Substations and Distribution Substations Overview (Photo by Ipco Group)
Introduction
High voltage substations are interconnection points within the power transmission and distribution systems between regions and countries.
Different applications of substations lead to high voltage substations with and without power transformers:
- Step up from a generator voltage level to a high voltage system (MV/HV)
- Power plants (in load centers)
- Renewable power plants (e.g., windfarms) - Transform voltage levels within the high voltage system (HV/HV)
- Step down to medium voltage level of a distribution system (HV/MV)
- Interconnection in the same voltage level
Scope
High voltage substations comprise not only the high voltage equipment which is relevant for the functionality in the power supply system.
The installations supplied worldwide range from basic substations with a single busbar to interconnection substations with multiple busbars, or a breaker–and–half arrangement for rated voltages up to 800 kV, rated currents up to 8,000A.
A and short circuit currents up to 100 kA.
Circuit configuration
High Voltage Substation
High voltage substations are points in the power system where power can be pooled from generating sources, distributed and transformed, and delivered to the load points.
Substations are interconnected with each other, so that the power system becomes a meshed network.
This increases the reliability of the power supply system by providing alternate paths for flow of power to take care of any contingency, so that power delivery to the loads is maintained and the generators do not face any outage.
The high voltage substation is critical component in the power system, and the reliability of the power system depends upon the substation. Therefore, the circuit configuration of the high voltage substation has to be selected carefully.
Busbars are the part of the substation where all the power is concentrated from the incoming feeders, and distributed to the outgoing feeders. That means that the reliability of any high voltage substation depends on the reliability of the busbars present in the power system.
An outage of any busbar can have dramatic effects on the power system.
As a result, the power flow shifts to the surviving healthy lines that are now carrying more power than they are capable of. This leads to tripping of these lines, and the cascading effect goes on until there is a blackout or similar situation.
The importance of busbar reliability should be kept in mind when taking a look at the different busbar systems that are prevalent.
Protective measures
The protective measures can be categorized as personal protection and functional protection of the substations.
Personal protection
- Protective measures against direct contact, i.e., through appropriate covering, obstruction, through sufficient clearance appropriate positioned protective devices, and minimum height
- Protective measures against indirect touching by means of relevant earthing measures in accordance with IEC 61936/DIN VDE 0101 or other required standards
- Protective measures during work on equipment, i.e., installation must be planned so that the specifications of DIN EN 50110 (VDE 0105) (e.g., five safety rules) are observed.
Functional protection
- Protective measures during operation, e.g., use of switchgear interlocking equipment
- Protective measures against voltage surges and lightning strikes
- Protective measures against fire, water and, if applicable, noise
Stresses
- Electrical stresses, e.g., rated current, short circuit current, adequate creepage distances and clearances
- Mechanical stresses (normal stressing), e.g., weight, static and dynamic loads, ice, wind
- Mechanical stresses (exceptional stresses), e.g., weight and constant loads in simultaneous combination with maximum switching forces or short circuit forces, etc.
- Special stresses, e.g., caused by installation altitudes of more than 1,000 m above sea level, or by earthquakes
Arrangement and modules
High Voltage Substation Elements (photo from Idec Group)
Arrangement
The system is off the enclosed 1-phase or 3-phase type.
The assembly consists of completely separate pressurized sections, and is thus designed to minimize any danger to the operating staff and risk of damage to adjacent sections, even if there should be trouble with the equipment.
Rupture diaphragms are provided to prevent the enclosures from bursting discs in an uncontrolled manner. Suitable deflectors provide protection for the operating personnel.
The modular design, complete segregation, arc-proof bushing and plug-in connections allow speedy removal and replacement of any section with only minimal effects on the remaining pressurized switchgear.
Busbars
All busbars of the enclosed 3-phase or the 1-phase type are connected with plug from the one bay to the next.
Circuit breakers
ABB - High voltage dead-tank circuit breaker 362 kV, max. 63 kA 362PMI
The circuit breakers operate according to the dynamic self-compression principle. The number of interrupting units per phase depends on the circuit breaker’s performance. The arcing chambers and the circuit breaker contacts are freely accessibly.
The circuit breaker is suitable for out-of-phase switching and designed to minimize overvoltages. The specified arc interruption performance has to be consistent across the entire operating range, from line-charging currents to full short circuit currents.
Opening the circuit breaker for service or maintenance is not necessary. The maximum tolerance for phase displacement is 3ms, that is, the time between the first and the last pole’s opening or closing.
A standard station battery that is required for control and tripping may also be used for recharging the operating mechanism.
The drive and the energy storage system are provided by a stored energy spring mechanism that holds sufficient energy for all standard IEC close-open duty cycles.
The control system provides alarms signals and internal interlocks but inhibits tripping or closing of the circuit breaker when the energy capacity in the energy storage system is insufficient or the SF6 density within the circuit breaker drops below the minimum permissible level.
Disconnectors
Transmission line disconnect switch (photo by Efrem Oshinsky @ Flickr)
All disconectors (isolators) are of the single-break type.
DC motor operation (110, 125, 220 or 250 V) which is fully suited to remote operation, and a manual emergency operating mechanism are provided. Each motor operating mechanism is self-contained and equipped with auxiliary switches in addition to the mechanical indicators.
The bearings are lubricated for life.
Earthing switches
High voltage outdoor earthing switch (126kV, 252kV) - Chint Electric Co.,Ltd.
Work-in progress earthing switches are generally provided on either side of the circuit breaker. Additional earthing switches may be used to earth busbar sections or other groups of the assembly.
DC motor operation (110, 125, 220 or 250 V) that is fully suited for remote operation and a manual emergency operating mechanism are provided. Each motor operating mechanism is self-contained and equipped with auxiliary position switches in addition to the mechanical indicators. The bearings are lubricated for life. Make proof high-speed earthing switches are generally installed at the cable and overhead line terminals.
They are equipped with a rapid closing mechanism to provide short circuit making capacity.
Instrument transformers
SF6 gas insulated high-voltage current transformer (72.5 - 800 KV) - Trench Group
Current transformers (CTs) are of the dry type design. Epoxy resin is not used for insulation purposes. Voltage transformers are of the inductive type, with ratings up to 200 VA.
Cable terminations
1-phase or 3-phase, SF6 gas insulated, metal enclosed cable end housing are provided. The cable manufacturer has to supply the stress cone and suitable sealings to prevent oil or gas from leaking into the SF6 switchgear.
Additionally, devices for safety isolating a feeder cable and connecting a high voltage test cable to the switchgear or cable will be provided.
Overhead line terminations
The terminations for connecting overhead lines come complete with SF6-to-air bushings but without line clamps.
Control and monitoring
As a standard, an electromechanical or solid-state interlocking control board is supplied for each switchgear bay. This fault-tolerant interlocking system prevents all operating malfunctions.
Mimic diagrams and position indicators provide the operating personnel with clear operating instructions. Provisions for remote control are included. Gas compartments are constantly monitored by density monitors that provide alarm and blocking signals via contacts.
To be continued…
References:
- SIEMENS Substations Guide
- Andreas Goutis, ‘Electrical drawing, Part 1’