ELECTRIC BUS WAYS CURRENT RATING BASIC INFORMATION

To apply busways properly in an electric power distribution system, some of the more important items to consider are the following.

Current-carrying capacity
Busways should be rated on a temperature-rise basis to provide safe operation, long life, and reliable service.

Conductor size (cross-sectional area) should not be used as the sole criterion for specifying busway. Busway may have seemingly adequate cross-sectional area and yet have a dangerously high temperature rise.

The UL requirement for temperature rise (55 deg C) (see ANSI/UL 857-1989) should be used to specify the maximum temperature rise permitted. Larger crosssectional areas can be used to provide lower voltage drop and temperature rise.

Although the temperature rise will not vary significantly with changes in ambient temperature, it may be a significant factor in the life of the busway. The limiting factor in most busway designs is the insulation life, and there is a wide range of types of insulating materials used by various manufacturers. If the ambient temperature exceeds 40 deg C or a total temperature in excess of 95 deg C is expected, then the manufacturer should be consulted.

Short-circuit current rating
The bus bars in busways may be subject to electromagnetic forces of considerable magnitude by a short-circuit current. The generated force per unit length of bus bar is directly proportional to the square of the short-circuit current and is inversely proportional to the spacing between bus bars.

Short-circuit current ratings are generally assigned in accordance with ANSI/NEMA BU1-1988 and tested in accordance with ANSI/UL 857-1989. The ratings are based on (1) the use of an adequately rated protective device ahead of the busway that will clear the short circuit in 3 cycles and (2) application in a system with short-circuit power factor not less than that given in table 13-1.


If the system on which the busway is to be applied has a lower short-circuit power factor (larger
X/R ratio), the short-circuit current rating of the bus may have to be increased. The manufacturer should then be consulted.
The required short-circuit current rating should be determined by calculating the available short-circuit current and X/R ratio at the point where the input end of the busway is to be connected. The short-circuit current rating of the busway must equal or exceed the available short-circuit current.
The short-circuit current may be reduced by using a current- limiting fuse or circuit breaker at the supply end of the busway to cut it off before it reaches maximum value. Short-circuit current ratings are dependent on many factors, such as bus bar center line spacing, size, strength of bus bars, and mechanical supports.

Since the ratings are different for each design of busway, the manufacturer should be consulted for speci√ěc ratings. Short-circuit current ratings should include the ability of the ground return path (housing and ground bar if provided) to carry the rated short-circuit current.
Failure of the ground return path to adequately carry this current can result in arcing at joints, creating a fire hazard. The ground-fault current can also be reduced to the point that the overcurrent protective device does not operate. Bus plugs and attachment accessories also should have adequate short-circuit interrupting and/or withstand ratings.

FAULT ARC PATH OF TRANSMISSION LINE BASIC INFORMATION

When lightning strikes a transmission line the field intensity stressing the insulation may exceed the ionization field intensity level (roughly 30kV/cm) and create an arc from the line to ground. A path now exists for current flow.

The resulting discharge current flow from the lightning stroke is usually over within a few milliseconds but the ionized path has been established and a 60Hz “follow” current flows. This current must be detected and interrupted by deenergizing the line with circuit breakers.

For the ionization path to dissipate, the voltage must be absent for a sufficient duration. The time during which the voltage is absent is commonly called “dead” time.

For transient faults to be successfully cleared, an adequate time for deionization must be afforded. Table 1 shows the minimum time required by voltage level and by probability of successfully reclosing and energizing the line.

Table 1. Minimum De-Ionization Time for Reclosing Breakers

System Voltage                                                         Cycles on 60-Cycle Basis
(line-line kV)                                                       95% probability    75% probability
23                                                                                    4
46                                                                                    5                             3.5
69                                                                                    6                              4
115                                                                                 8.5                            6
138                                                                                 10                            7.5
161                                                                                 13                            10
230                                                                                 18                            14

If sufficient motor load is still connected during the dead time the ionization path can/will be kept intact and a fault reignition will result when the utility breakers reclose. This occurs even though the fault is phase-ground and there is an interposing delta winding between the motor load and the fault.

Tapped motor load holds up the voltage as it decays. At the time of the reclose the voltage is roughly 50% of nominal. Oscillographic data has been obtained in the past showing transmission line voltage being maintained by tapped motor load during reclosing dead time.

Effect on Motors
Unsupervised high-speed reclosing on islanded motors (induction or synchronous machines) before their “residual” voltage has subsided below 25% may subject the motors and other equipment to damage. The motor should not be subjected to a reclose when the phasor difference between the source volts/Hz and the motor residual volts/Hz exceeds 1.33 per unit volts/Hz.

The available literature clearly indicates that reclosing on motor load should be delayed long enough for their residual voltage to decay to acceptable levels (or their contactors drop out) to prevent damage which may be immediate or cumulative. Alternatively, some means to ensure the two voltages are in-phase would be needed.

Damage may include shifting of stator coils, loosening of rotor bars, distortion of coil ends, shaft damage etc. In some cases torsional resonance can be established with resulting torques as high as 20
times normal.

When a motor is disconnected from its power supply it starts to slow down depending on its inertia and the characteristics of its connected load. For an open circuited induction (asynchronous) motor the voltage at its terminals will be a product of its speed, open circuit time constant, and its trapped rotor flux.

For a synchronous machine with field forcing it may take much longer for its voltage to decay. If not open circuited, the motors will experience an electrical interaction with other motors bussed with them as well (an electrical to-and-fro of energy).
From the moment the motors are disconnected from the power system they begin to slip out-of-phase with the power system and their voltage magnitude begins to decay. The voltage impressed across them at reclose will be a function of this internal residual voltage and the power system voltage at time of reclosing. If the two voltages were equal in magnitude and 180° out-of-phase the resulting voltage difference would be 2.0 per unit.

SERIES COMPENSATED TRANSMISSION LINES BASIC INFORMATION

Series compensation of long high-voltage and extra-high-voltage lines has become almost standard practice. The presence of series compensation affects the X0/X1 ratios of the system, with the reactance of the series capacitor appearing in all three sequence networks.

Therefore, temporary and transient overvoltages as a result of faults, as well as circuitbreaker recovery voltages and surge arrester operation, are different than those that would appear in the uncompensated system.

There have also been concerns about ferroresonant TOVs in series-compensation systems , but few if any cases of ferroresonance have been reported for operating transmission or subtransmission systems.

However, because of this concern, some utilities buying series capacitors have specified special subharmonic detection devices as part of the series capacitor bank. There are also concerns about subsynchronous resonance (SSR) of rotating machine mechanical systems with the series compensated electrical system.

Additional concerns have centered on fundamental-frequency resonance conditions during faults at critical locations in the transmission systems. But economical applications of series capacitors dictate that some means be supplied to limit the overvoltage appearing across the series capacitor during faults to voltages no higher than economical design levels.

Limiting this overvoltage virtually eliminates the possibility of high temporary fundamental resonant overvoltages.
The overvoltage protection for series capacitors applied to transmission systems has taken two forms. The earliest forms of overvoltage protection were spark-gap systems that limited voltage to the sparkover voltage of the gap setting, which was generally no more than 3.5 times the rated voltage across the series capacitor bank, but often less.

More recently, the protection has been achieved by metal-oxide varistors, somewhat similar to surge arresters but applied across the series capacitor and limiting the voltage to about two times the rated voltage across the series capacitor bank.

Both forms, when acting during a fault, can reduce temporary and transient overvoltages, the spark gap by electrically bypassing the capacitor during its arcing time, and the metaloxide varistors by limiting the overvoltage, inherently reducing the capacitive reactance, and inserting some value of equivalent resistance into the circuit until the fault is cleared.

The effect on temporary and transient overvoltages (and the possibility of SSR) as a result\ of using of series compensation with its overvoltage protection should be carefully studied.

CONNECTION AND TERMINATIONS OF METAL CLAD SWITCHGEARS BASIC INFORMATION

Bus connections
When the MC switchgear consists of several shipping sections, the main bus is necessarily disconnected before shipping. The main bus should be reconnected, with particular attention paid to the cleanliness of and pressure between the contact surfaces.

It is essential that the connections be securely bolted because the conductivity of the joints is dependent on the applied pressure. Refer to the manufacturer’s torque recommendations and any other special instructions.
Cable connections
Before the cable connections are made, the phasing of each cable should be determined in accordance with the connection diagram, and the cables should be tagged accordingly. The cable manufacturer’s instructions should be followed in forming cable terminations and during the installation of the cable.

It is essential that the connections be clean and torqued to manufacturer’s recommendations since the conductivity of the joints is proportional to the applied pressure. The terminating devices (where required) should be installed pursuant to the terminator manufacturer’s instructions.
Control connections
Control wires between shipping sections should be reconnected as marked by the manufacturer. Connections that are to be connected to terminals in apparatuses remote from the switchgear should be checked carefully against the connection diagram.

In making connections to terminals, care should be exercised to ensure that the connections are made properly.
Grounding
Sections of ground bus previously disconnected at shipping sections must be reconnected when the units are installed. It should be ensured that all secondary wiring is connected to the switchgear ground bus as indicated on the drawings.

The ground bus should be connected to the system ground with as direct a connection as possible and should not be run in metal conduit unless the conduit is adequately bonded to the circuit. The grounding conductor should be capable of carrying the maximum line-to-ground short-circuit current for the duration of a fault.

A reliable ground connection is necessary for every switchgear installation. It should be of sufficient capacity to handle any abnormal condition that might occur in the system and should be independent of the grounds used for other apparatuses.

A permanent low-resistance ground is essential for adequate protection and safety.

PAD MOUNTED FUSED SWITCHGEAR RATING INFORMATION

Overall ratings
The overall ratings of PMFSG shall include the following:
a) Rated power frequency;
b) Rated maximum voltage;
c) Rated lightning-impulse withstand voltage;
d) Rated power-frequency withstand voltage;
e) Rated short-circuit current.
Rated power frequencyThe rated power frequency shall be the frequency at which the PMFSG and its components are designed to operate. The preferred rated power frequency is 50 Hz or 60 Hz.

Rated maximum voltage
The rated maximum voltage of PMFSG shall be that of the way with the lowest rating. A three-phase PMFSG containing one or more ways with components, such as fuses, single-phase switches, or fused-loadbreak devices rated for phase-to-ground voltage (maximum voltage divided by 1.732), shall have the designation “Grd-Y” (grounded-wye) added to the rated maximum voltage.

The application of Grd-Y rated PMFSG should be limited to those three-phase applications where the recovery voltage, during switching or fault clearing across any Grd-Y rated way, does not exceed the phase-to-ground rating of components, and the three-phase system voltage does not exceed the rated maximum voltage of the PMFSG.
Rated lightning-impulse withstand voltage
The rated lightning-impulse withstand voltage shall be that of the way with the lowest rating.
Rated power-frequency withstand voltage
The rated power-frequency withstand voltage shall be that of the way with the lowest rating.
Rated short-circuit current
The rated short-circuit current shall be the lowest of the following ratings of any of the ways, and shall be expressed in both symmetrical (sym) and asymmetrical (asym) rms amperes (peak amperes may also be included or substituted for asymmetrical amperes):

a) The rated interrupting current of the fuses, if applicable (see IEEE Std C37.41-1994);
b) The rated momentary and short-time current of the switches, loadbreak devices (if applicable), and bus;

c) The rated fault-closing current of the switches and loadbreak devices.
NOTES
1—A PMFSG consisting of only a single switched way and ways containing fuses may have a rated short-circuit current equal to that of the fuses, if it can be demonstrated that the switch can withstand the fault-closing and momentary duty, as limited by the fuses.
2—Bushings, separable connectors, terminators, or cables may not have short-circuit capabilities as high as the rating of the gear, and could limit the application.

SUBSTATION SHIELDING AND GROUNDING PRACTICES BASIC INFORMATION

The following are recommendation based on IEEE STD 525-1992

Shielding practices
a) The cable for computer or high-speed data logging applications, using low-level analog signals, should be made up of twisted and shielded pairs. For noncomputer type applications, such as annunciators, shielding may not be required.
b) Twisting and shielding requirements for both digital input and digital output signals vary among different manufacturers of computerized instrumentation systems. Separation of digital input cables and digital output cables from each other and from power cables may be required.

Where digital inputs originate in proximity to each other, twisted pair multiple conductor cables with overall shield should be used or multiple conductor cable with common return may be permitted, and overall shielding may not be required.

Digital output cables of similar constructions may also be permitted. Individual twisted and shielded pairs should be considered for pulse-type circuits.
c) Cable shields should be electrically continuous except when specific reasons otherwise dictate. When two lengths of shielded cable are connected together at a terminal block, an insulated point on the terminal block should be used for connecting the shields.
d) Shields should be isolated and insulated except at their selected grounding point to prevent stray and multiple grounds to the shield.
e) At the point of termination, the shield should not be stripped back any further than necessary from the terminal block.
f) The shield should not be used as an electrical conductor except for neutralizing transformer excitation.
g) For signal circuits, the shield must not be part of the signal circuit. Furthermore, the use of shielded, twisted pairs into balanced terminations greatly improves transient suppression. It is never acceptable to use a common line return both for a low-voltage signal and a power circuit.
Grounding practices
a) All shields should be grounded in accordance with provisions above.
b) Signal circuits, if grounded, should be grounded at only one point.
c) Digital signal circuits should be grounded only at the power supply.
d) The shields of all grounded junction thermocouple circuits and the shields of thermocouple circuits intentionally grounded at the thermocouple should be grounded at or near the thermocouple well.
e) Multipair cables used with thermocouples should have twisted pairs with individually insulated shields so that each shield may be maintained at the particular thermocouple ground potential.
f) Each resistance temperature detector (RTD) system consisting of one power supply and one or more ungrounded RTDs should be grounded only at the power supply.
g) Each grounded RTD should be on a separate ungrounded power supply except as follows:
h) Groups of RTDs embedded in the windings of transformers and rotating machines should be grounded at the frame of the respective equipment for safety. A separate ungrounded power supply should be furnished for the group of RTDs installed in each piece of equipment.
i) When a signal circuit is grounded, the low or negative voltage lead and the shield should be grounded at the same point.