Power Electronics Technology that Supports Smart Grid

† Fuji Electric Co., Ltd following a parallel off (2) Isolated operation detection function An isolated operation detection function is essential for connecting to a power system. Isolated operation is the state in which an isolated system that has been disconnected from the power system is supplied with electricity from the output of a distributed power source only. In the isolated operating state, there is the possibility of electric shock or equipment damage, and this state must be detected as soon as possible and the relevant distributed power source s must be disconnected. (See explanation on page 152). The method used to detect isolated operation is either passive or active. The passive method detects sudden changes in voltage phase, frequency and the like resulting from imbalances between the generated output power and the load during the transition to isolated operation, while the active method continuously applies voltage and frequency fl uctuations and utilizes the fact that the fl uctuations become noticeable during transition to isolated operation. Presently, in power distribution systems, the load is larger than the generated power, and therefore the passive method operates reliably even when there is a transition to isolated operation. However, if the number of distributed power source s increases and the balance between the generated power and the load is realized within the power distribution system, then system fl uctuations will be smaller when transferring to isolated operation, and detection of isolated operation based on the passive method may not be possible. In the past, the passive method had provided the main protection, and the active method had been used as a backup. However, because of the risk of being unable to detect isolated operation with the passive method when a large number of distributed power source s are introduced, in recent years, the active method has been considered as the main protection. At present, especially for small-scale photovoltaic power generation, unifi cation toward an active method that is free of mutual interference is underway. Also, the trend of isolated operation of medium and large capacity power conditioning systems*2 (PCS) must be watched closely. 2.2 Function for accommodating power generation fl uctuations and power system fl uctuations In the past, generators have been controlled to absorb load fl uctuations and to stabilize frequency. If the amount of renewable energy generated fl uctuates, however, a balance between supply and demand is diffi cult to achieve with only generator control. For this purpose, the output at the renewable energy side must be adjusted to minimize the effect on the system. A power storage device is used to implement this function, and depending on the period of fl uctuation, the power storage method may need to be changed to storage cells, lithium ion batteries, electric double-layer capacitors, and the like, and appropriate discharge control technology for the storage method is also needed. If the fl uctuation in renewable energy power generation is to be adjusted with individual power stabilizers, then the same number of stabilizers as power generators (or power plants) will be needed. In contrast, an area-type stabilizer allows the fl uctuation to be averaged to that the total equipment capacity can be reduced, and is more economically effi cient than the individual approach. This area-type stabilizer controls the amount of power generation, including the amount of renewable energy, over a wide area (such as a town, city, prefecture or larger). For this purpose, the capacity of the stabilizer must be increased by expanding the individual device capacity of the inverters used in power storage systems and arranging them in parallel confi gurations. Additionally, in small-scale power systems at remote islands and the like, the generators have low inertia constant, and disturbances are likely to occur when a supply-demand imbalance arises due to a power fl uctuation. Such unstable states can be stabilized with a power storage device, and for this purpose, high-speed and high-precision control are required of the inverter. 3. Usage of Power Electronics in Smart Grids Recently, power electronics products incorporating the above technologies have become possible to manufacture, and the applicable range of power electronics technology has expanded. Additionally, complex control has become easier to implement in the distribution of energy, enabling more effi cient utilization of the public infrastructure. Figure 1 shows a conceptual diagram of a smart power distribution supply chain in a smart grid being promoted by Fuji Electric. In Fig. 1, sensors and smart meters monitor the system information, and power generation, distribution and consumption are optimized so that the system will operate more effi ciently. Fuji Electric has experience with many such examples for this purpose. Of the power electronics technology that may be used in a smart grid, implementation examples involving power generation and distribution are introduced below. 3.1 Usage of power electronics technology in power gen-