Transformers and Substations Handbook 2014

age or low-voltage level; in some cases even directly into the high voltage network. Another essential feature of renewables like wind power and photovoltaics is the stochastic availability. This has a great influence on the requirement for the network to manage the overall power, especially in terms of an existing distribution system. • Changed direction of energy flow • Changed cable load • Higher short-circuit currents • More difficulties with power quality • Additional demand for balancing energy • Changed requirements on the protection concept As electricity grids evolve, power consumption will tend to follow power generation rather than vice versa. Take the simple future exam- ple of electric cars that could be charged or operated at night drawing on inexpensive wind power. This means a ‘grid’ paradigm shift: from unidirectional energy flows to bidirectional power flows. This will not be possible without the key module of the future Smart Grid, namely the intelligent transformer substation that takes into account this new trend and still enables automatic and fast fault clearance whilst allow- ing active load management in secondary distribution systems. Statistics from power supply companies referring to supply inter- ruptions at the end customer show that most supply interruptions are caused by failures in the medium-voltage system. The grids are far from the ideal of 10 minutes of annual outage per cus- tomer. In reality, many regions of the world have outage times ranging from hours to days. Therefore, as secondary transform- er substations are usually not equipped with communication links to the network control centres, monitoring of faults as well as remote control are not possible. This can cause long supply in- terruptions, restricting the reliability and security of supply. Fault detection is also impaired by the long distances to the secondary transformer substations, which leads to even longer outage times. Generally, the proce- dure for fault clearance requires a lot of time and a large number of personnel. A highly qualified service expert has to drive to many substations to identify the fault prior to supply- ing all customers with power again. The re- sulting financial ineffi- ciencies for utilities by not supplying energy to households and companies mean that there is a great need for intelli- gent and automated Smart Grid solutions. The last section of this article delves a little deeper into the Intel- ligent Substation to give the uninitiated reader a flavour of the engi- neering issues involved to bring a higher degree of ‘intelligence’ to bear. Typical medium-voltage systems for the secondary distribution level now need to include a decentralised power supply on the medi- um-voltage and low-voltage side. The key data for the circuit breaker

Higher use of the power systems, variable load flows due to decen- tralised power generation from renewable energy sources and a growing need for information of the regulating authorities, place high- er demands on fault detection and acquisition of system operating data. This translates into higher intelligence in transformer substations moving from a merely passive substation to the future of complete automation. For example, with a gas insulated medium-voltage switch- gear, electrical engineering companies like Siemens offer the basis for intelligent transformer substations. These are optionally equipped with motorised operating mechanisms, short-circuit indicators and voltage detecting systems, as well as a variety of other sensors. They are plug connected to a Remote Terminal Unit (RTU) in a separated wallmount- ing cabinet, the switchgear fulfilling all preconditions for integration in an intelligent network infrastructure. So, Smart Grid technology allows further differentiation in intelli- gence levels and communication to the telecontrol system to meet the requirements of future intelligent transformer substations. In the past there was only one principle: “power generation follows load” resulting in the paradigm of one direction of energy flow: from the power plant to the consumer. As mentioned previously, much has changed, renew- able energies are generated in a decentralised way according to suita- ble locations. Energy is usually fed into the network at the medium-volt-

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Transformers + Substations Handbook: 2014