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.

LOW-SIDE BUS AND LINE-FAULT GROUND POTENTIAL RISE BASIC INFORMATION

Theoretically, LV faults can be handled in precisely the same manner as the HV faults. All differences between HV and LV side construction (transformer connections, line conductors, length, pole footing impedance, etc.) will reflect in the calculation of zero- and positive-sequence LV L-G and balanced fault currents.

As opposed to HV systems, which usually carry overhead ground wires, some LV lines, delta or wyeconnected, carry no neutrals. When neutrals are present on LV systems, LV bus fault calculations follow the same method, and LV line faults will be the same as HV line fault calculations.

When neutrals are not present on LV system, both LV bus and line-fault GPR can be calculated using the simplified method. For a LV bus fault, ZL will consist of the parallel combination of impedances-to-remote earth of all HV overhead ground wire-tower ladder networks only.

The rest of the method still applies.

For LV line faults, assuming a radial LV line, a single fault infeed can be assumed if no generation exists on the load side of the line. This assumption is correct for most cases, but it should be pointed out that, for instance, a large induction motor can become a zero-sequence current generator at the instant of the fault, due to the inertia of the rotor and the mechanical load.

If this can be neglected, the worst fault then occurs outside and near the station. The HV bus fault is modified by inserting, in series with Zs, the self-impedance of the faulted phase conductor, and inserting, in series with Za, a faulted pole footing impedance.

The methods described in this subclause should be used for hand calculation and estimation purposes only. For a more complicated network, that is, the network with a high number of ROWs, circuits, transformers, ground sources, and short lines such as could be found between generating and switching stations, hand calculations cannot be used for either exact or approximate solutions; a computer program shall be used.

In such a program, the theoretical approach should include the effect of other forms of grounding, such as rails, pipes, etc.; the effect of the length of lines; the effect of positive-sequence current phase shift in certain transformer windings; etc.

WORST FAULT LOCATION BASIC INFORMATION (GROUND POTENTIAL RISE CONSIDERATION

A complex power station may have a large number of rights-of-way (ROW) with multi circuit power lines on each ROW. These circuits may be operated at different voltage levels. A fault current study for an L-G fault at each transformer voltage level should be produced. Each fault current study should be examined as follows:

a) If the vectorial sum of all zero-sequence fault current contributions to the transformer bus fault from all transmission and distribution lines entering the station under study is greater than the sum of all current contributions from all grounded sources at that station (including generators, grounded transformers, shunt capacitors, etc.), then at the voltage level for which the fault current study is presently being examined, the bus fault will usually produce a worse GPR than the line fault.

b) If the reverse is true, that is, the vectorial sum of the line contributions is smaller than the local ground source current sum, the line fault will produce a greater GPR.

This is because the local ground current will return partially, in the case of the line fault, through the station ground impedance, adding to the GPR caused previously by the line current contribution.

In the bus fault case, the current merely circulates through the faulted transformer winding, the station ground bus, and the fault impedance.

Having determined the worst-fault location (bus versus out on the-line), to select that fault current study with the highest fault current is not appropriate.

Variances between grounding networks of lines with the various voltage levels may, for instance, cause the study showing lower zero-sequence fault currents to result in a GPR greater than that caused by the higher currents. Instead, all faults should be investigated for fault locations as determined above.

WORST FAULT TYPE BASIC INFORMATION (GROUND POTENTIAL RISE CONSIDERATION

Basically, three types of faults should be investigated:

a) Line-to-ground faults (L-G). These are predominant in terms of frequency of occurrence.  Zero sequence and positive-sequence currents will be required.

In practice, GPR is a function of zero sequence currents only, but positive-sequence currents are required to determine magnitudes of the individual zero-sequence currents ßowing in each phase of the faulted circuit.

b) Double line-to-ground faults (2L-G). These are statistically less frequent than L-G faults but could produce zero-sequence currents far exceeding those caused by L-G faults.

Theoretically, this is because of different connections of sequence networks during these faults. For an L-G fault, the positive-sequence, negative-sequence, and zero-sequence networks are connected in series and driven by the prefault voltage source; whereas, for a 2L-G fault, positive-sequence impedance is connected in series with the parallel combination of zero-sequence and negative-sequence impedances, with less overall impedance in the path of the fault current.

[For instance, many high MVA autotransformers may be added to power stations. These could have their primary-to-secondary, or primary-to-tertiary, zero-sequence reactance ratios so high that their primary current is small compared with the tertiary (ground) current.

In addition to this, if more such transformers are added to the station, the resulting tertiary currents will be very large due to further paralleling of reactances.]

c) Three-phase faults. These are statistically less frequent than L-G and 2L-G faults. Three-phase faults produce positive sequence currents, and detailed calculations are required to determine magnitudes of the individual zero-sequence currents ßowing in each phase of the faulted circuit.

If X1 and X0 are positive-sequence and zero-sequence reactances, respectively, of the system impedance at the point of fault and X1 is less than X0, the 2L-G fault will result in higher zero sequence fault currents, often twice as high as the L-G fault currents calculated at the same fault location.

The GPR produced by 2LG faults is not normally considered, due to its low probability. For an overview of the frequency of occurrence of different types of faults as a function of voltage levels on which they occur and other parameters.

HIGH-POTENTIAL (HIPOT) TEST REQUIREMENTS FOR EXCITATION SYSTEMS FOR SYNCHRONOUS MACHINES VOLTAGE STANDARDS

Exciter power circuit

The exciter power circuit includes all components not electrically isolated from the exciter output. For static exciters, this includes the rectifier and thyristor circuits, transformer windings, line filters, shaft current suppressors, and any auxiliary components connected to either the input or output of the rectifier/thyristor bridge.

For rotating exciters, it includes armature windings, commutators, and brushes. For rotating exciter rated outputs 350 V dc or less, the ac rms test voltage shall be 10 times the rated output voltage of the exciter, but with a minimum of 1500 V.

For static exciter rated outputs 350 V dc or less, the ac rms test voltage shall be the greater of 10 times the rated output voltage of the exciter, but with a minimum of 1500 V, or twice the rated ac rms input voltage of the exciter plus 1000 V.

For rotating exciter rated outputs greater than 350 V dc, the ac rms test voltage shall be 2800 V plus twice the rated output voltage of the exciter. For static exciter rated outputs greater than 350 V dc, the ac rms test voltage shall be the greater of 2800 V plus twice the rated output voltage of the exciter, or twice the rated ac rms input voltage of the exciter plus 1000 V.

The synchronous-machine field winding is not included as it is covered by ANSI C50.10-1990. The exciter rated output voltage (for determination of the test voltage) shall not be less than the voltage

required at the associated generator field terminals when the generator is operated at rated kilovolt amperes, rated power factor, and rated voltage with the generator field winding at

- 75°C for field windings designed to operate at rating with a temperature rise of 60°C or less, or

- 100°C for field windings designed to operate at rating with a temperature rise greater than 60° C.

The exciter rated input voltage shall not be less than the voltage at the exciter input terminals when the generator is operated at rated kilovolt-amperes, rated power factor, and rated voltage with the generator field winding at

- 75°C for field windings designed to operate at rating with a temperature rise of 60°C or less, or

- 100° C for field windings designed to operate at rating with a temperature rise greater than 60° C.

All other circuits (electrically isolated from the exciter power circuit)

For circuits rated above 60 V or above 60 VA and not greater than 600 V, the ac rms test voltage shall be 1000 V plus twice the rated voltage. For circuits rated above 600 V, the ac rms test voltage shall be 2000 V plus 2.25 times the rated voltage.

Circuits rated at 60 V or less and 60 VA or less need not be given a high-potential test.

GROUNDING GRID DESIGN IN DIFFICULT CONDITIONS BASIC INFORMATION AND TUTORIALS

In areas where the soil resistivity is rather high or the substation space is at a premium, it may not be possible to obtain a low impedance grounding system by spreading the grid electrodes over a large area, as is done in more favorable conditions.

Such a situation is typical of many GIS installations and industrial substations, occupying only a fraction of the land area normally used for conventional equipment. This often makes the control of surface gradients difficult.

Some of the solutions include

a) Connection(s) of remote ground grid(s) and adjacent grounding facilities, a combined system utilizing separate installations in buildings, underground vaults, etc. A predominant use of remote ground electrodes requires careful consideration of transferred potentials, surge arrester locations, and other critical points.

A significant voltage drop may develop between the local and remote grounding facilities, especially for high-frequency surges (lightning).

b) Use of deep-driven ground rods and drilled ground wells.

c) Various additives and soil treatments used in conjunction with ground rods and interconnecting conductors.

d) Use of wire mats. It is feasible to combine both a surface material and fabricated mats made of wire mesh to equalize the gradient field near the surface.

A typical wire mat might consist of copper-clad steel wires of No. 6 AWG, arranged in a 0.6 m × 0.6 m (24 in × 24 in) grid pattern, installed on the earth’s surface and below the surface material, and bonded to the main grounding grid at multiple locations.

e) Where feasible, controlled use of other available means to lower the overall resistance of a ground system, such as connecting static wires and neutrals to the ground. Typical is the use of metallic objects on the site that qualify for and can serve as auxiliary ground electrodes, or as ground ties to other systems. Consequences of such applications, of course, have to be carefully evaluated.

f) Wherever practical, a nearby deposit of low resistivity material of sufficient volume can be used to install an extra (satellite) grid. This satellite grid, when sufficiently connected to the main grid, will lower the overall resistance and, thus, the ground potential rise of the grounding grid.

The nearby low resistivity material may be a clay deposit or it may be a part of some large structure, such as the concrete mass of a hydroelectric dam.

BASIC ASPECT OF SUBSTATION GRID DESIGN BASIC INFORMATION AND TUTORIALS

Conceptual analysis of a grid system usually starts with inspection of the substation layout plan, showing all major equipment and structures. To establish the basic ideas and concepts, the following points may serve as guidelines for starting a typical grounding grid design:

a) A continuous conductor loop should surround the perimeter to enclose as much area as practical. This measure helps to avoid high current concentration and, hence, high gradients both in the grid area and near the projecting cable ends. Enclosing more area also reduces the resistance of the grounding grid.

b) Within the loop, conductors are typically laid in parallel lines and, where practical, along the structures or rows of equipment to provide for short ground connections.

c) A typical grid system for a substation may include 4/0 bare copper conductors buried 0.3–0.5 m (12–18 in) below grade, spaced 3–7 m (10–20 ft) apart, in a grid pattern. At cross-connections, the conductors would be securely bonded together.

Ground rods may be at the grid corners and at junction points along the perimeter. Ground rods may also be installed at major equipment, especially near surge arresters. In multilayer or high resistivity soils, it might be useful to use longer rods or rods installed at additional junction points.

d) This grid system would be extended over the entire substation switchyard and often beyond the fence line. Multiple ground leads or larger sized conductors would be used where high concentrations of current may occur, such as at a neutral-to-ground connection of generators, capacitor banks, or transformers.

e) The ratio of the sides of the grid meshes usually is from 1:1 to 1:3, unless a precise (computer-aided) analysis warrants more extreme values. Frequent cross-connections have a relatively small effect on lowering the resistance of a grid.

Their primary role is to assure adequate control of the surface potentials. The cross-connections are also useful in securing multiple paths for the fault current, minimizing the voltage drop in the grid itself, and providing a certain measure of redundancy in the case of a conductor failure.

RESISTANCE OF HUMAN BODY BASIC INFORMATION AND TUTORIALS

For dc and 50 Hz or 60 Hz ac currents, the human body can be approximated by a resistance. The current path typically considered is from one hand to both feet, or from one foot to the other one.

The internal resistance of the body is approximately 300 Ω, whereas values of body resistance including skin range from 500 Ω to 3000 Ω, as suggested in Daziel, Geddes and Baker , Gieiges, Kiselev [B94], and Osypka [B118].

The human body resistance is decreased by damage or puncture of the skin at the point of contact. Conducted extensive tests using saltwater to wet hands and feet to determine safe let-go currents, with hands and feet wet.

Values obtained using 60 Hz for men were as follows: the current was 9.0 mA; corresponding voltages were 21.0 V for hand-to-hand and 10.2 V for hand-to-feet.

Hence, the ac resistance for a hand-to-hand contact is equal to 21.0/0.009 or 2330 Ω, and the hand-to feet resistance equals 10.2/0.009 or 1130 Ω, based on this experiment.

Thus, for the purposes of this guide, the following resistances, in series with the body resistance, are assumed as follows:

a) Hand and foot contact resistances are equal to zero.

b) Glove and shoe resistances are equal to zero.

A value of 1000 Ω in Equation (10), which represents the resistance of a human body from hand-to-feet and also from hand-to-hand, or from one foot to the other foot, will be used throughout this guide. RB = 1000 Ω

BONDING, SHIELDING, AND GROUNDING RELATIONSHIP BASIC INFORMATION AND TUTORIALS

a. The simple grounding of elements of a communications facility is only one of several measures necessary to achieve a desired level of protection and electrical noise suppression. To provide a low impedance path for

(1) the flow of ac electrical current to/from the equipment and

(2) the achievement of an effective grounding system, various conductors, electrodes, equipment, and other metallic objects must be joined or bonded together.

Each of these bonds should be made so that the mechanical and electrical properties of the path are determined by the connected members and not by the interconnection junction. Further, the joint must maintain its properties over an extended period of time, to prevent progressive degradation of the degree of performance initially established by the interconnection.

Bonding is concerned with those techniques and procedures necessary to achieve a mechanically strong, low-impedance interconnection between metal objects and to prevent the path thus established from subsequent deterioration through corrosion or mechanical looseness.

b. The ability of an electrical shield to drain off induced electrical charges and to carry sufficient out of-phase current to cancel the effects of an interfering field is dependent upon the shielding material and the manner in which it is installed.

Shielding of sensitive electrical circuits is an essential protective measure to obtain reliable operation in a cluttered electromagnetic environment. Solid, mesh, foil, or stranded coverings of lead, aluminum, copper, iron, and other metals are used in communications facilities, equipment, and conductors to obtain shielding.

These shields are not fully effective unless proper bonding and grounding techniques are employed during installation. Shielding effectiveness of an equipment or subassembly enclosure depends upon such considerations as the frequency of the interfering signal, the characteristics of the shielding material, and the number and shapes of irregularities (openings) in the shield.

Interference-causing signals are associated with time-varying, repetitive electromagnetic fields and are directly related to rates of change of currents with time. A current-changing source generates either periodic signals, impulse signals, or a signal that varies randomly with time.

To cause interference, a potentially interfering signal must be transferred from the point of generation to the location of the susceptible device. The transfer of noise may occur over one or several paths. There are several modes of signal transfer (i.e., radiation, conduction, and inductive and capacitive.