Until recently, power distribution networks have operated in a simple, unidirectional flow. Rising overall consumption and a growing share of renewable generation are two factors changing these operational needs. Wind power and solar generation often take place closer to customers, making power networks multidirectional. To operate these growing unidirectional and multidirectional distribution networks in an efficient and economical way, a more comprehensive approach is essential that involves new smart grid technologies for monitoring and automating the distribution network.
Many power distribution networks are, or soon will be, under threat from unpredictable, inclement weather conditions or aging infrastructures. In addition, renewable power generation such as photovoltaic (PV), wind power and biomass are starting to directly feed into distribution networks. Consumers are evolving and the distribution network is changing into a multidirectional network that requires new automation to prevent overload situations and outages. There are, therefore, new challenges to address:
• Prevention of outages to avoid losses of earnings and/or penalties,
• Reduction of power losses,
• Maximized utilization of medium-voltage (MV) equipment (capacity utilization and life span),
• Low-voltage (LV) stabilization to stay within tolerances,
• Demand response or better modulation according to power production, and
• Asset management.
An ideal approach to distribution automation would not target only one of these challenges, but would address all major application issues.
Reduction of Brownouts or Blackouts
Two main factors should be considered: the reduction of the outage frequency and the reduction of the outage duration. Currently, the use of remote terminal units (RTUs) has reduced outage durations by nearly 50 percent on average. Adding spring-loaded or motor-driven load breakers can reduce the outage duration to minutes.
The RTU, combined with fault indicators, informs the substation automation system of the short circuit current flow on the faulty segment. Based on network topology and related algorithms, the substation automation system in the primary station will automatically isolate the faulty section and close the normal open switch in the ring structure. By reclosing the infeed switch at the primary substation, the procedure for service restoration will be completed. This usually takes just one to two minutes.
It is now known that using intelligent electronic devices (IEDs) with combined protection and RTU functionality is the next level of automation, and there are new systems that are able to clear a distribution fault and re-establish power from another source in less than 300 milliseconds.
The Need for “New” Protection Schemes
In a radial network, the load situation is normally very clear. With the installation of distributed power generation such as wind generators, solar panels or biomass, however, the load supplied by the substation is reduced. Today’s fault indicators are not able to handle such situations effectively, and traditional over-current protection relays can’t clearly operate.
Keeping in mind that more than 80 percent of outages are usually caused by the MV distribution network, a selective protection system should be applied using communication to verify infeed and output of distribution networks sections.
For these communications, a networking ability is required so the IED can send information to neighboring stations. The use of network-based communication and the International Electrotechnical Commission (IEC) 61850 architecture can do even more. Every IED in the network has all necessary network switch position information. With preconfigured algorithms, the IED will automatically reconfigure the network in case of a fault, and a service restoration is carried out within milliseconds.
Reduction of Losses and Maximized Equipment Utilization
IEDs can also help optimize the controller directly and record load curves for future planning. The knowledge of low and peak demand times in daily or monthly reports helps track the load and predict overload situations. IEDs provide integrated automatic transformer monitoring and will prevent transformers from being damaged by overloads, supply data for controlled overload situations and give a clear usage report.
PV solar panels and wind power increases decentralized power generation. The unpredictable production brings problems into the LV distribution network, and, at the same time, the power quality norms have narrowed the tolerances.
Two strategies currently apply to handle the situation:
• The use of distribution transformers with tap changer, and
• The use of controllable PV inverters.
Distribution transformers are simplified and cost driven but are unable to support the new situation with decentralized power production. New types of distribution transformers offer an electronically supported tap changer for uninterrupted tap changer control.
Since an IED is knowledgeable of the current network and load situation, it can operate tap changer control and transformer monitoring as an integrated solution. In addition, the IED can control PV inverters as a LV network master at the same time.
The advantage of using PV inverters is the ability to control active and reactive power, since they don’t need to operate at the same power factor. This also applies to MV distribution with large PV generation to take over some of the voltage regulating tasks from capacitor banks.
On average, both applications, tap changers and PV inverters, allow 25 percent more load handling on the primary equipment—allowing new investment in cable or overhead lines to be avoided or prolonged.
An IED with its existing capabilities is a desirable choice. All necessary measured values of the network can be sampled by using an IEC 61850 protocol from distributed sensors or directly from inverters or electronic meters.
For the communication network, a narrow-band power line carrier can be used for automatic meter reading. Alternatively, wireless communication systems can be used, including multiple address system (MAS) radio systems—Universal Mobile Telecommunications Service (UMTS) and General Packet Radio Service (GPRS)—and utility owned cell towers. Newer communication strategies include WiMAX and broadband power lines to interconnect areas with LV and MV distribution for feeder automation.
Demand response is a mechanism to manage consumption in response to supply conditions, which ideally drives consumer behavior to reduce their consumption at peak times. To use all the energy that solar or wind power can supply, electrical load has to follow generation as much as possible.
In the future, electricity storage will allow an IED to directly control solar and wind power based on actual demand, forecast and energy efficiency algorithms. In addition, an IED will be involved in electric vehicles (EVs) entering the grid, act as a microgrid controller to optimize load cycles in a decentralized way for home energy management, and act as a conduit for building energy management systems to get their input.
Because of the lack of available online data, asset management strategies bank on power distribution equipment’s installation date and the vendor’s specification, even though much of the installed equipment has reached or exceeded its lifespan. Missing but valuable information for condition monitoring are live values, such as trips or overload situations. With a thermal model, current load situation and temperature sensors, an IED is able to calculate hot spots. Having this information available, the overload situation can be handled properly without damage to the equipment. The additional information provided by an IED will give clearer indications to schedule maintenance and refurbishment plans, allowing utilities to keep maintenance costs at bay and prevent outages.
Distribution networks are diverse because of the technology available when they were built. This challenges today’s technology to cope with a variety of existing equipment, applying cost efficient solutions for sensors, actuators and especially communication to meet the needs of tomorrow. This means changing traditional RTUs into IEDs with local network management competence.
IEDs and their special applications are the answer for today’s needs, applicable for tomorrow’s requirements. With smart sensor interfaces they can integrate nearly every existing MV and LV installation. Smart actuators are able to upgrade the majority of MV switches to be operated remotely. Communication capabilities based on IEC 61850 architectures, Distributed Networking Protocol/Internet Protocol (DNP/IP), peer-to-peer to other IEDs, to remote sensors and PV controllers or EV charging stations provide the necessary grid intelligence needed to manage a modernized grid.