Substations and Distribution Substations Overview (Photo by Ipco Group)

http://electrical-engineering-portal.com/high-voltage-substations-overview-1 April 24, 2013

High Voltage Substations Overview (part 1)

Emmanouil Angeladas

IntroductionHigh voltage substat ions are interconnection points within the power t ransmission anddistribut ion systems between regions and countries.

Different applications of substations lead to high voltage substations with and withoutpower transformers:

1. 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)

2. Transform voltage levels within the high voltage system (HV/HV)

3. Step down to medium voltage level of a distribut ion system (HV/MV)

4. Interconnect ion in the same voltage level

Scope

High Voltage Substation

High voltage substat ions comprise not only the high voltage equipment which is relevant forthe funct ionality in the power supply system.

High voltage substations are planned and constructed comprising high voltage switchgear,medium voltage switchgear, major components such as high voltage equipment andtransformers, as well as all ancillary equipment such as auxiliaries, control systems, protect iveequipment and so on, on a turnkey basis or even as general contractor.

The installat ions supplied worldwide range from basic substat ions with a single busbar tointerconnect ion substat ions with mult iple busbars, or a breaker–and–half arrangement forrated voltages up to 800 kV, rated currents up to 8,000A.

A and short circuit currents up to 100 kA.

Circuit conf iguration

High voltage substations are points in thepower system where power can be pooled fromgenerat ing sources, distributed and transformed,and delivered to the load points.

Substat ions are interconnected with each other,so that the power system becomes a meshednetwork. This increases the reliability of thepower supply system by providing alternatepaths for f low of power to take care of anycont ingency, so that power delivery to the loadsis maintained and the generators do not faceany outage.

The high voltage substat ion is criticalcomponent in the power system, and thereliability of the power system depends upon thesubstat ion. Therefore, the circuit conf igurat ionof the high voltage substat ion has to be selected carefully.

Busbars are the part of the substat ion where all the power is concentrated from the incomingfeeders, and distributed to the outgoing feeders. That means that the reliability of any highvoltage substat ion depends on the reliability of the busbars present in the power system.

An outage of any busbar can have dramat ic ef fects on the power system.

An outage of a busbar leads to the outage of the transmission lines connected to it .

As a result , the power f low shif ts to the surviving healthy lines that are now carrying morepower than they are capable of . This leads to t ripping of these lines, and the cascading ef fectgoes on unt il there is a blackout or similar situat ion.

The importance of busbar reliability should be kept in mind when taking a look at the dif ferentbusbar systems that are prevalent.

Protective measures

The protect ive measures can be categorized as personal protect ion and funct ional protect ionof the substat ions.

Personal protection

1. Protect ive measures against direct contact , i.e., through appropriate covering,obstruct ion, through suff icient clearance appropriate posit ioned protect ive devices, andminimum height

2. Protect ive measures against indirect touching by means of relevant earthing measures inaccordance with IEC 61936/DIN VDE 0101 or other required standards

3. Protect ive measures during work on equipment, i.e., installat ion must be planned so thatthe specif icat ions of DIN EN 50110 (VDE 0105) (e.g., f ive safety rules) are observed.

Functional protection

1. Protect ive measures during operat ion, e.g., use of switchgear interlocking equipment

2. Protect ive measures against voltage surges and lightning strikes

3. Protect ive measures against f ire, water and, if applicable, noise

Stresses

1. Electrical stresses, e.g., rated current, short circuit current, adequate creepage distancesand clearances

2. Mechanical stresses (normal stressing), e.g., weight, stat ic and dynamic loads, ice, wind

3. Mechanical stresses (exceptional stresses), e.g., weight and constant loads insimultaneous combinat ion with maximum switching forces or short circuit forces, etc.

4. Special stresses, e.g., caused by installat ion alt itudes of more than 1,000 m above sealevel, or by earthquakes

Arrangement and modules

High Voltage Substation Elements (photo f rom Idec Group)

Arrangement

The system is off the enclosed 1-phase or 3-phase type.

The assembly consists of completely separate pressurized sect ions, and is thus designed tominimize any danger to the operat ing staf f and risk of damage to adjacent sect ions, even ifthere should be trouble with the equipment.

Rupture diaphragms are provided to prevent the enclosures f rom burst ing discs in anuncontrolled manner. Suitable def lectors provide protect ion for the operat ing personnel.

For maximum operating reliability, internal relief devises are not installed, because thesewould af fect adjacent compartments.

The modular design, complete segregat ion, arc-proof bushing and plug-in connect ions allowspeedy removal and replacement of any sect ion with only minimal ef fects on the remainingpressurized switchgear.

Busbars

All busbars of the enclosed 3-phase or the 1-phase type are connected with plug from theone bay to the next.

Circuit breakers

ABB - High voltage dead-tank circuit breaker 362 kV, max. 63 kA362PMI

The circuit breakers operateaccording to the dynamicself-compressionprinciple. The number ofinterrupt ing units per phasedepends on the circuitbreaker’s performance.The arcing chambers andthe circuit breaker contactsare f reely accessibly.

The circuit breakeris suitable for out-of-phaseswitching and designed tominimize overvoltages. Thespecif ied arc interrupt ionperformance has to beconsistent across the ent ireoperat ing range, f rom line-charging currents to fullshort circuit currents.

The circuit breaker isdesigned to withstandat least 10 operations(depending on the voltagelevels) at full short circuitrat ing.

Opening the circuit breakerfor service or maintenanceis not necessary. Themaximum tolerance forphase displacement is 3ms,that is, the t ime between the f irst and the last pole’s opening or closing.

A standard station battery that is required for control and tripping may also be used forrecharging the operat ing mechanism.

The drive and the energy storage system are provided by a stored energy spring mechanismthat holds suff icient energy for all standard IEC close-open duty cycles.

The control system provides alarms signals and internal interlocks but inhibits t ripping or closingof the circuit breaker when the energy capacity in the energy storage system is insuff icient orthe SF6 density within the circuit breaker drops below the minimum permissible level.

Disconnectors

Transmission line disconnect switch (photo by Ef rem Oshinsky @ Flickr)

High voltage outdoor earthing switch (126kV, 252kV) - Chint ElectricCo.,Ltd.

All disconectors(isolators) are of thesingle-break type.

DC motor operat ion(110, 125, 220 or 250 V)which is fully suited toremote operat ion, and amanual emergencyoperat ing mechanism areprovided. Each motoroperat ing mechanism isself -contained andequipped with auxiliaryswitches in addit ion tothe mechanicalindicators.

The bearings arelubricated for life.

Earthing switches

Work-in progress earthingswitches are generallyprovided on either side ofthe circuit breaker.Addit ional earthing switchesmay be used to earthbusbar sect ions or othergroups of the assembly.

DC motor operation (110,125, 220 or 250 V) that isfully suited for remoteoperat ion and a manualemergency operat ingmechanism are provided.Each motor operat ingmechanism is self -contained and equippedwith auxiliary posit ionswitches in addit ion to themechanical indicators. Thebearings are lubricated forlife. Make proof high-speedearthing switchesare generally installed at thecable and overhead lineterminals.

SF6 gas insulated high-voltagecurrent transf ormer (72.5 -

800 KV) - Trench Group

They are equipped with a rapid closing mechanism to provide short circuit makingcapacity.

Instrument transformers

Current transformers (CTs) are of the dry type design. Epoxyresin is not used for insulat ion purposes. Voltage transformersare of the induct ive type, with rat ings up to 200 VA.

Cable terminations

1-phase or 3-phase, SF6 gas insulated, metal enclosed cableend housing are provided. The cable manufacturer has tosupply the stress cone and suitable sealings to prevent oil orgas from leaking into the SF6 switchgear.

The cable end housing is suitable for oil type, gas-pressuretype cables with plastic insulation (PE, PVC, etc.).

Addit ionally, devices for safety isolat ing a feeder cable andconnect ing a high voltage test cable to the switchgear or cablewill be provided.

Overhead line terminations

The terminat ions for connect ing overhead lines come complete with SF6-to-air bushings butwithout line clamps.

Control and monitoring

As a standard, an electromechanical or solid-state interlocking control board is suppliedfor each switchgear bay. This fault -tolerant interlocking system prevents alloperat ing malfunct ions.

Mimic diagrams and posit ion indicators provide the operat ing personnel with clear operat inginstruct ions. Provisions for remote control are included. Gas compartments are constant lymonitored by density monitors that provide alarm and blocking signals via contacts.

To be continued…

Re fe re nce s:

- SIEMENS Substat ions Guide - Andreas Gout is, ‘Electrical drawing, Part 1’