Deformation and/or displacement of transformer winding are hazardous because electromagnetic force generated by a short-circuit current may increase. Frequency Response Analysis (FRA) has the possibility to detect these abnormalities with high sensitivity. In this paper, fundamentals and diagnosis method of power transformers by FRA are introduced. Examples of applications of FRA such as detection of deformation of winding due to short-circuit tests, detection of disconnection electrostatic shield, and detection of displacement of HV winding are introduced. Furthermore, recent researches related to interpretation of change of transfer functions are introduced.
Nowadays, a lot of sensitive electronic equipment is widely used in modern power systems such as power converters and adjustable speed drivers. Voltage sags have gained more interest due to their consequences on the performance of sensitive equipment (SE). Malfunction or failure of the equipment that leads to work or production losses can be caused by voltage sags. As a result, it is essential to have information on equipment sensitivity. If the magnitude and duration of voltage sag exceed the equipment sensitivity threshold, the equipment can malfunction, and such a consequence can affect an entire automatic process, resulting in high economical losses. Reclosers and fuses are the main overcurrent protection devices in distribution systems. Poor coordination may adversely impact on the sensitive equipment. This paper presents a method to analyze impacts of voltage sags and protection coordination on sensitive equipment. A fault position method and mathematical equations describing characteristics of protection devices are used to calculate voltage sags and the protection settings. Voltage tolerance thresholds and protective device characteristics are used to analyze protection and sensitive equipment coordination. Based on the results, new settings for protective devices can be adjusted for sensitive equipment in distribution systems. The Roy Billinton Test System (RBTS) bus 2 is used to analyze impacts of voltage sags and protection coordination.
In this paper, fundamentals of the transmission-line-modeling or matrix (TLM) method are briefly explained, and procedures to incorporate lumped-circuit elements such as a resistor, an inductor, and a capacitor in TLM computations are described. Then, the method is applied to analyzing surges on conductor systems including lumped-circuit elements. The surge waveforms calculated using the TLM method agree well with the corresponding waveforms calculated using the finite-difference time-domain (FDTD) method.
This paper describes experimental results of flashover characteristics of medium-voltage insulators to support an insulated cable which is used in Japanese power distribution lines. The experimental study is targeted for direct lightning strokes to a shielding wire or top of a reinforced concrete pole in the distribution line. Lightning impulse voltage is applied to a metallic cross arm to represent the direct lightning stroke. 50% flashover voltage, voltage vs. flashover time characteristic and a photograph of the discharge are measured. This paper investigates the flashover characteristics of three types of cable support system such as suspension support, dead-end support and strain support. The flashover characteristics are dependent on the supporting system. Surface discharge on the insulated cable due to the flashover is observed, causes the discrepancy of the flashover characteristics. This paper presents constants in the integration method, which is a flashover model and can consider the influence of applied voltage waveform in order for accurate lightning surge analysis.
PV, being one of the carbon-free energy supply technologies, has a feature of variation of its output which will have a substantial impact on the demand-supply balance of a power system. This paper proposes how we can modify the existing power system demand-supply balance planning technology to incorporate the PV penetration into the power system.
This paper presents a novel method to evaluate replacement plan for aged thermal power plants under uncertain circumstances through a real option approach. The most economical plan is selected among the three options: an option to operate an existing oil-fired thermal plant, an option to mothball it, and an option to abandon it and to construct an advanced gas combined cycle power plant (ACC) at the same time. Basic ideas of our model are: we use quadranomial approach in order to evaluate an option value consisted by two different uncertain assets; we consider cash flow with a dividend in order to reflect conditions of an aged oil-fired thermal plant and use the sequential compound option approach; we evaluate replacement time using quadranomial decision tree taking into account the options. We also analyze value and time of replacement using numerical examples. Our proposed method will be practically used for generation planning. For example it is possible to make priority quantitatively in replacements of aged thermal power plants by real option values. The target year of replacement may be set as a year when cumulative probability of replacement becomes over certain level.
Along with widespread distributed generators that use natural energy, power flow control of the distribution systems has become complicated. In particular, when a system failure occurs, distributed generators are disconnected and power flow changes rapidly. This causes concerns about supply voltage deviation from its regulated value, distribution line overload, etc. To meet this, it is required to have an operational control technology that allows rapidly taking measures for the power flow change in the event of a system failure while successively catching up with the generated output of distributed generators. The authors indicate a future concept that allows for distribution system operational management considering distributed generators, and propose, as a technology to achieve this concept, a method to estimate the operating status (whether in operation or shutdown) of co-generating systems. With this method, the operating status of a co-generating system installed by a consumer is estimated based on the power flow value measured at the consumer's inlet. The proposed method has two estimation methods, i.e. one is a “power factor method” for consumers without power factor correction capacitors and the other is a “power factor/total power method” for consumers with power factor correction capacitors. This has led to estimating the operating status of a co-generating system, which was impossible thus far, with a percentage of correct estimations of almost 90%.
We have investigated the performance of the silicone oil as alternative oil to the mineral oil that is used as an insulation medium of the oil immersed transformer. There are various methods of evaluating the performance, we especially investigated the breakdown characteristics and the streaming electrification characteristics. In the breakdown characteristics, the insulation performance under the influence of changing the temperature, and the electrode shape was investigated. Moreover, the insulation performance in the composite insulation system that was composed of the insulation oil and the oil immersed insulator was investigated. From these results, we found that in the oil gap model, the breakdown voltage of silicone oil was lower than that of mineral oil by 15%. In contrast, in the composite insulation system, breakdown voltage of combination with silicone oil is higher than that of combination with mineral oil. In the streaming electrification characteristics, the difference of the amount of electrification under the influence of changing the kinds of solid insulators and the temperature was investigated. As a result, we found that silicone oil has the maximum of the amount of electrification at a high temperature compared with mineral oil.