially when it is very pure, as is the case when it is used in certain devices such as capacitors or transformers, which are airtight devices.

However, circuit breakers have breathing holes and the oil is in contact with the arc. Thus, one finds in the breaker's oil a certain amount of impurities, in the form of moisture and miscellaneous dust, including carbon particles. This decreases its isolation properties significantly. It is imperative to monitor the state of the oil inside breakers in service, and to replace it periodically in function of the number of breaks performed by the device.

The criteria for oil replacement depend on the structure of the breakers and are indicated by the manufacturer.

Compressed air
Some of the advantages of air at atmospheric are that it has good insulation quality, it's always available and costs nothing. The insulating quality of air rises rapidly with its pressure. In practice, we can count on a disruption voltage of up to 90 kV between contacts separated by 1 cm at 10 bars pressure, and 1.5 times this value for the same distance at 20 bars pressure. Compressed air was mainly used for interruption in the earlier pneumatic circuit breaker designs. Later on it was used for insulation between the contacts after they opened, the latter being placed inside an insulating chamber designed to resist the air pressure. This reduced significantly the distance between the open contacts.

Air quality for pneumatic circuit breakers: It should be noted that the excellent quality of air is greatly affected by humidity. Indeed, it is important that any condensation in the insulators and air conduits be avoided, or internal tripping may occur. Installing the costly drying compression stations greatly raises the cost of operating air blast circuit breakers.

Sulfur hexafluoride (SF6)
A certain number of gases, called electronegative, have better insulating qualities than air. Among them is sulfur hexafluoride (SF6) which has seen a great deal of success in electrical apparatus design because of its excellent insulating properties and remarkable arc quenching abilities. It is five times heavier than air, odorless, colorless, nonflammable and non-toxic when new. Its dielectric strength is 3 times the air's dielectric.

When subjected to an electric arc, it partially decomposes. In the presence of moisture and impurities it produces acid by-products that attack metal and the insulating envelopes. An efficient way to reduce by-products is to use activated alumina inside the chambers containing the gas. SF6 is a gas at normal temperatures, and at atmospheric pressure it liquifies at -60°C, and at 20 bars it liquifies at 20°C, which is detrimental to its insulating qualities. For applications at very cold temperatures, it must be heated or mixed with other gases like Nitrogen or CF4.

The bulk oil circuit breaker consists of a steel tank partly filled with oil, through the cover of which are mounted porcelain or composite insulating bushings.

Contacts at the bottom of the bushings are bridged by a conducting cross head carried by a wooden or composite lift rod which, in common designs, drops following contact separation by spring action, thus opening the breaker.

An air cushion above the oil level serves as an expansion volume to prevent pressure from building up inside the chamber after the interruption of the short circuit current.

Regardless of improvements, the bulk oil circuit breaker presents many disadvantages including its great weight and bulk, risk of fire, a strong reaction to ground and frequent bushing failure, etc.

The minimum oil circuit breaker was developed for 170 and 245 kV systems, the highest voltages at the time, where the inherent problems of bulk oil breakers were the most severe, also to eliminate oil as insulating medium and thus reduce the quantity of oil in switchgear installations to an amount that would not cause any hazard. The excellent arc-quenching properties of oil, however, were used later in specially developed oil- and pressure-tight arc-interrupting chambers.

Minimum oil circuit breakers for high voltage are single-interrupter up to 170 kV and multiple-interrupter breakers for 230 kV and higher.

Contacts are placed in a cylindrical, insulating envelope, with connection terminals at either end, and placed on an insulating support.

Compared to a bulk oil breaker, ground isolation is considerably improved by the elimination of the vulnerable isolating bushings and metal tank in proximity of the arc. The oil no longer insulates to ground, and oil volume is reduced by a factor of 10 to 20.

They use an arc-control device in the arcing chamber which physically shortens the arc and the arcing time, thereby reducing the arc energy.

When the breaker interrupts small currents, the arc is extinguished by a forced axial flow of oil. In short-circuit range the arc is quenched as a function of current. The arc is blown by a jet of oil at right angles to its axis, and extinguished.

Most minimum oil circuit breakers are designed for fast reclose. They have to be able to interrupt short circuit currents twice in a row with 0.2 sec to 0.3-sec intervals, therefore the arc-control device has to contain enough oil to succeed in performing the second interruption.

Air blast circuit breaker
Until recently, the air blast circuit breakers have dominated the high and very high voltage applications. From 170 kV to 800 kV and breaking capacity from 20 kA to 100 kA.

Over 100 kV the breaker has multiple chambers connected in series. Each element is optimized to around 80 kV. At first, 800 kV breakers had 12 chambers in series per phase, now they have only 8 chambers per phase.

Although increasing the air pressure increases the speed of dielectric regeneration, it is still relatively slow. Insertion resistors are often used to reduce voltage surges.

Air blast circuit breakers adapt well to the high-voltage power network's new demands.

For example, adding insertion resistors, single or double step, to reduce closing voltage surges, is easily done. Also, it is capable of achieving very fast breaking times, 2 cycles and even less, improving the network's stability.

In general, air blast circuit breakers are high tech and robust equipment, with great electrical and mechanical endurance. Contact wear is low due to the short arc duration and low arc voltages.

The compressed air circuit breaker has two major disadvantages:

• installation of expensive compression stations
• high noise levels on operation

SF6 breakers are available for all voltages ranging from 14.4 kV to 800 kV, continuous current up to 4000 A, and symmetric interrupting ratings up to 63 kA.

SF6 circuit breakers are either the dead tank design, or the live tank design and the GIS design.

During recent years SF6 circuit breakers have reached a high degree of reliability, and they cope with all known switching phenomena.

Their completely closed gas system eliminates any exhaust during switching operations and thus adapts to environmental requirements. They may be installed horizontally or vertically, according to the structural requirements of the substation. The quick dielectric regeneration of the arc plasma in SF6 makes insertion resistors unnecessary, simplifying the apparatus.

Their compact design considerably reduces space requirements and building and installation costs. In addition, SF6 circuit breakers require very little maintenance..