Electrical loads both generate and absorb reactive power. Since the transmitted load often varies considerably from one hour to the next, the reactive power balance in a grid varies as well. This can result in unacceptable variations in voltage, including voltage depression or even voltage collapse.
Like SVC but faster, STATCOM continuously provides variable reactive power in response to voltage variations, supporting the stability of the grid. STATCOM operates according to voltage source converter (VSC) principles, combining unique PWM (pulse width modulation) with millisecond switching. STATCOM functions with a very limited need for harmonic filters, contributing to a small physical footprint. If required, switched or fixed air core reactors and capacitors can be used with the VSC as additional reactive power elements to achieve any desired range.
Installing a STATCOM at one or more suitable points in a grid will increase power transfer capability by enhancing voltage stability and maintaining a smooth voltage profile under different network conditions. Its ability to perform active filtering is also very useful for improvements in power quality.
Hitachi Energy's offerings in the high power STATCOM field include SVC Light STATCOM as well as PCS 6000 STATCOM, where PCS 6000 STATCOM is applicable for unit ratings up to 40 Mvar and SVC Light STATCOM is applicable for ratings exceeding 40 Mvar.
SVC Light® STATCOM
SVC Light is a VSC concept, based on a chain-link modular multilevel converter (MMC), particularly adapted for power system applications. Physically, SVC Light can be considered a voltage source behind a reactance. It generates and absorbs reactive power by electronically processing voltage and current waveforms in the VSC, rendering unnecessary to include physical capacitor and reactor branches for generating/absorbing reactive power. It is capable of yielding a high reactive power input to the grid more or less unimpeded by possible suppressed grid voltages, and with a high dynamic response.
This is particularly useful, for instance, to support weak grids and to improve the performance of large wind farms under varying grid conditions, as well as of grids loaded by a large percentage of air conditioners in hot and humid climates. SVC Light´s effective rating and speed of response is unequalled. For instance, for high speed rail systems and heavy industrial plants it is applied for voltage balancing of asymmetrical loads, mitigating voltage flicker created by electric arc furnaces, and active harmonic filtering.
IGBT (Insulated Gate Bipolar Transistors) are key components in SVC Light. The multilevel chain-link solution is built up by linking H-bridge modules in series with one another to form the phase legs of the VSC. Fig. 1a shows a single H-bridge with four IGBTs, and Fig. 1b shows a configuration in which four H-bridge modules make up each of the three phase legs.
SVC Light is available for system voltages up to 69 kV and converter ratings of up to -/+ 360 Mvar. For higher voltages, a step-down transformer is used to connect SVC Light to the grid. SVC Light provides a symmetrical operating range. For asymmetrical operations and in order to optimize performance, thyristor-switched reactors and capacitors are operated in parallel to form hybrid solutions.
A photograph of modular H-bridge units, two stacks of four modules each, is displayed in Fig. 2. The compactness of the IGBT valves, the absence of physical capacitor and reactor branches for generating/absorbing reactive power, plus the minute need for harmonic filters enable a very compact lay-out of SVC Light.
PCS 6000 STATCOM
PCS 6000 STATCOM is an efficient power system package specifically designed to be connected to demanding networks. The flexibility of the system allows it to be applied to a wide range of applications such as dynamic voltage stabilization, voltage balancing of asymmetrical loads, mitigating voltage flicker created by electric arc furnaces, and active harmonic filtering.
The PCS 6000 STATCOM is particularly competitive in terms of installation time and space requirements. Furthermore, the high efficiency and low maintenance lead to low operational costs. The PCS 6000 provides outstanding performance for both steady state and dynamic operation as a pure static device with no switched passive elements.
The PCS 6000 STATCOM solution is virtually maintenance free, due to the minimized number of moving mechanical parts. Outages for inspection can therefore be scheduled according to the customer’s needs and preferences. No rotating parts in the energy path result in extremely high reliability, availability and low operational costs combined with long life expectancy. The equipment is typically installed in a pre-assembled container. The STATCOM system including controls, power electronics and cooling unit are tailor made for fulfilling the customer specific requirements.
PCS 6000 converter
The PCS 6000 STATCOM uses advanced IGCT technology (Integrated Gate Commutated Thyristor) that has been developed by Hitachi Energy from a proven bipolar semiconductor background (GTO). The PCS 6000 converters are based upon IGCT PEBB (Power Electronic Building Block). This standardization allows systems to be realized with lower engineering and design costs and therefore affecting the customer’s end cost in a beneficial way. The innovative connection configuration allows for very good harmonic performance with typically no need for a harmonic filter.
Hitachi Energy has ensured that the design concept of the modular container system reduces on site construction time to a minimum. The converter equipment is pre-assembled and tested to the highest standards, and is shipped to site as a container package ready for immediate installation. Thanks to the high degree of standardization, the defined interfaces on site commissioning work can be reduced to a minimum. Due to the modular design the system can even be relocated to a new site within a short time.
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