New Technologies for Electric Power Distribution Systems 158

New Technologies for Electric Power Distribution Systems to Unify Protection/Control Units so They Can Be Incorporated into Intelligent

Substations

OVERVIEW: Recently, due to concerns about the liberalization of electricitysupply, deregulation, and global impact on the environment, securing areliable power supply has become an important social need worldwide. Toensure this need is fulfilled, detailed investigations and developments are inprogress on power distribution systems and the monitoring of apparatus.These are on (1) digital technology based on the application ofsemiconductor high-speed elements, (2) intelligent substations applying IT(information technology), and (3) system configurations aimed at high-speedcommunication. Incorporated in these are demands for the future intelligentcontrol of substations, protection, monitoring, and communication systemsthat have advantages in terms of high performance, functional distribution,information-sharing and integrated power distribution management. Todaysconventional apparatus also requires streamlining of functions,improvements in sensor technology, and standardized interfaces. Bypromoting these developments, the following savings for the whole systemcan be expected: (1) reduced costs in remote surveillance in the field ofapparatus monitoring, operation, and maintenance, (2) reduced maintenancecosts based on the integrated management of equipment, and (3) reducedcosts due to space saving as a result of miniaturizing equipment.

Hiroshi Kashiwazaki

Takayuki Wakida

Masakazu Sato

Yasuo Sato

INTRODUCTIONTHE upgrading of our 500-kV trunk transmissionsystem has almost been completed, and the electricitysystem has been considerably improved. Yet, costreductions are required to cope with the entry of IPP(independent power producer) and the introduction ofpower source distributors caused by the deregulationof electric utilities. To achieve this, each electricitysupply company is decreasing expenditure byefficiently using equipment, improving operations, andeffectively controlling plant-and-equipmentinvestment. The power distribution system alsorequires reductions in initial investments, such as theunit price of apparatus and miniaturization, andreduced costs for the whole life cycle, including theoperation/maintenance costs of the substation system.The construction of a new power distribution systemhas been considered to meet these requirements. It willadopt digital technology and IT-relatedtechnology, which has made rapid advances in recentyears.

This system aims at minimizing the total cost, not

only reducing of the unit price but also the cost ofinstallation, construction, operation, and maintenance.This article discusses the construction of intelligentsubstations in the power distribution system, as wellas protection/control-unified equipment as examplesof the new technology.

INTELLIGENT SUBSTATIONConcept of Intelligent Substations

In conventional substations, substation apparatus,such as switchgear and transformer, control, protectionand monitoring equipment is independent of everyother device, and connection is based on the signalscoming through the cable. On the other hand, anintelligent substation shares all information onapparatus, control, protection, measurement andapparatus monitoring equipment through one bus byapplying both digital technology and IT-relatedtechnology. Moreover, high efficiency andminiaturization can be achieved because the localcubicle contains unified control/protection andmeasurement equipment that is one integrated system

Hitachi Review Vol. 51 (2002), No. 5 159

(see Fig. 1). Since an optical bus shares the informationbetween the apparatus and equipment, the amount ofcable is sharply reduced. Moreover, as internationalstandards (IEC 61850 and 61375 etc.) are adopted and

the system conforms to the telecommunicationsstandard, equipment specifications can be standardizedfor different vendors.

Apparatus Monitoring SystemAll the data from each monitoring and measuring

device is transmitted and used for a higher-levelmonitoring system via an optical bus. The requireddata is accessed through the Intranet or the Internet atthe maintenance site of an electricity supply companyor a manufacturer and the apparatus can be monitoredfrom a remote location. The construction, analysis anddiagnosis of the database including trend managementand history management also become possible. As aresult, signs of abnormalities can be checked out wellin advance, and prompt action can be taken in timesof emergency. Maintenance plans can also be draftedto ensure reliability, by inspecting revision descriptionand parts management, efficient maintenance planningand reliability maintenance are also realizedsimultaneously.

DEVICES THAT CAN CONTRIBUTE TO ANINTELLIGENT SUBSTATIONSwitchgear and Transformer

The burden can be drastically decreased becausethe sensor signal from the PCT is digitized at the sensoroutput edge and the load on the PCT only reaches thatof an A-D (analog-to-digital) converter. Rogowski coilsare used as the current sensors and capacitive potentialdividers are used as the voltage sensors. These sensorsdrastically reduce the size of the switchgear (see Fig.2).

Fig. 1Intelligent Substation System Configuration (Image).The whole substation system is combined by optical LAN, andapparatus composition is simplified.

Fig. 2Gas Combined SwitchgearMiniaturization by DigitalCorrespondence Sensor.550-kV GCS (gas combinedswitchgear) is shown as an example.

Power companys maintenance site

Manufacturers maintenance site

Public circuit/Internet

Supervisory control

Serial bus

InterfacePC

Artificial satellite

GPS

Time server

Web server terminal cardWeb server terminal card

Protection card

Measuring card

Control card

Apparatus monitoring card

Mobile terminal card

Optical serial process bus

Optical serial process bus

PCT sensor/optical conversion

Apparatus control sensor/optical conversion

Apparatus monitoring sensor/optical conversion

Substation apparatus

Local cubicle

GPS: global positioning systemPCT: potential current transformer

5,16

0 m

m

7,340 mm

9,740 mm 8,050 mm

(a) Conventional PCT (b) Digital PCT with polymer bushing to reduce main parts

7,350 mm

PT

CT

9,06

0 m

m

7,88

0 m

m

1,78

0 m

m

3,65

0 m

m

1,57

5 m

m

GCB

CTPT

GCBGCB: gas circuit breakerCT: current transformerPT: potential transformer

New Technologies for Electric Power Distribution Systems 160

Present studies on miniaturizing conventionalequipment have so far been aimed at standardizingseries. Advanced miniaturization will be attempted bydigitizing this system, corresponding to its need.

Protection and ControlIntelligent substations require protection and

control equipment to be installed outdoors and thisneeds to be compact so that the local cubicle is able tocontain them. Outdoor installation requiresimprovements in insulation against heat andairtightness besides parts reliability. Compactprotection and control equipment will generate demandfor unified fabrication of protection/control and high-density components. The current protection/controlsystem that uses compact equipment is describedbelow.

LATEST PROTECTION AND CONTROLSYSTEMTrends in Protection and Control Systems

Due to the rapid progress in todays informationfield, applying digital technology and adding ITfunction to the protection/control system are possible,to support stable power supply, and improvemaintenance. In Japanese protection/control systems,digitization has made advances since the last half of

the 1980s. Digital technology has unique advantages,namely minimizing maintenance and improvingreliability, and it has speeded up the conversion fromindividual analog-type to digital-type relays.

Now, however, digitization is not only required forindependent single-function equipment, but for thesystematic operation and employment of the wholesubstation. Such systems have greatly improvedefficiency in employment and maintenance using IT.The key phrases to fulfill these needs are as follows:(1) Slimming of total system as a protection controlequipment Unification of equipment(2) High efficiency of employment/maintenancesupport using IT technology Extended employment/maintenance by remotecontrol(3) System directly linked to the equipment forprotection/control Distributed installation near the apparatus

Thus, there has been a need for constructing a highefficiency system through system-wide miniaturizationand integration of IT.

Unified Protection and Control UnitThe protection and control units of the substation

are designed and allocated with respect to individual

Fig. 3Example of Combination of Optical LAN Application and Protection/Control Equipment, andProblems of Large-scale System.The present substation system consists of exclusive use of LAN (local-area network) for every information unit.

Cable connection

Transmission line

CB

CB

Transformer

Bus lineBus disconnecting CB

Bus connecting CBDisconnecting switch

DAU

DAU

DAU

DAU

TP

CU

DAU CU

DAU

DAU

LP Transmission

CU

DAU BP

For transmission line

To an opposite substation

Problems with present large-scale systems

Optical cable

For bus line

For transformer

LAN for BP

LAN for control

Monitor

SC

SC

For supervisory contro