III Improvement of Reliability

3. Improvement of Fault Prevention

3-1 Prevention of Tree Contact

3-1-1 Tree Trimming

Unless all cable is completely insulated or installed underground, their is no technical alternative to prevent trees from contacting electrical facilities other than securing sufficient clearance between trees and electrical facilities. Against the backdrop, PLN has been trimming trees steadily by using outsourcing company or having workers climb trees to trim branches with saw or sickle. However, given the limites number of maintenance resources, trees are not trimmed in time, resulting in failures or power lines. In addition, trees are trimmed in an unorganized manner without any consideration of aesthetics. Since the distribution lines of Surabaya and Bali use connection, power can still be supplied if one phase is grounded.

However, if high voltage cable keeps contacting trees continuously of single phase ground fault is escalated to double line ground fault or line-to-line short circuit fault by snow or rain, power system facilities may be impacted significantly. Therefore, trees must be trimmed before they grow and in a range where trees may contact distribution lines if they fall down under snow or rain involving heavy rain to secure sufficient clearance with distribution lines.

When trimming trees, KEPCO considers aesthetics as well to reduce complaints of local neighbors and meet the permission requirements of authorities having jurisdiction. KEPCO has also engaged landscaping contractors from several years ago to overcome the constraints of its own maintenance resources and make it a rule to trim trees in February when trees have not yet started growing in full consideration of types and characteristics of trees to be trimmed.

3-1-2 Partial Use of Insulated Wire & Aerial Bundled Cable

In places where sufficient clearance between trees and distribution lines cannot be secured, insulated wire or aerial bundled cable is used instead of bare conductor.

1. Use of covered conductor (ACSR-OC, ACSR-AW-OC)
   Covered conductor does not guarantee complete insulation, however, it is an effective way to prevent fault by temporary contact with foreign object, and aerial cable is a completely insulated cable which can perfectly prevent fault by foreign object contact. As accessary materials including connectors are developed, it is effective to install covered conductor wire on the entire distribution lines and completely insulate live part by covers, however, covered conductor is expensive compared to bare conductor, For this reason, partial installment can be considered for areas where clearance cannot be secured by other ways.
2. Use of aerial bundled cable
   Aerial bundle cable is consisted of cable and messenger wire. They are twisted as a bundle and installed on a pole and the system uses fully insulated cable and equipment. While covered conductor is effectively preventing fault by temporary tree contact, it is not effective enough to prevent faults by continuous tree or branch contact.
   PLN already uses overhead cable but, in most cases, such overhead cable is installed as an alternative to configure additional lines in the face of line capacity shortage rather than to prevent faults arising out of contact with trees. Therefore, Guidelines for overhead cable installation and operation must be defined clearly and the connection between overhead wire and overhead cable is barely insulated, which is highly likely to result in safety accidents or faults. When using covered conductor or aerial cable, in order to enhance insulation effect it is more effective to use covers of dead-end clamp, sleeve, bushing, and stirrup on exposed live part.

3-1-3 Use of Curved Steel Pole

Curved steel pole was developed to secure clearance from buildings or trees. It can prevent pole with alley arm from leaning by continuous unbalanced load. It can also prevent excessive use of materials for alley arm assembling including cross arm and insulator, and help preserving beauty of the surroundings.

Another strength of curved steel pole is that it can avoid bumping into other pipes built under the ground. There are two types of curved steel poles, a pole whose lower part is curved and a pole whose mid part is curved. Both should maintain strength during its life span. SPS 400, SPS 490 are mainly used materials; materials should have enough bending strength and vertical strength. When installing curved steel poles, pole block should be attached.

3-2 Surge Arresting Arrangement

3-2-1 Concept of Lightning

Insulation levels of distribution facilities are lower than those of generation plant, substation, and transmission line, indirect stroke as well as direct stroke of lightening adjacent to distribution line may result in faults. Also, fault may be induced by reverse current of lightening stroke hitting load facilities as observed in distribution lines supplying loads on top of mountain. Lightening damages to distribution system can be classified into indirect stroke, direct stroke and reverse current of lightening.

3-2-2 Installation of Overhead Ground Wire

Overhead ground wire is intended to prevent reverse flashover and arrest indirect stroke when hit by lightening stroke. When directly hit by lightening stroke, overhead ground wire prevents accident by letting lightening current flow to the ground through itself. As for indirect stroke, it also arrests indirect stroke surge by combining current flowing along itself with phase conductor and creating cross-induced voltage. Therefore, overhead ground wire is effective in preventing such accidents as wire breaking or insulator damage induced not only by indirect stroke but also by direct stroke.

3-2-3 Installation of Surge Arrester

Surge arrester is intended to protect line equipment such as distribution transformer, switch and auto recloser, etc. and to prevent faults induced by indirect lightening stroke but not capable of preventing flashover. Surge arrester can divert indirect stroke surge up to 500kV.

As gap type Surge arrester’s discharging voltage is lowered if water finds its way into it and discharge may occur at normal operating voltage leading to momentary fault, gapless type is more advantageous. Polymer Surge arresters deserve consideration as they are also reliable.

3-2-4 Concurrent Installation of Overhead Ground Wire and Surge Arrester

To prevent flashover induced by direct/indirect lightening strokes, it is desirable to install Surge arrester and overhead ground wire together. While Surge arrester can provide full protection against indirect stroke, overhead ground wire alone can protect more than 38% of indirect strokes. As for direct stroke, Surge arrester cannot protect facilities from it while overhead ground wire can provide more than 30% of protection.

3-3 Improvement of Distribution Line Patrol, Inspection & Measurement

Line patrol is intended to prevent accidents or faults of distribution lines, to maintain safety and to perform required actions at accident or fault spots. Given the line conditions, PLN seems not to maintain a fault prevention patrol along its lines. The reason why patrol to prevent fault is not performed adequately in PLN seems to be that PLN’s line patrol focuses mainly on post factual accident and fault recovery.

3-3-1 Distribution Line Patrol

Scheduled line patrol is essential to fault prevention. To prevent faults efficiently, responsible persons for each line or arear need to be assigned and they need to be held responsible for inspecting lines in assigned area by pre-specified schedule, identifying issues and making sure that such issues are addressed depending on their urgency.

It is important that all employees patrol lines at scheduled interval to promote their understanding of significance of line patrol and reporting system be in place to ensure that fault-prone facilities are fixed in advance. In addition, when a fault occurs, it must be clarified if such a fault could have been prevented and line patrol or inspection was not done adequately to punish or reward responsible employees and adequate patrol techniques need to be trained on all relevant employees.

1. Maintain sufficient patrol resources
   Maintaining adequate level of patrol resources is critical to line patrol. Patrol manpower available at the moment does not seem to be sufficient to cover maintenance and fault prevention workload. To ensure effectiveness of maintenance activities, it seems necessary that patrol workload required annually be estimated and sufficient manpower be secured according to the estimated workload.
2. Introduce patrol jurisdiction system
   It is recommended that adequate line patrol jurisdiction be assigned and persons be held responsible for patrolling their own jurisdiction. It is effective to assign patrol jurisdictions by feeder. If a fault attributable to poor patrol occurs in given jurisdiction, penalty needs to be given to responsible employee and incentive given to him or her if he or she detects and prevents potential faults during patrol.
3. Develop patrol plan
   Annual and monthly patrol plan should be developed for each individual patroller and line patrol should be performed consecutively from feeder to terminal in given jurisdiction in accordance with applicable patrol plan. Patrol plan is expected to enhance patrol efficiency.
   Patrol should be performed at planned frequency and feedback from patrol reflected as required on patrol plan of following month or year.
4. Develop line patrol guideline
   It is critical to develop line patrol guideline to ensure systematic line patrol. Guideline can standardize work activities. Standardization by guideline is expected to further enhance effectiveness of line patrol activities.

3-3-2 Distribution Line Inspection

Inspection is performed to examine in more detail line or facility conditions that are hard to be checked by line patrol sufficiently. Inspection should be planned on a monthly or annual basis and performed according to schedule. In addition, inspection should be performed periodically.

Prior to facility inspection, tools and materials required for diagnosis of various distribution facilities, replacement or repair of poor facilities need to be prepared. Among diagnostic equipments used by KEPCO at the moment, thermo vision or spot thermometer is very effective in finding overheated spots. In addition, insulator detector is very instrumental in identifying defective insulators.

3-3-3 Distribution Line Measurement

Distribution line measurement is performed to maintain rated voltage and distribution line-connected equipment in normal conditions. Measurement should be performed periodically according to schedule or on a ad-hoc basis as required.

Voltage drop or voltage unbalance among phases is measured by voltage indicator or voltage recorder and voltage is adjusted according to measured data. Current is measured by probe current meter or hook-on current meter and load is adjusted according to measured data.

Other items such as grounding resistance are measured as specified in applicable guideline.

3-4 Use Facility Diagnosis Instruments Actively

Electrical facilities installed in distribution lines are bound to have their own service life and subject to failure once their service life expires. Therefore, replacing facilities adequately in advance before they fail due to deterioration over time is one of the techniques to reduce power system faults. PLN are short of maintenance manpower and resources required for patrolling and inspecting power system facilities in a systematic manner at the moment. Notably, as PLN maintains power system facilities on its own in many cases, it is difficult for PLN to assign time and efforts to power system patrol and inspection. Moreover, even if PLN engineers patrol power system facilities in accordance with applicable patrol and inspection standards, their effectiveness in detection of facility deterioration is limited as they rely only on visual inspection.

Deterioration diagnosis instruments are designed to detect energy radiated from the surface of power system facility in infrared waveform and display it in different colors subject to the intensity of heat radiated from the surface of power system facility. Such instruments were used for military applications initially and utilized for industrial use after the late 1950s. Since then, they have been extremely instrumental for preventative maintenance of power system facilities for several dozens of years. Deterioration diagnosis instruments are capable of inspecting power system facilities even when power cables are electrified and applicable to inspection of not only overhead facilities such as cable connection, insulator or COS but also ground-based facilities such as ground transformer, switch or connection of riser or underground cable.

3-5 Improvement of Protective Coordination System

Installation of protective devices on distribution line should be preceded by thorough review of their coordination. And, thorough understanding of time-current characteristic curve is a prerequisite to a review of protective coordination. In addition, sufficient knowledge of structure, actuation principles of such relays as OCR or OCGR and their actuation sequence is a must. Furthermore, line variables such as distribution of loads connected to distribution line, status of faults observed on distribution line and special or critical customers connected to distribution line should be studied in advance.

Reclosers installed in distribution lines can recover momentary interruptions automatically with reclosing function and reduce scope of interruption by isolating sections impacted by sustained interruptions that occur on load side of recloser. In that case, actuation time and reclosing function of CB should be considered together so that protection provided by recloser which is a primary protection device and CB which is a backup protection device may be well-coordinated.

3-6 Preventing the Theft of Grounding Wire & Neutral Conductor

Thieves are targeting grounding wire and neutral wire mostly made of copper which is more expensive than aluminum. As society evolves, theft of power system component such as electric wire may decrease gradually, but, not to the point of complete extinction. Notably, as theft of grounding wire or neutral conductor happens mostly in the field or rural area where population density is low than in urban area, it is hard to catch thieves. In addition, even if line patrol is strengthened, patrol area is expensive and theft usually happens at night when there are few people on the street, it is very difficult to find theft cases. Therefore, electric wire theft reporting compensation program is introduced herein as a reference for prevention of grounding wire and neutral conductor theft. Furthermore, to make sure that an electric wire theft reporting compensation program takes firm roots, placards announcing the introduction of electric wire theft reporting compensation program must be posted in electrical wire theft-prone areas to promote such program actively to the general public.

KEPCO’s electric wire theft reporting compensation program targets reports of theft of KEPCO’s power system component theft or equivalent event and compensation is payable when a criminal is caught. Compensation amount is determined up to 10% of the applicable value of lost electrical wire. The applicable value of lost electrical wire is estimated in reference to the unit contract price of new wire and exclusive of labor cost and other expenses by standard.

KEPCO’s electrical wire theft report accepting and processing process is as described in the following figure. When witnessing or knowing the theft of electrical wire, reporter files a report to KEPCO or police, notifying details. Police catches the thief and theft-impacted subsidiary valuates loss amount. In 30 days from the arrest of thief, compensation deliberation committee finalizes compensation amount and applicable branch pays corresponding amount to the reporter.

3-7 Preventing Faults Attributable to Facilities of Customer, Other Company

3-7-1 Outline of Propagation Fault

Propagation fault refers to fault propagated from electrical facilities owned by customers or other 3rd parties and they are broken down to temporary fault and momentary fault. Temporary fault refers to a fault that takes more than 5 minutes to recover from the moment when it is propagated to power system while momentary fault refers to a fault that takes less than 5 minutes.

Customers who are responsible for propagation faults must be notified in the name of the local PLN office director of the details of fault propagation, the cause of fault and the maintenance performed on power system facilities and that they may be deprived of electrical power unless they make improvements that they are supposed to make. Customers must report to PLN in writing within a month if they have performed improvements as notified and PLN must confirm such improvements on site to follow up on relevant faults.

Inspections of power-receiving facilities owned by customers per year must be planned by the frequencies and aligned to the frequency of propagation faults that broke out in the previous year to prevent propagation faults and power-receiving facilities including protection facilities of customers must be inspected to prevent fault propagation in advance. Customers who propagate faults frequently must be subject to in-depth inspection using thermal imaging instruments as well as visual inspection.

3-7-2 Conducting Preventative Diagnosis of Power-Receiving Facilities of High-Voltage Customers

1. Diagnosing customer’s power-receiving facilities with thermal imaging system
   First of all, KEPCO extends supports to customers in regard to the inspection of their power-receiving facilities, utilizing the thermal imaging diagnosis system. Many high-voltage customers are fairly reluctant to check their own facilities, paying separate costs and tend to respond to faults ex post factum, replacing or maintaining problematic facilities. If faults occur in the facilities of such customers, such faults propagate to KEPCO’s facilities as well.
   Therefore, PLN will be able to catch two birds with a stone by extending supports to such customers to realize customer satisfaction, reduce interruptions and improve supply reliability in the end. Before 2006, thermal imaging diagnosis service for customer’s power-receiving facilities was available mostly for high-voltage Important customers, but, KEPCO extended the service scope to include all high voltage customers in 2007.
   Depending on the vulnerability of high voltage customer, each branch selects customers independently and diagnoses their facilities by priority. Selection and prioritization criteria include frequency of fault occurrence, age of facilities and outdoor power-receiving facilities.
   Branches which own thermal imaging diagnosis systems utilize in-house resources as much as possible and other branches outsource diagnosis service to 3rd party contractors.
2. Conducting underground feeder cable deterioration diagnosis for high-voltage important customers

   For most high voltage customers, most feeder lines are installed as underground cables. Therefore, inspecting not only power-receiving facilities on the premise of customer but also feeder cables is important in preventing faults on customer’s side.

   Hence, KEPCO provides deterioration diagnosis service for underground feeder cables in high voltage important customers in order to enable customers to maintain their own facilities actively. Service scope includes underground feeder cables in high voltage important customers which have been in operation for 20 years or more since the beginning of transmission in consideration of the service life of feeder cable.

3. Diagnosing power-receiving facilities of fault-prone customers in depth

   KEPCO also extends more active supports to fault propagation-prone customers and customers who have caused temporary fault propagation in recent 3 years.

   Thermal imaging diagnosis is conducted on their power-receiving facilities in the same manner as other high voltage customers and the critical components in their power-receiving facilities are also inspected in depth additionally.

   In addition, they are provided with stronger inspection services including lightening arrester leakage current measurement in live wire state, etc. than ordinary high voltage customers as KEPCO tries hard to prevent faults propagated by customers. The service scope includes thermal imaging diagnosis of power-receiving facilities and live wire lightening arrestor leakage current measurement.

   As stated above, thermal imaging diagnosis is performed by in-house resources or 3rd party contractors and lightening arrestor inspection is conducted by KEPCO’s facility management department resources.

3-8 Preventing CSP Transformer Failure

CSP (Completely Self Protected) transformer is a new type of transformer developed by the U.S. Westing House in the 1970s. It has built-in fuse on the primary winding side inside to prevent transformer failure and built-in circuit breaker on the secondary winding side to block fault current from low voltage line. In addition, lightening arrestor is added to the outer case of transformer to protect transformer from surge on power side. Accordingly, CSP transformer was a fairly innovative technology back then and gained currency in other countries over time.

Korean transformer manufacturers began developing CSP transformer models as well and exported many of them to Southeast Asian countries such as Thailand.

KEPCO began to use self diagnosis transformer and completely self protected transformer in mid 1990s. Upfront, KEPCO was full of expectation that the new transformer models would reduce transformer failures drastically and shorten outages to improve supply reliability.

However, as the transformers were put in operation for multiple years, several issues began to emerge as opposed to the expectation. Notably, in case of self diagnosis transformer, supply reliability deteriorated because of the following reasons. Followings are the issued discovered in the operation of self diagnosis transformers.

(1) Poor Prevention of Internal Transformer Failure

Bimetal is designed to be activated based on the gap in thermal expansion coefficient between different metals (copper-tungsten). Blocking characteristics of bimetal based on the gap in thermal expansion coefficient required activation temperature to be sustained for a certain period of time and did not provide adequate protection against internal failure that progressed abruptly, resulting in leakage of insulation oil. In addition, bimetal was installed on the secondary winding side of transformer and somewhat inadequate in terms of preventing internal transformer failure on power side. And bimetal was supposed to be activated at a certain temperature based on bimetal calibration temperature plus ambient temperature and insulation oil temperature, but, it was not activated in some internal transformer cases when ambient temperature and load status were within prescribed parameter range.

(2) Lack of Effective Load Management

When self diagnosis transformer of higher capacity is used for a transformer with low utilization ratio, applying self diagnosis transformer of higher capacity which is expensive without insufficient internal failure prevention arrangement simply to prevent overloading just because load fluctuates significantly in the installation area is inefficient. Using self diagnosis transformer for a transformer whose utilization ratio is as high as 90 100% simply has transient effects and incurs additional costs for interruption attributable to tripping by overload and replacement to higher capacity transformer.

(3) High Defect Ratio

Self diagnosis transformer suffers higher defect ratio than other types of equipment/materials.

For example, in defect ratio of self diagnosis transformer in 2001, self diagnosis transformer suffered the most number of defects in KEPCO in 2001 and the defect ratio was about 3.7%, which is significantly higher than the defect ratios of other types of equipment/materials and inflicting a lot of impact on line operation.

Many of such issues emerged not only in KEPCO but also in the U.S. – the origin of self diagnosis transformer. Electrical utilities adopted self diagnosis transformer for many of its advantages in the early days, but, most utilities are not operating any more additional self diagnosis transformers these days. The underlying idea was fairly good, but, there still seem to be many technical challenges that are still outstanding. To sum up the operating results to date, KEPCO no longer uses self diagnosis transformer and nullified the specification as well. Self diagnosis transformers currently in operation will remain so until their service life expires.

And completely self protected transformers are used only in ABC lines limitedly. Therefore, PLN needs to analyze the failure status of CSP transformers in detail and, if CSP transformers are found to have built-in constraints, consider stopping the use of CSP transformers in line with the global trend.

3-9 Improving 20kV Bus Type

Bus type has influence on the maintenance of facilities on the power side of distribution line within substation. In case of PLN, the bus on the primary (transmission) side within distribution substation is 150kV and the one on the secondary (distribution) side after main transformer uses 20kV. 150kV transmission line bus uses double bus while 20kV distribution line bus uses radial bus. Bus configuration type is the most important foundational element in substation construction plan along with substation configuration. In a substation, bus is responsible for concentrating and distributing power flow and deserves careful selection in consideration of reliability, cost-effectiveness and flexibility in system operation, depending on the role and location that the applicable substation takes in power system.

As for substation bus configuration type, reliability, cost-effectiveness and flexibility in system operation and ease of maintenance must be reviewed comprehensively to ensure that bus function is exercised fully from power source system to distribution system and a type well aligned with power system configuration must be selected. Substation bus configuration types can be divided into radial bus, ring bus and double bus types.

Each bus type has its own merits and demerits respectively. To sum up, their pros and cons are as follows:

(1) Double Bus 1 CB Type

Applicable lines must be tripped for line circuit breaker to be inspected.

6 lines and half of bus are interrupted if circuit breaker fails to break and the whole substation is tripped if Bus Tie circuit breaker fails to break.

If 1 bus out of two fails, 6 lines and half of bus are interrupted but power system can be operated without line interruption after bus switching operation.

(2) Double Bus 1.5 CB Type

Line circuit breaker may be interrupted without involving applicable line interruption.

If circuit breaker on bus side fails to break, half of the bus connected with applicable line is tripped and, if central circuit breaker fails to break, 2 lines are tripped.

If 1 bus out of 2 fails, lines are not interrupted and half of bus is interrupted.

(3) Double Bus 4 Bus Tie Type

Applicable line must be tripped for circuit breaker to be inspected.

If circuit breaker fails to break, 3 lines and 1/4 of bus are tripped and, if Bus Tie circuit breaker fails to break, 6 lines and 1/2 of bus are tripped. Transmission line outlet must be placed in crossing fashion ( in case of 1000kV line in Japan) with bus sectionalizing circuit breaker in the center to prevent power system separation for route fault. In addition, if a Bus Tie circuit breaker fails and the other Bus Tie circuit breaker fails to break, 9 lines and 3/4 of bus are tripped.

If 1 bus out of 4 fails, 1/4 of bus is tripped and power system can be operated without line interruption after bus switching operation.

(4) Double Bus 2 CB Type

Circuit breaker can be inspected without interruption of applicable line.

If circuit breaker fails to break, lines and 1/2 of bus are always tripped and, if a circuit breaker on #1 bus side fails and the other circuit breaker on #2 bus side fails to break, the entire substation is tripped.

If 1 bus out of 2 fails, 1/2 of bus is tripped but without line interruption.

There seems to be no issue in relation to 150kV transmission bus since PLN has already adopted double bus configuration. However, since PLN uses radial bus for 20kV distribution line bus, it is sensible to migrate to double bus in the future which is relatively cost-effective and efficient. If it is difficult financially and technically to replace the existing radial bus in substation with double bus, it may be worth considering applying double bus to new substations over time. And migrating to double bus on 20kV distribution line side incrementally when necessary budget is available and there is no technical issue in the future will make significant contribution to improving supply reliability.