POWER SYSTEMS
It is being said that electrical power is somewhat like the air we all breathe. Power is just ‘there,’ meeting our every need, constantly. In an industrial plant for instance, it is only a matter of pushing a button and a 5,000 hp motor starts up - just like that! And nobody is thinking about it…
As one article in the internet puts it, “it is only during a power failure; when one walks into a dark room and instinctively hit the useless light switch, realizing how important power is in daily life. We use it for heating, cooling, cooking, refrigeration, light, sound, computers, entertainment… Without power, life can get somewhat cumbersome”. And production in manufacturing plants would come to a massive halt. Fact is that hitting the switch is actually invoking power from probably thousands of kilometers from a source passing through a maze of power distribution grids. Generally, the complete loop of an electric system may be composed of generation, transmission & distribution systems.
a) THE GENERATION SYSTEM
Generation of electric power is accomplished in a “power plant” where bulk power is produced through a system of prime movers coupled to generators turning out electricity. Generated voltages may vary from 4,000 to 24,000 volts (in most cases), or higher - depending on the size of the generating units. The prime movers may be in the form of internal combustion engines (ICE’s) or turbines (hydraulic, steam or gas). Generation in the Philippines is usually done by power producers such as the National Power Corporation (now known as Genco’s) and by Independent Power Producers known as IPP’s interconnected to common grids such as the Luzon, Visayas and Mindanao Grids.
For general discussion, generators can be classified as Turbo-Generators, Hydro-Generators, Industrial Generators or Induction Generators.
Turbo-Generators are driven by steam or gas turbines covering power outputs from a few MW up to the largest 1,300 MW steam turbine unit currently in existence. Information gathered by this author has it, that in two or three years time from now, the largest in the world will be the 1,600 MW 70-meter unit now being built by Siemens for a Finnish utility nuclear power plant. Operating at 3,600 rpm (2 poles) or 1,800 rpm (4 poles), these amazing machines are cooled by circulating air through shaft-mounted fans either drawn directly from external atmosphere or using an enclosed circuit having secondary air/air or air/water heat exchangers. For larger units typically in utility power plants of say 200 MW or larger, the rotor and stator cores may be cooled by circulating hydrogen and the stator windings by passing de-mineralized cooling water down the center of winding conductors.
Industrial plants however commonly use turbo-generators at sizes, 1,000 kW to 10,000 kW at 4,160v.
Hydro-Generators are driven by water turbines and are generally much slower than the steam or gas turbine counterparts. Depending on the available water head, these units are operating at 60 – 1,200 rpm. They have usually large diameters to accommodate a large number of salient pole pairs but with correspondingly short axial length. Generally such machines have vertical spindles with the generators mounted above the turbine. In the Philippines hydro-generators are usually found in utility power plants.
Industrial Generators are Diesel-Driven Generators widely used by industrial plants for standby power or for island generation. Standby power units at 1,750 rpm are typically the simple diesel fuel-fed units with ratings as large as 2,000 KW while island generation application engines are the more complicated bunker-oil fed units. Ratings of bunker-fed units may be up to 12 MW running at lower speeds of 300 – 700 rpm.
Operation with a Large System:
When the Industrial Plant decides to parallel their power plants with the grid, the following discussions are thus important:
When generators are connected to a large system as the grid, the action of the individual machine controls can not have significant effect on the system as a whole. Hence opening the throttle of the prime mover to increase power input to the machine can not affect system frequency but the power exported by the machine will be increased, while it continues to run at synchronous speed.
Similarly, increasing the machine excitation can not increase the voltage on the system but this simply increases the VARs exported by the machine to the system at the same time drifting the machine power factor further lagging. In the same way that reducing the machine excitation causes the power factor to move further leading however, reducing the excitation too far will cause the power factor to move more to the leading condition so that the machine could become unstable and pole-slip. Generally, to avoid any risk of pole-slipping leading power factor operation is limited to about 0.70 at zero load to 0.90 at full load. In the case of a generator connected to the system via a generator unit transformer, the change in reactive power and consequently the change in excitation are achieved by the operation of the on-load tap changer on the transformer.
b) THE TRANSMISSION SYSTEM
As learned from CESEEPS Power Books, power transformation may take place in several stages in sequence, starting at the generating plant where the voltage is raised to approximately 1,000 volts for every mile of transmission. Voltages are then progressively reduced to the voltage required for household, commercial or industrial use. A substation that has a step-up transformer increases the voltage whilst decreasing the current, while a step-down transformer decreases the voltage for domestic, commercial and industrial distribution.
Long distance transmission of energy over the feeders may take place at voltages as high as 500,000 volts. With sufficiently large load justifying the transmission, the farther the distance, the higher is the line voltage employed. Extra-High Voltages (EHV) reaching up to 1,200,000 volts are no longer uncommon in the United States, Sweden and Russia. In economic terms, this reduces conductor sizes while at the same time bringing down the line losses in transmission. However, more elaborate towers and higher insulation requirements are needed to accomplish this type of power conveyance.
The transmission line can therefore be considered as that part of the power system connecting one remote area with another remote area along with delivering bulk power over long distances. This is made possible by a system of towers carrying currents at high voltages. The towers usually of steel construction are now usually found crisscrossing the countryside. In Luzon Grid for instance, transmission voltages are accomplished at 500 KV, 350 KV, 230 KV and 115 KV. The Visayas Grid has 350 KV, 230 KV, 138KV, 115KV and occasionally 69 KV as sub-transmission lines while the Mindanao Grid at 138 KV and 69 KV. High Voltage DC transmission systems are now gaining popularity because of the tremendous economic advantage. The Philippines is no longer an exception, today the country has already this type of transmission at 350 KVDC.
Self power-generating industries normally omit the “transmission” component of the system because of the close proximity of the load centers from the power plant. In most cases, the distribution systems of the industrial plant also carry the function of transmission system.
c) THE DISTRIBUTION SYSTEM
The transmission lines end up at the proximate area of distribution. Electricity then is once more fed into substation transformers that reverse the process by lowering the voltage to some degree, normally in the Philippines at 69KV. Downstream of the system, another voltage transformation may still be necessary and be routed to various places for local distribution and utilization.
From the point of view of the power company, the distribution system is intended to cover a specific area such as towns, cities or directly to industrial plants usually through 60 footer wooden or concrete poles at 69 KV. These voltages may feed directly to large industrial plants while for towns or cities, another voltage transformation may be necessary to produce 13.8 or 23 or 34.5 KV. These feeders are usually called laterals.
In industrial plants, scenarios like self-generating power at 2.4 KV, 4.16 KV, 6.9 KV or 13.8 KV and distribute it directly to power centers which in turn transform the voltage to 480 V or 240 V are common installations in the country. Underground distribution or through over-head cable trays are likewise employed as common feeder management methods in industries.
d) THE SUBSTATIONS
The term ‘substation’ comes from the days before the distribution system became a grid. The first substations were connected to only one power station where the generators were housed, and were just ‘subsidiaries’ of that power station. Today, Substations carry several functions. They generally contain one or more transformers, and have switching, protection and control systems. In medium and large size substations, circuit breakers are used to interrupt any short circuit or overload currents that may occur on the network. Substations do not (usually) have generators, although a power plant may have a substation nearby. A typical substation will contain line termination structures, high-voltage switchgear, one or more power transformers, secondary switchgear, surge protection, controls & metering. Other devices such as power factor correction capacitors and voltage regulators may also be located at a substation.
Substations may be on the surface in fenced enclosures, or underground, or located in special-purpose buildings. Substations located within the buildings they serve are particularly a feature of high-rise buildings. Indoor substations are usually found in urban areas to reduce the noise from the transformers, and for reasons of appearance. Where a substation has a fence, it must be properly grounded (‘earthed’ in European terms) to protect people from high voltages that may occur during a fault in the transmission system.
Transmission Substations
A transmission substation is one whose main purpose is to connect together various transmission lines. The simplest case is where all transmission lines have the same voltage. In such cases, the substation contains high-voltage switchers that allow lines to be connected together or isolated for maintenance.
Transmission substations can range from simple to complex. A small "switching station" may be little more than a bus plus some circuit breakers. The largest substations can cover a large area with multiple voltage levels and a large amount of protection and control equipment.
Large transmission substations usually have complex configuration. Aside from the function of switching several incoming & outgoing circuits, redundancy as well as by-pass schemes are common in these types of substations. Set-up of circuit breakers and their related relaying, as well as arrays of HV switchers, surge arresters and disconnect switches make the system more complicated.
Bulk Distribution Substations
A distribution substation is one whose main purpose is to transfer power from the transmission system to the distribution system of some area. Note that it is uneconomical or impractical to directly connect electricity consumers to the main transmission network (unless they use huge amounts of energy, see Chapter 3 of this book). So the distribution station reduces voltage to a value suitable for connection to local loads.
The input for a distribution substation is typically at least two transmission or sub-transmission lines. Input voltage may be, for example, 115 kV or 138 kV, or whatever is common in the area. The output is a number of feeders. Distribution voltages are typically medium voltage, between 13.8, 24, 34.5 and 69 kV depending on the size of the area served and the practices of the local utility. The feeders will then run overhead, along streets (or under streets, in a city) and eventually power the distribution transformers at or near the customer premises.
Besides transforming the voltage, the job of the distribution substation is to isolate faults in either the transmission or distribution systems. Distribution substations may also be the points of voltage regulation, although on long distribution circuits (several kilometers), voltage regulation equipment may also be installed along the line.
Complicated distribution substations can be found in the downtown areas of large cities, with high-voltage switching, and switching and backup systems on the medium-voltage side. More typical small distribution substations have a disconnect switches, one transformer, and minimal facilities on the secondary voltage side.
As one article in the internet puts it, “it is only during a power failure; when one walks into a dark room and instinctively hit the useless light switch, realizing how important power is in daily life. We use it for heating, cooling, cooking, refrigeration, light, sound, computers, entertainment… Without power, life can get somewhat cumbersome”. And production in manufacturing plants would come to a massive halt. Fact is that hitting the switch is actually invoking power from probably thousands of kilometers from a source passing through a maze of power distribution grids. Generally, the complete loop of an electric system may be composed of generation, transmission & distribution systems.
a) THE GENERATION SYSTEM
Generation of electric power is accomplished in a “power plant” where bulk power is produced through a system of prime movers coupled to generators turning out electricity. Generated voltages may vary from 4,000 to 24,000 volts (in most cases), or higher - depending on the size of the generating units. The prime movers may be in the form of internal combustion engines (ICE’s) or turbines (hydraulic, steam or gas). Generation in the Philippines is usually done by power producers such as the National Power Corporation (now known as Genco’s) and by Independent Power Producers known as IPP’s interconnected to common grids such as the Luzon, Visayas and Mindanao Grids.
For general discussion, generators can be classified as Turbo-Generators, Hydro-Generators, Industrial Generators or Induction Generators.
Turbo-Generators are driven by steam or gas turbines covering power outputs from a few MW up to the largest 1,300 MW steam turbine unit currently in existence. Information gathered by this author has it, that in two or three years time from now, the largest in the world will be the 1,600 MW 70-meter unit now being built by Siemens for a Finnish utility nuclear power plant. Operating at 3,600 rpm (2 poles) or 1,800 rpm (4 poles), these amazing machines are cooled by circulating air through shaft-mounted fans either drawn directly from external atmosphere or using an enclosed circuit having secondary air/air or air/water heat exchangers. For larger units typically in utility power plants of say 200 MW or larger, the rotor and stator cores may be cooled by circulating hydrogen and the stator windings by passing de-mineralized cooling water down the center of winding conductors.
Industrial plants however commonly use turbo-generators at sizes, 1,000 kW to 10,000 kW at 4,160v.
Hydro-Generators are driven by water turbines and are generally much slower than the steam or gas turbine counterparts. Depending on the available water head, these units are operating at 60 – 1,200 rpm. They have usually large diameters to accommodate a large number of salient pole pairs but with correspondingly short axial length. Generally such machines have vertical spindles with the generators mounted above the turbine. In the Philippines hydro-generators are usually found in utility power plants.
Industrial Generators are Diesel-Driven Generators widely used by industrial plants for standby power or for island generation. Standby power units at 1,750 rpm are typically the simple diesel fuel-fed units with ratings as large as 2,000 KW while island generation application engines are the more complicated bunker-oil fed units. Ratings of bunker-fed units may be up to 12 MW running at lower speeds of 300 – 700 rpm.
Operation with a Large System:
When the Industrial Plant decides to parallel their power plants with the grid, the following discussions are thus important:
When generators are connected to a large system as the grid, the action of the individual machine controls can not have significant effect on the system as a whole. Hence opening the throttle of the prime mover to increase power input to the machine can not affect system frequency but the power exported by the machine will be increased, while it continues to run at synchronous speed.
Similarly, increasing the machine excitation can not increase the voltage on the system but this simply increases the VARs exported by the machine to the system at the same time drifting the machine power factor further lagging. In the same way that reducing the machine excitation causes the power factor to move further leading however, reducing the excitation too far will cause the power factor to move more to the leading condition so that the machine could become unstable and pole-slip. Generally, to avoid any risk of pole-slipping leading power factor operation is limited to about 0.70 at zero load to 0.90 at full load. In the case of a generator connected to the system via a generator unit transformer, the change in reactive power and consequently the change in excitation are achieved by the operation of the on-load tap changer on the transformer.
b) THE TRANSMISSION SYSTEM
As learned from CESEEPS Power Books, power transformation may take place in several stages in sequence, starting at the generating plant where the voltage is raised to approximately 1,000 volts for every mile of transmission. Voltages are then progressively reduced to the voltage required for household, commercial or industrial use. A substation that has a step-up transformer increases the voltage whilst decreasing the current, while a step-down transformer decreases the voltage for domestic, commercial and industrial distribution.
Long distance transmission of energy over the feeders may take place at voltages as high as 500,000 volts. With sufficiently large load justifying the transmission, the farther the distance, the higher is the line voltage employed. Extra-High Voltages (EHV) reaching up to 1,200,000 volts are no longer uncommon in the United States, Sweden and Russia. In economic terms, this reduces conductor sizes while at the same time bringing down the line losses in transmission. However, more elaborate towers and higher insulation requirements are needed to accomplish this type of power conveyance.
The transmission line can therefore be considered as that part of the power system connecting one remote area with another remote area along with delivering bulk power over long distances. This is made possible by a system of towers carrying currents at high voltages. The towers usually of steel construction are now usually found crisscrossing the countryside. In Luzon Grid for instance, transmission voltages are accomplished at 500 KV, 350 KV, 230 KV and 115 KV. The Visayas Grid has 350 KV, 230 KV, 138KV, 115KV and occasionally 69 KV as sub-transmission lines while the Mindanao Grid at 138 KV and 69 KV. High Voltage DC transmission systems are now gaining popularity because of the tremendous economic advantage. The Philippines is no longer an exception, today the country has already this type of transmission at 350 KVDC.
Self power-generating industries normally omit the “transmission” component of the system because of the close proximity of the load centers from the power plant. In most cases, the distribution systems of the industrial plant also carry the function of transmission system.
c) THE DISTRIBUTION SYSTEM
The transmission lines end up at the proximate area of distribution. Electricity then is once more fed into substation transformers that reverse the process by lowering the voltage to some degree, normally in the Philippines at 69KV. Downstream of the system, another voltage transformation may still be necessary and be routed to various places for local distribution and utilization.
From the point of view of the power company, the distribution system is intended to cover a specific area such as towns, cities or directly to industrial plants usually through 60 footer wooden or concrete poles at 69 KV. These voltages may feed directly to large industrial plants while for towns or cities, another voltage transformation may be necessary to produce 13.8 or 23 or 34.5 KV. These feeders are usually called laterals.
In industrial plants, scenarios like self-generating power at 2.4 KV, 4.16 KV, 6.9 KV or 13.8 KV and distribute it directly to power centers which in turn transform the voltage to 480 V or 240 V are common installations in the country. Underground distribution or through over-head cable trays are likewise employed as common feeder management methods in industries.
d) THE SUBSTATIONS
The term ‘substation’ comes from the days before the distribution system became a grid. The first substations were connected to only one power station where the generators were housed, and were just ‘subsidiaries’ of that power station. Today, Substations carry several functions. They generally contain one or more transformers, and have switching, protection and control systems. In medium and large size substations, circuit breakers are used to interrupt any short circuit or overload currents that may occur on the network. Substations do not (usually) have generators, although a power plant may have a substation nearby. A typical substation will contain line termination structures, high-voltage switchgear, one or more power transformers, secondary switchgear, surge protection, controls & metering. Other devices such as power factor correction capacitors and voltage regulators may also be located at a substation.
Substations may be on the surface in fenced enclosures, or underground, or located in special-purpose buildings. Substations located within the buildings they serve are particularly a feature of high-rise buildings. Indoor substations are usually found in urban areas to reduce the noise from the transformers, and for reasons of appearance. Where a substation has a fence, it must be properly grounded (‘earthed’ in European terms) to protect people from high voltages that may occur during a fault in the transmission system.
Transmission Substations
A transmission substation is one whose main purpose is to connect together various transmission lines. The simplest case is where all transmission lines have the same voltage. In such cases, the substation contains high-voltage switchers that allow lines to be connected together or isolated for maintenance.
Transmission substations can range from simple to complex. A small "switching station" may be little more than a bus plus some circuit breakers. The largest substations can cover a large area with multiple voltage levels and a large amount of protection and control equipment.
Large transmission substations usually have complex configuration. Aside from the function of switching several incoming & outgoing circuits, redundancy as well as by-pass schemes are common in these types of substations. Set-up of circuit breakers and their related relaying, as well as arrays of HV switchers, surge arresters and disconnect switches make the system more complicated.
Bulk Distribution Substations
A distribution substation is one whose main purpose is to transfer power from the transmission system to the distribution system of some area. Note that it is uneconomical or impractical to directly connect electricity consumers to the main transmission network (unless they use huge amounts of energy, see Chapter 3 of this book). So the distribution station reduces voltage to a value suitable for connection to local loads.
The input for a distribution substation is typically at least two transmission or sub-transmission lines. Input voltage may be, for example, 115 kV or 138 kV, or whatever is common in the area. The output is a number of feeders. Distribution voltages are typically medium voltage, between 13.8, 24, 34.5 and 69 kV depending on the size of the area served and the practices of the local utility. The feeders will then run overhead, along streets (or under streets, in a city) and eventually power the distribution transformers at or near the customer premises.
Besides transforming the voltage, the job of the distribution substation is to isolate faults in either the transmission or distribution systems. Distribution substations may also be the points of voltage regulation, although on long distribution circuits (several kilometers), voltage regulation equipment may also be installed along the line.
Complicated distribution substations can be found in the downtown areas of large cities, with high-voltage switching, and switching and backup systems on the medium-voltage side. More typical small distribution substations have a disconnect switches, one transformer, and minimal facilities on the secondary voltage side.
