INTERRUPTION OF SMALL INDUCTIVE CURRENTS POWER CIRCUIT BREAKER BASIC AND TUTORIALS

This occurs (see Fig. 10-67) when disconnecting unloaded transformers, reactors, or compensating coils. An arc is produced between the contacts when the circuit breaker is opened.

  FIGURE 10-67 Principleof small inductive- current interruption; equivalent circuit; typical shape of current and voltage.

The arc voltage is approximately constant at higher currents, since the arc energy is removed only be convection. With small currents, the arc voltage increases as a result of arc looping and a change in the cooling mechanism.

When approaching current zero, the arc current begins to oscillate as a result of interaction with the system; that is it becomes unstable.

As a result of the high oscillation frequency, the current interruption may occur prior to the natural zero passage, can be regarded as instantaneous, and is called current chopping. The chopping current is affected not only by the properties of the circuit breaker but also to a great extent by the system parameters.

Energy at the disconnected load side (L) oscillates with the natural frequency of the capacitances local to the circuit breaker. The maximum voltage is attained at the moment when all the energy is converted into capacitive energy.

As a result of the resistive losses, the voltage on the disconnected load side decays to zero. During current chopping, the breaker is stressed by the supply-side voltage on one side and by the load voltage on the other side.

The supply side voltage is at a maximum, since the load is highly inductive. The load side voltage is the oscillating voltage as the energy exchanges from inductive energy to capacitive energy.

This load side voltage will have a high frequency of up to several thousand cycles per second. During this increasing stress, reignition across the breaker may occur. However, the arc is immediately extinguished again because of the low current and the process begins anew.

Hence, the reignition also helps reduce the energy stored in the disconnected circuit.

TERMINAL FAULT, SHORT LINE FAULT AND OUT OF PHASE SWITCHING INTERRUPTING CONDITIONS OF POWER CIRCUIT BREAKERS BASIC INFORMATION AND TUTORIALS

Terminal Fault.

After interruption of short-circuit current, the recovery voltage oscillates toward the service frequency driving voltage via an initial peak. The natural frequency is determined by the inductance and capacitance of the driving system.

The dc component of the short-circuit current depends on the time constants of the network components like generators, transformers, cables, and high-voltage lines and their reactances of the zero-sequence and the positive sequence networks.

The recovery voltage will accordingly vary depending on the location of the circuit breaker within the network.

Short-Line Fault.

In the case of a short-line fault, a section of line lies between the breaker and the fault location. After the short-circuit current has been interrupted, the oscillation at the line side (L) of the breaker assumes a superimposed “saw-tooth” shape.

The rate of rise of this line oscillation is directly proportional to the effective surge impedance and the time rate of change of current\ (di/dt) at current zero. The component on the supply side (S) basically exhibits the same waveform as a terminal fault.

The circuit breaker is stressed by the difference between these two voltages. Because of the high frequency of the line oscillation, the transient recovery voltage has a very steep initial rate of rise.

Since the initial rate of rise increases with increasing rate of current change, the limiting interrupting capability of many breaker designs is determined by the short-line fault.

Out-of-Phase Switching.

Two network systems with driving voltages E1 and E2 are connected via a high-voltage transmission line. Since the circuit is closed via the closed circuit breaker, the resulting driving voltage is equal to the sum of the two system voltages.

Driving voltage E2 may, for example, exceed voltage E1 by the voltage drop across the transmission line. After opening the breaker, the transient recovery voltages of the disconnected networks oscillate independently.

The circuit breaker is stressed by the difference of these two voltages. In the case of disconnection of long lines, the recovery voltage across the breaker could be increased because of the Ferranti effect, where the voltage of the receiving end can be up to 15% higher than the sending end if the line is lightly loaded.

ALTERNATING CURRENT (AC) INTERRUPTION OF POWER CIRCUIT BREAKERS BASIC INFORMATION

How Power Circuit Breaker Interrupt Alternating Current?

AC interruption occurs at current zero. During the following half-cycle, the recovery voltage will build up across the circuit breaker main contacts. The typical appearance of recovery voltage will differ in inductive, resistive, and capacitive circuits (see Fig. 10-62).

  

FIGURE 10-62 Typical shape of recovery voltage on interruption: (a) induction current; (b) resistive current; (c) capacitive current.

When opening an inductive circuit, the recovery voltage will rise suddenly at a high rate because current interruption occurs at the moment of system voltage peak. This case requires fast building of dielectric strength of the open contact gap.

When interrupting resistive load, current and voltage pass through zero at about the same moment. The recovery voltage will therefore rise at a moderate rate and no particular problems are imposed on the circuit breaker.

At the moment of interruption of capacitive current, the capacitance is fully charged. The recovery voltage rises slowly during the first half cycle but continues to rise to a value twice the system voltage.

This may lead to restrikes, undesired network oscillations, and over voltages. At the moment of fault current interruption the two sections—source side (S) and line side (L)—of the network are decoupled and oscillate independently about their driving voltage.

The difference of these two transients appears across the open contacts of the breaker pole. The behavior of this transient recovery voltage is determined by the circuit parameters.

The still-moving or already fully-open breaker contacts must be able to withstand the recovery voltage.

The most severe stress for the open contact gap is the initial peak and the rate of rise (kV/#s) of the recovery voltage.

If the recovery voltage exceeds the gap insulation, the arc will restrike and current will continue until the next current zero, when interruption will again be attempted. The rate of rise of recovery voltage is a function of the constants of the circuits which supply power through the breaker.

The larger the adjacent capacitance to ground before the major inductance limiting the fault current, the slower will be the rise of the recovery voltage. Some breakers modify the recovery voltage characteristics by limiting the current, modifying its power factor, and so on.

SHORT CIRCUIT DUTY OF POWER CIRCUIT BREAKERS BASIC INFORMATION AND TUTORIALS

The short-circuit duty is determined by the maximum short-circuit that the rotating machinery connected to the system at the time of short circuit can pass through the breaker to a point just beyond the breaker, at the instant the breaker contacts open.

The short-circuit current is determined by the characteristics of synchronous and induction machines connected to the system at the time of the short circuit, the impedance between them and the point of short circuit, and the elapsed time between the starting of the short circuit and the parting of the breaker contacts.

In calculating short-circuit currents of high-voltage ac circuit, it is ordinarily sufficiently accurate to take into account only the reactance of the machines and circuits, whereas in low-voltage circuit resistance as well as reactance may enter into the calculation. In dc circuit, resistance only is ordinarily sufficient.

For first approximations, the reactance and typical time-decrement curves of the synchronous machines may be used. For close calculations, the actual reactances and time characteristics of the equipment should be used, and calculation made for single- as well as 3-phase faults.

The “per unit” impedance system and the “internal voltage” method, using “symmetrical components,” are often used in more exact calculations. Programs are available for digital computer studies of system shortcircuit currents, both balanced 3-phase and phase-to-ground.

The interrupting capacity, in kilovolt amperes, is the product of the phase-to-ground voltage, in kilovolts, of the circuit and the interrupting ability, in amperes, at stated intervals and for a specific number of operations. The current taken is the rms value existing during the first half-cycle of arc between contacts during the opening stroke.

PARTS OF SF6 POWER CIRCUIT BREAKERS BASIC INFORMATION

SF6 gas circuit breakers were first developed in the early 1950s by Westinghouse Corporation, following the discovery of the excellent arc quenching and insulating properties of SF6 gas.

Both live tank and dead tank designs were introduced from the late 1950s into the 1960s. SF6 remains the dominant insulating and arc-quenching medium at higher voltages (72.5 kV and above) even today.

Dead tank SF6 gas circuit breakers were incorporated into gas-insulated substations (GIS) up to 800 Kv from the mid-1960s through the present. Gas insulated substations offer space savings and environmental advantages over conventional outdoor substations, using the reduced insulation gap requirements of SF6 gas.

SF6 gas circuit breakers were initially of the two-pressure type, in which high pressure gas for interruption is compressed and stored for later interrupting duty.

Later designs employed the puffer principle, in which interrupting pressure is developed during the contact motion itself, and no high pressure gas is stored.

The latest designs of SF6 gas circuit breakers utilize the arc thermal energy itself to develop the interrupting pressure; these designs are referred to as self-blast or thermal-assist circuit breakers.

Below are the parts of Dead-tank SF6 circuit breaker.

POWER CIRCUIT BREAKERS BASIC INFORMATION AND TUTORIALS

What Are Circuit Breakers?

Courtesy of Siemens

Circuit breakers are mechanical switching devices capable of making and breaking currents under either normal or specified abnormal (short circuit) conditions on the power system. 

Though circuit breakers are primarily defined by their protective capabilities and ratings under abnormal short circuit conditions, they also perform switching duties under a myriad of other system conditions, each of which has its own set of switching stresses.

Circuit breakers are rated primarily by power frequency voltage, insulation levels (BIL, switching impulse, hi-pot voltage), continuous current, short-circuit current, and interrupting time. Reference is made to IEEE C37.1001 for definitions of ratings subjects, and to IEEE C37.042 and IEEE C37.063 for values of ratings typically applied to circuit breakers.

Circuit breakers employ a variety of media for high voltage insulation and/or current interruption. The type of media employed in a specific design is often designated as a prefix in the naming of the circuit breaker, for example, vacuum circuit breaker, or sulfur hexafluoride (SF6) gas circuit breaker.

Circuit breakers are often categorized as being of either “dead tank” or “live tank” design. In the dead tank case, the interrupting contact system is enclosed in a grounded tank, typically surrounded by an insulating fluid (oil) or gas (SF6) (see Figs. 10-53 and 10-54 for examples of dead tank circuit breakers).

The electrical current enters the tank through high voltage entrance bushings, passes through the contact system, and then exits through another high voltage entrance bushing. In the live tank case, the interrupting contact system is supported by insulators at some height above ground potential, but is not contained within a grounded tank system.

There is no grounded tank or enclosure surrounding the live parts. Dead tank design allows the placement of current transformers, which are necessary for protective relaying input signals, around the high voltage entrance bushing.

Live tank design offers no location to place current transformers, and therefore must be independently placed adjacent to the circuit breaker.

WHAT ARE FUSES? BASIC DEFINITION AND TUTORIALS

A fuse is a calibrated weak link in a circuit that will predictably and reliably melt when a predetermined magnitude of current is reached for a designated duration. When the fuse element melts, the circuit is interrupted and the current will cease to flow. There is an inverse-time relationship between the size of the current and the time it takes to blow: the higher the current, the faster the fuse will open.

Fuses are sized according to their rated current and voltage. The current rating of a fuse should be between 0 and 30% higher than the continuous operating current in the circuit, depending on the type of fuse and the standard to which it complies.

Fuses used in North America typically comply with Underwriters Laboratories (UL) and/or Canadian Standards Association (CSA) standard 248-14 for low-voltage fuses (under 600V), while those used in Europe comply with International Electrotechnical Commission (IEC) standard 60127-2. UL and CSA standards are harmonized but they differ from IEC standards.

Table 9.1 shows the allowable continuous operating current for various types of fuses at 23°C.

If the rated current of a fuse is undersized, then it is subject to nuisance tripping due to fluctuations and spikes in the line voltage. If it’s oversized, it can be a potential fire hazard or a hazard to personnel by allowing too much current to flow.

When you are replacing a fuse, it is important to use the same fuse type, since UL and CSA ratings are different from IEC ratings. For a 250V fuse, for example, a 1.4-amp UL/ CSA fuse is approximately the same as a 1-amp IEC-rated fuse.

Therefore, if a fuse manufactured to UL standards is replaced with a fuse manufactured to IEC standards, then the circuit will no longer be protected properly. And it goes without saying that it’s never a good idea, regardless of the circumstances, to bypass a fuse with a chewing gum wrapper or any other conductive material.

It is also very important that the fuse is rated at or higher than the circuit voltage, or there is a risk of arcing across the open fuse terminals, thus bypassing the overcurrent protection. Furthermore, a fuse with the wrong voltage rating will work just fine until the fuse link blows and an arc is generated across the terminals.

Therefore, it is extremely important to pay close attention to the current and voltage ratings of replacement fuses. A properly rated fuse is designed to withstand the open circuit voltage for 30 seconds after the fuse blows or to have an interrupt resistance of at least 1 k ohms.

There are several different types of fuses that are classified according to how quickly or how slowly they will open in a fault or overcurrent situation. A fast-acting normal fuse will blow more quickly than a timedelayed or time-lag fuse will; although, you can overload a fast-acting normal fuse by 50% and it might still take several minutes to blow.

Time-delayed fuses are used in situations where the inrush current is high and the steady-state operating current is lower. Examples include discharge lamps, motors, transformers, and other highly capacitive or inductive loads. In the power distribution systems that we typically deal

with, we come across fuses on a regular basis in certain applications.

The secondary sides of feeder transformers, for example, are sometimes fused. So are the inputs to some large permanently installed dimmer racks. They sometimes use large bolt-in fuses filled with quartz sand to quench the arc and absorb the heat generated by stopping large currents. This type of fuse is occasionally called by the trade name Amp-Trap.

Of course, many luminaires are fused with miniature fuses, and some connectors in the UK have built in fuses. But the BS-546:1950 15 A connector is commonly used in theatres in the UK precisely because it doesn’t have a fuse.

And since every circuit is already protected by a fuse or circuit breaker in the dimmer, a second fuse in the connector is redundant and unnecessary. In fact, a second fuse in the same circuit makes it more difficult and time consuming to troubleshoot the circuit.