CIRCUIT BREAKERS EQUIPPED WITH PARALLEL IMPEDANCE

Impedances in parallel with the arc may be either capacitors or resistors, or both, in various combinations. Although such impedances modify the shape of the specified inherent transient recovery voltage, the type and degree of modification in the synthetic test should be the same as in the direct test.

For example, the insertion of a resistor equal to the surge impedance of the line will reduce the line side rate-of-rise to half value. The effect is not as pronounced for a bus fault where a large number of lines are in parallel because their combined surge impedance is much lower than the resistance in parallel with the arc.

Where the shunt impedance is a resistor, particularly if the ohmic value of the resistor is low, the actual peak transient recovery voltage (TRV) in a synthetic test may not attain the value it would in a direct test because of the limited energy available from the voltage source.

Furthermore, the shunt resistor may cause a too rapid decay of the dc voltage following the TRV crest.

In some cases, to meet the TRV requirements of ANSI/IEEE C37.09-1979 [3], it may be possible

1) To adjust the parameters of the voltage circuit to provide the necessary additional energy absorbed by the shunt resistor

2) To switch over to an additional ac voltage source capable of maintaining voltage across the resistor.

An equivalent transient recovery voltage waveform across the terminals of the test circuit breaker can be produced by replacement of resistance at other appropriate places in test circuits.

SHORT-CIRCUIT CURRENT INTERRUPTION PROCESS BASIC INFORMATION AND TUTORIALS

How power circuit breaker works?

Circuit Breaker Performance During Interruption; Basic Intervals

The Circuit breaker has two basic positions: closed and opened. In the closed position the circuit breaker conducts full current with negligible voltage drop across its contacts. In the open position it conducts negligible current but with full voltage across the contacts.

This defines two main stresses, the current stress and the voltage stress, that are separated in time. However, the main function of the circuit breaker is neither to conduct nor to isolate. It performs its main function in changing from one condition to the other, that is, the switching operation.

If closer attention is paid to the voltage and current stresses during the interrupting test (figure below), three main intervals can be recognized.

1. High-Current Interval

The high-current interval is the time from contact separation to the significant change in arc voltage preceding the interaction and high-voltage intervals.

2. Interaction Interval

The interaction interval is the time from the significant change in arc voltage prior to current zero to the time when the current including the post arc current, if any, ceases to flow through the test breaker.

3. High-Voltage Interval

The high-voltage interval is the time from the moment when the current including the post arc current, if any, ceases to flow through the test breaker to the end of the test.

State of Interrupting Process

The three intervals described in 1 to 3 follow each other immediately, that is, they cover the whole interrupting process without any discontinuities, even though it might be difficult to establish precisely the moment when one interval ends and the other begins. However, this accuracy may not be required.

State of Interrupting Process During Three Basic Intervals

The quantities determining the physics of the interrupting process change considerably during the circuit breaking operation. In fact, the prevailing physical conditions have different importance during the three time intervals.

High-Current Interval

During the high-current interval, short-circuit current is flowing through the circuit breaker with a relatively small voltage drop across the contacts. A large amount of energy is supplied to the arc establishing the state of ionization, temperature, dynamic pressure, etc, important for the switching function.

Interaction Interval

During the interaction interval, the short-circuit current stress changes into high-voltage stress and the breaker performance can significantly influence the currents and voltages in the circuit. As the current decreases to zero, the arc voltage may rise to charge parallel capacitance and distort current passing through the arc.

After the current zero the post arc conductivity may result in additional damping of the transient recovery voltage and thus influence the voltage across the breaker and the energy supplied to the ionized contact gap.

The mutual interaction between the circuit and the circuit breaker immediately before and after current zero (that is, during the interaction interval) is of extreme importance to the switching process.

High-Voltage Interval

During the high-voltage interval, the gap of the breaker is stressed by recovery voltage. The circuit breaker is now a passive element in the circuit.

POWER CIRCUIT BREAKER INTERRUPTION OF CAPACITIVE CURRENTS AND CLOSING ON FAULTS BASIC INFORMATION AND TUTORIALS

Capacitive currents occur during line drooping as well as during disconnecting unloaded cables or capacitor banks. Although, switching of capacitor banks is regarded as a special application, disconnecting of charged lines is a frequent switching operation.

Current chopping may occur at a low instantaneous current value during interruption of capacitive currents, but this does not lead to overvoltages. After interruption of current, the voltage at the line capacitance (L) remains at the peak value of the power frequency voltage, whereas the voltage on the source side (S) oscillates about the driving voltage.

The difference between the two voltages appears across the circuit breaker with an amplitude of more than double the rated voltage. If the circuit breaker cannot withstand this higher voltage restriking may occur.

Restriking is similar to closing transmission lines with trapped charge. After restiking, a transient current flows through the circuit breaker, which is of higher frequency than that of the system and which can again be interrupted during the reignition process.

After reextinction, the line is charged to the potential of the peak value of the equalizing process, whereas the circuit-breaker terminal on the source side (S) recovers to the system voltage. A very high differential voltage appears across the breaker, which may lead to renewed restriking and even switching failures.

Restrike-free interruption of capacitive currents is thus of the utmost importance. Basically, the same phenomenon occurs during disconnection of capacitor banks. To determine the voltage stresses of the circuit breaker, however, the grounding condition of the supply system and capacitor bank and the arrangement of the bank have to be taken into account.

Closing on a Fault.

This directs the stress onto the circuit breaker contact system, particularly as regards the electrodynamic and thermal forces. The current and voltage stress is different during closing on (a) symmetrical or (b) asymmetric short-circuit current.

The deciding factor is the moment of contact touch relative to the phase angle of system voltage. In case contact touch and consequently ignition of the arc occurs at the voltage maximum, the short circuit current will appear symmetrical.

The other extreme case takes place with the moment of closing at voltage zero. Here the asymmetrical short-circuit current contains the maximum dc component. A contact system designed for fast closing operation will be subjected to a shorter arcing time and consequently to reduced contact burning when closing on symmetrical currents. Fast operation is therefore not only important for opening but also for circuit breaker closing.

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.

SYMMETRICAL CURRENT BASIS OF POWER CIRCUIT BREAKERS BASIC INFORMATION AND TUTORIALS

It has become a widely adopted practice to determine the interrupting capability of circuit breakers in kiloamperes symmetrical. The rated short-circuit current in rms kiloamperes is referred to the rated maximum voltage in kilovolts.

The ratings structure and tables of ratings for ac high-voltage circuit breakers are found in IEEE C37.042 and IEEE C37.063.

The short-circuit current interrupting process is characterized first by an arc appearing between the breaker contacts. The arc contains a high conductivity plasma column originating from the high temperature and related gas ionization (in the case of gas-blast interrupters).

Interruption will occur at current zero and in this case is first determined by successful cooling of the arc (through gas flow) to eliminate the ionized gas conductive path, and then the race to build up dielectric strength of the open contact gap faster than the rise of the power system recovery voltage.

Several specific problems are encountered during the interrupting process of gas-blast interrupters:

1. Arc plasma temperatures exceeding 20,000 K.

2. The turbulent supersonic flow of the quenching gas in a changing flow geometry with speeds ranging from a few hundred meters per second to several thousand meters per second.

3. The interrupter-moving system and its drive accelerates the moving masses in the few thousandths\ of a second to speeds as high as 10 m/s while simultaneously compressing the quenching gas.

4. The stress places on the network system by the current interruption and the recovery voltage.

The interrupting principle of an SF6 puffer-type interrupter is sketched in Fig. 10-60. On opening, the fixed and moving contacts are pulled apart by the operating mechanism. Thus, the fault current is forced to flow along the arc plasma.

  

The contact movement combined with the compression cylinder movement in the opposite direction compresses the quenching gas inside the cylinder. The quenching gas is consequently forced to flow through the contact system, and the insulated nozzle toward the exhaust.

This intensive flow of quenching medium along the arc rapidly removes the energy converted within the arc plasma and transforms the path between the open contacts into an insulating gap.

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.

SHORT-CIRCUITED FIELD TURNS AND FIELD INSULATION POLARITY TEST OF SYNCHRONOUS GENERATORS

The purpose of these tests is to check for field-coil short-circuited turns, for number of turns/coil, or for short-circuit conductor size. Besides tests at standstill, a test at rated speed is required, as short circuited turns may occur at various speeds.

There are DC and alternating current (AC) voltage tests for the scope. The DC or AC voltage drop across each field coil is measured.

A more than +2% difference between the coil voltage drop indicates possible short-circuits in the respective coils. The method is adequate for salient-pole rotors.

For cylindrical rotors, the DC field-winding resistance is measured and compared with values from previous tests. A smaller resistance indicates that short-circuited turns may be present.

Also, a short-circuited coil with a U-shaped core may be placed to bridge one coil slot. The U-shaped core coil is placed successively on all rotor slots.

The field-winding voltage or the impedance of the winding voltage or the impedance of the exciting coil decreases in case there are some short-circuited turns in the respective field coil.

Alternatively, a Hall flux probe may be moved in the airgap from pole to pole and measures the flux density value and polarity at standstill, with the field coil DC fed at 5 to 10% of rated current value.

If the flux density amplitude is higher or smaller than that for the neighboring poles, some field coil turns are short-circuited (or the airgap is larger) for the corresponding rotor pole. If the flux density does not switch polarity regularly (after each pole), the field coil connections are not correct.

SYNCHRONOUS GENERATORS SHORT-CIRCUIT RATIO (SCR)

The short-circuit ratio (SCR) of a generator is the inverse ratio of saturated direct axis reactance in per unit (P.U.):

SCR = 1/ Xd (sat)

The SCR has a direct impact on the static stability and on the leading (absorbed) reactive power capability of the SG. A larger SCR means a smaller xd(sat) and, almost inevitably, a larger airgap.

In turn, this requires more ampere-turns (magnetomotive force [mmf]) in the field winding to produce the same apparent power.

As the permissible temperature rise is limited by the SG insulation class (class B, in general, ΔT = 130°), more excitation mmf means a larger rotor volume and, thus, a larger SG.

Also, the SCR has an impact on SG efficiency. An increase of SCR from 0.4 to 0.5 tends to produce a 0.02 to 0.04% reduction in efficiency, while it increases the machine volume by 5 to 10%.

The impact of SCR on SG static stability may be illustrated by the expression of electromagnetic torque te P.U. in a lossless SG connected to a infinite power bus:

te = SCR x E0 x Vg x 1sin δ

The larger the SCR, the larger the torque for given no-load voltage (E0), terminal voltage V1, and power angle δ (between E0 and ΔV1 per phase). If the terminal voltage decreases, a larger SCR would lead to a smaller power angle δ increase for given torque (active power) and given field current.

If the transmission line reactance — including the generator step-up transformer — is xe, and V1 is now replaced by the infinite grid voltage Vg behind xe, the generator torque te′ is as follows:

te' = SCR x E0 x Vg x 1sin δ'/(1 + Xe/Xd)

The power angle δ′ is the angle between E0 of the generator and Vg of the infinite power grid. The impact of improvement of a larger SCR on maximum output is diminished as xe/xd increases.

Increasing SCR from 0.4 to 0.5 produces the same maximum output if the transmission line reactance ratio xe/xd increases from 0.17 to 0.345 at a leading power factor of 0.95 and 85% rated megawatt (MW) output.

Historically, the trend has been toward lower SCRs, from 0.8 to 1.0, 70 years ago, to 0.58 to 0.65 in the 1960s, and to 0.5 to 0.4 today. Modern — fast response — excitation systems compensate for the apparent loss of static stability grounds. The lower SCRs mean lower generator volumes, losses, and costs.