Integrating renewable energy sources into smart grids – Part three

July 19th, 2019, Published in Articles: Energize

There is global momentum to diversify electricity generation sources for economic, environmental and security reasons. Distributed generation (DG) enables more companies to enter the electricity market stimulating competition, encourages the development of renewable energy (RE) and limits the impact of events such as cyber-attacks. However, conventional electricity distribution networks are poorly matched to the needs of DG because they were designed to meet the demands of large generating capacity sited close to population centres.  

This is the final part of the article and deals with the use of synchrophasors, islanding prevention, communications and HV support. Part one was published in the June issue; and part two was published in the July issue.

Industrial and domestic users are already compromising power supply quality in contemporary grids. Automatic circuit reclosers (ACRs) can assist utilities in meeting their power quality obligations – which will become increasingly tougher as more renewable energy powered distributed generation (RE DG) is added to the grid. The ACRs can record power supply data for customer-determined durations, sections of feeder and user base. Once the data collected from the ACR is analysed, waveforms are displayed and harmonics identified for all three phases, allowing the utility to quickly react to problems.

Synchrophasors

ACR manufacturers are working hard to incorporate synchrophasor technology into their products. Such technology will be critical if ACRs are to become practical alternatives to substations for interfacing RE DG to the grid. Synchrophasor technology is not yet commercially available but is likely to be a standard feature of the next generation of ACRs.

Synchrophasors have become increasingly relevant since a 2004 US-Canada investigation recognised that many of North America’s major blackouts have been caused by “inadequate situational awareness” for grid operators and recommended the use of synchrophasors to provide a real-time, wide-area grid visibility.

A synchrophasor is a time-synchronised measurement of a quantity described by a phasor (which includes magnitude and phase information). A phasor is a complex number that represents both the magnitude and phase angle of voltage and current sinusoidal waveforms at a specific point in time. (See Fig. 1.) Synchrophasors provide a real-time measurement of electrical parameters from across the power system and can be combined to provide a precise and comprehensive overview [19].

Fig. 1: Synchrophasors are time-synchronised measurements of magnitude and phase information.

Phasor measurement units (PMU)

These devices measure voltage and current and then derive parameters such as frequency and phase angle. Data reporting rates are typically 30 to 60 Hz but may be higher. In contrast, current supervisory control and data acquisition (Scada) report rates are slower (4 to 6 Hz). The precise timing of synchrophasor measurements allows rapid identification of details such as oscillations and voltage instability that cannot be seen directly from Scada measurements.

Synchrophasors enable utilities to reduce the number of outages and restore power more quickly in the event of failure. In a 2014 study by the California Energy Commission forecast that the use of synchrophasor technology would save US$260-million in net present value annualised benefits, taking into account avoided customer outages and reduced electricity costs [20].

Islanding prevention

Other applications of synchrophasors include adaptive protection, real time-monitoring and -control, but perhaps the important application of the technology is islanding detection. Traditional methods for islanding detection use local voltage and frequency information. However, local detection schemes cannot detect islanding in a timely manner if the power (real and reactive) mismatch between the source and the local load is small.

Other traditional schemes rely on circuit breaker status communication, open-phase detectors and trip commands to detect islanding and isolate the source. Such schemes are simple in concept but must be adapted to topology changes in the power system. These adaptation requirements can result in a system with many communications links and poor reliability.

Another limitation to traditional approaches is the inability to scale with future requirements. For example, present standards require disconnection for sagging voltage under high demand. With a small amount of generation, such a requirement is reasonable, but disconnecting a high-density PV generation source would aggravate the low voltage level rather than mitigate the problem.

In contrast, synchrophasors provide precise wide-area measurements and a means for detecting islanding under nearly all load and generation conditions. (ACRs equipped with synchrophasor technology would detect early signs of islanding and switch to swap generation sources or shed loads to prevent an island forming.) Operators gain better situational awareness and can better detect oscillations and more closely track voltage stability.

Synchrophasor technology can also be extended to allow PV generation to improve low-voltage conditions under heavy loading or to provide power for an islanded set of customers – an important consideration as greater PV generation is incorporated into the grid [21].

Communications

Reliable communications are vital for the precise control of the data acquisition (DA) that will be needed to integrate RE into the electricity grid. Smart grids leverage established wired- and wireless-communication technologies such as the cellular network and the Internet in addition to local area networks (LAN) like Ethernet and Wi-Fi.

For example, Noja Power’s RC15 Scada-ready controller for its OSM series ACRs incorporates a cellular network modem which supports 2G (such as GSM), 3G (UMTS) and 4G (LTE) mobile communications network technologies. This cellular integration enables utilities to communicate with the RC15 controller over long distances on cellular networks to operate or interrogate the ACR, change settings or download new firmware. Long-distance communication is a key requirement for smart grid implementations, particularly for isolated installations. In addition, cellular connectivity enables the RC15 controller to automatically integrate with other Scada systems.

The RC15 cubicle also incorporates Wi-Fi wireless connectivity allowing multiple substation-based ACRs to be linked into the substation’s wireless LAN (WLAN) to accelerate set up or software upgrading. Finally, the RC15 controller includes GPS capability providing mapping co-ordinates which can then be used for automatic population into Scada mapping systems.

Utilities are already making use of communication technology as they develop their smart grids. For example, Colombian utility Electrocaquetá used ACRs to overcome the challenge of installing smart grid infrastructure in remote and hard to access areas by linking the devices to control centres using the cellular network. (See Fig. 2.) The ACRs are connected (via routers) to the cellphone network and then through the Internet using virtual private networks (VPN) to a server installed in the client’s control centre [22].

Fig. 2: A Colombian utility used Noja Power’s ACRs to overcome the challenge of installing smart grid infrastructure in remote areas by linking the devices to control centres using the cellular network. (Source: PTI S.A.)

While using existing communication infrastructure, smart grids require specialised protocols to ensure rapid, reliable, secure communication between Scada master stations (or control centres), remote terminal units (RTU) and intelligent electronic devices (IED). Because smart grids are still under development, globally accepted protocols are still evolving, but there is growing support for IEC 61850 and IEE 1815 (DNP3). Major DA manufacturers are steadily introducing support for these protocols which will ensure communication interoperability between the key elements of tomorrow’s smart grids.

IEC 61850 is a family of international standards that specify the use of a set of communication protocols for the integration of all protection, control, measurement and monitoring functions in a smart grid. The protocol builds on earlier protocols including Manufacturing Message Specification (MMS) and Generic Object-Oriented System Event (GOOSE). MMS provides the vertical supervisory and control functions that allow devices to record data and then report that data to other equipment while GOOSE is a horizontal process coordination function used for high-speed sharing of information.

The standard is now being extended beyond the original scope of substation automation into the domains of managing wide-area electrical transmission and distribution systems and the control of DG including RE.

Communication security is a key concern. There is a realisation by authorities that smart grids’ reliance on Internet Protocol (IP) technology for communication makes it possible for hackers and other malevolent forces to disrupt control systems and disable critical infrastructure. A report [23] conducted by California State University for the California Energy Commission, for example, concluded that smart grids were increasingly vulnerable to cyber security issues such as confidentiality of user information, integrity of demand response systems, integrity and availability of Scada systems, and integrity and availability of EVs. The report suggested that smart grids should be designed with measures to counter these vulnerabilities.

In part to defend against unauthorised access, the Institute of Electrical and Electronic Engineers (IEEE), formally adopted DNP3 in July 2010, defining the protocol in IEEE 1815-2010 [24]. DNP3 already plays a crucial role in Scada systems employed to monitor and control contemporary grids where it is used by Scada control centres, RTUs and IEDs. The protocol was designed with an emphasis on security and reliability making it a natural choice for smart grids. The latest version, DNP3-SAv5, defines a security architecture that uniquely identifies devices or multiple individual “users” of a device, provides for separate update keys for each device or user and supports encryption. An update supports symmetric or asymmetric public key infrastructure mechanisms.

Many IEDs support DNP3-SAv5, including Noja Power’s RC10 controller, which ensures secure, interoperable communications with other IEDs on the smart grid.

High voltage (HV) support

Raising voltage and significantly lowering current reduces losses when transmitting high power over long distances. This has seen, for example, the development of 72 kV distribution lines to replace 36 kV lines for offshore wind farm electricity delivery. In the future, HV DC is likely to be the preferred transmission mode for long distances and commercial solutions are already established. For example, the Chinese have installed an HV DC transmission project, the Xiangjiaba line, terminating in Shanghai, which operates at 800 kV and delivers 6 GW of power over 2000 km. High-voltage transmission is likely to lead in turn to distribution lines carrying even higher voltages than the 72 kV AC systems already deployed for some RE DG installations.

Today’s ACRs have been developed to operate on medium voltage distribution systems with Noja Power’s products, for example, designed to operate on lines operating up to 38 kV under normal conditions. However, development is under way that will allow the products to deal with the higher AC (and future) DC voltages that are likely to result as RE DG penetration increases.

Ready for the future

Commercial, regulatory and environmental pressures are rapidly changing the way the world generates electricity. The old model of centralised generation is not flexible enough to meet these rising demands. Instead utilities are turning to DG to reduce reliance on large, capital intensive, fossil-fuelled power stations and take advantage of RE resources, including an increasing percentage from micro-generating “prosumers” (consumers with wind or PV micro-generation capacity who periodically return power to the grid).

DG with a large proportion of RE demands a radical restructuring of electricity generation, transmission and distribution to deal with the challenges of the cost of new infrastructure, variability, power quality, protection and long-distance transmission and distribution that currently limit the potential of new technologies.

Smart grids with modern DA such as ACRs can overcome these challenges if governments commit to reform the regulatory environment to stimulate competition and utilities commit to investment in the equipment and staff training required to implement smart grid technologies.

The next generation of intelligent ACRs is likely to include the protection, monitoring, communication and high voltage features which will make the products a cost-effective alternative to substations for interfacing RE DG to the network. Key among these developments is the addition of synchrophasor technology to enable the precise wide-area monitoring and control that will be vital if ACRs are to replace substations as the interface between RE DG and consumers.

Huge benefits would flow from investments in smart grids that encourage RE DG, primarily a reduction on reliance on fossil-fuelled power stations and stimulation of development of RE technologies and associated industries such as energy storage and EVs, resulting in a major reduction in carbon emissions.

References

[1] JA Pec ̧et al: “Integrating distributed generation into electric power systems: A review of drivers, challenges and opportunities,” Electric Power Systems Research, 9 October 2006.

[2] www.resourcesandenergy.nsw.gov.au/energy-consumers/energy-sources/electricity/removal-of-electricity-price-regulation, retrieved 14 December 2015.

[3] RK Pachauri and LA Meyer (eds.): “Climate Change 2014: Synthesis Report,” contribution of Working Groups I, II and III to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change, IPCC, 2014.

[4] “Electricity network transformation roadmap. Interim Program Report,” ENA/CSIRO, December 2015.

[5] http://af.reuters.com/article/commoditiesNews/idAFB9N0XZ02620150519, retrieved 14 December 2015.

[6] http://www.bbc.com/news/science-environment-35073297, retrieved 14 December 2015.

[7] “EU energy, transport and CHG emissions: Trends to 2050,” European Commission, December 2013.

[8] Presentation at Noja Power Distributor Conference, Energex, October 2015.

[9] “Electricity generation,” Institute for Energy Research, September 2014.

[10] “Lessons learned in the development of Moree Solar Farm,” Fotowatic Renewable Ventures.

[11] “Integrating Renewable Energy Electricity on the Grid,” American Physical Society.

[12] H Holtinen: “Estimating the impacts of wind power on power systems—summary of IEA Wind collaboration,” 2008.

[13] http://www.eia.gov/electricity/annual/html/epa_04_03.html,  retrieved 2 February 2016.

[14] https://en.wikipedia.org/wiki/High-voltage_direct_current

[15] Muhamad Reza, et al: “Evaluation of 72 kV collection grid on Offshore Wind Farms,” ABB PS Consulting, 2012.

[16] “Smart grids and renewables: A guide for effective deployment,” International Renewable Energy Agency, March 2013.

[17] “Priorities for inverter energy system connection standards,” Clean Energy Council, Australian Government, June 2015.

[18] “The Australian long- term power quality monitoring project”, University of Wollongong, 2008.

[19] “Using Synchrophasor Data during System Islanding Events and Blackstart Restoration,” North American Synchrophasor Initiative.

[20] R Bush: “Timing Is Everything,” T&D World, 2016.

[21] M Mills-Price and B Flerchinger: “Smart Anti-Islanding Using Synchrophasor Measurements,” North American Synchrophasor Initiative.

[22] Juan Carlos Quijano and Miguel Fuertes: “Cellular networks bring remote smart grid installations to life,” PTI SA, June 2015.

[23] “Smart Grid Cyber Security, Potential Threats, Vulnerabilities and Risks,” California State University, Sacramento, May 2012.

[24] “IEEE Standard for Electric Power Systems Communications – Distributed Network Protocol (DNP3),” IEEE, 2010.

Contact John Dykes, Noja Power, johnd@nojapower.co.uk

 

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