Substation Design Complexity
Substation design is a very complex process that involves many professional engineers in many different areas. However, there are always few people who are leading the whole design process both from the technical and economical aspects. This technical article covers the selection of substation type (GIS or Air-insulated) and the detailed design of Air-insulated Substation.
Many nations have their own safety rules and, because of their importance and legality as considered at the beginning of this article. The three component parts of the substation are defined as follows: Primary, Secondary, and Auxiliary System.
Let’s remind ourselves of these three basic substation systems:
- Primary System: The primary system comprises all equipment that, in whole or in part, is in service at the highest operating voltage of the system.
- Secondary System: The secondary system comprises all equipment that is used for the control (local and remote), protection, monitoring automation and measurement of the primary system.
- Auxiliary System: Auxiliary systems are those which are required to enable the primary and secondary equipment to operate.
- Selection Of Substation Type (GIS/AIS)
- Choosing Substation Layout:
- General Criteria and Rules:
- Safety Rules – Definitions
- Overvoltage and Insulation Levels
- Current Rating and Overcurrents
- Electrical Clearances
- Mechanical Forces:
- Equipment weight
- Equipment wind loading
- Combinations of Forces
- Corona and Radio Interference
- Acoustic Noise
- Water Contamination
- Substation Civil Design:
- Substation Fire Protection
- Substation Security
- Earthing for Personnel Safety
- Direct Lightning Stroke Shielding
The selection of substation type is, in most cases, largely dependent upon economic factors. As far as HV equipment is concerned an air-insulated substation costs less than an equivalent in GIS, but, as GIS allows a much wider choice of site, the distance to the load centre, site preparation costs and reduced maintenance costs may balance the difference.
In recent years reduction of the HV equipment price gap and increasing pollution and environmental concerns have made GIS more attractive. As a consequence, air-insulated, indoor, HV substations are not installed at voltages above 145 kV.
That makes them a good solution in cases of:
- Urban areas: High land cost and unavailability of required area for an air-insulated substation.
- Mountainous zones: High site preparation costs.
- Environmental concerns: GIS can be easily disguised and indoor (or, exceptionally, underground) installed if necessary. It should he noted, however, that it is not always possible for incoming feeders to be overhead lines.
- Very high pollution levels: GIS phase-to-phase and phase-to-ground insulations are not affected by pollution, but the metallic envelopes may suffer from corrosion problems in harsh environments. in case of outdoor GIS substations.
- Altitude above 1000 m: The internal GIS insulation is independent of atmospheric pressure, so the need for special insulators is limited to the bushings.
- Uprating, refurbishment or replacement of air-insulated substations in restricted areas.
The disadvantages of GIS substations are that the small area occupied can lead to difficulties concerning maximum step and touch voltages, so earth conductors may have to be extended beyond the substation limits (IEEE 80).
If possible, HV equipment in a GIS must be compatible, and extensions and replacements for the next 20 or 30 years must be considered at the time the initial order is placed. That’s what we call a good planning!
Suggested reading – Mastering GIS control circuits: AC/DC auxiliary circuits and circuit breaker closing circuit
Although it is impossible to supply construction details for various types of layouts corresponding to a given bus bar scheme, it is advisable to suggest a few general guidelines with reference to the more usual design situations.
Layouts with conductors disposition with only two-level are preferred, e.g.: the upper for the bus-bars system, the lower for the switchgear connections. See Figure 1 and notes on the solutions employing rigid or flexible conductors.
Figure 1 – AC substation with flexible or rigid conductor
Heavy components, such as circuit breakers and measuring transformers should be installed in line and on the side of appropriate routes for mounting, dismounting and maintenance. Usually, the maintenance should be carried out by means of equipped vehicles. In this case, it is very important to fix the width of the route and its distance from the bay taking into account the safety distances between the operator handling work tools and the live parts.
See Figure 2.
Figure 2 – Safety distances in AC substation
In order to avoid long service interruptions due to clamps or conductors failures, it is advisable that energy conductors or earth wires span do not overpass more than one busbars system, See Figure 3.
Figure 3 – Overpass with two spans
A good solution is, when possible, the adoption of line-bays on both sides of the substation. See Figure 4.
Figure 4 – Arrangements of circuit breakers (a) With circuit breakers at one side of the substation; (b) With circuit breakers at both sides of the substation
Line and transformer bays sequence should, if possible, be fixed minimising the possibility of overloading busbars or connection conductors.
It is necessary to consider in the layout design, the possibility of extension of the substation. This topic is more important in the case of substations with ring scheme busbars. A particular design allows for an easy transformation from a ring scheme to a double busbar with a 1 ½ circuit-breaker scheme.
See Figure 5.
Figure 5 – Mesh arrangement
Lightning protection is necessary to protect a substation against direct lightning strokes. This protection can be arranged in either overhead earth wires or lightning rods. It is easier to get an efficient protection bay using earth wires.
Special attention has to be paid to the elimination of the risk of earth wire fall down on the switchgear.
In order to minimise the electromagnetic interference between primary circuit and control equipment and for economic reasons (especially when cables are employed), an alternative is to decentralise the control, protection and automation apparatus.
In Figure 6, a disposition example of secondary circuits concerning control and protection systems is illustrated, taking into account the technology trends regarding energy management systems.
Figure 6 – Mesh arrangement, Section a (see Figure 7)
At present, the substation design with voltage level up to 500kV, rigid conductors is preferred as it is more simple and economic. In this case, aluminium alloy tubes are employed. For higher voltage levels, it can cause more difficulty to set up by tube bundle conductors with an equivalent diameter suitable to contain the corona effect to an acceptable limit.
Of course, the choice of the best solution is influenced by the availability of materials that change from country to country, and by different company experiences.
In the following paragraphs, the main advantages and disadvantages of both solutions are listed.
- Simplicity, easy reading of operation configuration
- Plant disposition with only two levels
- Easy access to the transformers or to the switchyard for maintenance.
- Easy use of pantograph or semi-pantograph disconnectors
- Easy substation extension
- Easy verification of electrodynamic forces effect
- Short erection time
- Lower grounding area for plant installation
- Uneasy temporary bypass of circuit-breakers on both sides of busbars.
- Possibility of mechanical resonance between the tube structure and the wind gust frequency. Can be prevented by suitable damping devices
- The difficulty with the availability of tubes and the support material in some countries.
Figure 7 – Rigid busbars
- Use of the same material employed for overhead lines
- Bundle Multiple conductors with appropriate diameter to reduce corona effect in ultra-high voltage substations are easy fulfilled
- The complex layout also for simplex schemes
- Difficult verification of withstanding to electrodynamic forces.
- Busbars overpasses are to be provided
- Considerable environmental impact consequent to three levels of conductors in the substation
- Considerable construction cost.
- Difficulty in employing pantograph and semi pantograph disconnectors
- Difficulty in substation extension.
Figure 8 – Flexible conductors
Step Voltage – The difference in surface potential experienced by a person bridging the distance of a human step without contacting any other conductive part [IEEE 80].
Touch Voltage – The maximum potential difference between the accessible earth surface and the dead part which can be touched by a hand of a person standing on the surface [IEEE 81].
Safe Current – The current which can flow through the human body without threat to the life and health of the exposed person [IEC 60479-1, -2]. Maximum step and touch voltages are set to levels that will limit the current flowing through an exposed person to the safe current level.
The proposed methods of service and repair work must be considered in the design of a substation. In most countries, the minimum clearance between live parts and personnel is standardised.
The following parameters are usually defined:
- Minimum height of live parts above the accessible surface.
- Minimum height of the lowest parts of insulators above the accessible surface.
- Minimum horizontal distance between a live part and protective rails, fences, etc.
- Minimum distance between a live part and a human body(or conductive tools) during the work in the substation.
It is also dangerous to handle a long rod with a portable earthing conductor in the vicinity of live parts. Therefore earthing switches are preferable in substations above 145 kV. Portable earthing devices are only used for additional earthing e.g. protection against induced voltage on long busbars.
Suggested reading –
All equipment installed in a substation must be designed by taking the rated power frequency voltage of the network into account. The temporary overvoltages at power frequency are often caused by the sudden loss of load or earth faults, switching overvoltages and lightning overvoltages.
In order to determine its capability it is subject to voltage tests as follows:
- Lightning Impulse Withstand Voltage. (1.2/50)
- Switching Impulse withstand voltage (250/2500)
- Power frequency (50 or 60 Hz) (wet and/or dry).
The necessary insulation level depends on the insulation co-ordination, i.e. on the properties of different parts of the network (mainly lines), the protection used against overvoltages (surge arresters ZnO are very effective), on altitude and also on the required reliability of the substation (permissible probability of flashover) and may vary in different parts of the same substation.
Suggested reading – Insulation coordination study for lightning overvoltages in 420 kV power substation
The instantaneous load flow within a substation depends on the state of the entire electrical network. Usually, a complete network analysis including the development of the network in future is required to determine the nominal values of currents flowing in an individual substation circuit.
It is theoretically possible for maximum current flow to occur with a relatively low total production of electricity in the network. e.g. the supply to a pumped storage power station or when utilising the by-pass facility within a substation.
While designing a substation it is necessary to consider the following two aspects of the effect of current:
- The thermal effect (including induced currents).
- The mechanical effect on conductive items of plant and their support structures.
It may be possible to assign a short-term current rating in excess of the nominal but the analysis leading to this must be to ensure that no “hot spots” (transformers, terminals, busbar support points) are overlooked.
Methods of calculating short-circuit current values are given in IEC Standard 909 and the effects of short circuit current can be evaluated in accordance with IEC Standard 865-1.
Further study – Practical guidance to calculation of prospective short-circuit currents on T&D systems
It is not possible to test the whole HV installation by corresponding test voltages. Therefore minimum clearances in the air between live parts or between live and dead parts in the air are stated, to obtain the required insulation level in arrangements that have not been tested. As the clearances are stated universally, they must assume the insulation in the worst case of spark-gap with sufficient reliability.
Smaller clearances are permissible if the particular arrangement has been tested by the prescribed insulation test (IEC 60071-1). The values of minimum distances to live parts in the air also depend upon practical experience and therefore, some differences can be found when comparing rules in different countries.
The specified electrical clearances must be maintained under all normal conditions. Exceptionally reduced electrical clearances may be allowed. For example, in the case of conductor movement caused by short-circuit current or by an extremely strong wind.
Suggested reading – The art of the switchyard design: Handpicked details you must consider without fail
In addition to the normal weight of apparatus, conductors, structures etc, temporary loads must be considered, especially the weight of frost and ice (depends on local climate) and loads imposed by maintenance staff access. The strain during erection must also be considered (lifting of structures, the asymmetric pull of conductor etc.).
The wind pressure may substantially influence the strain exerted on structures and footings and may also reduce the clearances between conductors (in the case of turbulent wind) or between the conductors and grounded structures (consideration should be given to insulators in V formation where problems occur).
Standard values for wind speed are recommended by IEC but local conditions must always be considered. When calculating the wind loads on bundle conductors the screen effect from the other subconductors may be taken into consideration.
Special care should be taken against the effects of aeolian vibration in rigid tubes. These vibrations are due to Von Karman vortex shedding and can be managed (damped) using either external dampers or in some cases by installing a flexible cable inside the tubes. The flexible cable acts as an impact damper (efficient if the acceleration of the movement is greater than the gravity).
Figure 9 – Panels of layered, fibreglass-reinforced plastic. Upon impact, the layers delaminate, slowing and absorbing a speeding wind impact.
Suggested video – Ballistic walls
Earthquakes occur in different parts of the world. Substation planners should consider the probability and the expected severity of a possible earthquake. Because the horizontal acceleration is about 0,3-0,5g (the vertical acceleration is less than 50% of the horizontal) and the frequency of the earthquake is 0,5-10 Hz, 275 – 500kV equipment that resonates in this frequency band may be damaged.
Bus support insulators are particularly vulnerable.
Tubular aluminium conductors are also thought to resonate during earthquakes and, if required, “dampers” or “slide supports” may be fitted. Equipment in the substation is liable to suffer from damage where the ground conditions are not stable.
Therefore, particular attention should be paid to site preparation to ensure ‘solidity’ of the ground.
Figure 10 – Circuit breakers on isolated supports after the earthquake
Figure 11 – Substation damage due to earthquake
Generally, equipment is type tested in a short-circuit laboratory to determine its dynamic behaviour with the exception of support insulators. Insulators are generally tested to meet loads in static mode. The short-circuit strength of the busbars is usually only calculated, the calculation methods being verified by testing of typical busbar arrangements.
IEC865-1 can be used both for rigid and flexible connections, the background to the calculation methods is detailed in CIGRE Brochure N105, published in 1996. Equations presented in these publications do not take into account the flexibility of insulators and this may result in over-specification of the required strength of insulators, especially where the application is of non-ceramic insulators. Advanced methods can be used for specific arrangements, to validate simple evaluations, or to limit short-circuit tests to the minimum number of configurations.
They give access to much more comprehensive data than simple methods (for example dynamic effects, supporting structure stress and strain, spacer compression, connection to apparatus, insulator and supporting structure flexibility, etc.)
In the case of insulator supports, apparatus and transformer bushings, resonance may occur. This occurs especially in substations designed for a voltage of around 150 kV, but other installations may be affected, The use of damping devices may be considered.
Always consider the two-phase isolated fault condition for clearances and the two or three-phase fault condition for maximum inter-phase stress. Where bundle conductors are used the pinch effects must be considered. Aas a minimum, reduce sub-conductor distances and avoid a ratio of sub-span length to bundle diameter in the range of 10 to 60.
Suggested course – Power System Analysis Course: Load Flow and Short Circuits
The probability of simultaneous occurrence of various mechanical forces will be dependent upon local conditions. Calculations should normally include specified combinations of :
- Wind loads on conductors and equipment
- Ice loads
- Short circuit loads
- Earthquake loads whenever necessary
- Maintenance and/or erection loads
- Weight of equipment and reaction forces
- Static conductor tension
Additionally, mechanical loads due to low ambient temperatures and circuit-breaker operations should be taken into account.
Figure 12 – Electrical workers remove the ice on a 35-kilovolt transmission tower on Qiyue Mountain in Lichuan city of Central China’s Hubei province
All devices must satisfy the specified level of radio noise. The limits of radio noise are stated by national standards. International rules are IEC-CISPR Publication 1 and IEC-CISPR Recommendation No 30.
Further study – Seven Bad Effects Of Corona On Transmission Lines
National regulations or standards generally give the permissible acoustic noise level. An acoustic study for the planned substation should be carried out to determine the acoustic conditions of the various items of equipment, chiefly the transformers and shunt reactors and their cooling equipment and, if necessary, the circuit-breakers. Within the framework of this study, the nature, distribution and number of sources of noise for the final installation and at intermediate stages has also to be considered.
If the acoustic study shows that the natural attenuation of the sound level is not enough to meet the permissible agreed noise criterion, three courses of action are possible:
- Use of a low-noise transformer
- Modifying the layout of equipment in the substation, for example by relocating the transformers or cooling equipment.
- Provision of one or more noise attenuation devices.
Suggested reading – Problems with audible substation noise and what you can do about it
All noxious materials in the substation must be used and handled without leakage. The vessels of power and instrument transformers, capacitors, coils etc. must be, where possible, leakproof. In most countries, additional measures against detrimental materials are required.
Oil pits are designed to catch some proportion of the oil (or other liquid) and to prevent oil from burning. If a central underground tank is used, it must be large enough to contain the volume of the largest oil-filled equipment, any rainwater collected since the tank was last emptied and the volume of water resulting from the operation of the water spray fire protection system (where used).
When no oil leakage occurs, the rainwater may be drained off. Otherwise, decontamination is necessary by such means as mechanical separation, filtering or chemical cleaning.
Suggested video – Substation Hardening for Future Storm Resilience
The civil design includes supporting structures, foundations, facilities (internal roads, rails, site surface…) fencing and buildings.
Supporting structures include terminal gantries and support structures for circuit-breakers, disconnectors, instrument transformers and post insulators. Whereas reinforced concrete may be used for HV substations, supporting structures of UHV substations are commonly made from welded or bolted open profile steel lattice, or tubes.
Calculation of loads is usually covered by national standards and regulations which specify safety factors and load combinations.
Suggested reading – Substation and switchyard support structures for electrical equipment
Calculation methods are given by national or company standards. Foundation dimensioning is carried out according to the loads on the structures and additional forces such as the dynamic stresses imposed by circuit-breaker operation.
Depending upon the type of soil and the loads, foundation types can be:
- Poured concrete with or without steel reinforcement
- Prefabricated reinforced concrete
- Concrete slab (mostly used in indoor substations or for GIS)
- Drilled (suitable in hard soil)
- Auger bored piles
Steel stubs or anchor bolts, to which the structures are attached, are usually cast into the foundation, a template often being used to locate such fixings prior to the concrete being poured.
Civil works for transformers or oil-filled reactors have four main purposes:
- To support the transformer during service and enable it to be moved in and out of its service position (rails may be needed depending upon transformer type).
- To retain any leakage of transformer oil. Additionally, by filling the oil containment area with gravel covered by an upper layer of broken stones or by connecting it to an underground tank, extinguishing of oil fires can be assisted.
- To reduce the risk of fire propagation (firewalls and fire stops in trenches are recommended).
- Where necessary, to reduce the propagation of acoustic noise.
Facilities for maintenance and operational needs must be taken into account in substation design. Where access by crane or trucks has to be provided for installation, maintenance or replacement operations, roads or tracks have to be constructed. The surface of the site will also influence access.
Usually, stone chippings or grass are used to reduce dust levels. Stone chippings are also effective in limiting ‘touch’ and ‘step’ voltages!
Figure 13 – Stone chippings in substation are effective in limiting ‘touch’ and ‘step’ voltages
External fencing reduces the possibility of entry to the site by unauthorised persons. Special measures are usually defined in national standards. Special attention should be paid to ‘touch’ voltages where metallic fencing is used. Internal fencing is used mostly for defining areas where access is restricted, rails or wire fencing can be used for this purpose.
Fences, buildings, transformer civil works and supporting structures can influence the aesthetic impact of the substation on the environment.
Figure 14 – Substation fence that reduces the possibility of entry to the site by unauthorised persons
The design of buildings has to conform, where relevant, to national and utility standards. Their main role is to contain and give shelter to protection relays, SCADA equipment, auxiliary equipment, battery systems, fire protection pumps etc.
For economic reasons (reduction of the length of cable runs, reducing auxiliary supply voltage, minimising first investment) several dispersed buildings rather than one central building can be built in a substation. Whether a substation is manned or unmanned will determine the extent of the facilities required locally for the operators.
Normally a substation building is provided with a septic tank. Access to the substation and the maintenance practices of the utility will determine whether or not to install a building suitable for transformer detanking.
Suggested reading – Civil engineering in installation of substation buildings and switchboard rooms
The use of fire protection systems and/or measures is mainly based on :
- Minimising the hazard for the operators and the public and protecting the environment.
- Limiting the damage to power transformers and to adjacent apparatus, equipment and buildings.
- Minimising the loss of customer’s service.
Water spray protection is generally preferred for outdoor installations, predominantly to protect the power transformers, CO2 for indoor installations. Halon is now being phased out of use in many countries because of its environmental impact. Smoke detectors (indoor), bimetal detectors and quartzoid bulbs with a detecting pipe system pressurised with compressed air at 0.25 – 0.8 MPa are the most common fire detecting elements.
Provision may be made to aid the extinguishing of burning oil under the transformer, such as :
- A 200 – 300 mm layer of broken stones on the grid above the oil-containing pit.
- A stone-filled pit.
- A steel or concrete chamber connected to the oil retaining area by pipe(s), e.g. 5 m long by 200 mm diameter.
Fire protection of cables in indoor and outdoor HV substations is usually only by passive measures to reduce the fire propagation – fire stops (concrete, steel, mineral wool, sand, silicone) and/or fire-resistant painting. In installing fire barriers care must be taken to ensure that hot spots are not introduced.
Power and control cables should be installed along separate routes e.g. separate cable racks or separate trenches.
Suggested reading – What happens when a transformer fire occurs in a substation?
To control entry to a substation by unauthorised persons, the site should be equipped with security measures. These measures serve two purposes: protection of public and personnel safety and protection of assets against loss and/or damage. Substations are a high voltage environment that can present potential safety hazards to untrained and/or unaware people.
Furthermore, the theft of copper grounding wires from perimeter fencing or from inside a substation could affect the integrity of the substation grounding system, thus potentially compromising the safety of the intruder as well as the substation staff.
Substation security measures may include:
- Fences (in some cases with electrified wires),
- Entrance/equipment locks,
- Photoelectric motion-sensing equipment,
- Video surveillance systems,
- Computer security systems,
- Lightning or landscaping.
For each substation, an assessment should be made to determine which measure and/or combination of measures are most appropriate.
Figure 15 – Fence on Walney Substation, UK
An earthing system, engineered for personnel safety, ensures that a person is not exposed to a dangerous electric voltage gradient, when in the vicinity of facilities that are connected to the earth. A personnel safety earthing system controls the effects of the temporary earthing path, established by a person exposed to a voltage gradient in, or in the vicinity of the substation.
Moreover, hazardous voltages may develop between metal structures or equipment frames that are connected to the earth, and nearby surfaces on which a person may stand.
The following circumstances can contribute to hazardous voltage gradients and risk to personnel:
- Relatively high earth fault current
- High soil resistivity
- Distribution of earth fault currents such that a significant ground-return current flows.
- Presence of an individual at such a point, time, and position that the body is bridging two points with a voltage difference
- Insufficient contact resistance to limit the current through the body to a safe value under the above circumstances
- A fault duration such that the duration of the flow of current through the human body is for sufficient time to cause harm
The safety earthing system should be engineered to limit the magnitude of the human body current by limiting the step voltage and the touch voltage during earth faults. High-speed clearing of earth faults reduces the probability of exposure to electric shock and reduces the duration of current flow through the body, which limits the severity of the bodily injury.
The allowable earth fault current value may therefore be based on the clearing time of primary protective devices or that of the backup protection.
Suggested book – Handbook for application of neutral earthing resistors (NERs) at the substation
A substation should be protected against direct strikes by lightning, where there is a significant probability that lightning discharges will occur. Please note that there is no known method of providing 100% shielding short of enclosing the equipment in a solid metallic enclosure.
The following are characteristics of the lightning phenomena that make it difficult to engineer direct stroke protection:
- The unpredictable, probabilistic nature of lightning
- The lack of data due to the infrequency of lightning strokes in substations
- The complexity and economics involved in analysing a system in detail
The uncertainty, complexity, and cost of performing a detailed analysis of a shielding system have historically resulted in simple rules of thumb being utilised in the design of lower voltage facilities.
Suggested video – Lightning storm approaching a substation
Source: General Guidelines For The Design Of Outdoor AC Substations by CIgre Working Group 23.03