3,200 views
100 kA Superconducting Transformer
In a superconducting transformer the windings, made of a high temperature superconducting material (HTS), are cooled with liquid nitrogen at about 77K so that the resistance is almost negligible. Load losses, even after adding losses from nitrogen processing, can be reduced by 50%.
The use of HTS transformers on a larger scale is economically justified and will become more attractive as cooling systems improve and the cost of liquid nitrogen production falls. Another important factor is progress in the processing of long lengths of HTS conductors.
These transformers have smaller weight and volume and are more resistant to overload but cost about 150% to 200% of the price of conventional transformers. So, in applications where weight is crucial (railway vehicles), transformers are much more “squeezed” (by forced cooling) to cut the weight. So efficiencies are much lower, and saving weight saves energy twice.
In our opinion, HTS transformers are suitable only in applications where the load losses make up a high proportion of the total losses, but are not yet ready for general use.
.
Related articles
31,129 views
Substation, Its Function And Types
An electrical sub-station is an assemblage of electrical components including busbars, switchgear, power transformers, auxiliaries etc.
These components are connected in a definite sequence such that a circuit can be switched off during normal operation by manual command and also automatically during abnormal conditions such as short-circuit. Basically an electrical substation consists of No. of incoming circuits and outgoing circuits connected to a common Bus-bar systems. A substation receives electrical power from generating station via incoming transmission lines and delivers elect. power via the outgoing transmission lines.
Sub-station are integral parts of a power system and form important links between the generating station, transmission systems, distribution systems and the load points.
MAIN TASKS
…Associated with major sub-stations in the transmission and distribution system include the following:
- Protection of transmission system.
- Controlling the Exchange of Energy.
- Ensure steady State & Transient stability.
- Load shedding and prevention of loss of synchronism. Maintaining the system frequency within targeted limits.
- Voltage Control; reducing the reactive power flow by compensation of reactive power, tap-changing.
- Securing the supply by proving adequate line capacity.
- Data transmission via power line carrier for the purpose of network monitoring; control and protection.
- Fault analysis and pin-pointing the cause and subsequent improvement in that area of field.
- Determining the energy transfer through transmission lines.
- Reliable supply by feeding the network at various points.
- Establishment of economic load distribution and several associated functions.
TYPES OF SUBSTATION
The substations can be classified in several ways including the following :
- Classification based on voltage levels, e.g. : A.C. Substation : EHV, HV, MV, LV; HVDC Substation.
- Classification based on Outdoor or Indoor : Outdor substation is under open skv. Indoor substation is inside a building.
- Classification based on configuration, e.g. :
- Conventional air insulated outdoor substation or
- SF6 Gas Insulated Substation (GIS)
- Composite substations having combination of the above two
- Classification based on application
- Step Up Substation : Associated with generating station as the generating voltage is low.
- Primary Grid Substation : Created at suitable load centre along Primary transmission lines.
- Secondary Substation : Along Secondary Transmission Line.
- Distribution Substation : Created where the transmission line voltage is Step Down to supply voltage.
- Bulk supply and industrial substation : Similar to distribution sub-station but created separately for each consumer.
- Mining Substation : Needs special design consideration because of extra precaution for safety needed in the operation of electric supply.
- Mobile Substation : Temporary requirement.
NOTE : - Primary Substations receive power from EHV lines at 400KV, 220KV, 132KV and transform the voltage to 66KV, 33KV or 22KV (22KV is uncommon) to suit the local requirements in respect of both load and distance of ultimate consumers. These are also referred to ‘EHV’ Substations.
- Secondary Substations receive power at 66/33KV which is stepped down usually to 11KV.
- Distribution Substations receive power at 11KV, 6.6 KV and step down to a volt suitable for LV distribution purposes, normally at 415 volts
SUBSTATION PARTS AND EQUIPMENTS
Each sub-station has the following parts and equipment.
- Outdoor Switchyard
- Incoming Lines
- Outgoing Lines
- Bus bar
- Transformers
- Bus post insulator & string insulators
- Substation Equipment such as Circuit-beakers, Isolators, Earthing Switches, Surge Arresters, CTs, VTs, Neutral Grounding equipment.
- Station Earthing system comprising ground mat, risers, auxiliary mat, earthing strips, earthing spikes & earth electrodes.
- Overhead earthwire shielding against lightening strokes.
- Galvanised steel structures for towers, gantries, equipment supports.
- PLCC equipment including line trap, tuning unit, coupling capacitor, etc.
- Power cables
- Control cables for protection and control
- Roads, Railway track, cable trenches
- Station illumination system
- Main Office Building
- Administrative building
- Conference room etc.
- 6/10/11/20/35 KV Switchgear, LV
- Indoor Switchgear
- Switchgear and Control Panel Building
- Low voltage a.c. Switchgear
- Control Panels, Protection Panels
- Battery Room and D.C. Distribution System
- D.C. Battery system and charging equipment
- D.C. distribution system
- Mechanical, Electrical and Other Auxiliaries
- Fire fighting system
- D.G. Set
- Oil purification system
An important function performed by a substation is switching, which is the connecting and disconnecting of transmission lines or other components to and from the system. Switching events may be “planned” or “unplanned”. A transmission line or other component may need to be deenergized for maintenance or for new construction; for example, adding or removing a transmission line or a transformer. To maintain reliability of supply, no company ever brings down its whole system for maintenance. All work to be performed, from routine testing to adding entirely new substations, must be done while keeping the whole system running.
Perhaps more importantly, a fault may develop in a transmission line or any other component. Some examples of this: a line is hit by lightning and develops an arc, or a tower is blown down by a high wind. The function of the substation is to isolate the faulted portion of the system in the shortest possible time.
There are two main reasons: a fault tends to cause equipment damage; and it tends to destabilize the whole system. For example, a transmission line left in a faulted condition will eventually burn down, and similarly, a transformer left in a faulted condition will eventually blow up. While these are happening, the power drain makes the system more unstable. Disconnecting the faulted component, quickly, tends to minimize both of these problems.
.
Related articles
5,176 views
Figure 1: Three generations of transformer substations
Early transformers were located at the top of pylons and could achieve powers of up to 1000kVA. Column-type transformer substations provided the interface between overhead and underground networks. These were equipped essentially with air-insulated MV switchgear, a liquid- insulated transformer and a low voltage distribution switchboard.
These were fabricated from bricks, and thanks to the chimney effect provided by the column the substations had good airflow, and consequently there were no problems with overheating of the equipment.
However the next generation of substations for underground networks also built from brick, had a reduced height and no chimney effect, and for the first time the equipment designers had to confront overheating problems.
This second generation of transformer substations was also the subject of the first internal fault tests, intended to provide operating personnel and the general public with greater levels of safety. The next step was the introduction of factory assembled prefabricated transformer substations.
Figure 2: 630kVA qualified by EDF, validated against internal faults in the air and easily integrated into buildings.
This third generation of transformer substations were subject to the first international regulations. These substations were characterised by the use of more compact, environmentally insensitive equipment, factory assembled and standardised which means that Utilities can be supplied with series produced products, the production and performance of which are guaranteed by the tests carried out by the manufacturers in accredited laboratories.
For the third generation substations the first Utility specifications demanded small surface areas, leading to a standardised layout of the substations (Figure 1).
This enclosure can be metallic, GRC (Glass Reinforced Concrete), GRP (Glass Reinforced Plastic), SFRC (Steel Fibres Reinforced Concrete). The world wide trend is for reinforced concrete enclosures for the following reasons :
- Improved mechanical strength
- Reduced effect of solar radiation
- Reduced condensation
- Improved fire behaviour
- Weathering
- Improved aesthetic.
Finally, a fourth generation of transformer substations (Figure 2) has recently appeared, where the detailed technical specification has been replaced by functional specification.
AUTHORS: Thierry CORMENIER, ALSTOM-France; Robert DIDES.
.
Related articles
23,315 views
Paralleling Three-Phase Transformers
Two or more three-phase transformers, or two or more banks made up of three single-phase units, can be connected in parallel for additional capacity.
In addition to requirements listed above for single-phase transformers, phase angular displacements (phase rotation) between high and low voltages must be the same for both.
The requirement for identical angular displacement must be met for paralleling any combination of three-phase units and/or any combination of banks made up of three single-phase units.
CAUTION:
This means that some possible connections will not work and will produce dangerous short circuits. See table 2 below.
For delta-delta and wye-wye connections, corresponding voltages on the high-voltage and low-voltage sides are in phase.
This is known as zero phase (angular) displacement. Since the displacement is the same, these may be paralleled. For delta-wye and wye-delta connections, each low-voltage phase lags its corresponding high-voltage phase by 30 degrees. Since the lag is the same with both transformers, these may be paralleled.
A delta-delta, wye-wye transformer, or bank (both with zero degrees displacement) cannot be paralleled with a delta-wye or a wye-delta that has 30 degrees of displacement. This will result in a dangerous short circuit.
Figure 20 – Delta-Wye and Wye-Delta Connections Using Single- Phase Transformers for Three-Phase Operation.
Note: Connections on this page are the most common and should be used if possible.
Table 1 shows the combinations that will operate in parallel, and table 2 shows the combinations that will not operate in parallel.
Table 1 – Operative Parallel Connections of Three-Phase Transformers
.
Table 2 – Inoperative Parallel Connections of Three-Phase Transformers
Wye-wye connected transformers are seldom, if ever, used to supply plant loads or as GSU units, due to the inherent third harmonic problems with this connection. Delta-delta, delta-wye, and wye-delta are used extensively at Reclamation facilities. Some rural electric associations use wye-wye connections that may be supplying reclamation structures in remote areas.
There are three methods to negate the third harmonic problems found with wye-wye connections:
- Primary and secondary neutrals can be connected together and grounded by one common grounding conductor.
- Primary and secondary neutrals can be grounded individually using two grounding conductors.
- The neutral of the primary can be connected back to the neutral of the sending transformer by using the transmission line neutral.
In making parallel connections of transformers, polarity markings must be followed. Regardless of whether transformers are additive or subtractive, connections of the terminals must be made according to the markings and according to the method of the connection (i.e., delta or wye).
CAUTION:
As mentioned above regarding paralleling single-phase units, when connecting additive polarity transformers to subtractive ones, connections will be in different locations from one transformer to the next.
.
