A SURVEY OF ADVANCED TECHNOLOGIES ON THE MODERN GRID Kabeed Mansur and Kevin Porter, Exeter Associates Draft: NOT FOR CITATION OR ATTRIBUTION Introduction A fully modernized power grid is essential for providing electric service that is reliable, economical, and secure. We define the modern grid for the purposes of this paper as a two dimensional system. These two dimensions are: The Physical Dimension – this refers to the physical infrastructure (physical grid) of the modern grid, which is comprised of three sectors, namely: Generation, Transmission, and Distribution. The Operational Dimension – this refers to system operations (operational grid) which is the domain of the system operator. The primary function of the system operator is to keep the system in balance, that is, ensuring that supply (i.e., generation) and demand (i.e., load) are in equilibrium at all times. The goal of this paper is to analyze the set of advanced technologies currently deployed on the modern grid and understand how they relate to the physical grid as well as how they support the operational grid. Our analysis is organized into three sections: Section I – Overview of the Energy Infrastructure: This section examines at the physical infrastructure of the modern grid, and focus on the functional and physical characteristics (i.e., hardware, equipment, devices, etc.) of each sector. Section II – Overview of System Operations: In this section we will define system operations, and, the functional role of the system operator. Section III – Overview of Advanced Grid Technologies: This section will be the crux of our analysis. The core technologies deployed on the modern grid, with respect to sensing and measurement technologies, advanced control methods, and communication technologies.
Transcript
A SURVEY OF ADVANCED TECHNOLOGIES ON THE MODERN GRIDKabeed Mansur and Kevin Porter, Exeter Associates
Draft: NOT FOR CITATION OR ATTRIBUTION
Introduction
A fully modernized power grid is essential for providing electric service that is reliable, economical, and secure. We define the modern grid for the purposes of this paper as a two dimensional system. These two dimensions are:
The Physical Dimension – this refers to the physical infrastructure (physical grid) of the modern grid, which is comprised of three sectors, namely: Generation, Transmission, and Distribution.
The Operational Dimension – this refers to system operations (operational grid) which is the domain of the system operator. The primary function of the system operator is to keep the system in balance, that is, ensuring that supply (i.e., generation) and demand (i.e., load) are in equilibrium at all times.
The goal of this paper is to analyze the set of advanced technologies currently deployed on the modern grid and understand how they relate to the physical grid as well as how they support the operational grid.
Our analysis is organized into three sections:
Section I – Overview of the Energy Infrastructure: This section examines at the physical infrastructure of the modern grid, and focus on the functional and physical characteristics (i.e., hardware, equipment, devices, etc.) of each sector.
Section II – Overview of System Operations: In this section we will define system operations, and, the functional role of the system operator.
Section III – Overview of Advanced Grid Technologies: This section will be the crux of our analysis. The core technologies deployed on the modern grid, with respect to sensing and measurement technologies, advanced control methods, and communication technologies.
I. Overview of the Energy Infrastructure
The electric grid is comprised of three sectors: generation, transmission, and distribution. Generation produces electricity, transmission carries this electricity via high voltage power lines, and distribution delivers the electricity to end use customers. Figure 1.1 shows how these three sectors are connected on the grid: Figure 1.1
Source: University of Idaho, Principles of Sustainability (Ch.6)
Generation:
The generation sector is responsible for the bulk production of electric power. Generators produce electricity by converting primary energy sources like fossil fuels, nuclear, hydro, wind, and solar power into electric energy.
Generators are connected to the transmission grid via a ‘step up’ transformer. Step up transformers are responsible for increasing the voltage of the electricity exiting the generation station in order to match the voltage level of the transmission line.
With respect to system operations, generating units are classified into three categories: Baseload, Intermediate, and Peaking units. Baseload units are used to meet the constant (i.e. base) power needs of the system and to this end they are inflexible (that is, their volumetric electric output cannot be changed) and run continuously (24 hours a day). Intermediate units are typically operated for an extended period of time to cover morning (mid-morning to evening). These units are used because their operational flexibility allows them to be ramped up and down in response to load fluctuations. Peaking units are typically operated/brought on-line when the system demand is near its peak. Peaking units are similar to intermediate units, in terms of operational flexibility; however, due to higher variable costs, these units are only used during peak demand hours (early afternoon to early evening). Peaking units
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Figure 1.1 - Energy Infrastructure of the Modern Grid
run for a limited number of hours per year and may be restricted in the number of operating hours due to environmental restrictions. Figure 1.2 shows which generation resources are used most often, with respect to each of the categories outlined above:
The transmission system is responsible for carrying bulk electric power over long distances. In this process electricity is moved from a central generating unit to an interconnection with an electrical distribution system. To this end, we can think of the transmission system acting as the electrical highway connecting supply (generation) to demand (load).
The transmission system (see Figure 1.3) is composed of high voltage power lines (>60 kV), stations, and substations. Stations and substations typically house the following equipment/devices:
Transformers are used to change the voltage level up or down. Generation side transformers increase the voltage level of the electrical output, while demand side transformers decrease the voltage;
Switchgears includes circuit breakers and other types of switches that can be used to turn on or off parts of the transmission network in order to protect the system and maintain reliability;
Measurement Instrumentation are sensors that are used to collect voltage, current, and power data for monitoring and control purposes; and
Communications Equipment are used to transmit the data collected from sensors and measurement instruments to the system operator, and, this equipment can also be used to remotely control switchgears.
With respect to topology, the transmission system has a mesh network type configuration – this means that there exists multiple pathways between any two points on the transmission network (see Figure 1.4).
Figure 1.4 Topological Illustration of Transmission Network Mesh
The mesh topology of the transmission system allows for greater redundancy, this redundancy in turn allows the power system to be able to supply power to loads in the event that a transmission line or generating unit goes offline.
The amount of power a transmission line carries is not unlimited. There exists three primary constraints with respect to the capacity of a transmission line:
• Thermal Constraints1 refer to the max temperature a transmission line can handle. To this end, the temperature in a transmission line is a function of that lines’ loss rate. Losses increase the temperature of a transmission line causing the line to stretch and sag, at some maximum temperature, the sag is sufficient enough to reduce the lines’ capacity factor;
• Voltage Stability Constraint2 voltage stability is defined as the ability of a power system to maintain steady voltages at all buses after a disturbance event. The voltage stability constraint arises due to reactance of a transmission line, this in turn causes the voltage at the far end of the line to drop below some allowable level; and
• Transient Constraint3 refers to the tolerance threshold of a transmission line with respect to changes in the power flowing through, if a transmission line exceeds its transient tolerance it will cause generators to fall out of synch with each other.
Figure 1.5 illustrates graphically the limiting factors most commonly associated with these three constraints, with respect to short, medium, and long range transmission lines.
1 Source: MIT, Future of the Electrical Grid, Ch. 22 Ibid3 Ibid
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Figure 1.5 Common Limits to Transmission Carrying Capacity
_______________Source: MIT, Future of the Electrical Grid, Ch. 2
Distribution (work in progress)
The distribution system serves acts as the interconnector between the transmission system and end-use consumption. Distribution typically refers to electric systems with voltages lower than 60kV. A distribution system is composed of the following elements:
Distribution Lines Transformers Voltage Regulators Witches Circuit Breakers Automatics Reclosers Power Capacitors Monitoring Systems Service Drops
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II. Overview of System Operations
The objective of system operations is to ensure that the electrical system is operating reliably; to that end, system operators must keep supply (generation) and demand (load) in balance at all times, as well as maintain and control system voltages and frequencies. In order to carry out these functions system operators must:
Forecast demand in the day-ahead; Schedule generation (or its applicable demand response) to match forecasted demand; Schedule reserves and other ancillary services; Schedule use of the transmission system; Communicate schedules to neighboring operators so that power flows across interconnections
can be anticipated; Manage and control the electrical system by correcting supply and demand imbalances in real
time; and Correct any system disturbances, and restore power in the event an outage occurs.
Load Forecasting and Scheduling
System operators are responsible for scheduling generation in order to meet the expected system demand. To do this, the scheduling process starts by first forecasting system demand, otherwise known as load forecasting. Generally, system operators prepare load forecasts using statistical models based on historical demand and current weather forecasts. System operators then use the output from these models in order to develop hourly, day-ahead demand forecasts. The models are rerun during the operating day such that forecasts can be continually adjusted based on changes in weather or other exogenous variables that affect demand.
Once the system operator has prepared the load forecast, they then take an inventory of all available generation resources. Based on this inventory, system operators schedules available generation on an hour-by-hour basis in order to match the expected system load as well as meet system reserve4 requirements.
The system operator schedules the available generation based on a principle known as least-cost dispatch5. Figure 2.1 presents a simple example to help illustrate/model this process.
4 Reserves – Generation capacity that is available to the system operator if needed, but that is not currently generating electricity.5 Least-Cost Dispatch - The operation of generation facilities to produce energy at the lowest cost to reliably serve consumers, recognizing any operational limits of generation and transmission facilities.
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Reserves
Reserves may be divided into spinning (synchronized to the grid and able to operate within a short period of time, such as 10 minutes) and non-spinning (not synchronized to the grid but able to respond and reach full output within a defined period of time, such as 30 minutes). The amount of receivers is usually set to meet a reliability standard, such as minimizing unserved energy to one day in ten years. Reserve levels vary, but a general range is from 10-15 percent of peak demand.
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Figure 2.1 - SIMPLE EXAMPLE OF SCHEDULING
Unit 1400 MW @ $45/MWh
Unit 2100 MW @ $35/MWh
Unit 3100 MW @ $40/MWh Unit 4
200 MW @ $25/MWh
Unit 5200 MW @ $55/MWh
400 MW
Load Forecast
In this simple model, system operations has forecast 400 MW of load for the hour. In attempting to optimize scheduling, the system operator would prefer to schedule:
Unit 4 $25 200 MW
Unit 2 $35 100 MW
Unit 3 $40 100 MW
Unfortunately, this is clearly not feasible given the limited capacity of transmission line B (only 200 MW). So the optimized dispatch, subject to constraints will be:
Generation Schedule
Unit 4 $25 200 MW
Unit 2 $35 100 MW
Unit 3 $40 100 MW
Transmission Schedule
Line A 200 MW
Line B 200 MW
In addition to the units scheduled for energy, the operator will also need reserves. Since the greatest single loss contingency is 200 MW on Line B, the operator will need to schedule 200 MW of reserves. The safest place to obtain the reserves is from Unit 5, since the loss of Line A would create a situation where reserves from Unit 1 would not be available to the system. If Unit 5 is scheduled for reserves, this would also necessitate scheduling Line C for 200 MW to ensure the transmission capacity is available if supply from Unit 5 is needed.
III. Sensing and Measurement Technologies
In order for system operators to carry out their operational functions they require continuous information regarding the state of the electric system. To this end, the efficient operation of the modern electric grid is made possible through a series of sensors and measurement devices which collect information from different parts of the physical grid and relay this information to the system operator. In Table 3.1 we identify the critical information collected by sensors and measurement devices installed on the physical grid.
Table 3.1 Grid Information Collected by Service and Measurement Devices
Generation Transmission Distribution
Generation equipment information regarding availability (e.g., online, not online)
Information from sensors monitoring the state of high-voltage power lines
Power usage information collected from customer meters
Information from sensors monitoring the interconnections with the transmission grid
Information from sensors monitoring the state of devices in the transmission substations
Information from sensors monitoring the state of devices in the distribution substations
Available capacity for individual generators particularly important with respect to variable generation (i.e., wind and solar).
Information from phasor measurement units (PMUs) monitoring power flow on the transmission grid
Information from sensors monitoring the state of distribution feeders (a transmission line carrying electricity to a distribution point)
_______________Source: CISCO, Smart Grid – Leveraging Intelligent Communication to Transform the Power Infrastructure
This section focuses on two specific sensing and measurement technologies which are widely used on the modern electric grid, namely:
1. Supervisory Control and Data Acquisition Systems (SCADA)2. Phasor Measurement Units (PMUs)
SCADA Systems
SCADA systems are used by system operators to collect real time data in order to monitor and control generation, transmission and distribution equipment. This section will consider only the data acquisition dimension of SCADA systems – the supervisory control dimension will be covered in the section corresponding to advanced control methods and technologies.
The data acquisition portion of SCADA allows system operators to remotely monitor electrical quantities such as voltage and current in real time. SCADA systems accomplish this function by using sensor devices installed on generators and at distribution substations to measure state variables such as voltage, current, and power levels. This data is then collected by devices known as remote terminal units (RTUs).
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SCADA systems also contain a backend software application known as a state estimator. The state estimator receives the measurement data from the SCADA RTUs. The state estimator then processes the using advanced algorithms and “calculates all the load flows and critical voltage points in the system, and calibrates them to real time values….[the] state estimator uses all available measurements, known facts, and other relevant information to calculate the best possible estimate of the true status (“state”) of the power system. For example, the state estimator is used to calculate new power flow conditions, such as voltages and currents, to help system operators predict “what if” scenarios”.6 Figure 3.2 presents a conceptual model of the SCADA data acquisition process.
Figure 3.2 Data Needed for SCADA Systems
_______________ Source: CISCO, Smart Grid – Leveraging Intelligent Communication to Transform the Power Infrastructure
PMUs
PMUs are used to “estimate voltages and currents at substations, generators, and load center…system frequency and other quantities are also measured.” Combined with known line characteristics, PMU measurements can determine instantaneous power flows throughout the system…PMUs [can also provide much faster] data than SCADA systems, which results in higher-resolution information about the status of the grid. Therefore, measurements from all PMUs can be synchronized using GPS time signals, [thus] allowing for a more accurate assessment of the status of the grid.”7
PMUs also measure the electrical waves on high voltage AC transmission lines. To understand how this metric helps the system operator carry out its function with respect to reliable operation, it is important to understand what a phasor is. To this end, a phasor is a complex number which represents both the magnitude and phase angle of the sine waves generated by the flow of electrons through a high voltage AC transmission line. Thus, if a large number of PMUs can be installed on the transmission grid, system operators can compare - in real time - the shapes of these electrical waves at various points on the transmission grid. System operators can then use this data to help measure the state of the power system as well as respond to system conditions in a rapid and dynamic way. 8
6 Source: Blume, Electric Power System Basics, Ch. 97 Source: MIT, Future of Electric Grid, Ch. 28 Source: Fang, Smart Grid – The New and Improved Power Grid: A Survey.
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Advanced Control Methods and Technologies
Advanced control methods and technologies refer to devices and algorithms that use backend analytics to help diagnose, evaluate and predict different conditions on the electric grid. Advanced control methods and technologies also have the ability to autonomously take corrective actions to mitigate, eliminate and prevent outages or other grid reliability issues.9
In this section two prominent control methods and technologies currently in use are reviewed:
1. SCADA systems2. Governor Control (GC)
SCADA Systems
In the prior section we looked at the data acquisition dimension of SCADA systems, in this section we consider the systems supervisory control dimension. The basic function of the SCADA system, with respect to supervisory control, is to operate all critical equipment in each substation from a single control center.
Figure 3.3 – Inputs and Outputs to a SCADA System
9 Source: Enose, Advanced Technologies Implementation Framework for a Smart Grid
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_______________Source: Modern SCADA Philosophy in Power System Operation].
SCADA systems in operational control centers allow for the centralized monitoring and control of various devices and equipment on the physical grid, specifically equipment housed at distribution substations. The control function of the SCADA system relies on the data acquisition process. In general, the supervisory control process begins with the system operator receiving actionable information from the state estimator and using this information to send either an automated or operator-driven supervisory command to an RTU, the RTU then passes this command onto the specified field devices. Field devices, also called remote station control devices, have the ability to control local operations such as opening and closing valves or circuit breakers based on a set of received SCADA control commands.10
Governor Control (AGC)
The electrical grid can become unstable if the balance between supply and demand, i.e., the voltage and frequency may exceed allowable bounds which can in turn result in damaged equipment as well as service interruptions. To this end, the balance between supply and demand, in the short run, is maintained by generators equipped with governor control.
The governor is a device that controls the mechanical power driving a generator via the valve limiting the amount of steam, water, or gas flowing to the turbine. The governor acts in response to locally measured changes in the generator’s output frequency relative to some established system standard; the standard in the U.S. is 60 Hz. If the electrical load on the generator is greater than the mechanical power driving it, the generator maintains power balance by converting some of its kinetic energy into extra output power—but slows down in the process. On the other hand, if the electrical load is less than the mechanical power driving the generator, the generator absorbs the extra energy as kinetic energy and speeds up. This behavior is known as “inertial response.”
10 Source: Modern SCADA Philosophy in Power System Operation
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The frequency of the AC voltage produced by a generator is proportional to its rotational speed. Therefore, changes in generator rotational speed can be tracked by the generator’s output frequency. A decreasing frequency indicates that real power consumption is greater than generation, while an increasing frequency indicates that generation is exceeding power consumption. Any changes in frequency are sensed within a fraction of a second, and the governor responds within seconds by altering the position of the valve— increasing or reducing the flow to the turbine. If the frequency is decreasing, the valve will be opened further to increase the flow and provide more mechanical power to the turbine, hence increasing the generator’s output power, bringing demand and supply in balance and stabilizing the speed of the generator at this reduced level. The speed of the generator will stay constant at this level as long as the mechanical power driving it balances its electrical load. 11
the power system.
Communication Technology (work in progress)
Communication technologies refer to the specific communication networks used to connect various parts, systems, and devices on the modern. Communication networks allow for real time exchange of system data between the physical grid and the operational grid. Therefore, without proper communication networks, system operators would not be able to effectively monitor and control the power system.
We will organize our analysis here into three subsections:
Communication System Elements Communication System Schemes
Communication System Elements
Communication systems for the modern power grid are designed to carry out a single core function, namely, transfer data and information between various layers of the power system. To this end, a typical communication system contains three elements:
Transmitter - a set of equipment used to generate and transmit electromagnetic waves carrying messages or signals
Channel - is a particular type of media through which a message is sent and received Receiver – a device that receives and extracts the information contained in the electromagnetic
signals sent by a transmitter information
Figure 3.4 shows the information flow through a typical communication system.
Figure 3.4
11 Source: MIT, Future of the Electric Grid, Appendix B.2
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Survey of Communication Systems on the Modern Grid
In this section we look at the communication system that are currently used on the modern power grid, to this end, we will consider, in turn, the following systems:
Power Line Carrier Packet Switching
Power Line Carrier (PLC)
PLC is a communication system that uses power lines (transmission and distribution) as the primary communication channel to send and receive data. A PLC network structure is divided in two main parts.
1. PLC network parallel to the medium voltage grid 2. PLC network parallel to the low voltage grid.
The border and end components of the network are shown in figure 3.5.
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Medium Voltage PLC Network
Low Voltage PLC Network
Medium Voltage Head End: It enables the communication between the Backbone or the main communications network and the PLC network.
Medium Voltage Modem: it is the interface between a Medium Voltage PLC Network and a Low Voltage PLC network on the Medium Voltage side.
Low Voltage Head End: It represents the end of the Low Voltage PLC network and is a gateway to the Medium Voltage network which can be PLC or otherwise. The low voltage head end is normally placed on the distribution transformer which acts as a natural low pass filter for the high frequency signal injected in the network.
Low Voltage Repeater: In case of lines of significant distances between the Head End and the Network Termination Unit it will be necessary to place Repeater Units along the line in order not to lose the high frequency signal.
Network Termination Unit (NTU): It is the interface between the client equipment and the low voltage PLC network. The NTU is normally placed at the client premises.
Packet Switching Network
A packet switching network (PSN) is a communication system that works by grouping and sending data from a transmission node to a receiving node in the form of small packets, where each packet contains specific details like a source IP address, destination IP address. Moreover, PSN uses the broadband Ethernet as its communication channel. It is the most commonly used communication system on the modern power grid because of its ability to provide real time support for SCADA systems.
