During the past 10 years, the Korea Electric Power Corp. (KEPCO) has developed and installed two types of distribution automation systems (DAS). The first was a small DAS with traditional functionality, such as remote control, remote data acquisition and remote setting capability. This relatively simple structure, designed for small cities or rural area applications, proved successful and was installed in 173 low-grade business offices from 1998 to 2001.
In 2000, KEPCO developed a DAS with increased functionality for metropolitan areas. This total DAS was designed with a dual server structure for hot standby backup and includes more remote functions, such as feeder automation, simulation using real-time data, feeder reconfiguration for loss minimization and relay coordination.
Distribution Networks in Korea
The distribution network in Korea has an operating voltage of 22.9 kV, designed with multipoint feeder grounding that limits the fault current magnitude of each feeder to 8000 A. At present, each 22.9-kV distribution feeder has three or four automated switches, with an increase of switches per feeder planned. The total number of distribution feeders currently in commission is 6010, of which 3726 feeders (62%) will be equipped for remote automation by the end of 2003. Table 1 shows the rate at which KEPCO has installed DAS since 1998.
System designers can select the optimal communications media — optical fiber, telephone wire, wireless data communications, CDMA, twisted pair cable and trunked radio service — depending on the geographical conditions. At present, the ratio of communications media for DAS is telephone wire 48%, wireless data communication 23%, TRS 14%, optical fiber 12%, twisted pair cable 2% and CDMA 1%.
The configuration of the small DAS is simple. The control system comprises one industrial PC, two 21-inch (53-cm) monitors, and one laser printer, HUB and LAN. As the DAS has no commercial database or GIS tool, system cost is low. The system is designed for remote operation of less than 200 feeder switches. The functions of the small DAS include:
Remote supervision: open/close, lock/unlock, remote/local, gas pressure normal/abnormal, battery voltage normal/abnormal, fault indicator (instant mode/permanent mode), open line (phase missing), phase unbalance.
Remote control: open/close, lock/unlock, battery test, fault indicator reset.
Remote measurements: current (A, B, C, N), voltage (A-N, B-N, C-N), fault current (A, B, C, N).
Remote setting: minimum pickup current (phase, ground) and time delay for inrush current.
Total DAS is a complex configuration system with dual servers and raid 5 (redundant arrays of inexpensive disks) clustering dual HDD (hard disk drives). The operating software of both systems is Windows 2000. It uses database management system (MS SQL server) and middleware (BASE star) but not graphic tools for low cost. MMI displays of the small DAS are schematic diagrams, but the total DAS has such displays on geographic information system (GIS) based on a standard geographical map. Basically, total DAS has a scalable structure so the central control system can be configured using one computer or several computers (Figure 1).
Total DAS has the distributed object-oriented structure using middle-ware software; therefore, the number of client nodes has no limitation. Because the small DAS is scalable, it can be upgraded easily to total DAS.
Configuration of Total DAS
DAS equipment includes a central control system (CCS), front-end processor (FEP), communications media, feeder remote terminal unit (FRTU) and automated switches. The typical difference of configuration between small DAS and total DAS is whether or not it uses the exclusive communications equipment, such as FEP. The structural difference of CCS is whether or not it adopts the dual server system to increase the reliability. The diverse application programs, such as feeder automation program, simulation program, reporting program and MMI program, are for the user's convenience.
This system includes the dual server with clustering to prohibit data failure, dual 100M LAN, several MMI nodes, switching HUB, FEP and printers. There are two client nodes for two operators and one maintenance node for data update. One of two client nodes is used as the online operation mode, and the other is used as the simulation mode. Similar to the small DAS, there are several communication medias for total DAS, such as dedicated metallic line, fiber-optic cable, wireless data packet and trunked radio communications links.
There are many main boards in the FEP, and each of them communicates through each media individually. Should the need arise to use other communication equipment, a main board can be added. Figure 2 shows the system configuration of single server total DAS.
The basic functions for remote operation of total DAS and small DAS are identical, but the total DAS can perform the feeder automation to restore feeder faults in 2 to 3 minutes. Additionally, the simulation function includes programs to train system operators and a reporting function to edit data. Total DAS, which is designed for metropolitan areas, includes:
Remote supervision: close/open, lock/unlock, open line (phase missing), phase unbalance, fault indicator (instant/permanent), battery status, gas pressure condition.
Remote control: close/open, lock/unlock, fault indicator reset, battery test.
Remote measurements: current (A, B, C, N), voltage (A-N, B-N, C-N), fault current (A, B, C, N).
Remote setting: minimum pickup current (phase, ground), delay time for inrush current, angle of phase unbalance, voltage magnitude low limit.
System networking: SCADA-DAS for substation data, new distribution information system (NDIS)-DAS for distribution network data, total DAS-small DAS for remote control.
Other functions: fault location, isolation, service restoration, simulation for operator training, overload detection and load-transfer processing.
Interface Between DAS and SCADA
The SCADA system is designed for distribution substation facilities, and the data required to operate the DAS includes maximum transformer loads, circuit breaker status and feeder supply current. There is a direct interface between SCADA and DAS, so additional RTUs are not required. By interfacing with SCADA, the status of all facilities is obtained including the changing status of each event and the analog data acquisition of voltage and current. This information transmits one direction from SCADA to DAS.
The initial DAS systems were not capable of improving system performance because they lacked real-time data. Therefore, KEPCO optimized operations' programs to improve the efficiency of DAS, focused on decision capability and cost savings. The suite of optimal operations' programs includes: distribution diagram auto-creation, outage restoration program, protection coordination, data error detection, and otimal network reconfiguration program.
|Number of small DAS||17||66||61||29||-8||165|
|Number of total DAS||1||1||1||9||8||20|
|Number of automated switches||896||4390||2951||4519||2489||2500||17,745|
|Ratio of automated distribution feeder||5%||23%||42%||45%||50%||62%||6010|
The features and capability of these programs are illustrated by the following applications:
Single-line feeder diagram auto-drawing from GIS network map. Generally, system operational staff uses the GIS-based distribution network diagram, but it's complex and difficult to follow. This program derives a single-line diagram from the geographic network map and DAS status information (Figure 4).
Feeder automation program. In general, KEPCO's distribution feeders are divided into four sections and interconnected to three adjacent feeders by automatic switches. This program can detect the faulty section via the information received from the fault indicator (FI) installed on the distribution network, identifying the location of the faulty section on single line diagram (Figure 5). It also provides the autoload transfer function for no fault areas by initiating a switching schedule of automated switches for service restoration. This load transfer decision is provided by fuzzy logic based on the switching numbers, margin of power supply capacity, load balance and voltage drop limitation.
Protection coordination program. The control system acquires real-time data from automated devices on distribution networks. The database manages all types of data that includes the length and characteristics of the feeder, the load on each section, and the operating times of protective relays and reclosers to ensure protection coordination. In the past, operators had to assume the level of protection coordination using expertise and experience to overcome the incomplete data records. However, total DAS calculates the fault current and automatically sets the timing of relays and reclosers to establish coordination of the feeder protection. Setting the operating times for protection devices can be changed by remote control via the diagram for protection coordination and the topological map on the control center monitor screen. The time settings for the majority of automatic devices can be obtained by remote controlling techniques, by the system control operator remotely setting the parameters, or by confirming the results from the inverse time/current characteristics of protection devices (Figure 6).
Optimal feeder reconfiguration program. To minimize the distribution network power loss, the program database must contain the cross-sectional area and length of distribution line, and the load on each section of the feeder between automated switches. Furthermore, the loss minimization program calculates the load current loss of each section, calculates the total feeder loss, and searches for the loss minimization solution of the distribution network. This process results in the closure of the normal open feeder points and the creation of “temporary open points” pending location and repair of the feeder fault. Finally, this program presents the operator at the control center with a list of switching operations to change the normal open point. During the process, the program considers voltage drop limitations and feeder loadings to identify the optimal solution.
To minimize power loss and load balance, algorithms based on natural selection and laws of inheritance are used for the application. The optimal feeder reconfiguration program follows this sequence:
Estimation of networks: calculation for power loss and voltage drop.
Load equalization operating between lines: load balance by network reconfiguration.
Power loss minimization of distribution network: power loss minimization by network reconfiguration.
Result of analysis output: operation sequence of switches, comparison with load amount and power loss after network reconfiguration.
As a result of undertaking six switching operations, the network power loss on six feeders supplying the Gangdong business office was reduced from 178.4 kW to 139.4 kW (22%) — a significant cost savings.
Total DAS, developed in August 2000, is now in operation at 12 branch offices in a large city following two years of field testing. This system will be expanded to another eight business offices by the end of 2003 by upgrading from the small to total DAS. The reliability performance of these feeder systems will be enhanced with the installation of application programs, including a feeder reconfiguration program for loss minimization and optimization of system capacity.
Although the basic functions of remote monitoring and remote control provide no measurable cost benefit, the suite of application programs developed and now being commissioned by KEPCO will continue to offer this utility substantial savings in system operational costs, improved system reliability and customer service. Because KEPCO has been responsible for developing this DAS, it has the potential for nationwide application with minimal capital investment.
Bok-nam Ha received the MSEE degree from Chungnam National University, Korea, and joined KEPCO in 1978. He is currently working for KEPRI, the Research Institute for KEPCO, where he has been the principal member and leader of the project on distribution automation since 1990.
Ieel-ho Seol is qualified as a professional engineer in the fields of generation, transmission and distribution. He joined KEPRI in 1987, where he is a senior researcher. Ieel-Ho is also a senior member of the Distribution Technology Group.