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XYLENE POWER LTD.

ELECTRICITY TRANSMISSION CONCEPTS

By Charles Rhodes, P.Eng., Ph.D.

INTRODUCTION:
Electricity transmission lines are used to guide electrical energy over long distances. Most of the transmitted energy is actually contained in the propagating electromagnetic field energy in close proximity to the transmission line conductors rather than within the conductors. Usually transmission lines guide electromagnetic field energy unidirectionally from an energy source (usually a generator), to energy sinks (usually substations in municipalities). The direction of energy flow may change when there is transmission connected energy storage system, because an energy storage system can act as either an energy source or an energy sink. Bidirectional energy flow introduces various design complications.
 

TRANSMISSION TYPES:
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.
 

AC TRANSMISSION:
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.

Most galvanized steel lattice transmission towers support either one or two three phase AC transmission circuits. Usually 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 towers are taller (~ 198 feet), the insulators are longer, the conductor spacing is greater and the individual conductors are usually composed of 4 wires held in close proximity to each other by spreaders that control the interwire spacing. The purpose of this multi-wire 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 /conductor X 2 conductors = 1,500,000 kW = 1500 MW.
The third conductor in effect acts as a common return for the other two conductors.

The direct cost of building a two circuit 500 KV 3 phase AC transmission line, involving a single line of tall lattice towers each supporting two circuits (six main 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 most 500 kV transmission runs parallel and/or adjacent to a major highway such as highways 400, 401 or 407.

AC electricty systems generally have partially inductive loads such as transformers and motors that tend to cause phase separation between the sinusoidal voltage and current waveforms. In order to make an AC transmission line function most efficiently the phase angle between the voltage and current waveforms should be zero. Hence large AC transmission systems utilize either switched capacitors and/or special generators to provide phase angle correction.

High voltage AC transmission has the disadvantage that it cannot be routed under water.
 

TRANSMISSION RELIABILITY:
In order for transmission to be reliable it must be possible to shut down any one transmission circuit for maintenance without shutting off load customer electricity service. Hence in a dual circuit arrangement each 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.

The system should be designed to have a 20% safety margin between maximum operating power and maximum capacity. Thus with two parallel circuits each circuit normally carries up to 40% of its peak capacity. If there are three parallel circuits that fraction can in principle be increased to 53%.

Sometimes there are four transmission circiuts supported by two lines of lattice towers that follow the same path. In that case the normal power transmission per circuit can theoretically be increased from 40% of maximum circuit capacity to 60% of maximum circuit capacity while still maintaining the capability of taking any one circuit out of service for maintenance while not exceeding the 80% of maximum circuit capacity on the three remaining circuits. This arrangement realizes significant capital cost savings. However, there is a serious reliability tradeoff because an ice storm, earthquake, natural gas line rupture fire, etc. can easily take out all four circuits leaving no capacity to support even the most critical loads such as nuclear reactor cooling pumps and hospitals. Load customers are simply without electricity until the transmission is repaired or replaced. From a power system reliabiity perspective it is better to limit the number of circuits that follow a common path to two and accept the extra land acquisition cost.
 

TRANSMISSION EFFICIENCY:
A major advantage of normally operating transmission lines at 40% of maximum power capacity is in improved efficiency. As a transmission line moves from 40% of maximum capacity to 80% of maximum capacity the energy loss via heating quadruples. Thus an energy loss fraction that is normally 6% becomes 24%, which has a major impact on power system economics. This is another issue that is often not adequately appreciated by parties that question the need for redundant transmission lines.
 

AC TRANSMISSION COST:
It is generally accepted that the capital cost of rural dual circuit 500 kVAC transmission is about:
$3,000,000 / km-3000 MW peak = $1.00 / km-kW peak

Hence the actual capital cost of most 500 kV AC transmission is given by:
$1.00 / km-kW peak X (1 / 0.5) X (1 / 0.8) = $2.50 / 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 $2.50 / km-kW = $.50 / 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 $.50 / kW-km-year = $500.00 / kW-year

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 of:
($500.00 / kW-year) / (7889.4 kWh / kW-year) = $.063 / kWh

If the transmission distance is 500 km the corresponding cost of nuclear electricity transmission is:
($250 / kW-year) / (7889.4 kWh / kW-year) = $0.032 / kWh

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:
($500.00 / kW-year) / (2629.8 kWh / kW-year) = $.190 / kWh

If the transmission distance is 500 km the corresponding cost of transmitting unconstrained wind energy is:
($250 / kW-year) / (2629.8 kWh / kW-year) = $0.0950 / kWh

Hence 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 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 storage and then to the load center. That transmission path is frequently very long.
 

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 so geographically 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. Newfoundland is building a DC link to 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 instead of three. However, DC has the disadvantage that the termination equipment is relatively expensive. Hence it is usually not economical to tap the DC 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 system cause a DC ground current that can corrode buried metal piping within a wide radius around the termination equipment.
 

ENERGY FLOW DIRECTION:
An issue that is not adequately appreciated outside the electrical transmission community is that it is extremely difficult to design, install, operate and safely maintain a high voltage electrical transmission system in which energy flow is permitted to be bidirectional. It is far simpler and less expensive to design voltage regulation and electrical protection for a system with unidirectional energy flow. When a fault occurs that fault must be instantly isolated. Knowing which isolation switches to trip is not so easy if power flow is bidirectional. Each isolation switch must be sized for the lowest possible power source impedance, which again becomes complicated and expensive if the power flow is bi-directional. If proper design rules are not followed, only at generators and at loads is there certainty as to power flow direction.

The simple solution to this issue is to divide generation into three classes, transmission connected generation, distribution connected generation and behind the meter generation. Transmission connected generation always exports power to the transmission system, so there is no unusual protection problem. Distribution connected generation should be controlled such that the distribution system always remains a net load. Behind the meter generation should be controlled such that the load customer always remains a net load. Thus power always flows unidirectionally from transmission connected generators to the transmission system, from the transmission system to local distributors and from local distributors to their load customers.
 

This web page last updated December 11, 2013.

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