Integrating renewable energy sources into smart grids – Part two

June 30th, 2019, Published in Articles: EE Publishers, 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 part two of a three-part article. Part one was published in the June issue; part three will be published next month.

Smart grids enable utilities to overcome the technical challenges of RE DG integration by dramatically improving the control and flexibility of electrical distribution. While there is no official definition of the smart grid, the US National Institute of Standards and Technology (NIST) offers a concise description. NIST says the smart grid is “a modernised grid which enables bidirectional flows of energy and uses two-way communication and control capabilities which will lead to an array of new functionalities and applications”.

To make this vision of smart grids a reality requires the introduction of computerised digital technology, automated control and autonomous systems to electricity distribution. Such investment would provide the foundation for a grid which is more reliable than conventional infrastructure, offering fewer and briefer outages, “cleaner” power and “self-healing” properties.

Implementation includes fitting each device on the network with sensors to gather data and adding bidirectional digital communication between the devices in the field and the utility’s network operations centre. Another key feature of smart grids is the automation technology which lets the utility adjust and control each individual device from a central location.

Smart grids automatically monitor, protect, and optimise the operation of their interconnected elements from the central and distributed generators through the high-voltage transmission- and distribution networks, to industrial users and end-use consumers. These enhanced networks promise improved efficiency which reduces total energy demand by limiting line losses and encouraging consumers to reduce consumption. This improved efficiency and decreased consumption, together with greater use of efficient fossil fuel- and RE-power sources, reduces the generation of carbon emissions and other pollutants.

A smart grid’s continuous monitoring allows automated systems or operators to detect and act upon dangerous situations or security breaches which threaten reliable and safe operation of the network. In addition, cyber security and privacy protection for customers is significantly enhanced.

Residential consumers can take advantage of smart metering which will offer greater choice and control over electricity use. Consumers will also be able to buy “intelligent” appliances which can autonomously determine when to operate based on the cost of power at a particular time. In addition, consumers will be able to operate as micro-generators, feeding power back to the grid through bidirectional distribution lines.

The smart grid will use the same HV (i.e. above 100 kV) transmission lines used for contemporary long distance- and high capacity-transmission. Substations will then convert HV to MV for distribution and secondary distribution. The distribution lines in a smart grid will also be equipped with distribution automation (DA) devices. Such units will be essential for protecting the integrity of the grid, isolating faulty lines, re-routing power to communities affected by line failures (by reversing power flow if necessary) and switching in RE resources (when they are able to provide power) to cover demand peaks.

Smart grids can do much to help meet the challenge of integrating RE generation together with a wide range of diverse conventional electricity resources. For example, imagine PV generation and a set of commercial and industrial electricity consumers all tied together with smart grid communication and control technologies. In one scenario, rapid communication technologies will warn operators of approaching clouds allowing a smooth transition from PV to conventional supply using DA to switch the direction of power flow and back again when the sun reappears.

In a second scenario, customers might be prepared to subscribe to an “interruptible tariff” whereby they would be prepared to accept reduced power flows (in the event of cloud cover) in return for a cheaper electricity price. Smart meters would record the interruptions and in the future the system could even be extended to warn intelligent appliances of impending power reductions allowing them to switch to stored power sources (such as the consumer’s EV battery) for the duration of the interruption. Later, when the sun goes down and the evening wind picks up, a smart grid could switch to wind turbine power to recharge the EV’s battery ready for the following morning’s commute.

Smart grids also help resolve the challenges of RE DG. The technology can provide system operators with continual, real-time information on how these systems are operating and allow precise control. Such close monitoring and control will encourage utilities to consider RE DG as an alternative to traditional large-scale power plants. Moreover, detailed data on RE DG output and performance can help the utility place an accurate figure on the value of the RE DG. Similarly, the data can help the utility determine the proper price to pay the RE DG system owners or operators for their systems’ output.

Overcoming variability

Smart grids enable utilities to manage the variability challenges which come with large RE generation contributions to the electricity supply. Today, RE variability is handled almost exclusively by ramping conventional reserves up or down on the basis of forecasts. However, smart grids will allow seamless augmentation of RE DG by easing installation of energy storage on the grid, using fast-acting conventional generation when RE resources are expected to decrease, and enhancing long-distance transmission such that a utility can access larger pools of RE resource to balance regional and local excesses or deficits.

Energy storage can help resolve many of the challenges relating to the existing grid. While it helps to deal with variability, it can also play a key role in managing power flows. Commonly deployed electrical energy storage technologies include electrochemical storage (batteries), flywheels, pumped hydro storage, specialised storage systems such as compressed-air energy storage, superconducting magnetic energy storage, supercapacitors and fuel cells.

The role of the ACR in a smart grid

Implementing a smart grid will require extensive use of DA to allow faster and more precise control than is typical for conventional systems. Enhanced control will be essential for resolving the challenges of integrating RE.

Fig. 6: ACRs are fundamental components of smart grids.

Automatic circuit reclosers (ACR or “auto-recloser”) are fundamental components of the DA underpinning future smart grids (Fig. 6). The ACRs perform voltage measurements on all six bushings, current measurement on all three phases and provide extensive power quality and data logging capability as well as many of the switching, bidirectional protection, control and communication capabilities required to integrate RE generation into the electricity grid.

The primary role of an ACR is to act as a switch and circuit breaker, enabling power re-routing, either in the event of fault protection, isolation and restoration (FPIR) events or the interfacing of local sources of RE to supplement base load in times of peak demand, demanded by a smart grid to maintain high availability.

ACRs operate in cases of faults, for example, when a phase-to-phase or phase-to-ground fault increases the current in the feeder above normal levels. The ACR can also be triggered by a fault resulting in a current lower than normal levels, as, for example, in the case of a fallen conductor touching a high resistance surface such a concrete. By re-routing the supply via remote switching of other ACRs, the utility can quickly restore power to customers affected by the original fault.

Smart grid building blocks

In addition to their primary role as switches and circuit breakers, modern ACRs are designed as building blocks for smart grids. As such they have many capabilities which make the devices ideal for smart grid applications like the integration of RE DG.

Noja Power, for example, has included sectionaliser functionality in its firmware platform for the OSM series ACRs. The functionality allows utilities to configure the ACR as a conventional auto-recloser, a sectionaliser or to function as either device depending on the application. The ACR can perform all three functions in either direction of the electricity feeder.

To ensure that utilities take full advantage of the capabilities of DA such as ACRs, technicians need to be familiar with the equipment. This can be challenging for personnel which have previously worked with traditional utility equipment. Equipment suppliers are working to ease this transition. For example, the company offers its Smart Grid Automation (SGA) software, a PC-based software suite which enables engineers to develop, test and debug smart grid functionality for ACRs.

SGA software multiplies smart grid customisation options and adds the capability of distributing the resulting applications simultaneously across groups of ACRs. Such applications help make smart grid implementations easier to develop, deploy and modify.

The software uses IEC 61499 as the basis for constructing the applications. IEC 61499 is an open standard for distributed industrial automation systems aiming at portability, reusability, interoperability and reconfiguration of distributed applications. The IEC 61499 model includes processes and communication networks as an environment for embedded devices, resources and applications.

Economic benefits

There are currently no easy fixes to the challenge of widely-varying and somewhat vague specifications for the connection of greater than 30 kVA capacity RE DG to the grid. However, as the penetration of RE DG climbs, the issue is under careful consideration. In Australia, for example, the Clean Energy Council has produced a report which describes the challenge and recommends the priorities which should be addressed to simplify “IES” (Inverter Energy Systems such as solar and wind generation) connections.

Among these recommendations are to develop standardised protection requirements for IES connections, post installation, commissioning/testing and maintenance requirements for IES, standardised utility technical assessments for IES connection, and standards and utility guidelines for the connection of hybrid/IES.

As RE DG systems become more prevalent, the need for standardised connection will become more pressing. Safety and reliability are the highest priority for utilities and the absence of guidelines has encouraged them to take a conservative approach, leading to expensive connection schemes which can make some RE DG schemes economically unviable. NOJA Power is working with utilities to draw up practical RE DG connection guidelines in the absence of standards which provide satisfactory safety and protection without excessive design and engineering costs.

Once pragmatic RE DG connection specifications have been agreed, the use of ACRs as the primary connection device can lower the cost of connection. ACR functionality already includes bidirectional protection, power quality monitoring and communication technology. With the addition of extended bidirectional-, rate of change of frequency- and vector shift-protection (particularly combined with synchrophasors) tomorrow’s products could operate as a highly cost-effective interface between RE DG and the distribution grid. Next-generation ACRs with these capabilities promise to make today’s uneconomic RE DG projects viable because the cost and complexity associated with building substations would be eliminated.

Bidirectional power flow and protection

In conventional grids power typically flows in a single direction, from centralised power generation to industrial, commercial and domestic consumers. However, smart grids are able to support bidirectional flows which will be common as consumers become producers when the sun shines or the wind blows. Smart grids must offer this support while still maintaining a high level of reliability and network protection. ACRs are the key component in ensuring that the smart grid can meet these criteria.

Power quality

As power generation becomes more diversified and RE DG contributes an increasing proportion of the total supply, monitoring of power quality will become more critical. Utilities will be responsible for ensuring high power quality when switching in RE generation and for isolating the supply rapidly if problems occur, to limit the risk of damage to network and consumer assets. Modern ACRs can help utilities meet these obligations. For example, the OSM series ACRs can measure harmonic distortion, interruptions, and sags and swells, helping to prevent power contamination. The power quality information is available via a display for onsite technicians, or remotely accessible for staff based at a distant location.

Part three of this article, to be published next month, will deal with the use of synchrophasors, islanding prevention, communications and HV support.

References

The references for this article will be published with the online version in August 2019.

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

 

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