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Electricity transmission systems are used to transmit energy over long distances and to enhance electricity supply reliability by providing tolerance to local equipment failure. Ideally transmission systems should be designed so that a random failure of any single generator or any single transmission line segment has little or no impact on load customers. Including scheduled equipment shutdowns for maintenance the electricity system should be able to tolerate at least two adjacent equipment outages without load customers experiencing extended loss of electricity service. The ability of the electricity grid to keep operating in the face of both planned maintenance outages and unplanned equipment failures is known as Grid Resiliency.
A transmission system should be designed to have a 20% safety margin between maximum circuit operating power and maximum circuit capacity. This margin allows for unplanned changes in load and power factor. However, in order for a transmission system to be reliable it must be possible to shut down any one transmission circuit for maintenance without shutting off load customer electricity service. Hence each transmission circuit normally operates at less than 40% of maximum capacity so that during maintenance periods a single circuit can safely meet the maximum normal load of two circuits.
Sometimes there are two lines of transmission pylons that follow the same path. This arrangement realizes significant capital cost savings in terms of reduced transmission right-of-way requirements. However, there is a reliability tradeoff because an event such as an ice storm, earthquake, natural gas line rupture fire, etc. can potentially take out both lines of pylons leaving no capacity to support even the most critical loads such as nuclear reactor cooling pumps, air traffic control centers and hospitals. Under these circumstances load customers are simply without electricity until the transmission is repaired or replaced. From a power system reliabiity perspective it is best to limit the number of transmission circuits that follow a common corridor to two or three and accept the extra cost of geographically separate energy transmission corridors.
The issues related to electricity transmission lines and forest fire control have been well known for at least 70 years.
1) Build transmission towers big enough that wind cannot cause either phase to phase or phase to ground shorts.
2) Build an access road alongside the transmission which enables foresters to easily cut and remove vegetation. That road also doubles as a temporary fire break. Such roads often need fences and cattle gates.
3) Pay foresters to biannually cut vegetation. In suitable conditions trees can grow a lot in two years and can overwhelm a power line in four years.
4) Maintain a forest service with watch towers that have line of sight view of the entire transmission right-of-way.
5) Maintain a force of large water bombers that are ready to go at literally 5 minutes notice. If the fire is not suppressed during the first hour it can grow very big very quickly.
6) Maintain a force of smoke jumpers or helicopter borne fire fighters to completely extinguish the fire. Again they must be ready to go 24/7.
7) Maintain a force of daytime fire fighters fully equipped to back up the smoke jumpers.
All of the aforementioned measures cost serious money. From time to time various parties have tried to cut corners on these costs, always with long term devastating results. There always seems to be some politician or smart-ass CEO who thinks that he/she is going to save the taxpayers/ratepayers/shareholders money by cutting funding related to one or more of the aforementioned measures. The bottom line is that these measures must be adequately funded which means that power transmission and forest fire suppression are not free, despite claims to the contrary by various parties.
The whole concept of politicians being elected or CEOs being chosen to cut these costs is fundamentally wrong. All that happens is that the costs are shifted onto someone else. In the private sector the root of the problem is compensating CEO's based on short term profitability. CEO share based compensation should be delayed by at least five years.
A major advantage of normally operating transmission lines at 40% of their maximum power capacity lies in improved transmission efficiency. As a transmission line moves from 40% of its maximum capacity to 80% of its maximum capacity the energy loss via resistive heating quadruples. Thus an energy loss fraction that is normally 6% becomes 24%, which has a major impact on power system economics. This is an issue that may not be adequately appreciated by parties that question the need for redundant transmission lines.
Usually transmission lines are designed to guide energy from an energy source (usually a generator) to energy sinks (usually local distribution substations in municipalities). Most of the transmitted energy is contained in the propagating electromagnetic fields that are in close proximity to the transmission line conductors.
ENERGY FLOW DIRECTION:
The direction of net energy flow along a transmission line may change over time when there is a transmission connected energy storage system, because an energy storage system can alternately act as either an energy source or an energy sink. Likewise, if there is distribution connected generation a distribution system can potentially be either an energy source or an energy sink for the transmission system. However, bidirectional energy flow introduces numerous complications.
An issue that is often not adequately appreciated is that it is both difficult and expensive to design, install, operate and maintain an electricity transmission system in which bidirectional net energy flow is permitted. It is simpler and less expensive to design voltage regulation and electrical protection for a system with unidirectional net energy flow. When a fault occurs that fault must be immediately isolated. Determining which isolation switches to trip when bidirectional power flow is permitted is not easy. Each isolation switch must be sized for the lowest possible power source impedance, which is complicated and expensive to implement if the permitted power flow is bi-directional.
For electricity system reliability it is helpful to provide isolation switches at both ends of each transmission line segment. Then a transmission line segment can be fed from either end. However, at any instant in time only one end switch is closed so that the net power flow at any point along the transmission line remains unidirectional.
Note that due to non-ideal power factor, even if the net power flow is unidirectional the instantaneous power flow may be bi-directional. At any point on the transmission line the total power consists of an incident power propagating in one direction and a reflected power propagating in the opposite direction. Generally the transmission system is designed to minimize the ratio of the reflected power to the incident power. Usually the reflected power winds up being dissipated as heat.
Generally generation is divided into three classes, transmission connected generation, distribution connected generation and behind the meter generation. Transmission connected generation always exports power to the transmission system. Distribution connected generation should be constrained so that net power always flows from the transmission system to the distribution system. Behind the meter generation should be constrained so that net power always flows from the distribution system to the load customer. Thus net power should always flow unidirectionally from transmission connected generators to the transmission system, from the transmission system to local distribution and from local distribution to load customers. This arrangement provides equipment protection and improves safety for maintenance personnel.
There are parties that argue that bi-directional net energy flow should be permitted, that load customers should be paid for power exported to local distribution and local distributors should be paid for power exported to transmission.
These parties fail to realize that in a non-fossil energy system kWh have little monetary value and that electricity rates are dominated by kVA related costs. A sizable fraction of the per kVA cost is the cost of fault isolation switchgear which is much more expensive if random bi-directional net energy flow is permitted.
If load customers are permitted to net generate the short circuit disconnect capacity of each nearby branch isolation switch must be increased. Under present legislation there is no simple way of recovering the required switchgear upgrade costs.
Further, if load customers are permitted to randomly export power to the local distribution grid, utility maintenance workers lack certainty that a particular branch feeder thought to be de-energized will remain fully de-energized.
ELECTRICITY SYSTEM RELIABILITY:
Electricity systems with transmission that is sized as set out herein and with four unit generation stations that have peak capacities of about 200% of their normal peak load provide sufficient redundancy for high electricity supply reliability provided that the causes of generation and transmission equipment failure are statistically independent. One generation unit can be scheduled out of service for major maintenance and one additional unit can randomly fail before there is reliance on transmission for obtaining additional power from other generating stations.
If due to random equipment failure one generation station cannot meet its local load other nearby generation stations usually have sufficient excess capacity to meet that load.
The probability of two simultaneous adjacent unscheduled equipment outages in addition to a scheduled equipment outage is remote but may be still tolerable from the load customer perspective. However, three adjacent random equipment outages in addition to a scheduled equipment outage will likely cause prolonged loss of service for some load customers.
This probabilistic analysis of electricity service reliability is not valid for intermittent renewable generation because the major causes of loss of renewable generation failure are not statistically independent. In any probabilistic analysis the power capacity assigned to wind and solar generators must be zero because from time to time wind and solar generation both go to zero over a wide area.
Long distance electricity transmission is always done at a high voltage to minimize resistive line losses. Transmission circuits are usually designed for either balanced 3-wire 3-phase AC or balanced 2 wire DC operation.
Overhead balanced 3-wire 3-phase AC is most commonly used for power transmission because it provides relatively economical medium distance power transmission with capability of economical servicing of small communities along the transmission route. In commercial-industrial applications 3-phase is advantageous because it delivers power to the load at a constant rate and because it provides motors with excellent starting torque and speed control characteristics.
When the voltage and current wave forms are not exactly in phase then there is reflected power which increases transmission losses and reduces the transmission system's net power transfer capability.
Most galvanized steel lattice transmission towers support either one or two three phase AC transmission circuits. Often there is another smaller wire interconnecting the tops of the lattice towers that provides ground potential lightning protection. In Ontario typical phase to phase AC transmission voltages are 115 kV, 230 kV and 500 kV.
At 500 kV as compared to 230 kV the lattice pylons are taller (~ 198 feet), the insulators are longer, the phase conductor spacing is greater and the individual phase conductors are usually each composed of 4 wires held in close proximity to each other by spreaders to control the interwire spacing. The purpose of this multi-wire phase conductor construction is to reduce the peak radial electric field at the conductor surface. With a single wire conductor at 500 kV RMS the peak radial electric field can cause ionization of the surrounding air. Such ionization causes radio interference and power loss.
The maximum power transfer capacity of one 500 kV 3-phase AC circuit is given by:
500 kV RMS X 1500 A RMS /phase conductor X 2 phase conductors = 1,500,000 kW = 1500 MW.
The third phase conductor in effect acts as a common return for the other two phase conductors.
Typically two three phase circuits are supported by a single line of pylons. Hence the maximum power capacity of an energy transmission corridor with a single line of pylons is:
2 circuits X 1500 MW / circuit = 3000 MW
The direct cost of building a two circuit 500 KV 3 phase AC transmission line, involving a single line of tall galvanized steel lattice pylons each supporting two circuits (six phase conductors per lattice tower), through a rural area is typically about $5 million per mile ($3.1 million / km). The cost of building 500 kV transmission through an urban area is generally prohibitive due to the cost of right-of-way acquisition. In Ontario much of the 500 kV transmission runs parallel and/or adjacent to major highways such as highways 400 and 407.
An AC transmission line has distributed inductance and capacitance which gives the transmission line a characteristic impedance at 60 Hz. For optimum power transmission it is desirable to maintain a nearly constant voltage on the transmission line which allows efficient power insertion and power extraction. A constant voltage and a constant impedance imply a constant power. However, when transmission lines are used to transmit wind power from one region to another there is considerable power variation over time. The change in transmission line behavior with change in power leads to grid instabilities.
At the AC line frequency the distributed inductance on an overhead open wire power transmission line dominates the distributed capacitance. There are also partially inductive loads such as transformers and motors. These net inductances tend to cause a phase angle separation between the sinusoidal voltage and current waveforms on the same conductor. The cosine of this phase angle difference is known as the power factor. The transmission line has peak voltage and peak current constraints. To maximize the transmission power capacity the phase angle between the voltage and current waveforms on the same conductor must be zero. Long AC transmission lines frequently utilize synchronous capacitors at intervals along the line to provide power factor correction. Power factor correction works well for point to point transmission lines but becomes very complex when there are branching circuits with variable loads.
A synchronous capacitor is basically an over excited and unloaded synchronous motor. The grid is connected via a transformer to the motor stator. This motor has fly wheel attached to its rotor which adds mechanical inertia to the grid. If the DC current exciting the motor's rotor is less than a critical value the assembly acts as an inductor. If this DC current is greater than the critical value the assembly acts like a capacitor. Hence by varing the synchronous motor rotor excitation current the local transmission line power factor can be adjusted to unity.
High voltage open wire AC transmission has the disadvantage that it cannot be routed under water. Above ground open wire 500 kV AC transmission conductors are usually supported by 65 m to 100 m high transmission pylons to minimize transmission loses due to the fringing electromagnetic field interacting with water in the ground.
DC transmission has the advantage that it permits twice as much power to be transmitted over the same towers, conductors and energy transmission corridor as are used for AC transmission. In an AC system the peak operating voltage rating is:
1.41 X 500 kV = 705 kV.
If the six conductors on a two circuit 500 kV AC tower are repurposed for DC operation there is potential for 3 balanced DC circuits. Each conductor can operate at 705 kV with respect to ground. On each DC circuit the differential voltage across the circuit is:
2 X 705 kV.
Hence the maximum power capacity becomes:
2 X 705 kV X 1500 A X 3 circuits = 6,345,000 kW
= 6,345 MW
DC transmission has the advantage that with sufficient cable insulation it can be routed under water.
DC transmission has the disadvantage that it requires equipment at the energy source end to convert the AC source power to DC for transmission and at the load end to convert the DC back to AC. This conversion equipment, known as rectifiers and power inverters, is inefficient and expensive, and is generally only used in circumstances where AC equipment is technically unsuitable. The four common circumstances are: very long point to point transmission runs, underwater transmission runs, interconnection of separately controlled power zones and grid stabilization in the presence of multiple power transmission paths of substantially differing lengths.
In a DC transmission system care must be taken to ensure that the current through the positive conductor is always precisely equal to the current through the negative conductor. The difference current will flow through ground causing corrosion of buried metal piping in the vicinty of the DC terminal stations.
AC TRANSMISSION COST:
It is generally accepted that the 2007 capital cost of rural dual circuit 500 kVAC transmission is about:
$3,600,000 / km-3000 MW peak = $1.20 / km-kW peak
Hence including redundancy for reliability the actual capital cost of 500 kV AC transmission is given by:
$1.20 / km-kW peak X (1 / 0.5) X (1 / 0.8) = $3.00 / km-kW peak
The corresponding annual blended cost of interest, capital amortization and maintenance for a transmission capacity of 1 kW is given by:
0.2 / year X $3.00 / km-kW = $.60 / kW-km-year
When the distance from the generator to the load is 1000 km, the annual cost of that transmission capacity becomes:
1000 km X $.60 / kW-km-year = $600.00 / kW-year / 1000 km
If, as in the case of nuclear generation, the generator capacity factor is 90%, this cost is amortized over:
0.9 X 8766 kWh / kW-year = 7889.4 kWh / kW-year
leading to a transmission cost per kWh per 1000 km of:
($600.00 / kW-year) / (7889.4 kWh / kW-year) = $.076 / kWh / 1000 km
If the transmission distance is 500 km the corresponding cost of nuclear electricity transmission is:
($300.00 / kW-year) / (7889.4 kWh / kW-year) = $0.038 / kWh / 500 km
Thus in the case of nuclear generation transmission over distances greater than 1000 km is usually uneconomic.
If, as in the case of an unconstrained wind generation, the generator capacity factor is only 30%, the cost of 1000 km transmission must be amortized over:
0.3 X 8766 kWh / kW-year = 2629.8 kWh / kW-year
leading to a transmission cost per kWh of:
($600.00 / kW-year / 1000 km) / (2629.8 kWh / kW-year) = $.228 / kWh / 1000 km.
If the transmission distance is only 500 km the corresponding cost of transmitting unconstrained wind energy is:
($300 / kW-year / 500 km) / (2629.8 kWh / kW-year / 500 km) = $0.114 / kWh / 500 km
Thus in the case of intermittent wind generation transmission over distances exceeding 300 km is usually uneconomic.
Note that the cost of transmission is proportional to the transmission distance and is inversely proportional to the generator capacity factor. This issue heavily impacts the cost of energy transmission to urban load centres from remote wind generation. The actual transmission path from a wind generator to a load center is from the generator to a hydraulic dam with energy storage and then to the load center. That transmission path is frequently very long. In addition there are energy losses due to transmission and energy storage inefficiency. Clearly if the source of the wind energy is not close to the load it is less expensive to build a nuclear reactor close to the load than it is to build distant distributed wind generation.
APPLICATION OF DC TRANSMISSION:
In Ontario DC transmission is primarily used for system isolation from neighbouring jurisdictions in the extreme east and extreme west to provide power system phase stability.
Ontario is geographically so large that if AC power propagated around the Great Lakes, following an uncertain route through the USA, without DC isolation, that AC power could come back to where it started with substantial phase error. Such phase error could cause uncontrollable electricity grid voltage and power oscillations.
In other jurisdictions DC is used for very long distance point to point power transmission and for transmitting power under major water bodies. For example, China has recentlly commissioned a DC transmission system to bring bulk Hydro power from western Mongolia to China's densely populated east coast. BC Hydro uses DC to transmit power from the BC mainland to Vancouver Island. Hydro Quebec uses DC to transmit power under the wider portions of the St. Lawrence River as well as to isolate itself from neighbouring jurisdictions. Newfoundland has a sub-ocean DC link from generation in Labrador.
Overhead DC has the advantage that with identical towers and cables it can operate at a higher power than AC while using only two main conductors per circuit instead of three. However, DC has the disadvantage that the termination equipment is relatively expensive. Hence it is usually not economical to tap DC bulk power transmission lines to service small communities along the DC transmission route.
A further complication with DC is a requirement for almost perfect conductor current balance. Unbalanced conductor currents in a DC transmission system cause a DC ground currents near terminal equipment that can corrode buried metal pipelines for kilometers around the terminal equipment.
ELECTRO-MAGNETIC PULSE (EMP) ATTACK:
A large nuclear explosion near the surface of the sun or a nuclear explosion in Earths upper atmosphere can induce a major electro-magnetic pulse in extended power transmission lines and can directly destroy sensitive electronic equipment via the radiation pulse.
The electromagnetic energy can be picked up by extended transmission lines that may be hundreds of km long. This energy pulse seeks to find ground and will destroy almost anything in its path.
Thus the conducted energy is an even greater problem than the radiated EMP energy. The EMP radiation can be attenuated by a suitable Faraday which is simply a conducting metal enclosure. However, any radial wire penetrating that shield is a path for an energy pulse seeking ground.
This issue is normally addressed in electronic circuit design by use of high voltage shunt breakdown devices such as varistors. However, varistors used in practical circuit designs seldom have an energy pulse rating of more than a few joules. If a varistor is hit with too big a pulse it can fail open, in which case it will not protect against subsequent voltage pulses or it can fail closed in which case the circuit that it was protecting will not function properly.
These same principles apply to lightning protection in power distribution.
The real threat in this matter is major power transformer failure. It is very difficult to design an efficient major 60 Hz AC power transformer that will stop the propagation of an extremely high common mode voltage pulse. The transformer windings must be insulated to withstand that voltage pulse. Normal lightning protection may not be sufficient because the propagating common mode voltage pulse can jump the transformer via the lightning protection.
In these circumstances the design objective should be to protect the major transformers which are expensive and time consuming to replace and live with some propagating pulse damage.
Then to withstand EMP we need terminal equipment that is lightning protected, Faraday shielded and that has the highest possible common mode voltage isolation that can be economically manufactured. In this respect the focus should be on local distribution transformers and their primary lightning protection and grounding. From a national security perspective it would be prudent for local distribution companies (LDCs) to maintain a large stock of such enhanced local distribution transformers as spares. The normal operating life of local distribution transformers is in excess of 50 years, so the LDCs will be in no hurry to make such a capital investment. Replacing these transformers in the normal course of events will likely take at least two human generations.
It would be prudent for energy system planners to realize that any workable plan for EMP protection likely involves re-specification and replacement of all the local distribution transformers and related lightning protection and grounding. There are likely more than 20 million such transformers in the USA. There would also need to be a replacement inventory of about 4 million spares. Increasing the common mode voltage isolation probably requires making the transformers physically larger.
The alternative is to face the reality that in the event of an EMP attack most of the line connected electrical and electronic devices in the affected area will be seriously damaged.
If the world public fully understood the scope and potential consequences of this problem they might forgive the USA for dropping a nuke on any party that threatens an EMP attack.
In our modern digital society the consequences of an EMP attack on grid connected devices would be extremely serious. There is something to be said for the robustness of old fashioned electrical-mechanical control systems.
TIE LINES TO THE USA:
All the Canadian provinces adjoining the USA have transmission connections to the USA. Quebec has the highest voltage lines that operate at 735/765 kV. The Quebec links to the USA and to Ontario are DC isolated for electrical stability reasons. Quebec’s James Bay development is about 1500 km north of the USA border and most of the Quebec load is near the Canadian/USA border so the lines do not have much load along the route. Quebec has 5 tie-lines to the USA with a total 4,250 MW capacity. Quebec also has 2,700 MW of tie-lines to Ontario and can wheel power through Ontario to New York and Michigan.
The Quebec interconnections and HVDC lines are described at the following link:
Quebec Tie Lines
A description of all the Canadian connections to the USA is at:
Canadian Tie Lines
There is considerable concern within the US government about foreign malevolent parties potentially tampering with control of the US public electricity system. On May 1, 2020 US president Donald J. Trump signed the following executive order relating to foreign sources of equipment used for control of the US public electricity system.
In the view of this author the underlying problem is more basic. Communications and control of the electricity system should be physically totally independent of all other services. The present practise of sharing packet data communication facilities with other services simply invites problems. There were no such problems when the electricity system was controlled via a dedicated microwave system. Today there should be a dedicated fiber optic system for electricity system control. The public electricity system should not rely on any packet data network communication components used by other parties.
The problem is that packet data is software controlled. A sufficiently sophisticated adverse party can potentially tamper with the programming of any software controlled system that it can access. The certain solution is to do all necessary to ensure that there are no data communication paths between the electricity control system and other systems. Similarly electricity control system software updates must be rigerously controlled.
Whenever it is physically possible for remotely loaded software to change the packet data management the control system is not secure against sophisticated attacks by malevolent parties.
This web page last updated July 1, 2020.
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