There is a power and telecommunications industry concern that a high percentage of people knowledgeable in this subject area have either retired or are approaching retirement, and these industries are losing valuable expertise for proper application solutions. Personnel in public safety, natural gas, water, public utilities, and others involved in assuring that critical telecommunications circuits are reliable under lightning conditions should find benefit from reading this book.
What is not covered in this book are the details surrounding each type of telecommunications circuit such as digital subscriber line DSL , high speed DSL, T1, plain old telephone service, the difference between two-wire and four-wire circuits, and so on. The focus here is to explain how such critical circuits are protected from lightning strikes and power faults, plus the safe handling of such circuits should a power fault or lightning strike occur at the time of construction or maintenance.
The reader should also be aware that sophisticated computer programs and consulting services are readily available for guidance with unique circumstances that may not be covered in this book. This book does provide the knowledge and background necessary to work closely with consultants or to understand the principles of sophisticated computer programs. Chapter 2 discusses the fundamentals of electric power systems to help readers who are unfamiliar with HV equipment become better acquainted with HVPT applications.
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This chapter explains how substation ground grids, protective relaying, fault clearing devices, lightning arresters, and telecommunications equipment are used to clear power faults to minimize equipment damage and maximize electric service reliability. The critical telecommunications circuits found in HV environments are discussed in Chapter 4.
The standard service performance objectives SPOs and standard levels of protection are discussed in this chapter. Chapter 5 presents discussions about the actual HV protection equipment used in these critical circuit situations. Chapter 6 discusses proper installation and testing procedures of high voltage interface HVI equipment.
The design and installation objectives, both theoretical and physical, for reliable HV isolation are the focus points of this chapter. Photographs of actual installed equipment configurations are included in this chapter to give the reader a practical perspective.
Safety in working with HV equipment is the main topic of Chapter 7. The safety of personnel who work with HV protection equipment is a paramount concern for supervisors and managers responsible for technicians and field personnel. The reader will be made aware of these safety concerns and how and when to properly use PPE. This book is not intended to be a legal or other expert advice resource and should not be used in place of highly qualified professional consultants.
The information in this book is considered accurate and helpful but is not to be considered exhaustive and complete. Steven W. Blume Acknowledgments I would personally like to thank several people who have contributed to the success of my career and the success of this book. To my wife Maureen who has been supporting me for over 40 years, thank you for your guidance, understanding, encouragement, and so much more.
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I would also like to thank Positron Inc. He has worked in both the telecommunications industry City of Los Angeles Police and Fire critical telecommunications and electric power industry Sierra Pacific Power Company [now NV Energy] in planning, design, operations, and construction of high voltage power facilities and telecommunications systems. Steve has been teaching high voltage protection for telecommunications HVPT courses for over 15 years.
His combined knowledge, experience, and recognized ability to explain complex subjects in the simple-tounderstand terms presented in this book will be useful to those interested in gaining a working knowledge in HVPT applications. For more information on online, instructor-led, and private custom training opportunities, please visit the Web site www. There are various means of properly protecting telecommunications facilities equipment and personnel. The goal of high voltage protection for telecommunications HVPT is to provide the design engineer with safe, reliable, and costeffective installations when exposed to unexpected HV events such as power faults and lightning strikes.
Power faults are HV flashovers of insulation, the breakdown of equipment used in HV systems, or when something happens to HV equipment causing it to discharge large amounts of electrical energy into its surroundings. When personnel are working in HV environments such as electric power substations, power plants, cell sites on power towers, and other potentially dangerous locations where an HV event is possible, properly protecting critical telecommunications facilities is essential.
Copper telecommunications cables can transfer dangerous potentials from remote locations due to their insulated jackets and remote connections. All dielectric optical fiber systems, on the other hand, offer electrical isolation due to the nonconductive properties of glass. This chapter summarizes the potential problems with telecommunications circuits in HV environments, the industry solutions, and the recommended methods to work in these environments safely. The terms are almost synonymous when it comes to protecting telecommunications circuits from HV conditions.
Both terms apply to HV exposure conditions where circuits need to protect themselves from damage. Hence, both HV protectors and HV isolation equipment may be required at HV environments such as power substations, personal communications system PCS cell sites located on electric power towers, stand-alone mountaintop telecommunications antenna towers, and emergency call centers.
Power faults and lightning strikes cause high currents to flow through metallic paths to earth-grounded objects. The portion of faulted current that flows through the earth itself, returning to voltage sources, can have harmful effects on telecommunications cables and equipment. For example, Figure 1. Notice that the cable shield is grounded on both ends grounded is the term used to describe how the connection is made between the metal cable shield and the metal conductors buried in the earth.
For the sake of illustration, intermediate grounds of the cable shield are not shown. In this case, the earth serves as a natural conductive body that can potentially conduct electrical current should a voltage appear between the grounded objects. In the normal state, the earth has zero potential between these two grounded objects, and no current is flowing through the cable shield. These GPR voltages can differ on the order of tens of thousands of volts at the location where the fault occurs. Figure 1. The same is true with all ESLs.
Since the fault is located at the substation, the CO side is also referred to as the remote ground location. In other words, when a copper telecommunications cable is connected between the CO and a substation and both ends of the cable are grounded, the CO side is referred to as remote ground location, and the station side of the cable is referred to as the ESL. Further, personal injury can also occur if the person comes in contact with both potentials at the same time. These situations are discussed in more detail throughout this book. Combining the conditions of cable grounding at both ends, as in Figure 1.
Although the copper shield of the telecommunications cable is typically jacketed with insulation, the conductive shield will most likely fail the cable and create safety concerns. The essence of this book is to explain how this undesirable situation can be prevented and how to design reliable telecommunications circuits in the event of a high GPR condition.
Locations where these HV events can occur are referred to as HV environments. Aside from wireless and other telecommunications systems that provide GPR isolation by nature, there are two IEEE recommended practices of protecting telecommunications cables from the adverse effects of GPR. The two IEEE standards are 1. IEEE Std. There are acceptable variations of the recommended practices, usually resulting in additional protection; however, minimum conditions must be met to assure equipment protection, personnel safety, and reliable circuit operations.
The cable shield is grounded only at the remote ground location V point, CO side and isolated from all grounded conductors at the ESL and everywhere in the middle see Fig. Note that the cable shield is not connected directly to the substation ground grid. The high voltage interface HVI is the telecommunications equipment that provides isolation from the voltage across the cable when the GPR occurs.
Lightning arresters can be part of the HVI equipment. When the GPR exceeds the breakdown voltage level of the arrester, the arrester conducts and limits or clamps the voltage across the HVI. The arrester is connected to the copper telecommunications cable between the cable shield and the station ground grid. Lightning arresters are often installed at the substation end. The purposes of the lightning arrester are to limit the voltage potential across the HVI and to utilize the remote ground to help dissipate some lightning energy during extreme lightning strike conditions.
During extreme lightning strike conditions, the lightning arrester conducts fires and helps dissipate lightning energy directly to the earth ground electrodes the station ground grid in this example. Normally, the magnitude of the GPR is less than the firing voltage of the arrestor, and therefore the arrestor does nothing. Its purpose is to provide a secondary path for extreme lightning energy should the substation side experience an unusually high lightning event.
Additionally, the arrester limits the voltage across the HVI, thus protecting the HVI from voltages exceeding its insulation capability. The copper cable-type HVI accomplishes circuit isolation two ways; either through high dielectric strength transformer action coupling through electromagnetics or short-reach fiber optics coupling through an optical interface. The insulation properties of a short section of glass are used to isolate the HV potentials. Note that the figure shows that the optical fiber cable is not of the metallic shielded type. Only all dielectric optical fiber cable is recommended, where there is no copper or conductive shield present in the optical fiber cable as recommended by IEEE Voltage Voltage is the potential energy source in an electrical circuit that causes current to flow, work to be performed, and energy to be produced and consumed.
Voltage is sometimes called electromotive force or EMF. The basic unit of voltage is the volt. Voltage does nothing by itself, but it has the potential to do work. Voltage is a push or pull force that appears between two points. Voltage is either constant direct , alternating, or in a transient state. The common voltage classes used in the electric power industry are summarized in Table 2. Electrons are pushed and pulled by voltage through an electrical circuit that is sometimes called a closed-loop path. The electrons flowing in a conductor always return to their voltage source.
Current is measured in amperes, usually called amps one amp is equal to 6. The number of electrons is constant in a loop or circuit. Therefore, when a complete circuit path or closed loop is provided, voltage will cause current to flow. As the electrons flow in a circuit, the potential energy voltage is converted into kinetic energy. Kinetic energy is then used by the load consumption device where it is converted into useful work. Current flow in a conductor is similar to ping-pong balls lined up in a tube. Referring to Figure 2. The pressure source battery collects the balls exiting the tube and reenters them into the tube in a circulating manner i.
The number of balls traveling through the tube per second is analogous to current flow as measured in amps electrons per second. In interconnected power systems, currents can range from a few amps a residential electrical appliance to several thousand amps during a power fault short circuit event.
Short circuit implies a sudden abnormal very low resistance path that allows very high levels of current to flow. The high current flows that are returning to their source or sources can travel in several paths simultaneously: earth soil, power company transmission and distribution lines, metal facilities water pipes, rails for transit systems, etc.
Note that current flows in all closed-loop conductive paths when a voltage is applied. Note also that most of the current flows in the path of least resistance but a proportion of the current flows in all available paths. The engineer and technician are always concerned about the amount of current that flows in the earth during a power system fault or lightning strike. Currents flowing in the earth cause stray voltages to occur on the surface of the earth.
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Large stray voltages from large power faults or lightning strikes can be great enough to cause equipment damage and personal injuries. DC occurs when the voltage is kept constant, as shown in Figure 2. A battery, for example, produces a direct current when connected to a circuit.
The electrons that leave the negative terminal of the battery move through the circuit, returning to the positive terminal of the battery. This characteristic shape of a sudden DC rise and slower decay can negatively affect telecommunications circuits the effect of transient voltage conditions on telecommunications circuits is discussed later in this book.
Alternating Current AC Voltage and Current When the terminals of the potential energy source voltage alternate between positive and negative, the current flowing in the electrical circuit likewise alternates positive and negative. Figure 2. Note DC has no frequency; therefore, frequency is a term used only for AC circuits.
Asymmetrical Peak and Insulation Level When voltage transients occur on a power system, the sine wave is distorted. The asymmetrical peak voltage APV is considered the highest voltage that occurs on the power system during a transient condition. The insulation level of a device, conductor, air gap, and so on must be sufficient to withstand the peak asymmetrical voltage transient, or an insulation failure could result. There must exist a safety margin of protection between the APV and the BIL or flashover or cable insulation puncture is possible. Switching Surges HV electric power systems can produce switching surges that can cause voltage and current transients on telecommunications equipment through a concept called induction.
For example, closing a kV circuit breaker in a substation energizing a transmission line will put a voltage and current transient on the electric power system. The sudden increase in current can induce a voltage onto the telecommunications cable. Switching transients can be as high as 1. Transients on power systems are identified as high frequency and short-duration voltage fluctuations having a decaying amplitude characteristic. In some cases, repeating pulses can occur. Longitudinal-induced voltages on telecommunications cables, caused by continuous AC current flowing on the power lines, can be elevated by switching transients, power faults, and lightning strikes.
DC voltage sources cause continuous heating of the load, while AC voltage sources cause heating to increase and decrease during the positive and the negative parts of the cycle. The heat is averaged in an AC circuit from the continual cycling effect. It is important to note that there is an equivalent AC voltage and current that will produce the same heating effect in electrical load as if it were a continuous DC voltage and current.
The reason this concept is important to the subject of high voltage protection for telecommunications HVPT is that all electric power systems rate transmission and distribution voltages and current in rms quantities and HVI protection is applicable to APVs. For example, electrical insulation is usually rated in BIL a peak voltage , and power system voltage is usually rated in rms. Note that conversion factors are used when calculating GPR because fault currents that are typically provided by the power company are in rms values, but the engineer must calculate GPR in peak asymmetrical in order to determine the V point in the ZOI.
The V point is also referenced in peak asymmetrical this is explained in more detail later in this book. All electrical outlets are rated in rms. Power Voltage by itself does not do any real work. Current by itself does not do any real work. However, voltage and current together can produce real work.
Power is used to produce real work. The basic unit measurement of power is the watt. Electrical power can be used to create heat, spin motors, light lamps, and so on. The amount of time a load is on i. The common measurement for electrical energy is watt-hours. The more common units of electrical energy in power systems are kilowatt-hours kWh, meaning watt-hours for residential applications and megawatt-hours MWh, meaning 1,, watt-hours for large industrial or power company applications.
Impedance The concept of circuit impedance is a little challenging to comprehend. Impedance is the term used to describe the total opposition to current flowing in AC circuits. It is composed of resistors, capacitors, and inductors. Electrical impedance extends the concept of resistance to AC circuits. When the circuit is DC only, there is no distinction between impedance and resistance. When the circuit is AC, capacitors and inductors have different resistances for different frequencies.
Therefore, the AC impedance of a circuit varies depending on frequency. The impedance of an AC circuit is measured in ohms. The impedance of a DC circuit i. Resistance The electrical resistance of an element measures its opposition to current flow. It is similar to friction in mechanical movement. The resistance of a body stays the same whether the current flowing is AC or DC. In other words, a resistor maintains its value of resistance over all frequencies. It is measured in ohms. Inductance There are two common forms of inductance: self-inductance and mutual inductance.
Mutual inductance describes the voltage induced in an electrical circuit by the rate of change of the electrical current flowing in another circuit. This term becomes prevalent when copper communications cables parallel HV power cables. Capacitance Capacitance describes the ability of an object to store an electric charge. A capacitor is a device that provides capacitance in an electric circuit by storing energy in the dielectric material between two conducting bodies.
The energy is placed in the dielectric material by an electric field. This book refers to impedance and resistance from time to time as they pertain to grounding electrodes, station ground grids, and their contribution to GPR. When analyzing AC situations having relatively large values of inductance and small values of resistance, the term impedance Z is used. When analyzing situations having small values of inductance and large values of resistance, the term resistance R is used.
However, if only DC is considered, then only resistance applies. This knowledge will help ensure the safety of personnel working on telecommunications HV isolation equipment. Engineers and technicians that understand the behavior of power systems during normal and abnormal conditions are able to apply proper design clearances, construction procedure details, personnel protective equipment requirements, and so on while installing and maintaining these systems safely.
High Voltage Protection for Telecommunications (IEEE Press Series on Power Engineering)
Further, this knowledge enables one to quickly identify improper installations that could produce undesirable situations equipment failures and personnel injuries if not corrected. A full-scale actual interconnected utility electric power system the grid is much more complex than that shown; however, the basic principles, concepts, theories, equipment, operations, and terminologies that affect telecommunications and power system reliability are the same.
Starting with generation power plants, wind turbines, etc. HV EHV transmission lines are more efficient for transporting power long distances compared with medium voltage MV distribution power lines. MV distribution lines are an efficient means for distributing electrical energy to several localized consumers such as industrial, commercial, and residential consumers.
Service transformers are then used on distribution lines to convert MV distribution electrical energy to service voltages that are suitable for residential, commercial, and industrial loads. The use of low current minimizes system losses, making EHV the cost-effective means for energy transportation over long distances. The electrical energy at the far end of the transmission line is then converted back to MV and current electrical energy for distribution.
The distribution voltages are suitable for service transformers to be used to further reduce the electrical energy to usable service voltages for residential, commercial, and industrial consumption. Note that it is impractical not cost-effective to provide transformers capable of connecting service voltage consumers directly to EHV transmission lines. The next section discusses electrical terminology and concepts used in the electric power industry. Later chapters apply these terms frequently when explaining the reasons behind the special design, construction and maintenance methods and procedures of HVPT.
The fact that fault current flows back to voltage sources through earth is the main reason for engineers to provide HV isolation equipment. This section discusses how three-phase AC voltage is generated and connected to transmission and distribution lines and eventually consumed. It is easier to understand the meaning and significance of this statement through graphs, pictures, and animations.
In essence, it is saying that if one takes a coil of wire and puts it next to a moving or rotating magnet changing magnetic field , a voltage will be produced in that coil that can be measured across its leads. This voltage is then distributed throughout the electric power system as an electrical energy source. The rotor is spinning by a steam, wind, or hydro turbine. The means by which the magnetic field in the rotor is actually changed will be discussed later in this chapter when the second physical law for power systems is addressed.
Single-Phase AC Voltage Generation Placing a coil of wire in the presence of a changing magnetic field produces a voltage, as discovered by Faraday. This principle is graphically presented in Figure 2. Also realize that increasing the number of turns loops of conductor in the coil increases the resulting output voltage.
Three-Phase AC Voltage Generation When three coils are placed in the presence of a changing magnetic field, three voltages are produced. When the coils are spaced degrees apart in a degree circle, three-phase AC voltage is produced. Threephase voltage generation was selected due to its several advantages over single-phase sources.
For example, system losses are less, current does not flow in the neutral when the currents are balanced in the phases, and other benefits. More than three phases do not add significantly to the benefits, and they would require more wires, right of way, and so on. Generator Connections Three-phase generators and power transformers have three coils and six wires leads for connections.
There are two ways to connect these three windings that have a total of six leads symmetrically. Generator stator windings can be connected in either a delta or wye configuration. The generator nameplate specifies which winding configuration type is used on the stator. Delta Delta configurations have all three windings connected in a series as shown in Figure 2. The phase leads are connected to the three common points where windings are joined. Therefore, three wires are available for connections in three-phase delta configurations.
Wye The wye configuration in Figure 2. Therefore, four wires are available in three-phase wye configurations. The neutral is often grounded to the station ground grid for voltage reference and stability. Grounding the neutral is discussed later in this book. Delta and Wye Generator Connections Electric power plant generators use either wye or delta connections. Figures 2. The rotor uses an electromagnet as opposed to a permanent magnet. As shown in Figure 2.
Applying voltage to the field winding on the rotor creates a magnetic field in the rotor winding. Power Transformer Connections Power transformers and generator stator windings provide voltage sources to transmission and distribution lines. What is important to the telecommunications protection engineer is whether the source is wye or delta, meaning is there an earth return grounded neutral configuration.
A fault on a delta power line to a grounded telecommunications cable shield could result in a telecommunications cable failure by providing a current path for high electrical energy discharge to ground this is discussed in more detail later in this book. With regard to HVPT and HVI equipment, what is important to determine is the maximum amount of current entering the earth creating the GPR and not necessarily whether the source is wye or delta, transformer or generator, single or three phases, and so on.
As discussed in the next section, wye or delta transmission and distribution lines can have an impact as to how the telecommunications engineer grounds or isolates the aerial cable on joint poles. Delta and Wye Characteristics The wye and delta configurations have advantages and disadvantages in three-phase power systems.
This type of wye configuration on distribution lines is called the MGN-wye and is used in most distribution systems. Delta distribution systems are also used but not as often. Most aerial telecommunications cables are installed on MGN-wye electric distribution lines. Note that aerial telecommunications cables are installed differently depending on whether the power line is configured delta or wye.
In the case of the MGN-wye configuration, all carrier wires, guy wires, shields, and so on are commonly bonded to the electric power line grounds. The reason for not grounding the telecommunications cable to the power system grounds in a delta configuration is to avoid power faults from using the telecommunications conductors as current paths to earth return sources.
This can damage telecommunications cables and other equipment. Equipment can be connected either L-L or L-N. The L-N voltage is less than the L-L voltage. The neutral side of the L-N voltage is normally connected to earth by means of the MGN grounding wires and rods. For example, Neutral versus Ground versus Bond The center connection of the three-phase wye electrical circuit is called the neutral. Unbalanced neutral currents also flow through the earth connection and can cause annoying stray voltages and currents, leading to other problems. Power companies try to balance the currents on three-phase systems to minimize neutral current unbalanced for improved power quality performance, among other things.
Telephone companies also prefer balanced neutrals to minimize induction and cable pair noise. Bonding minimizes potential differences in equipment grounds and is not intended to be the direct path of fault or lightning current to enter earth soil. Bonding helps provide an equipotential situation to minimize electrical problems during power fault and lightning events.
Operations control of this major equipment is often accomplished using telecommunications circuits and therefore should be protected by HVI equipment if GPR is an issue. The purpose, function, design characteristics, and key operating properties of the major equipment that can be affected by telecommunications reliability are discussed in this section.
The proper operation of this substation equipment affects how power faults are cleared, how lightning arresters minimize disturbances, and how properly protecting telecommunication circuits from faults can improve power system operations, reliability, and safety. For example, transmission substations have higher system voltages and require greater dimensional clearances.
For example, a distribution substation could have a higher available fault current hence higher earth return current , higher GPR, and a greater distance to the V point than a transmission substation. Power consisting of voltage and current can flow either direction through a transformer. Generator step-up transformers, for example, increase the power voltage at the same time, reduce power current , enabling power to flow more efficiently over long distances.
For the same amount of power and conductor resistance, high current power has higher losses than low current power. Power transformers in substations convert the EHV and low current power in transmission lines to lower HV and higher current power distribution lines to then serve as electrical energy feeders to consumers. Service transformers are then used on distribution lines to further reduce voltage in order to provide the consumer with usable service voltages.
Customer service voltage is provided as single-phase, three-phase, wye, and, sometimes, delta configurations, depending on the need. Transformers can be single-phase, three-phase, or configured as a bank of single-phase transformers that operate as a single threephase unit. Another important note regarding power transformers with respect to telecommunications circuits is that the LV side of a transformer can have the higher available fault current than the HV side.
Meaning, faults on the LV side of a power transformer inside a substation can cause higher GPR situations than faults on the transmission side! Similarly, faults on distribution lines can have higher GPRs than faults on transmission lines! The protection engineer determining the effects of GPR on the telecommunications equipment should know the maximum available fault current possible flowing through the earth return in a substation, regardless of whether it is from transmission or distribution, when designing telecommunications protection isolation systems.
Circuit-Opening Devices Circuit-opening devices such as fuses, circuit breakers, reclosers, and protective relays are used to clear power faults and lightning strikes. Circuit breakers are normally filled with sulfur hexafluoride SF6 gas, oil, or vacuum, or use air blast assist to minimize arcing during current interruption as the breaker opens its contacts. Circuit breakers, such as those shown in Figures 2. Fuses, on the other hand, do not rely on protective relays, battery banks, control signals, and so on; instead, they melt open with sufficient current and melting time.
Most distribution lines and some transmission lines are equipped with automatic reclosing. The automatic reclose operation re-energizes the line. When the fault is still present, the line trips again. Automatic reclosing can occur several times before the line is finally locked out and deemed deenergized. Note that the GPRs recur each time the circuit breaker is reclosed and the fault is still present.
Some distribution lines extend great distances where end-of-line faults do not draw enough current from its sources to trip the circuit breaker or melt a fuse. In those rare cases, GPRs can exist for extended periods of time without tripping circuit breakers. Reliable telecommunication circuit operation depends on faults being cleared quickly. Faults that do not clear quickly will have sustained GPRs.
Properly designed telecommunications protection systems should not be affected by prolonged or repeated power fault situations. Reclosers such as the one shown in Figure 2. In contrast to breakers that rely on protective relays, battery banks and control schemes reclosers incorporate their own protective relaying functionality and normally have automatic reclosing equipment enabled.
Reclosers can be programmed to automatically reclose the circuit once, twice, or three times after trips and have different dead time intervals. Repeated operation of certain protector devices e. Power System Lightning Arresters Lightning arresters are designed to limit the line-to-ground voltage exposure in the event lightning or other excessive transient voltage condition occurs. Some older gap-type lightning arresters actually short-circuited the power line or equipment, forcing the circuit breaker to trip.
In these cases, GPRs are produced as a normal means of opening circuit breakers after a lightning strike. The newer lightning arresters use gapless metal oxide semiconductor materials to clamp or limit the high voltage transient. These newer designs offer better voltage control and have higher energy dissipation characteristics. For example, suppose an kV lightning arrester was installed on a 7. The arrester limits or clamps the HV transient and prevents the equipment from experiencing an insulation failure.
Aside from the continuous voltage rating of an arrester, arresters fall into energy dissipation classes depending on how much energy the arrester is able to dissipate. Station class arresters see Fig. They are usually located adjacent to large substation power transformers. Distribution class arresters see Fig. They can be found near distribution transformers, overhead to underground transition structures, and along long distribution lines.
Intermediate class arresters are normally used in substations that do not have excessive fault current. Residential and small commercial customers may use lower voltage secondary class arresters to protect large motors, sensitive electronic equipment, and other voltage transient sensitive devices connected to their electrical service panel. This continuous rating identifies their voltage class. The older gap-type lightning arresters were also used to protect the HVI equipment and several are still in service today. The newer metal oxide gapless arresters handle significantly more energy, are nonfragmenting, and are therefore installed inside the control building or cabinet, or next to or included as part of the HVI equipment.
Substation Ground Grids The ground grid as shown in Figure 2. All underground copper ground grid connections are exothermically connected together. The fence is connected to the grid, as well as all structures see Fig. Having all metal components of a substation connected together forming a ground grid, an equipotential situation results when a power fault occurs in the substation. Ground rods are typically installed at the base of each footing, slab, fence, and large area. When power faults occur, these ground grids help provide a connection to earth soil where fault current can return back to its sources.
Once a fault occurs, it is up to the protective relays to sense the fault and trip the corresponding circuit breakers. Grounding all metal equipment protects personnel by providing an area of equipotential. Theoretically, everything connected together experiences approximately the same voltage potential during a lightning strike or power fault. Anything the person or animal touches, when in the area of equipotential, does not experience excessive potential differences. Ground grids help dissipate power fault and lightning energy better than simply using ground rods.
The lower the ground grid resistance, the higher the earth return fault current, thus causing the protective relays and circuit breakers to operate faster. Note that protective relays are normally designed to trip faster as fault current increases. Control Building Equipment Control buildings are commonly found in the larger substations.
They are used to house the equipment associated with the monitoring, control, and protection of the substation equipment. The control buildings such as the one shown in Figure 2. The types of equipment found in control buildings that use critical telecommunications circuits having a high reliability requirement include the following: 1. Phones 5. Telecommunications equipment should operate on batteries and chargers so that loss of AC power does not interrupt telecommunications service and the remote control of equipment. Electric power control centers restore power to customers via telecommunications circuits.
Therefore, telecommunications circuits must operate even when there is a loss of AC power. The two conductors per phase option is called bundling. Power companies bundle conductors in order to double, triple, and so on the power transport capability of the power line and to improve the electrical performance of the line. This line also has two static wires on the very top to shield the conductors from lightning.
Most of the time, static wires are directly connected to the grounded metal towers so that lightning strike energy is immediately directed to earth. These shield wires help prevent the main power conductors from experiencing direct hits from lightning. Transmission towers play a very important role in HVPT. Wireless communications radio equipment sites are often located on HV power towers as shown at the top in Figure 2.
Wireless equipment sites face the same electrical GPR issues as do substations or other electric supply locations. Insulation flashovers on power structures from power faults or lightning strikes cause great GPR situations that require HV isolation of telecommunications equipment. Most distribution systems in the United States operate at three-phase grounded-wye primary voltages between Some operate at Wye Configuration Wye-connected three-phase primary distribution lines consist of three phases and a neutral, as shown in Figure 2. The common practice is to ground the neutral at every pole most systems.
When the neutral is continually grounded, the line is said to have an MGN. One of the transformer bushing connections will be grounded, and the other bushing is connected to a phase conductor. Examining the transformer bushing wire connections closely helps determine if the transformer is connected line to ground or L-L. Some wye-connected transformers only have one HV bushing, and the other connection is internally connected to ground via the transformer tank ground lug.
Regarding wye power line configurations, telecommunications equipment grounds are normally connected together with power company grounds. Delta Configuration Delta three-phase primary distribution lines use three conductors one for each phase and no neutral. Single-phase transformers must have two HV bushings, and each bushing is connected to different phases. Since delta primaries do not have primary neutrals, the transformer tank and lightning arrester grounds must be connected to ground rods at the base of the pole. Notice that there is no neutral.
Regarding delta power line configuration, telecommunications equipment grounds are normally kept separate from the power company grounds. This configuration usually involves small isolators in the guy wires. The main reason for separating the grounds is to minimize or eliminate power fault current flow through the telecommunications cable shield conductors. Should telecommunications cable shields share the same ground rods as the power company grounds without MGN, fault current will seek all available grounds, and telecommunications cable and electronic equipment damage is possible.
The importance of telecommunications circuits in electric power systems is crucial, and reliability is essential. The purpose of this section is to provide basic information about protective relaying to give the reader insight as to the importance of communication circuit reliability. Keep in mind that the goals for protective relaying are to clear power faults, trip only the line or equipment in despair, and stop any violent energy discharge as quickly as possible. One of the many important examples for the use of critical telecommunications is transfer trip.
Proper relay coordination calls for power faults on transmission lines to have the near-end circuit breaker trip open instantaneously while at the same moment have the near-end protective relay send a transfer trip signal over a secured reliable telecommunications circuit to the remote-end circuit breaker for a fast trip that would otherwise be time delayed. Power system protective relays monitor voltages and currents for real-time conditions that are over or under preset programmed quantities.
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Protective relays react to overcurrent situations, for example, by sending a DC station battery trip signal to the corresponding HV AC circuit breaker. The protective relay gets its analog sensing information from instrument transformers. Instrument transformers are composed of current transformers CTs and potential transformers PTs. The telecommunications circuits described earlier are considered critical telecommunications circuits to power company personnel.
They are so critical that backup systems are provided just in case the primary communication system fails. SCADA discussed in more detail later are computer systems involving two-way telecommunications circuits used by system control operators to monitor and control the HV equipment located in remote locations such as substations, power plants, and other power grid facilities. The analog SCADA information from the instrument transformers is then upscaled at the system control center computer where the information is displayed to the operators in actual values and in effect real time.
The current flowing in the main HV power conductor is scaled down by the CT for the instrument load. The power company protection engineer determines the proper CT ratio required for the instrument load. Taps or connection points on the coil are used to select the turns ratio option suitable for the main conductor current flows and instrument requirements. The secondary of a CT can also be grounded to provide safe working conditions. Note: never open-circuit an energized CT secondary, for it can produce extreme HVs.
Most CTs are located on transformer and circuit breaker bushings as shown in Figure 2. PTs Similarly, PTs are used to scale down very high levels of voltages to levels that are safer and more practical. The protective relays or metering equipment might use these scaled-down voltage quantities. Similar to most transformers, taps are used to allow turns ratio or scale factor selection to best match system operating voltage and instrument transformer load. Note: never short-circuit the secondary side of a PT, for it can produce extremely high currents.
High Voltage Protection for Telecommunications
When system problems are detected and breakers are tripped, alarm indications are sent to the system control center via SCADA. As a result, power equipment is de-energized and taken out of service or de-energized immediately and the system control operator is informed. De-energized equipment may result in consumers being out of power; however, there is minimal power equipment damage if the circuitry is fast and reliable. The operation of protective relays is the stabilizing force against the unwanted destabilizing forces that occur when unanticipated power faults and lightning strikes impede the power grid system.
Electromechanical relays are of the older style and are composed of coils of wire, magnets, spinning disks, and moving electrical switch contacts and are often considered one-function devices. They are very mechanical in nature. The newer solid-state electronic or microprocessor type relays have no moving parts, are more reliable, offer enhanced telecommunications options, and are sometimes considered multifunctional. Most utilities are replacing the older electromechanical relays with solid-state or microprocessor relays. The solid-state relays have several advantages over the traditional electromechanical relays.
The basic differences between electromechanical and solid-state relays are the following: Solid state see Fig. Electromechanical relays see Fig. Most overcurrent relays are designed to follow the inverse current—time curve as shown in Figure 2. In other words, the time to trip a circuit breaker shortens as the amount of fault current increases. This has many implications; lower station ground grid resistances trip breakers faster, power faults that are closer to power sources trip faster, and high resistive faults trip slower.
These implications will be referenced later in this book. The breaker can receive a trip signal from the local protective relay or from a transfer trip telecommunications circuit. Some older style breakers take nine cycles or more to trip. Note the GPR remains until the line is de-energized by the open circuit breaker. The telecommunications protection engineers and technicians should understand the importance of protective relays, transfer trip telecommunications, SCADA concepts, and applications to best serve the needs of the power utility.
The curve in Figure 2. When the current exceeds the minimum pickup setting, the relay sends a trip signal to the breaker after a programmed time delay that follows the inverse current—time curve. Relay coordination is the term used to create a situation where the most downstream clearing device from the source trips the line. In other words, the clearing device closest to the power fault actually stops the fault current.
Upstream clearing devices are used for backup with additional time delay settings. Note that failure of a critical telecommunications circuit to bypass time delays can cause excess electrical energy discharge and possible equipment damage, GPRs that are prolonged, customers who are out of service longer, and other undesirable effects. Table 2. The telecommunications circuit can be exposed to multiple GPR conditions during a permanent fault event when automatic reclosing is used.
GPRs are present during the fault and disappear after the breaker clears the fault and the breaker is successfully reclosed. The GPR is not present during the intervals between reclosing operations. Normally, transmission lines are cleared well under a second unless there is an abnormal operation. Distribution circuit breakers controlled by protective relays or fuses melting open typically clear faults in 0.
Backup protection is rarely over 3. Reclosing automatic or manual recreates the GPR situation. GPR duration is considered longer when automatic reclosing is used due to the repetitiveness of the fault condition. Having fuses clear faults instead of breakers helps to sectionalize the outage and minimize the number of customers experiencing a prolonged outage. A VDC battery bank and charger system separate from a telecommunications battery system is usually found in the control building and is the same DC source used for the protective relays and SCADA.
The relays also use this DC source to operate the trip and close coils in circuit breakers. When any contact in the trip circuit closes, the breaker opens. Note in the figure that the supervisory trip contact is closed. This implies that the SCADA control command has completed the circuit and is tripping the circuit breaker. Note that without the SCADA telecommunications circuit being operational, the system operator would not be able to trip the circuit breaker remotely, nor would the operator be alarmed that the breaker was open or closed. Permissive contacts help assure proper operating conditions exist before a high power breaker can be closed.
Transmission versus Distribution Protection Transmission protection is much different from distribution protection simply because distribution systems are radially fed meaning single source to consumer loads , and transmission systems normally have multiple lines or power sources to substations. Distribution systems normally use overcurrent relays only to trip local breakers. Transmission protection requires more sophisticated relay schemes due to the need to identify the actual faulted transmission line when there are multiple lines feeding the fault.
To complicate matters, some transmission lines have generation sources at both ends that combine for the total fault. There are radial transmission lines too where protective relay schemes are similar to overcurrent distribution protection. The up-and-coming smart grid and distributed automation technology utilizes high reliability telecommunications circuits to accomplish its energy efficiency and reliability objectives. This fast-developing power system enhancement smart grid is an expansion of SCADA and therefore requires reliable telecommunications circuits at all times. Zones of Protection The purpose of this section is to provide the reader with a better understanding of how highly reliable telecommunications service plays a vital role in protection coordination in large-scale power systems.
Transmission protection schemes incorporate overlapping zones of protection to achieve fast fault detection, clearing, and backup. These overlapping protection zones are shown in Figure 2. These overlapping zones protect against faults on the transmission lines, substation bus, generators, and transformers. Overlap is accomplished using CTs located on opposite sides of circuit breakers, transformers, or standalone CTs. Note the location of the fault on the transmission line closer to one substation than the other. A fault on one of the transmission lines requires just the breakers on both ends of that one line to trip even though there are multiple AC power sources to the fault.
Also, zone relaying provides backup tripping protection should a primary protection scheme fail to operate properly. In this example, each breaker has three protection zones. The following zone relay settings are typically used: Zone 1 Relays. Zone 2 Relays. Bestselling Series. Harry Potter. Popular Features.
New Releases. Description There is growing concern that new engineers, planners, and field technicians are not aware of the danger and reliability issues surrounding proper protection of telecommunications circuits. Using a practical, hands-on approach, High Voltage Protection for Telecommunications combines all the essential information and key issues into one book. Designed for professional training and self-study, the text will help guide managers, engineers, planners, and technicians through the process of planning, designing, installing, and maintaining safe and reliable data and voice communications circuits that are exposed to High Voltage events.
Other books in this series. Add to basket. Optimization Principles N. Electricity Markets Jeremy Lin. Electric Distribution Systems Abdelhay A. Back cover copy Sets forth the steps needed to protect critical telecommunications circuits from power faults and lightning in high voltage environments The need to protect telecommunications circuits from power faults and lightning has never been greater: when power outages or system disturbances occur, reliable telecommunications are essential.
With this book as their guide, readers will know what they need to do and not do to protect critical telecommunications circuits and equipment located in high voltage environments such as electrical power plants, substations, and power towers. Moreover, the book explains how to get the job done safely, detailing the proper implementation of safety procedures and the use of protective equipment to eliminate or minimize risk. Setting the foundation, the book begins with an overview of the problem, key issues, industry standards, and safety concerns.
Next, it covers: Definitions and fundamentals of electric power systems, with regard to HVPT applications Causes, boundary conditions, and calculations of ground potential rise Critical telecommunications circuits that must be protected in high voltage environments Protection schemes and equipment to resolve ground potential rise problems Effective installation and testing of high voltage interface equipment Personal safety for telecommunications personnel working with equipment in high voltage environments Throughout the book, the author refers to accepted industry standards and best practices.
Written by one of the most respected professional trainers in the telecommunications industry, High Voltage Protection for Telecommunications can be used for professional training courses or self-study. It will enable readers to protect critical data and voice communications circuits from high voltage events and protect themselves during equipment installation and maintenance. Review Text "It is certainly a worthwhile book for anyone who needs to understand the theory of basic telecommunication circuit protection and the practical protection methods in use today.
Review quote It is certainly a worthwhile book for anyone who needs to understand the theory of basic telecommunication circuit protection and the practical protection methods in use today. About Steven W. Blume has extensive experience in power systems planning, design, construction, and operations of major power plants, lines and substations.