• <tr id="yyy80"></tr>
  • <sup id="yyy80"></sup>
  • <tfoot id="yyy80"><noscript id="yyy80"></noscript></tfoot>
  • 99热精品在线国产_美女午夜性视频免费_国产精品国产高清国产av_av欧美777_自拍偷自拍亚洲精品老妇_亚洲熟女精品中文字幕_www日本黄色视频网_国产精品野战在线观看 ?

    A Grid as Smart as the Internet

    2020-10-20 08:19:38YnliLiuYixinYuNingGoFelixWu
    Engineering 2020年7期

    Ynli Liu*, Yixin Yu Ning Go Felix Wu*

    a School of Electrical and Information Engineering, Tianjin University, Tianjin 300072, China

    b Department of Electrical and Electronic Engineering, The University of Hong Kong, Hong Kong, China

    c Department of Electrical Engineering and Computer Sciences, University of California, Berkeley, CA 94720, USA

    Keywords:Distributed intelligence Electric grid

    A B S T R A C T A new era of electricity is dawning that combines the decarbonization of the grid with the extensive electrification of all sectors of society.A grid as smart as the internet is needed to harness the full potential of renewables, accommodate technology disruptions, embrace the rise of prosumers, and seamlessly integrate nano-,mini-,and micro-grids.The internet is built upon a layered architecture that facilitates technology innovations, and its intelligence is distributed throughout a hierarchy of networks. Fundamental differences between data flows and power flows are examined. The current operating paradigm of the grid is based on the conviction that a centralized grid operator is necessary to maintain instantaneous power balance on the grid.A new distributed paradigm can be realized by distributing this responsibility to sub-grids and requiring each sub-grid to maintain its net power balance.A grid as smart as the internet based on this new paradigm, as well as a hierarchical network structure and a layered architecture of operating principles, is presented.

    1. Introduction

    Global climate change, largely due to greenhouse gases from human burning of fossil fuels, is already taking place. The concentration of greenhouse gases(primarily carbon dioxide)in the atmosphere has increased to levels unprecedented on earth in about a million years. The world is currently 1 °C warmer than preindustrial levels.Greenhouse gases remain in the atmosphere unabated,causing the warming effect to be cumulative.Leaders from around the world collectively agreed in 2015 in Paris that human behavior-induced climate change is a threat to all of humanity and global action is needed to substantially reduce greenhouse gas emissions in an effort to limit the temperature increase in this century to 2 °C, while pursuing means to limit the increase to 1.5 °C. The world’s leading climate scientists recently warned [1]that the half a degree difference between 1.5 and 2 °C will significantly worsen the risk of severe droughts,floods,extreme heat,and poverty for hundreds of millions of people, and that actions must commence immediately to limit the temperature rise to within 1.5°C before the window of opportunity closes around 2030.Every citizen of the world in every profession has the responsibility to do whatever he or she can, in large or small measures, immediately and urgently, to combat global climate change in order to save humanity for future generations.

    Economic development is accompanied by higher energy consumption. Improvement in living standards is propelled by the increased use of energy. Most of the world’s population still lives in countries that are considered to be developing,where the average energy consumption per person is very low.In the next decade or so, billions of people—a large proportion of which are in Asia—will be lifted either from poverty or from low-income levels to mid-income levels. Global energy demand will rise. In its Sustainable Development Goals, the United Nations (UN) has explicitly recognized that ending poverty and building economic growth must go hand-in-hand with tackling climate change and environmental protection. To put it simply, the world will need more energy but less carbon.

    The decarbonization of electricity by switching from fossil fuels to renewable and other non-fossil sources, coupled with the increased electrification of other sectors of the economy,is considered to be an effective pathway to sustainable development [2,3].However, the introduction of new renewable sources fundamentally changes the characteristics of the electric energy system,commonly referred to as the grid.Wind and solar generation is best located in regions where the environmental conditions (i.e., wind speed,solar radiation)favor it.It possesses little economy of scale;hence,it is mostly deployed in a dispersed and distributed manner,and can be installed and used on the consumer’s premises. As a result of these changes,the characteristics of the new electric grid will be more like those of the internet,where information,vis-à-vis energy,is generated and shared everywhere and anytime throughout the network.This apparent similarity has inspired some people to envision a future electric grid in which hundreds of millions of people produce their own energy from renewables, store it locally in batteries in their homes,offices,and factories,and then share it with others through the grid[4].The internet is smart.Will the grid be as smart as the internet to facilitate the massive sharing of energy?

    This paper addresses the following issues:

    · Why would we like the grid to be as smart as the internet?

    · What makes the internet smart?

    ·Why are previous attempts to make electric grids internet-like unsuccessful?

    · How to make the grid as smart as the internet?

    2. The future is electric

    A new era of electricity is coming. The step-by-step decarbonization of electricity with renewables will provide humanity with more energy and less carbon,and will place the power sector in the vanguard of emissions reduction efforts. Further intensified electrification of other sectors of the economy will lead to a fasttrack pathway to combat global climate change.

    Government policies and technology advancements have made tremendous cost reductions in renewable energy. Data from 2017 shows that 179 countries have set renewable energy targets and 57 countries have set 100% renewable electricity targets [5]. At the 2019 UN Climate Action Summit, 77 countries and more than 100 cities committed to net-zero carbon emissions by 2050.Hopefully,other countries,including India,China,and the United States,will soon follow suit. China—the largest carbon emitter in the world—is well on the way to fulfilling the commitment of reaching at least 20% non-fossil energy by 2030. Although the United States—the second largest emitter—is walking away from the Paris Agreement under the Trump administration, many states and private sectors are nevertheless aggressively trying to meet the goals of Paris and beyond. For example, California signed a bill in 2018 mandating 60% of the state’s electricity to be powered by renewable resources by 2030, while calling for a path toward 100%zero-carbon sources for electricity by 2045. The state of New York is pushing to reach 100% zero carbon five years earlier. Various government policies aiming at providing encouragement and incentives for renewable energy development and utilization have stimulated intensive research and development (R&D),technology innovation, and business entrepreneurship. The resulting technology advancements, economies of scale, and increasingly automated production processes have driven the cost of renewable energy down drastically.

    Wind energy is currently one of the cheapest sources of electricity,and it is getting cheaper[6].Progress in materials research has resulted in astonishing cost reduction in solar panels—more than 99%decline in 40 years,from 77 USD per watt in 1977 to 0.64 USD in 2017[7].The average installed cost of residential solar generation in the United States has dropped by 60%and that of utility-scale solar generation has dropped by 77%from 2010 to 2017[8].The costs of renewable power are forecasted to continue declining.Renewables accounted for two thirds of the new power added to the world’s grids in 2017(of which half is in China)[9],including both rich and poor countries. The share of generation from renewables may rise to become the world’s main power source as early as 2040[10].

    At present, about a fifth of the world’s total energy use goes through electricity and more than three quarters of the energy used by key non-power sectors (i.e., transport, buildings, and industry) comes from fossil fuels. Decarbonization of the power supply coupled with greater electrification of these sectors holds the potential to significantly reduce fossil fuel use and carbon emissions [2,3]. The transport sector presents the most sizeable near-term electrification opportunity. Electric vehicles (EVs) have progressed rapidly in the last couple of years and more than 3 million EVs are on the road worldwide, although the market share of EVs is still low.China alone added more than 500 000 EVs in 2017—a 72%increase over 2016—in addition to 370 000 electric buses and 250 million electric two-wheelers. The emergence of low-cost shared mobility services with autonomous(i.e.,driverless)vehicles is expected to be predominately based on EVs. The International Energy Agency (IEA) forecasts that the number of EVs on the road could reach 125 million, or even as high as 220 million, in 2030[11].Buildings are already electrified to some extent.The industry sector,which is the biggest consumer of fossil fuels beyond power,is more difficult to electrify due to the heterogeneity of its users.Aggressive government policies, technology innovation, and business entrepreneurship, similar to the cases of renewables and EVs, hold the keys to drive down costs for further electrification of these sectors.

    Conventional fossil fuel generation is controllable to supply fluctuating load demands on the grid. Variable renewable energy(VRE),which depends on wind and solar radiation,is intermittent,variable,and stochastic.The massive introduction of VRE is changing the landscape of the grid. A variety of new energy storage systems, including batteries, flywheels, compressed air, thermal storage, and hydrogen storage, are swiftly developed in recent years to smooth out the variability of renewables and to assist in the instantaneous power balance required by the grid. In particular, battery technologies have advanced tremendously, thanks to technology innovations and market expansion due a large extent to the demand of batteries for EVs. Battery costs have decreased by more than a factor of four from 2010 to 2016 [12]. Batteries can be deployed both on the grid and at individual consumer premises. Large-scale installation of batteries in front of the meter assists grid operators in maintaining power balance and a variety of other applications. Local solar panels and batteries behind the meter, on the other hand, may lead to grid defection, leaving less controllability for grid operators. From the grid perspective, coupling battery storage with renewable generation is a weak substitute for conventional fossil fuel plants [13]. The magnitude and quality that are required solely of energy storage systems to accommodate fast-growing levels of VRE are deemed to be technically and economically exorbitant.

    Another option to help deal with the variability of renewable generation is to shift consumption to other periods when supply is more abundant, such as when the sun shines and the wind blows.This kind of demand response can be considered as a virtual energy storage system, where customers’ energy demand is used as‘‘storage.”Digital connectivity allows appliances and equipment(e.g., smart home appliances, smart thermostats, building energy management systems,smart industry boilers,etc.)to be monitored and controlled continuously in order to shape demand to optimally match it with available supply.Greater automation,the diffusion of internet-of-Things (IoT) devices in the residential and commercial sectors,and higher deployment of EVs and smart charging systems will expand the demand response capability. Some estimates reckon that 20% of electricity consumption in 2040 will be technically available for demand response [14].

    For over 100 years, since Thomas Edison lit up the homes in New York, the grid is delivering electricity to 6 billion people every day to enjoy the economic benefits and opportunities.For the remaining 1 billion people on the planet today who lack electricity, most of whom are very poor and live in remote villages, the cost of connections is prohibitive and their governments are too poor to subsidize them. The rise of new technologies in the last decade, coupled with the emergence of private-sector entrepreneurship, has radically changed the economics of energy delivery to remote rural dwellers, allowing poorer and more remote people to have access to electricity faster and cheaper than has previously been possible. As mentioned earlier, the colossal drop in the costs of solar panels and batteries has spurred the localized generation and storage of electricity in homes or communities. Other technological mega-trends, such as more energy-efficient appliances (e.g., light-emitting diode(LED) lighting and mobile phones), the IoT for remote monitoring and servicing, and mobile money, have enabled entrepreneurs to provide viable energy services, such as selling generation/storage kits that range from a few watts to a couple of hundred watts,commercialized through the pay-as-you-go (PAYG) business model. An increasing number of entrepreneurs are actively testing a range of business models and helping to move renewablebased mini-grid sector to maturity. In India alone, more than 200 mini-grids were installed during 2016–2017. Through the deployment of either off-grid solar systems or renewable-based mini-grids, these distributed renewables for energy access(DREA) systems are providing electricity access to more than 360 million people worldwide [5,15].

    The combined demand for electricity access in developing countries and for further electrification for emission reduction in developed countries could result in a staggering 60%–90%global growth in electricity by 2040 [3]. This is happening at a time when the electrical energy system itself is experiencing its most dramatic transformation since its creation more than a century ago. Major changes include:

    (1)Increasing share of VRE.The more successful decarbonization of the grid is, the more electrification will come, resulting in more global carbon reduction. Grid operation must endeavor to accommodate a continually increasing share of VRE, and increasingly improve its utilization of VRE.

    (2)Rise of prosumers.Today,residential customers own about one third of global solar photovoltaic(PV)capacity.Battery storage will likely be similar. This continuing trend has two implications:First, there will be thousands or even millions of smallgeneration sources that are dispersed and distributed throughout the system; second, users of the grid will be both producers and consumers, or will become prosumers.

    (3)Intelligent periphery.In the digital era,data,analytics,and connectivity are abundantly available to prosumers at the periphery of the grid (i.e., distribution system and beyond) to intelligently schedule, manage, and control their own variable renewable generation, battery storage systems, EV charging,various demand response systems, and so forth. Controllability and intelligence are no longer the monopoly of grid operators.

    (4)Proliferation of nano-, mini-, and micro-grids.Microgrids that are largely self-sufficient have become popular [16].Renewable-based mini-grids and off-grid nano-grids (with combined solar and battery) in the developing countries are mushrooming. At the same time, an increasing amount of grid defection of prosumers in developed countries who possess their own solar and battery storage systems is threatening the survival of power companies (the so-called ‘‘utility death spiral”).

    (5)Fast pace of technology innovation.Explosive advancements of solar PV and battery technologies in recent years are hard-pressed on conventional grid operations.More new and innovative technologies, some of which will be disruptive, will emerge at some point in the future. Grid operation must take full and timely advantage of available innovations.

    The physical composition and characteristics of the grid are changing significantly, especially on the periphery [17]. But the operating paradigm of the grid remains unchanged. The operating principles, control architecture, and basic tools that are used for grid operation today were developed in the middle of the 20th century, during the last great grid expansion. Intelligence was enhanced in control centers when the first generation of digital computers came along. It is a tall order to ask the same operating paradigm to work in a new and different environment.

    How well is the grid coping with the changes so far?Let us first look at how well it handles the integration of VRE into the grid. In 2016, China—the country with the largest installed VRE capacity in the world—had much more solar and wind capacity in terms of percentage (13.7%) than in terms of the energy it generated(5.3%).The United States—the second largest country in VRE installation—was slightly better (10.7% in capacity and 6.9% in energy)[18].These numbers indicate that the utilization(i.e.,energy generated) of the VRE asset (i.e., capacity) is lower than the average of today’s grid (which consists of mostly a conventional base and peaking units). The data from Ref. [19], summarized in Table 1, is even more telling, as it shows that the utilization of wind energy in terms of the capacity factor(i.e.,percentage of energy output to what could have generated if available around the clock)worldwide is fairly low and does not improve with greater capacity.

    It is common knowledge in grid operation that VRE resources are frequently subject to curtailment due to constraints imposed by the grid [20–22]. Curtailment rate is defined as the percentage of energy curtailed to the energy generated. The curtailment rate of wind,for example,could be 10%or more,which means that tens of terawatt hours of energy are wasted annually.These constraints are brought about by the conventional grid operation protocols,even in cases where the transmission capacity is adequately sized.The grid was built to facilitate the transmission of energy from sources to consumers. In the era of VRE, the grid is no longer an enabler; it becomes a blocker.

    The electric grid is considered to be the greatest invention of the last century, while the internet is the greatest innovation of this century. The internet is smart and can readily accommodate fastchanging landscapes of continuous disruptive information revolutions. In the new era of electricity, we would like the grid to be as smart as the internet! A grid as smart as the internet should be able to harness the full potential of renewables, incorporate technology innovations, embrace the rise of prosumers, and integrate nano-, mini-, and micro-grids.

    3. The internet

    To the users of the grid and the internet, both networks are ubiquitous (available everywhere and all the time) and heterogeneous (passing through any form of energy/data). The internet is smarter because of the way the intelligence is deployed.

    3.1. Distributed intelligence

    Any form of data—text, voice, or video—is transmitted through the internet without central control or coordinating facilities,relying solely on the endpoints of transmissions at local nodes that handle the processing to complete the job. There is no extendedglobal reach by a single authority. Intelligence is distributed throughout the network and the responsibility of ensuring successful transmission—as well as data integrity, reliability, and authentication—is shared among the nodes. Distributed intelligence and decentralized control render the internet resilient to disturbances and disruptions. The complex system is made to work flawlessly and efficiently by the design of the network structure and a layered architecture of operational protocols [23,24].

    Table 1 Wind energy capacity and capacity factor.

    3.2. Internet structure: A hierarchy of sub-networks

    To understand the structure of the internet,let us take a simple example:User A in Tianjin wants to send an e-mail to User B in San Francisco (Fig. 1). User A’s computer is connected to a local area network (LAN) and her e-mail is served by a local internet service provider (ISP) in China (Fig. 2). The e-mail may have to hop through a few intermediate points in the ISP’s backbone network(e.g., local ISP to regional ISP) to reach a network service provider(NSP)of a large international network of the global internet;similarly for User B. But User A’s and User B’s ISPs may not belong to the same NSP network. The e-mail will have to exchange through a network access point(NAP)between the two NSP backbone networks.The point-to-point path along which the e-mail travels can be traced from A’s computer in Tianjin to an LAN, an ISP (possibly more intermediate points),an NSP(or more points in between),an NAP, another NSP (or more points in between), another ISP (or more points in between), another LAN and finally to B’s computer in San Francisco.The internet is a network of(sub-)networks structured in a hierarchical manner with a number of tiers.At the top is the global internet, followed by several tiers, including NSP backbones, ISP backbones, and so forth, LANs or users at the bottom.

    3.3. Routers

    The e-mail is routed in the internet through a number of routers.Routers are usually used to connect different networks.An ISP and NSP may have several routers as part of their backbone networks.The logical view of the internet is, therefore, a network of routers arranged in a hierarchy. A router is a specialized computer that directs data traffic. Each router has the knowledge of its own network and of all the sub-networks below it.When the e-mail arrives at a router, it examines the recipient’s address and sends it to the next correct router according to a simple rule:If it is within its own sub-networks,it will be sent to the sub-network router;otherwise,it will be sent to a default router, which is usually one up in the hierarchy and has a larger set of sub-networks.

    Since each router is associated with a sub-network to which it belongs and over which it holds responsibility,an alternative view of the path of data transmission in terms of sub-networks, which will be useful in Sections 4 and 5, is provided here. User A’s computer is connected to the router of an LAN. When it is connected to the internet via an ISP router,it becomes part of the ISP network,and so on. Therefore, instead of tracing the e-mail from node to node (i.e., from router to router) in the internet, we may say that the e-mail moves from network to network. The e-mail travels from an LAN in Tianjin, to a Chinese ISP network containing the LAN,to an NSP network containing the ISP,to an NAP in the global internet, to another NSP network, to another ISP network in the United States, and finally to the LAN in San Francisco. The path of the e-mail,up and down the hierarchy of sub-networks,depends on the locations of the users and the sub-networks they belong to.

    Fig. 1. An example of sending an e-mail through the internet.

    Fig. 2. The structure of the internet. NAP: network access point; NSP: network service provider.

    3.4. Layered internet architecture

    The e-mail from User A to User B must be translated from text into electronic signals,routed through the internet,and translated back into text.The information exchanged between routers is governed by rules and conventions that are set out in communication protocol specifications. In modern design, protocols are layered to form a protocol stack. Layering is a design principle that divides the design task into smaller steps, each of which accomplishes a specific subtask and interacts with other subtasks only in a small number of well-defined ways.It allows the decomposition of a single and complex task into simpler,clear,and cooperating subtasks.Layering is also a functional decomposition;each layer solves a distinct class of communication problems.

    The International Organization for Standardization defines seven layers of networking protocols,called Open System Interconnection (OSI) reference model, which can be simplified into four layers, as shown in Fig. 3. The simplified version roughly corresponds to the protocol stack used on the internet known as the Transmission Control Protocol/Internet Protocol (TCP/IP) stack.The functions of the four layers are briefly described below:

    ·Application layer.Users interact with the application layer.Electronic mail (Simple Mail Transfer Protocol, SMTP) is one of the internet applications. Others include World Wide Web (Hypertext Transfer Protocol, HTTP) and file transfer(File Transfer Protocol, FTP). The application program passes the message to the transport layer for delivery.

    ·Transport layer.A message is usually divided into smaller packets, which are sent individually along with a destination address.The transport layer ensures that packets arrive without error and in sequence.

    ·Network layer.The network layer handles communications between machines. The packets are encapsulated in datagrams. A routing algorithm is used to determine whether the datagram should be delivered directly or sent to a router.

    ·Physical layer.The physical layer takes care of turning packets containing text into electronic signals and transmitting them over the communication channel.

    Fig. 3. Transmission Control Protocol/Internet Protocol (TCP/IP) stack.

    The message—in this case, the e-mail—starts at the top of the protocol stack on the sender’s computer and works its way downward (Fig. 3). An upper layer uses the functions available in the next layer and instructs the next layer what to do.The instruction is coded as a header added to the front of the message. Each layer adds a header on the way down. This process reverses on the receiving end.Each layer reads and interprets the instruction from the header and moves the message up or down after stripping the header intended for this layer from the message. Fig. 4 shows another example of the paths of an e-mail up and down the protocol stacks with two intermediate routers. Here, it is assumed that router D is the main server of the ISP and performs the storeand-forward function.

    The internet is smart because the layered architecture provides division of labor and the distributed control empowers sharing of responsibility. The responsibility of sending a message from A to B is shared by a number of routers along the path. The required intelligence of each router is simple and specific, i.e., forwarding the message to the next recipient correctly.Functional decomposition in a layered architecture makes it possible for new applications or functions to be added by utilizing and configuring existing lower layer functions. Innovation becomes more readily achievable.Distributed control and layered architecture also make the internet resilient to disturbance and adaptable to technology advancement.

    4. Data flow and power flow

    Previous attempts to make the grid internet-like have focused on the role of the router in the internet as a ‘‘switch” in passing incoming‘‘packets”to the next router[25–28].A so-called‘‘energy router” has been developed, with the help of modern power electronics,to direct or limit the power flow from a micro-grid or prosumer to the grid[26].A different‘‘energy router”uses alternating current (AC)/direct current (DC)/AC converters to regulate the power flow [27,28], taking advantage of the fact that power electronic circuits have better control of the power flow in the DC portion of the device, and essentially changing an AC grid into many hybrid AC–DC–AC sub-grids. All of these efforts are laudable and contribute to the advancement of power system technologies;however, they fall short of making the grid internet-like. We shall examine the underlying physics that makes data flows on the internet different from power flows on the grid.

    4.1. Data flow

    Voice, video, and other data signals are typically superimposed on a wave of some kind suitable for transmission over the chosen medium [29]. In communication networks, a physical medium is the transmission path over which a signal propagates or data flows.Many different types of communication media—wired or wireless—are used, including phone lines, cables, optical fibers, microwaves, and radio. A high-frequency sine wave is usually used as a carrier wave, but it can be DC or pulse chain depending on the application. In modern radio communications, such as orthogonal frequency-division multiplexing(OFDM)or code-division multiple access(CDMA),multiple or a spread of carrier waves of various frequencies are used.The carrier wave,be it electromagnetic,optical,or radio, involves the physical movement of electrons or photons.The process of modifying the carrier wave to carry signals from the transmitter is called modulation. At the receiving end, signals are recovered through demodulation. Modulation is important to transfer signals over long distances,since it is not possible to send low-frequency signals for longer distances. Modulated waves in communications can be considered as adding data flows onto the flow of electrons or photons of the carrier wave.

    High-capacity communication media can be divided into distinct segments (i.e., bandwidths) for non-overlapping modulated carrier waves, which can then be leased to and operated independently by different companies.Moreover,multiple signals are usually combined into one signal over a shared medium for transmission. This process is called multiplexing. Multiplexing divides a communication channel further into several logical channels through frequency division,time division,or others,and allots each one to a different and independent set of data flows.Multiple transmitters and receivers can share a common medium,resulting in multiple-access channels in communications.

    In summary, data flows are added to the flows of electrons or photons in communication media and can be directed to flow from one node to another node (or alternatively from one sub-network to another sub-network) in a communication network. Take the example shown in Fig. 4: The data generated in A is sent to B through intermediate routers, that is, from A to C, C to D, and D to B.

    4.2. Power flow

    Power is carried directly by electrons in the power flow [30].More power means moving more electrons. The flow of power must obey physical laws—that is, Kirchhoff’s and Ohm’s laws.These physical laws can be summarized as the requirement that power must be balanced at all times and everywhere on the grid.Any addition or reduction of consumption of electric power must be accompanied simultaneously by the addition or reduction of generation somewhere on the grid.The distribution of power flows on the grid is the consequence of the physics of power balance.Any change in the supply and demand of electricity will result in a redistribution of power flows in the interconnected grid. From a technical perspective, one learns from the first course on power systems that power flows on the grid are calculated mathematically after solving the so-called power flow (or load flow) equations, which are the mathematical manifestation of the fact that real and reactive power must be balanced at each node of the grid.The power flow equations are derived from Ohm’s law and from Kirchhoff’s current and voltage laws.

    It should be pointed out that in an AC power system, which is the grid we have today, the power includes both real (or active)and reactive powers. The real power is the average power that is generated or consumed.The reactive power is associated with voltage: Sufficient reactive power is necessary to maintain a desired voltage level. Moreover, power must be balanced in both steady states and transients during grid operation. A sudden change (i.e.,disturbance) of power balance on the grid will cause the system to move to a new equilibrium of power balance. During the transient, no device for overload protection should trigger further disruption to the grid operation (such as a blackout). Power system stability refers to the ability of the grid to continuously maintain the power balance after a disturbance without causing overload or other abnormal conditions.In the following discussion,for brevity,the term‘‘power”is used loosely to cover both real and reactive powers,while‘‘power balance”is used to cover both steady states and transients.

    As power must be balanced anywhere on the grid, for any subgrid or area of the grid,the net power—counting the power flowing into(import)and out of(export)all lines crossing the boundary of the sub-grid—must be balanced.Conversely,if the net power is balanced on any of the sub-grids of the grid whose union covers the whole grid, power will be balanced on the whole grid.

    Power cannot be directed from one node to another node, as data flows on the internet. Nevertheless, it is logically possible to trace the power flowing from generation to consumption through sub-grids, just as data flows can be traced through sub-networks,as described in Section 3.2. For example, suppose A sells power to B,where A and B are assumed to be in the same distribution substation, as shown in Fig. 5. The additional power from A to B represents a change in power balance, which will affect power flows on the parts of the grid that are connected to both A and B, i.e.,the sub-grid of the distribution substation.The sub-grids to which A and B belong are defined as A and B,respectively,and are also the sub-grids of substation D. Let us assume that an additional subgrid C is defined (see Section 5.2 for its selection) and attempt to trace out the sub-grids that are affected by the change of power flows.Since(sub-grid)A must keep its net power balance,the extra power from A must export to C,where C is the sub-grid of the feeder containing A in this example.Similarly,the extra power import to C must export to D in order for C to maintain its net power balance. D can maintain its power balance since B will consume the extra power from C, and both C and B are inside D. Of course, all of these power flows occur simultaneously and instantaneously.

    5. Come to grids with intelligent periphery (GRIPs)

    Fig. 5. Power flows from A to B.

    Energy management systems(EMSs),which are placed on highvoltage transmission systems covering hundreds of generators and substations, have been tremendously successful for decades in managing and controlling power systems to ensure their economical and reliable operation [31]. Extending this system, along with its underlying centralized operating paradigm, to distribution systems and beyond for thousands and millions of prosumers in future grids will stretch the limit to the point of being inefficient,unwise, and untenable. A recent study has concluded that, as the future grid becomes ever more complex, its reliability and resilience will be under tremendous stress if the current centralized operating paradigm continues[32].The internet has demonstrated that distributed intelligence and decentralized control provide the most effective means to enhance the reliability and resilience of the system.A new operating paradigm of the grid with distributed intelligence and responsibility sharing is developed. A grid with intelligent periphery (GRIP) that makes the grid as smart as the internet by empowering the periphery of the grid to embrace the new operating paradigm is proposed [33,34]. GRIP focuses on the periphery and requires no scrapping of the successful EMS or the practices of grid operation of the transmission system that are functioning well,only to make them simpler (without the responsibility for the periphery). Nevertheless, future transmission systems have the freedom to evolve into more decentralized operation consistent with the new paradigm.

    5.1. Distributed intelligence

    The conventional operating paradigm leaves the fundamental responsibility of system operation, i.e., maintaining instantaneous power balance without overload and abnormal conditions of the whole grid, to a single centralized decision-maker: the grid operator.As noted in Section 4,power is balanced(with no overload and abnormal conditions)on the grid if and only if the net power is balanced (with no overload and abnormal conditions) on any of the sub-grids of the grid. The latter principle can be used as the foundation of a new distributed operating paradigm for the future grid.

    Let us call a connected sub-grid of the grid,consisting of a cluster of prosumers with the intelligence to manage and control its net power balance,a cluster.With this definition,all of the following are examples of clusters:

    · An interconnected transmission system with an independent system operator(ISO)operating an EMS to control the generation and consumption of its members in its defined jurisdiction;

    ·A transmission company or authority with EMS operating as a‘‘control area” [35];

    ·A distribution company that has a modern distribution automation system to control power flows and voltage regulation (i.e., reactive power flows) on feeders and laterals and/or an advanced metering infrastructure (AMI) system to control customer loads;

    ·A micro-grid, mini-grid, or nano-grid;

    ·A smart community with its own EMS can be made into a cluster by adding the capability of managing and controlling its net power balance;

    ·A smart building with a building EMS can be made into a cluster by adding the capability of managing and controlling its net power balance;

    · A smart home can also be made into a cluster by adding the capability of managing and controlling its net power balance using the smart meter;

    ·An aggregator can sign up a group of smart homes, smart buildings, smart communities, and micro-grids on the same distribution company to form a cluster by managing and controlling the power generation and consumption of the aggregation.

    5.2. GRIP structure: A hierarchy of clusters

    Two clusters cannot partially overlap because, if they do, none of them can manage their own net power balance. Therefore, two clusters are either non-overlapping or one is completely contained inside the other, just like an LAN is viewed as being inside an ISP sub-network of the internet (Fig. 2). A cluster may contain many clusters or none at all.A cluster that is itself part of a larger cluster may contain several clusters. This leads to a natural hierarchy of clusters arranged in tiers (Fig. 6). At the top of the hierarchy is the whole interconnected grid. One tier below could be a power authority, or a company with its own EMS operated as a ‘‘control area” in the interconnection. The bottom tier could be a smart home.

    Let us revisit the example in Fig.5,where A sells power to B and A,B,C,and D are all clusters.We will view the path of transaction in terms of power flow in clusters;more in tune with the perspective of data flows through a number of sub-networks associated with internet routers, as described in Section 3.3. Cluster A sees no extra power demand inside the cluster to balance the additional power generated; it exports (sends) the power to the cluster it is connected to that is one tier above—namely, cluster C. Similarly,the extra power to C is sent to D. The extra demand of B, which is inside cluster D, now balances off the extra power flowing in from C to D. The path of power flow is from A to C, C to D, and D to B. The task of managing the transfer of additional power from A to B is therefore distributed among a set of clusters; each sends the power either to the cluster one tier up in the hierarchy or to the receiver,if it is inside the cluster.Physically,A generates additional power and B simultaneously takes the same amount of power. As long as all clusters—A, B, C, and D—maintain net power balance,the transfer of power happens instantaneously.

    The responsibility of maintaining the power balance of the grid is thus shared among the clusters that are affected; each has the responsibility of maintaining its own net power balance. Fig. 5 is redrawn in Fig.7 to show the path of power flows in the hierarchy of clusters.

    5.3. Energy router or cluster EMS

    A cluster must have the intelligence to manage and control its net power balance. The energy management system of a cluster(CEMS),consisting of all necessary hardware and software to manage and control its net power balance, is equivalent to a router in the internet and can legitimately be called the energy router (Erouter) in GRIP. An E-router, or CEMS, must have sensors to monitor power flows across the boundary of the cluster; powerconditioning devices (e.g., current limiters) to control the power flows;and information and communication technology(ICT)capability to manage generation and load—as well as the power flow—in the cluster.

    Fig. 6. The nested hierarchy of clusters.

    Fig. 7. Power delivery from A to B in the hierarchy of clusters.

    5.4. Layered architecture of GRIP

    A layered architecture of GRIP consisting of three layers—namely, market layer, scheduling layer, and balancing layer—is proposed (Fig. 8). Users of the grid—that is, prosumers—interact with the electricity market (sometimes called the power market)to share or trade electricity. The transaction must be scheduled and realized physically on the grid and the net power of the cluster must be balanced at all times.

    (1)Market layer.Various forms of day-ahead,hour-ahead,realtime, and other types of electricity markets are operating under different rules and regulations in different countries and regions.Bilateral trading is the simplest form of market activity for energy sharing that can be accommodated in any market. Physical realizability may require a more general multilateral trading scheme

    Fig. 8. Layered architecture of GRIP.

    [36]. Multilateral trading has been successfully implemented in an interconnected grid with five regional grids[37].Modern blockchain technology, which provides an open and distributed ledger,may be gainfully employed to facilitate multilateral trading. The trading of electricity in the market must be realizable for implementation in the scheduling layer. Off-line analysis is used to translate cluster operating limits into constraints on acceptable transactions the clusters can engage in. For clusters on the distribution system,where the networks are mostly radial and the limits are line loading and allowable voltage bands,this may be relatively simple, compared to the heavily meshed transmission system. For complex transmission systems, time-tested standard protocols for control-area operation can be used.

    (2)Scheduling layer.A prosumer may participate in one or more of the day-ahead,hour-ahead,and real-time markets to maximize his/her benefit. Preparation work for scheduling must be done in advance to ensure,first of all,that the cluster has the ability to maintain net power balance at the time of execution of the transaction. Since unexpected events or disturbances, such as outages of generation or lines, happen all the time on the grid,the cluster must have the ability to withstand and have sufficient reserve available to fill in power imbalance caused by any credible disturbance in the cluster.A cluster’s reserve is the additional controllable generation or load that is instantly available to compensate for the imbalance caused by the disturbance. The ability to withstand disturbances is called security in power system terminology. For a cluster with multiple generation and consumption,a dispatch of supply and demand may be conducted to share the resources in the most efficient and economic manner. A comprehensive tool for the scheduling function, called risk-limiting dispatch (RLD), which takes scheduling, dispatch, security, and economic considerations into account, is proposed [38,39]. RLD is built on a multi-stage stochastic optimization framework (Fig. 9)and has the following features:

    ·The stages correspond to the scheduling stages of the markets,i.e., day-ahead, hour-ahead, real-time, and so forth.

    · Stochastic nature of VRE, other types of generation,and loads is considered.

    · The control variables are the schedulable and dispatchable generation and demand in the cluster.

    · The allowed terminal states are restricted to those states whose risk of not achieving net power balance is limited to an acceptable level(similar to risk management in the banking sector).

    · Security constraints are imposed on cluster operation.

    · The objective function of the optimization is the economic benefit of the cluster.

    The symbols used in Fig. 9 are defined in Table 2.

    RLD is thus an evolutionary extension of the long list of traditional power system operational tools, including economic dispatch, unit commitment, optimal power flow, securityconstrained economic dispatch, stochastic optimal power flow,and so forth,adapted for the operation of the future grid.The adaptation of RLD by existing EMSs on the transmission system can be gradual and incremental,but simpler versions of the methodology can be developed and implemented readily in smaller clusters on the distribution system and beyond.

    Fig. 9. Multi-stage stochastic optimization framework of RLD (represented by π).

    Table 2 Symbols used in Fig. 9.

    (3)Balancing layer.The scheduling layer ensures that the net power of the cluster can be balanced within the time-step of the dispatch,which may be seconds or less.Deviations from the scheduled power balance within the time-step,due to fluctuations either in generation or load in a cluster, must be smoothed out to maintain instantaneous net power balance. An electric spring (ES) for the periphery cluster, where there are no speed-governors and exciters in synchronous generators that are used in the transmission grid for smoothing out fluctuations, is invented [40,41].ES is a power electronic device that uses load, which is prevalent in periphery clusters, to do the job. Some of the loads, such as water heaters, air conditioners, and non-essential lightings, are considered to be non-critical in the sense that they can withstand temporary reduction or increase of power with no noticeable adverse effect. The ES is connected in series with, or embedded in, non-critical loads and the combination is connected in parallel with the rest of the loads(critical loads),as shown in Fig.10.The ES dumps temporary power imbalance to non-critical loads,i.e.,when there is an over/under supply of power,the ES lets the non-critical loads consume more/less power in order to absorb the imbalance in the cluster. An ES-embedded smart load may be considered as an advanced demand response system.

    To illustrate the various functions performed by different layers of the clusters, the same example in Fig. 5 (or Fig. 7) is used again in Fig.11,where it is assumed that a re-dispatch of the generation/load in the cluster is necessary by the distribution substation cluster D.

    Fig. 10. An ES-embedded smart load.

    The responsibility of maintaining power balance in a GRIP is shared among all clusters; that means that each cluster must maintain its net power balance, with all the responsibilities of scheduling,dispatching,balancing,and security.Each cluster must operate as an autonomous unit,sticking precisely to its announced schedules, even in the event of a disturbance such as generation failure in the cluster.Each cluster must be able to prevent unscheduled power flows and acquire necessary reserve from within or from the cluster one tier above.The reserve from the cluster above will be able to maintain that cluster’s net power balance and prevent the effect of the disturbance from traveling further.The interconnected grid at the top of the tiers no longer serves as the last resort to supply or absorb imbalances, as is the practice today.There are no ‘‘free rides” for micro-grids or prosumers connected to the grid only for reliability assurance.

    In a layered architecture, the lower layer carries out tasks instructed by the upper layer, and the interface must be well defined. The market layer expects the scheduling layer to implement any transaction that is deemed acceptable by the scheduling layer. What constitutes an acceptable transaction must be well defined.The scheduling layer knows precisely the level of capability the balancing layer has in smoothing out fluctuations.There are no back-and-forth negotiations between the layers.

    6. Conclusions

    The fundamental requirement of grid operation is to maintain instantaneous power balance on the grid. The current operating paradigm is based on the assumption that a centralized grid operator is necessary to maintain power balance.In the coming new era,prosumers will have full controllability and digital intelligence to better manage their own resources and smart devices, and will no longer need to cede the authority to a grid operator. A new distributed operating paradigm can be realized by distributing the responsibility of power balance to sub-grids of the grid on the periphery to maintain individually their net power balance.A grid as smart as the internet based on this new paradigm—the GRIP—is presented.

    A GRIP has the following features and is suitable for serving the grid of the future:

    ·Better utilization of VRE.The distributed operating paradigm will lead to maximal utilization of VRE resources because the operation of VRE will rest on the hands of local stakeholders who have better knowledge to forecast, schedule, and control the resources.

    ·Empowering prosumers.Prosumers will have complete control over the operation of their own generation and load, and will have the incentive to install and operate the most efficient and effective facilities, such as solar PV, battery storage systems, EV charging systems, and ICT hardware and software.

    ·Responsibility sharing with the periphery.In the new digital era,the periphery has a similar level of intelligence and capability as the grid operator in managing its own sub-grid, as hardware gets cheaper and software gets smarter.

    Fig. 11. Power delivery from A to B in the layered architecture of clusters.

    ·Seamless integration of nano-, mini-, and micro-grids.The responsibility-sharing feature of the new paradigm is compatible with the autonomous or semi-autonomous philosophy of today’s nano-, mini-, and micro-grids, and assists their seamless integration. Moreover, the allowance of semi-autonomous operation of clusters will prevent the total grid defection of prosumers.

    ·Fast adaptation of technology innovations.The layered architecture of GRIP makes it easy to incorporate innovative new technologies.

    GRIP can evolve from the existing grid at any pace once the new operating paradigm is adopted. A transmission grid with its own EMS can be the first cluster of the interconnection. More clusters of distribution companies, each covering a sub-grid of a distribution substation, can be added. Clusters of smart homes, smart buildings, smart communities, micro-grids, and so forth can also be added. Other clusters can be flexibly formed and added, since a cluster may be part of an existing cluster and may contain any number of pre-existing clusters. The GRIP requires no scrapping of successful EMS or effective traditions in the operation of the transmission grid and focuses on adding more intelligence and responsibility to currently mostly passive distribution systems and beyond. It can be built on the success of existing grid operations and strengthens the periphery of the grid to embrace innovative new technologies.

    The energy internet is a recent trend toward integrating and managing multiple energy systems, including electricity, thermal,gas, and transport, by identifying and coordinating synergies among them in order to achieve optimal solutions for each individual sector,as well as for the overall system[42].Using excess heat from electricity production and industry for district heating has been around for decades.Three-way conversion among electricity,thermal,and gas,as well as two-way conversion between electricity and EV batteries, adds more flexibility in energy management.Different forms of energy all have the same underlying physical law—namely,the conservation of energy.We believe that the same idea of decomposing the system into a hierarchy of clusters, each maintaining its own net energy balance, will lead to an energy internet as smart as the internet.

    Acknowledgements

    The research is supported by the National Key Research and Development Program of China(2017YFB0903000),Basic Theories and Methods of Analysis and Control of the Cyber Physical System for Power Grid.

    Compliance with ethics guidelines

    Yanli Liu, Yixin Yu, Ning Gao, and Felix Wu declare that they have no conflict of interest or financial conflicts to disclose.

    精品一区二区三区视频在线| 一区二区三区乱码不卡18| 中文天堂在线官网| 一级av片app| 亚洲欧美日韩卡通动漫| 精品人妻视频免费看| 网址你懂的国产日韩在线| 国产精品一区二区在线观看99| 老司机影院毛片| 日韩在线高清观看一区二区三区| 日韩成人伦理影院| 欧美日韩综合久久久久久| 日韩视频在线欧美| 丝袜喷水一区| 亚洲成人久久爱视频| 免费看av在线观看网站| 在线亚洲精品国产二区图片欧美 | 又爽又黄a免费视频| 免费大片黄手机在线观看| 一个人看的www免费观看视频| 五月伊人婷婷丁香| 99久久中文字幕三级久久日本| 天天躁日日操中文字幕| 中文字幕制服av| 免费看光身美女| 亚洲自拍偷在线| 嫩草影院精品99| 51国产日韩欧美| 久久久久久久亚洲中文字幕| 永久网站在线| 男女边吃奶边做爰视频| a级毛色黄片| 欧美高清成人免费视频www| 国产免费福利视频在线观看| 亚洲av免费在线观看| 亚洲高清免费不卡视频| 久久久a久久爽久久v久久| 人体艺术视频欧美日本| 99热网站在线观看| 国产精品嫩草影院av在线观看| 久久久久久久大尺度免费视频| 精品少妇黑人巨大在线播放| 亚洲熟女精品中文字幕| 毛片一级片免费看久久久久| 在线亚洲精品国产二区图片欧美 | 97热精品久久久久久| 欧美极品一区二区三区四区| 看十八女毛片水多多多| 99久久中文字幕三级久久日本| 午夜老司机福利剧场| 亚洲图色成人| 亚洲色图av天堂| 精华霜和精华液先用哪个| 18禁在线播放成人免费| 嘟嘟电影网在线观看| 亚洲国产精品999| 亚洲av欧美aⅴ国产| 国产精品人妻久久久影院| 2021少妇久久久久久久久久久| 国产亚洲av片在线观看秒播厂| 五月开心婷婷网| 日韩伦理黄色片| 免费观看无遮挡的男女| 国产91av在线免费观看| av天堂中文字幕网| 久久人人爽人人爽人人片va| 99热6这里只有精品| 老师上课跳d突然被开到最大视频| 国产91av在线免费观看| 高清av免费在线| 青青草视频在线视频观看| 王馨瑶露胸无遮挡在线观看| 啦啦啦啦在线视频资源| 波多野结衣巨乳人妻| h日本视频在线播放| 在线看a的网站| 26uuu在线亚洲综合色| 18禁裸乳无遮挡动漫免费视频 | 国产成人aa在线观看| 又爽又黄a免费视频| 在线播放无遮挡| 两个人的视频大全免费| 国产成人91sexporn| 成人特级av手机在线观看| 免费观看的影片在线观看| 如何舔出高潮| 午夜免费观看性视频| 99精国产麻豆久久婷婷| 亚洲精品自拍成人| 日韩三级伦理在线观看| 亚洲综合精品二区| 成人二区视频| 午夜视频国产福利| 在线a可以看的网站| 亚洲精品亚洲一区二区| 亚洲国产欧美在线一区| 丰满少妇做爰视频| 国产亚洲一区二区精品| 美女视频免费永久观看网站| 国产精品麻豆人妻色哟哟久久| av专区在线播放| 大香蕉久久网| 午夜福利在线在线| 亚洲综合色惰| av在线播放精品| 简卡轻食公司| 亚洲国产精品国产精品| a级一级毛片免费在线观看| 国产成人精品福利久久| 欧美日韩一区二区视频在线观看视频在线 | 日韩一区二区三区影片| 在线精品无人区一区二区三 | 欧美激情在线99| 国产久久久一区二区三区| 插逼视频在线观看| 国产国拍精品亚洲av在线观看| 成年人午夜在线观看视频| 日韩欧美一区视频在线观看 | 精品99又大又爽又粗少妇毛片| 精品视频人人做人人爽| 亚洲精品自拍成人| 亚洲国产精品成人综合色| 亚洲av不卡在线观看| 国产精品一区二区三区四区免费观看| 国产男人的电影天堂91| 男人爽女人下面视频在线观看| 看黄色毛片网站| 日产精品乱码卡一卡2卡三| 国产精品女同一区二区软件| 尤物成人国产欧美一区二区三区| 成人毛片a级毛片在线播放| 国产高清国产精品国产三级 | 人妻制服诱惑在线中文字幕| 日本wwww免费看| 禁无遮挡网站| 国产毛片在线视频| 一级毛片 在线播放| 色婷婷久久久亚洲欧美| 成人午夜精彩视频在线观看| 日韩精品有码人妻一区| 熟女人妻精品中文字幕| 99re6热这里在线精品视频| 免费播放大片免费观看视频在线观看| 午夜爱爱视频在线播放| 色综合色国产| 亚洲国产欧美在线一区| 国产欧美日韩一区二区三区在线 | 狂野欧美白嫩少妇大欣赏| 亚洲国产精品999| 日韩伦理黄色片| 插阴视频在线观看视频| 美女被艹到高潮喷水动态| 久久久午夜欧美精品| 视频中文字幕在线观看| 国产久久久一区二区三区| 国内精品美女久久久久久| 成人特级av手机在线观看| 又爽又黄无遮挡网站| 免费av不卡在线播放| 在线观看三级黄色| 香蕉精品网在线| 高清午夜精品一区二区三区| 18禁在线播放成人免费| 直男gayav资源| 国产女主播在线喷水免费视频网站| 嫩草影院精品99| 国产91av在线免费观看| 在线看a的网站| 偷拍熟女少妇极品色| 熟妇人妻不卡中文字幕| 国内少妇人妻偷人精品xxx网站| 日韩成人av中文字幕在线观看| 蜜桃久久精品国产亚洲av| 日本免费在线观看一区| 免费观看性生交大片5| 乱码一卡2卡4卡精品| 在线观看av片永久免费下载| 好男人视频免费观看在线| 大又大粗又爽又黄少妇毛片口| 亚洲精品日韩在线中文字幕| 亚洲国产最新在线播放| 中国美白少妇内射xxxbb| 国内揄拍国产精品人妻在线| 51国产日韩欧美| 国产精品.久久久| 亚洲国产欧美在线一区| 中文字幕免费在线视频6| 永久网站在线| 综合色丁香网| 国产色婷婷99| 亚洲av成人精品一二三区| 毛片女人毛片| 中国美白少妇内射xxxbb| 99热网站在线观看| 国产精品久久久久久精品古装| 波野结衣二区三区在线| 超碰av人人做人人爽久久| 直男gayav资源| 国产永久视频网站| 日韩欧美精品免费久久| 欧美+日韩+精品| 一级毛片 在线播放| 秋霞在线观看毛片| 最近中文字幕2019免费版| 亚洲国产欧美人成| 97在线人人人人妻| 免费观看a级毛片全部| 最近手机中文字幕大全| 内射极品少妇av片p| 国产欧美另类精品又又久久亚洲欧美| 国产v大片淫在线免费观看| 午夜福利在线观看免费完整高清在| 欧美性感艳星| 大又大粗又爽又黄少妇毛片口| 国产 一区精品| 国内精品宾馆在线| 成人高潮视频无遮挡免费网站| 国产精品人妻久久久影院| 国产精品久久久久久精品古装| 国产熟女欧美一区二区| 嫩草影院新地址| 国产淫语在线视频| www.色视频.com| 最近2019中文字幕mv第一页| 天堂中文最新版在线下载 | 久久久精品免费免费高清| 又大又黄又爽视频免费| 久久久久性生活片| 日本免费在线观看一区| 六月丁香七月| 欧美激情国产日韩精品一区| 亚洲国产欧美人成| 免费少妇av软件| av国产免费在线观看| 人妻少妇偷人精品九色| 色网站视频免费| 亚洲精品国产色婷婷电影| av在线app专区| 免费高清在线观看视频在线观看| 最后的刺客免费高清国语| 国产毛片在线视频| 国产精品人妻久久久影院| 免费看日本二区| 边亲边吃奶的免费视频| 久久午夜福利片| 亚洲精华国产精华液的使用体验| 一级片'在线观看视频| 欧美成人精品欧美一级黄| 国产成人精品久久久久久| 久久精品久久精品一区二区三区| 免费看不卡的av| 日日撸夜夜添| 国产久久久一区二区三区| 午夜福利在线在线| 在线 av 中文字幕| 国产黄片美女视频| 中国国产av一级| 男女那种视频在线观看| 精华霜和精华液先用哪个| 亚洲欧美中文字幕日韩二区| 国产有黄有色有爽视频| 亚洲婷婷狠狠爱综合网| 国产伦理片在线播放av一区| 国产亚洲午夜精品一区二区久久 | 噜噜噜噜噜久久久久久91| 中文在线观看免费www的网站| 亚洲自拍偷在线| 男人爽女人下面视频在线观看| 一个人看的www免费观看视频| 噜噜噜噜噜久久久久久91| 男女那种视频在线观看| 国产精品一及| 日韩亚洲欧美综合| 黄色配什么色好看| 日本av手机在线免费观看| 国产伦精品一区二区三区四那| 高清日韩中文字幕在线| 高清毛片免费看| 只有这里有精品99| 99热6这里只有精品| 国产av码专区亚洲av| 综合色丁香网| 高清视频免费观看一区二区| 亚洲精品国产色婷婷电影| 真实男女啪啪啪动态图| 日韩精品有码人妻一区| 欧美极品一区二区三区四区| 国产 一区精品| 国产中年淑女户外野战色| 三级男女做爰猛烈吃奶摸视频| 在线观看美女被高潮喷水网站| 可以在线观看毛片的网站| 亚洲天堂av无毛| 日韩 亚洲 欧美在线| 人体艺术视频欧美日本| 亚洲婷婷狠狠爱综合网| 看十八女毛片水多多多| 亚洲,一卡二卡三卡| 久久亚洲国产成人精品v| 国产亚洲91精品色在线| 真实男女啪啪啪动态图| 国产成人免费观看mmmm| 日本三级黄在线观看| 亚洲天堂av无毛| 国产中年淑女户外野战色| 国产成人精品久久久久久| 国模一区二区三区四区视频| 成年版毛片免费区| 久久97久久精品| 亚洲无线观看免费| 2018国产大陆天天弄谢| 久热久热在线精品观看| 在线观看av片永久免费下载| 在现免费观看毛片| 男女无遮挡免费网站观看| 免费观看a级毛片全部| 久久精品国产亚洲网站| 欧美精品一区二区大全| 大香蕉97超碰在线| 插逼视频在线观看| 插阴视频在线观看视频| 九九在线视频观看精品| 精品久久久久久久末码| 波野结衣二区三区在线| 丰满少妇做爰视频| 国产老妇女一区| 国产成人a∨麻豆精品| 久久久久久久久久成人| 久久人人爽av亚洲精品天堂 | 亚洲精品久久久久久婷婷小说| 久久国内精品自在自线图片| 成人亚洲欧美一区二区av| 一级毛片久久久久久久久女| 欧美日本视频| 美女xxoo啪啪120秒动态图| 一级毛片aaaaaa免费看小| 亚州av有码| 国产在线一区二区三区精| 中文欧美无线码| 国产中年淑女户外野战色| 亚洲精品aⅴ在线观看| av国产久精品久网站免费入址| 久久久久久久久久久免费av| 黄色怎么调成土黄色| 国产欧美日韩精品一区二区| 日韩av免费高清视频| 又黄又爽又刺激的免费视频.| 视频区图区小说| 国产69精品久久久久777片| 中文天堂在线官网| 在线免费观看不下载黄p国产| 天堂中文最新版在线下载 | 亚洲,欧美,日韩| av一本久久久久| 可以在线观看毛片的网站| 自拍欧美九色日韩亚洲蝌蚪91 | 免费看av在线观看网站| 少妇人妻一区二区三区视频| 国产一级毛片在线| 看非洲黑人一级黄片| 干丝袜人妻中文字幕| 日韩av不卡免费在线播放| 午夜爱爱视频在线播放| 日韩av不卡免费在线播放| 熟女人妻精品中文字幕| 欧美亚洲 丝袜 人妻 在线| 好男人在线观看高清免费视频| 亚洲成人av在线免费| 日日摸夜夜添夜夜爱| 国产成人freesex在线| 听说在线观看完整版免费高清| 99热这里只有是精品50| 我的女老师完整版在线观看| 丝袜喷水一区| 亚洲av福利一区| 在线观看一区二区三区激情| 最近最新中文字幕免费大全7| av在线app专区| 日本wwww免费看| 亚洲精品乱码久久久v下载方式| 欧美日韩精品成人综合77777| 亚洲精品第二区| 欧美三级亚洲精品| 色婷婷久久久亚洲欧美| 日韩av在线免费看完整版不卡| 久久久久九九精品影院| 欧美xxⅹ黑人| 国产精品一区www在线观看| 国产免费福利视频在线观看| 春色校园在线视频观看| av.在线天堂| 日本一二三区视频观看| 成人亚洲欧美一区二区av| 亚洲,一卡二卡三卡| 五月开心婷婷网| 一个人观看的视频www高清免费观看| 亚洲经典国产精华液单| 最后的刺客免费高清国语| 国产精品一区二区在线观看99| 一区二区三区免费毛片| 亚洲成人av在线免费| 日本一二三区视频观看| 1000部很黄的大片| av女优亚洲男人天堂| 成人毛片a级毛片在线播放| 男人爽女人下面视频在线观看| 欧美日韩一区二区视频在线观看视频在线 | 在线亚洲精品国产二区图片欧美 | 日韩一区二区视频免费看| 日本猛色少妇xxxxx猛交久久| 一级毛片aaaaaa免费看小| 有码 亚洲区| 久久久欧美国产精品| 嫩草影院精品99| 久久精品国产亚洲av涩爱| 中国国产av一级| 国产成人freesex在线| 久久97久久精品| 亚洲色图av天堂| 精品久久久精品久久久| 1000部很黄的大片| 天堂俺去俺来也www色官网| 国产真实伦视频高清在线观看| 欧美成人精品欧美一级黄| 26uuu在线亚洲综合色| 亚洲图色成人| 大码成人一级视频| 一级毛片aaaaaa免费看小| a级毛片免费高清观看在线播放| 亚洲性久久影院| 久久精品久久精品一区二区三区| 久久女婷五月综合色啪小说 | 黄色配什么色好看| 欧美日韩国产mv在线观看视频 | 日本猛色少妇xxxxx猛交久久| 久久久精品免费免费高清| 我要看日韩黄色一级片| 特级一级黄色大片| av国产久精品久网站免费入址| 高清午夜精品一区二区三区| 欧美xxxx性猛交bbbb| 2021天堂中文幕一二区在线观| 18禁动态无遮挡网站| 久久久久久久久大av| 国产一区有黄有色的免费视频| 夫妻午夜视频| 人人妻人人爽人人添夜夜欢视频 | 日韩制服骚丝袜av| 成人美女网站在线观看视频| 亚洲精品国产色婷婷电影| 不卡视频在线观看欧美| 一个人看视频在线观看www免费| 久久久精品94久久精品| 国产伦在线观看视频一区| 91在线精品国自产拍蜜月| 亚洲经典国产精华液单| videossex国产| 寂寞人妻少妇视频99o| 亚洲欧美一区二区三区国产| 少妇人妻一区二区三区视频| 亚洲精品一区蜜桃| 国产精品人妻久久久影院| 国产伦精品一区二区三区四那| 视频中文字幕在线观看| h日本视频在线播放| 能在线免费看毛片的网站| 我的老师免费观看完整版| 51国产日韩欧美| 欧美日韩在线观看h| 亚洲精品乱码久久久v下载方式| 国产精品国产三级国产av玫瑰| 亚洲av中文字字幕乱码综合| 狂野欧美激情性xxxx在线观看| 国产成人freesex在线| 欧美xxxx黑人xx丫x性爽| 日本-黄色视频高清免费观看| 国产精品一区www在线观看| 国产黄频视频在线观看| 精品99又大又爽又粗少妇毛片| 亚州av有码| 波多野结衣巨乳人妻| 国产高清有码在线观看视频| 99视频精品全部免费 在线| 国产淫片久久久久久久久| 男女边摸边吃奶| 亚洲精品中文字幕在线视频 | 精品国产一区二区三区久久久樱花 | 在线观看免费高清a一片| 精品国产露脸久久av麻豆| 久久久色成人| 日产精品乱码卡一卡2卡三| 久久久久久久国产电影| 五月玫瑰六月丁香| 欧美激情国产日韩精品一区| 精品久久久精品久久久| 男人舔奶头视频| 中文字幕亚洲精品专区| 欧美精品国产亚洲| 久久久久久久久久成人| 最后的刺客免费高清国语| 国内少妇人妻偷人精品xxx网站| 久热久热在线精品观看| 免费不卡的大黄色大毛片视频在线观看| 国产又色又爽无遮挡免| 69av精品久久久久久| 一级毛片黄色毛片免费观看视频| 特大巨黑吊av在线直播| 国产成人免费观看mmmm| 欧美日本视频| 欧美成人一区二区免费高清观看| 男的添女的下面高潮视频| 亚洲精品456在线播放app| 美女国产视频在线观看| 另类亚洲欧美激情| 国产免费视频播放在线视频| 三级经典国产精品| 97精品久久久久久久久久精品| 91aial.com中文字幕在线观看| 久久精品国产a三级三级三级| 亚洲内射少妇av| 特级一级黄色大片| 午夜福利视频1000在线观看| 黄片无遮挡物在线观看| 乱码一卡2卡4卡精品| 在线观看国产h片| 99久久九九国产精品国产免费| 久久国内精品自在自线图片| 国产黄频视频在线观看| 涩涩av久久男人的天堂| 噜噜噜噜噜久久久久久91| 久久久久久久久久人人人人人人| 麻豆成人午夜福利视频| 五月伊人婷婷丁香| 久久久久久久亚洲中文字幕| 18禁在线播放成人免费| 一级毛片黄色毛片免费观看视频| 啦啦啦啦在线视频资源| 最近最新中文字幕免费大全7| 亚洲国产精品999| 亚洲欧美清纯卡通| 欧美亚洲 丝袜 人妻 在线| 中文在线观看免费www的网站| 国产视频首页在线观看| 欧美日韩综合久久久久久| 人妻系列 视频| 久久久久久伊人网av| 中国美白少妇内射xxxbb| 亚洲精品国产av成人精品| 汤姆久久久久久久影院中文字幕| 中文在线观看免费www的网站| 夫妻午夜视频| 成人亚洲欧美一区二区av| 麻豆精品久久久久久蜜桃| 少妇人妻 视频| 国产精品国产av在线观看| 欧美日韩国产mv在线观看视频 | 一级毛片电影观看| 成人亚洲精品一区在线观看 | 国产一级毛片在线| 国产成年人精品一区二区| 国产成人一区二区在线| 国产精品熟女久久久久浪| 欧美日韩在线观看h| 夜夜爽夜夜爽视频| 亚洲国产精品成人综合色| 午夜福利高清视频| 亚洲精品乱码久久久v下载方式| 日韩强制内射视频| 美女脱内裤让男人舔精品视频| 久久精品久久久久久久性| 亚洲精品色激情综合| 日韩国内少妇激情av| 国产人妻一区二区三区在| 日韩精品有码人妻一区| 波多野结衣巨乳人妻| 一级毛片我不卡| 久久99蜜桃精品久久| 国产色婷婷99| 国产av国产精品国产| 中文字幕av成人在线电影| 熟妇人妻不卡中文字幕| av一本久久久久| 欧美区成人在线视频| 性色av一级| 自拍欧美九色日韩亚洲蝌蚪91 | 精品一区二区三卡| 国精品久久久久久国模美| 1000部很黄的大片| 亚洲av不卡在线观看| 国产视频首页在线观看| 高清午夜精品一区二区三区| 国产精品久久久久久精品古装| 啦啦啦在线观看免费高清www| 国产黄片美女视频| 少妇熟女欧美另类| 国产成年人精品一区二区| 白带黄色成豆腐渣| 久久人人爽人人片av| 五月玫瑰六月丁香| 少妇高潮的动态图| 人妻系列 视频| 亚洲av免费在线观看| 女人久久www免费人成看片| 一级黄片播放器| 免费不卡的大黄色大毛片视频在线观看| 麻豆成人av视频| 久久久久网色| 在线观看一区二区三区| 欧美老熟妇乱子伦牲交| 热99国产精品久久久久久7| 免费人成在线观看视频色| 视频中文字幕在线观看| 国产高潮美女av| 欧美成人午夜免费资源| 久久综合国产亚洲精品|