Jong-Beom Lee

Multitime-interval scheduling for daily operation of a two-cogeneration system with evolutionary programming

Abstract This paper proposes a multitime-interval scheduling for the daily operation of a two-cogeneration system connected with auxiliary devices, which include auxiliary boilers, heat storage tanks, electricity chargers and independent generators. The efficiency of a cogeneration system depends on the production of thermal and electrical energy which is modelled with a quadratic equation obtained by the least-squares method. Evolutionary programming (EP) is used to establish operation scheduling for the cogeneration system. For this complex scheduling problem with multivariables in multitime-intervals, the optimal operational cost for the cogeneration system obtained with EP is much lower than the initial feasible solution obtained by trial and error.

Characteristics and Reduction of Sheath Circulating Currents in Underground Power Cable Systems

Sheath current can cause sheath loss and reduce the permissible current of a power cable transmission system. High sheath current is usually caused by mixed cable burying formation, different length between sections, poor connection of the bonding leads, and connection of distribution cable onto transmission systems. This paper analyses the characteristics of sheath circulating current at various conditions and presents a useful method to reduce it in a mixed burying formation system. It can effectively reduce the current by up to 90%. The method is validated by practical measurements and simulations using ATP, and has been implemented in actual systems. In addition, a special equipment is designed to measure and analyze the sheath circulating current. It can measure and analyze nine currents at a same time.

Protective relay testing and characteristic analysis for high impedance faults in transmission lines

Abstract This paper describes the relay testing method and the results for detecting availability of protective relays under high impedance faults (HIFs) in transmission lines. Mho type distance relays, quadrilateral type distance relays and PCM type current differential relays are selected for relay testing. Four kinds of model power systems including actual power systems to be used for relay testing are established in PSCAD/EMTDC. Each protective relay is tested using RTDS and RTPS. The boundaries of each operating zone are surveyed by testing. Availability of HIFs detection on each relay are compared to each other. Impedance variation characteristics at the relay point are investigated using EMTDC in the case that source zero sequence impedance and load impedance are varied. The results shown in this paper can be used as an effective reference to install protective relays in real power systems.

A daily operation scheduling of cogeneration systems using fuzzy linear programming

This paper describes a daily operation scheduling method for industrial cogeneration systems that are connected to auxiliary devices. Such systems either sell electricity to a utility or buy electricity from the utility. Auxiliary devices connected to cogeneration systems include a heat storage tank, an electricity charger, an auxiliary boiler and an independent generator. The auxiliary devices can be effectively operated during the time interval of the thermal peak load or the electrical peak load. Simulation is performed in the case of bottoming cycle which produces preferentially thermal energy over electrical energy. In this paper, the modeling of cogeneration systems and auxiliary devices is described and the effectiveness of the modeling is evaluated by means of detailed simulations. A daily operation scheduling scheme is established under the operation cost of minimum level. Simulations are carried out using fuzzy linear programming.

Sheath current characteristic and its reduction on underground power cable system

Sheath current causes the sheath loss and can reduce the permissible current of the power cable transmission system. High sheath current is usually caused by the mixed burying formation of the cable, different length between sections, poor connection of the bonding leads, and connection of distribution cable on the transmission system. This paper analyses the characteristics of the sheath circulating current at various conditions and presents a useful method to reduce the sheath current in a mixed buying formation system. It can effectively reduce the current by up to 90%. The method is validated by the practical measurements and simulation using ATP, and also adopted in actual system. In addition, a special equipment is designed in this paper to measure and analyze the sheath circulating current. It can measure and analyze nine currents at a single time.

Development of digital distance relaying algorithm in combined transmission lines with underground power cables

This paper presents a digital distance relaying algorithm in combined transmission lines with underground power cables. When underground power cables are used as part of a transmission line, the design and setting of the protective relay will have to consider the different impedance characteristics because of the configuration characteristics of the cables. A power cable system consists of the conductor, a sheath, several kinds of joints, grounding types of such joints, cable covering protection units and grounding wires with resistance. Simulation for an actual complex transmission line with 154 kV XLPE cables was carried out to develop a correct distance relaying algorithm using modeling which was, in detail, established using EMTP and ATP Draw. This paper describes the accurate calculation algorithm of line impedance with a compensation factor at the relay point as well as a method to discriminate the fault section between both sections when a single line-to-ground fault occurs. Simulation results reveal that the accuracy of the line impedance obtained by the proposed algorithm has been increased compared with the impedance obtained using the general distance relaying algorithm.

Technical Review on Parallel Ground Continuity Conductor of Underground Cable systems

Abstract – The Induced sheath voltage is significantly increased at single point bonding section when the ground fault occurs on power cable system because there is no return path of fault current. For solving this problem, therefore, many researchers recommend the PGCC (Parallel Ground Continuity Conductor). In this paper, the characteristics of PGCC are extensively analyzed for reducing the level of induced sheath voltage at the single point bonded section for Korea underground power cable system Keywords: Parallel ground continuity conductor, Overvoltage, Underground cable 1. Introduction Generally, single point bonding section consists in arranging for the sheaths of the three cables to be connected together and earthed at one point only. In Korea, in each single point bonded section, the SVL (sheath voltage limiter) units should be connected between the unbounded ends of the cable sheaths. In this section, during a ground fault, the zero sequence fault current carried by the cable conductor cannot return by any path because single point bonded cable sheath is ground at one position only or the sheaths are separated by SVLs. Specially, the induced sheath voltage is significantly increased at this point when the ground fault occurs on power cable system because there is no return path of fault current. For solving this problem, therefore, many researchers recommend the PGCC (Parallel Ground Continuity Conductor) in single point bonded section. The PGCC is very effective for suppressing an induced sheath overvoltage because of providing an external fault current path. Accordingly, a single point bonded cable installation with PGCC which is earthed at both ends of the route is recommended. The spacing between PGCC and phase conductor should be closed to limit the voltage level during a single phase ground fault. The size of PGCC should be also adequate to carry the full expected fault current for the cable system. However, in Korea, the PGCCs are not applied for single point bonded section till now. The sheaths are just separated by SVLs. Recently, the installed SVLs in single point bonded section were exploded by single line to ground fault, which caused heavy damage to joint box. Therefore, in this paper, the characteristics of PGCC are extensively analyzed for reducing the level of induced sheath voltage at the single point bonded section for Korea underground power cable system. Firstly, the optimal installation section is estimated by evaluating of economical and technical efficiency. Next, the adequate conductor size of PGCC will be decided by maximum fault current considering an expected maximum circuit breaker capacity. The additional various cases are also considered including the dimension and the position of PGCC, spacing between PGCC and phase conductors, faulty phase, cable construction types and two PGCCs usage. Korea electric power company, KEPCO, has now plan for adopting PGCC which is suitable on Korea underground power cable system based on the results of this paper. Accordingly, the PGCC will expect to be applied on real power cable system in Korea, and it also contributes to safety service.

Evaluation of Two Lagrangian Dual Optimization Algorithms for Large-Scale Unit Commitment Problems

Lagrangian relaxation is the most widely adopted method for solving unit commitment (UC) problems. It consists of two steps: dual optimization and primal feasible solution construction. The dual optimization step is crucial in determining the overall performance of the solution. This paper intends to evaluate two dual optimization methods - one based on subgradient (SG) and the other based on the cutting plane. Large-scale UC problems with hundreds of thousands of variables and constraints have been generated for evaluation purposes. It is found that the evaluated SG method yields very promising results.

Sheath Circulating Current Analysis of a Crossbonded Power Cable Systems

The sheath in underground power cables serves as a layer to prevent moisture ingress into the insulation layer and provide a path for earth return current. Nowadays, owing to the maturity of manufacturing technologies, there are normally no problems for the quality of the sheath itself. However, after the cable is laid in the cable tunnel and is operating as part of the transmission network, due to network construction and some unexpected factors, some problems may be caused to the sheath. One of them is the high sheath circulating current. In a power cable system, the uniform configuration of the cables between sections is sometimes difficult to achieve because of the geometrical limitation. This will cause the increase of sheath circulating current, which results in the increase of sheath loss and the decrease of permissible current. This paper will study the various characteristics and effects of sheath circulating current, and then will prove why the sheath current rises on the underground power cable system. A newly designed device known as the Power Cable Current Analyser, as well as ATP simulation and calculation equation are used for this analysis.

Analysis of Frequency Response Characteristic Considering Semiconducting Screen in Underground Distribution Cables

This paper describes frequency response and propagation characteristics considering semiconducting screen in distribution cables. In CIGRE WG 21-05, Simplified Approach(SA) and Rigorous Approach(RA) which can revise the permittivity considering semiconducting screen propose for more detailed EMTP model and frequency dependant analysis. In this paper, the frequency dependent characteristics of complex permittivity are variously analysed by cole-cole function of RA. The attenuation, propagation velocity and surge impedance according to frequency range(1 kHz to 500 MHz) and cable length are also analysed by SA and EMTP simulation in distribution cables. The propagation velocity considering semiconducting screen is slower, and it is saturated over the range of 1 MHz. The signal is significantly attenuated as frequency range is high.