Transmission
Many large generators require easy access to their fuel supply and cooling water, so they cannot necessarily be sited close to areas of major consumption. Environmental constraints may also preclude siting close to areas of consumption. A bulk power transmission system is therefore needed between the generators and the consumers.
Large generating plant produces output ranging from 100 MW to 2000 MW and for economic reasons this normally operates with phase-to-phase voltages in the range 10 kV to 26 kV. In order to reduce transmission losses so that transmission circuits are economic and environmentally acceptable, a higher voltage is necessary. Phase-to- phase transmission voltages of up to 765 kV are used in sparsely populated large countries, such as Brazil, USA and Canada, but 380–400 kV is more prevalent in Europe. The standard voltages recommended by IEC are 765 kV, 500 kV, 380–400 kV, 345 kV, 275 kV, 220–230 kV, 135–138 kV and 66–69 kV.
Most transmission circuits are carried overhead on steel pylons. An example is shown in Fig. 13.1. They are suspended from insulators which provide sufficient insulation and air clearance to earth to prevent flashovers and danger to the public. A typical suspension-type insulator is shown in Fig. 13.2. Each country has tended to have its own acceptable tower and conductor design. At higher voltages, Aluminium Conductor Steel Reinforced (ACSR) conductor is used, a core of steel strands providing the required strength. A typical cross section for an ACSR conductor is shown in Fig. 13.3. For voltages over 200 kV two or more conductors per phase are used. This results in lower losses because of the large conductor cross section and lower radio interference and corona because of the lower voltage stress at the conductor surface.
In cases where an overhead line route is impossible because of congestion in an urban area or for environmental amenity reasons, buried cables may be employed, but the cost is 15–20 times higher than that of an equivalent overhead line. On sea
crossings an underwater cable is the only solution, but these are often dc, for reasons explained in section 13.3.1.1.
A high-voltage transmission system interconnects many large generators with areas of high electricity demand; its reliability is paramount, since a failure could result in loss of supply to many people and to vital industry and services. The system is there- fore arranged as a network so that the loss of one circuit can be tolerated. This is shown in Fig. 13.4. In many countries, three-phase lines are duplicated on one tower, in which case a tower failure might still result in a partial blackout. Mixed-voltage systems are often carried on a single tower, but this is not the practice in the UK.
In order to achieve flexibility of operation, circuits are marshalled at substations. The substations may include transformers to convert from one voltage level to another,
and switchgear to switch circuits and interrupt faults. Substations are normally outdoors and they occupy an extensive secure area, although compact indoor substations using SF6 (as described in section 7.5.3.2) have become more prevalent recently because of their improved reliability in adverse weather and a more compact layout.
An interconnected transmission network can comprise many substations which are all remotely controlled and monitored to ensure rapid reconnection after a disturbance or to enable maintenance.
Principles of design
The two main requirements of transmission systems are:
● the interconnection of plant and neighbouring systems to provide security, economic operation and the exchange of energy on a buy–sell basis
● the transport of electrical energy from remote generation to load centres
These objectives are met by selection of the most economical overhead line design, commensurate with the various constraints imposed by environmental and national considerations. The design and approval process for new lines can take many years; following public enquiries and judicial proceedings before planning, permission is granted and line construction begins. Typical objections to new overhead lines, particularly in industrialized countries are:
● the visual deterioration of open country areas
● the possibility that electromagnetic field propagation may cause interference with television, radio and telecommunications, with an increasing awareness of public health issues
● emission of noise by corona discharges, particularly at deteriorating conductor surfaces, joints and insulation surfaces
● the danger to low-flying aircraft
● a preference for alternative energy supply, such as gas and local environment- friendly generation including solar cells and wind power, or measures for reducing electricity demand, such as better thermal insulation in houses and commercial premises, lower energy lighting, natural ventilation in place of air conditioning, and even changes of lifestyle
Power system planners are required to show that extensive studies using a range of scenarios and sensitivities have been carried out, and that the most economical and least environmentally damaging design has been chosen. Impact statements are also required in many countries to address the concerns regarding the many issues raised by local groups, planning authorities and others.
Technically, the key issues which have to be decided are:
● whether to use an overhead line or an underground cable
● the siting of substations and the size of substation required to contain the necessary equipment for control of voltage and power flow
● provision for future expansion, for an increase in demand, and in particular for the likelihood of tee-off connections to new load centres
● the availability of services and access to substation sites, including secure communications for control and monitoring
HVDC transmission
Direct current is being used increasingly for the high-voltage transport of electrical energy. The main reasons for using dc in preference to ac are:
● dc provides an asynchronous connection between two ac systems which operate at different frequencies, or which are not in phase with each other. It allows real power to be dispatched economically, independently of differences in voltage or phase angle between the two ends of the link
● in the case of underground cables or undersea crossings, the charging current for ac cables would exceed the thermal capacity of the cable when the length is over about 50 km, leaving no capacity for the transfer of real power. A dc link overcomes this difficulty and a cable with a lower cross section can be used for a given power transfer
Where a line is several hundred kilometres long, savings on cost and improvements in appearance can be gained in the dc case by using just two conductors (positive and negative) instead of the three conductors needed in an ac system. Some security is provided should one dc conductor fail, since an emergency earth return can provide half power. The insulation required in a dc line is equivalent only to that required for the peak voltage in an ac system, and lower towers can therefore be used with considerable cost savings.
Against these reductions in cost with a dc line must be set the extra cost of solid state conversion equipment at the interfaces between the ac and dc systems, and the corresponding harmonic correction and reactive compensation equipment which is required in the substations. It is normally accepted that the break-even distance is 50 km for cable routes and 300 km for overhead lines; above these distances dc is more economical than ac.
As many ac systems already exist, and because the trading of energy across national and international boundaries is becoming more prevalent, dc transmission is being increasingly chosen as the appropriate link. An added advantage is that a dc infeed to a system allows fast control of transients and rapid balancing of power in the case of loss of a generator or other supply, and it does not contribute to the fault level of the receiving system. It is important here that when a short circuit occurs on the ac system side, the current that flows can be safely interrupted by the ac-side circuit breaker.
The accepted disadvantages of a dc infeed, apart from the already-mentioned extra cost of conversion equipment, are the lack of an acceptable circuit breaker for flexible circuit operation, and the slightly higher power losses in the conversion equipment compared with an equivalent ac infeed.
With careful design of the transmission and conversion components, the reliability of a dc and ac systems is comparable.
A typical HVDC scheme providing a two-way power flow is shown schematically in Fig. 13.5. Each converter comprises rectifying components in the three-phase bridge connection, and each of the rectifying components consists of a number of thyristors connected in series and parallel. Increasingly, Gate Turn Off (GTO) thyristors are being used because of the greater control they allow. The current rating of each bridge component can be up to 200 A at 200 kV and bridges may be connected in series for higher voltages up to 400 kV or even 600 kV ± to earth, each bridge being supplied from a three-phase converter transformer. By triggering the thyristors, the current flow through the system can be controlled every few electrical degrees, hence rapid isolation
can be achieved in the event of a fault on the system. Similarly, by delay triggering, the current can be easily controlled, the direct voltage being best set by tapchange on the converter supply transformers. The triggering of the thyristors may be by a light pulse which provides voltage isolation. Inversion, which enables power flow from the dc system to the ac system, depends on the ac back-emf being available with a mini- mum fault level in the receiving systems, so inversion into an isolated system is not possible unless devices with turn-off capability are available.
A further feature of dc converter substations is the need for ac transmission filters to produce an acceptable ac sinusoidal waveform following the infeed or outfeed of almost square-wave blocks of current. Such filters for 6n ± 1 harmonics (where n is the number of bridges and substations) can add 25 per cent to the cost of a substation, although they can also be used to provide some of the VAr generation which is necessary to control the power factor of the inverter.
AC system compensation
As ac power systems become more extensive at transmission voltages, it is desirable to make provision for flexible operation with compensation equipment. Such equip- ment not only enables larger power flows to be accommodated in a given rating of ac circuit, but also provides a means of routing flows over the interconnected system for economic or trading purposes.
Compensation equipment consisting of fixed or variable inductance and capaci- tance can be connected in series with the circuit, in which case it must be rated to carry the circuit current, or it may be connected in shunt, and used to inject or absorb
reactive power (VArs) depending upon requirements. In the same way, that real power injected into the system must always just balance the load on the system and the sys- tem losses at that instant, so too must the reactive power achieve a balance over regions of the system.
Transmission circuits absorb VArs because their conductors are inductive, but they also generate VArs because they have a stray capacitance between phases and between phase and earth. The latter can be particularly important with high-voltage cables. The absorption is proportional to I 2X, where I is the current and X is the reactance of the circuit, and the generation is proportional to V 2B, where V is the voltage and B is the susceptance of the circuit. When I 2X is equal to V 2B at all parts of the circuit, it is found that the system voltage is close to the rated value. If V 2B is greater than I2X, then the system voltage will be higher than the rated value, and vice versa. Designers therefore need to maintain a balance over the foreseeable range of current as loads vary from minimum to maximum during the day, and over the season and the year.
Compensation may be provided by the following three main methods:
● series capacitors connected in each phase to cancel the series inductance of the circuit. Up to 70 per cent compensation is possible in this way.
● shunt inductance to absorb excessive VArs generated by the circuit stray capacitance or (exceptionally) to compensate for the leading power factor of a load.
● shunt capacitance to generate VArs for the compensation of load power factor or excessive VArs absorbed under heavy current flow conditions on short over- head line circuits.
A combination of these arrangements are possible, especially in transmission sub- stations where no generators are connected; generators are able to generate or absorb VArs through excitation control (see section 5.3.2).
FACTS devices
Recently, Flexible AC Transmission Systems (FACTS) devices have become available. In these, the amount of VAr absorption by inductors is varied through the control of thyristors connected in series with the inductor limbs. A typical shunt controllable unit is shown in Fig. 13.6. This is known as a Shunt Variable Compensator (SVC). Other FACTS devices are variable series capacitors, variable phase-shifters and Universal Power Controllers (UPC), in which energy is drawn from the system in shunt and injected back into the system in series at a controlled phase angle by means of GTO thyristors.
The FACTS devices offer instantaneous control over voltage, current and impedance. Reactive power can be generated or absorbed and the flow of real power can be varied between alternative paths. The FACTS devices can therefore be used for voltage control, power flow control between parallel paths (particularly following a circuit outage), and by fully utilizing the instantaneous capability they can stabilize intermachine or interarea power oscillations. The disadvantages of FACTS are the additional cost, including filters to reduce the associated injection of undesirable harmonic currents into the grid.
Although a wide variety of FACTS devices have been proposed and investigated, at present the SVC is the main device which has achieved widespread adoption. The SVC offers a modern, cost-effective alternative to the rotating synchronous condenser
(an over-excited synchronous generator), providing a dynamic reactive power source for voltage control in parts of a grid remote from synchronous generators.
System operation
A transmission system may be vertically integrated, in which case the generating plant belongs to the same utility, or more commonly it may be unbundled, in which case it has only transmission capacity, with no generation plant. In either case, the main tasks for the transmission operator are to maintain a constant frequency and voltage for all consumers, and to operate the system economically and securely. Security in this context means maintaining voltage within limits, staying within a prescribed stability margin and operating all circuits within their thermal rating. This requires adequate monitoring of all the transmission components, with sufficient communication and control facilities to achieve these desired goals. Most transmission systems will, there- fore, have a coordinating room and possibly a number of manned outstations for local or regional devolvement of responsibility.
For frequency control, some of the synchronized generators are equipped with sensitive governors which use a frequency signal rather than a speed signal. The output of these generators is dependent upon the balancing power required to achieve a steady frequency over the whole system. The transmission system operator, backed up by computer forecasts of load variations and knowledge of the available plant and their offer prices, may have the authority to instruct generators to start up or to shut down (unit commitment) and to set their output (loading or dispatching) so that over a prescribed hourly, daily or weekly period they generate energy to meet the consumer demand at the minimum overall cost. In the UK however, the system operator only has the authority to select offers and bids from generators and energy purchasers to effect a balancing market, whereas the bulk unit commitment and dispatch of generation and demand is accomplished by a bilateral market. In a bilateral market, generators contract directly with energy purchasers and are responsible for their own output scheduling. The balancing market operates over a short time period (one hour in the UK) imposing
any adjustments necessary to obtain balanced supply and demand and technical satisfaction of any transmission system constraints.
There is a considerable scope for minimization of the losses in an interconnected system through the control of the compensation devices described in section 13.3.1.2. This control is guided by the use of optimal load flow programs, security assessments and calculations of transient stability margin. One of the main concerns is to arrange patterns of generation, including some plant which may otherwise be uneconomic, to maintain voltage despite outages of circuits and other components for maintenance, extension and repair. Safety of utility personnel and the operation of the system to avoid risk to the public is at all times paramount.