1. 1 © A. Kwasinski, 2015 Cyber Physical Power Systems Fall 2015 Week #1

2. 2 © A. Kwasinski, 2015 Cyber-physical systems From NSF: Cyber-physical systems (CPS) are engineered systems that are built from, and depend upon, the seamless integration of computational algorithms and physical components. In this course the cyber-physical system under consideration is built by combining power and communications infrastructures (physical components) through integrated sensing, data management and control (implicit computational algorithms).

3. 3 © A. Kwasinski, 2015 Historical Perspective Competing technologies for electrification in 1880s: Edison: dc. Relatively small power plants (e.g. Pearl Street Station). No voltage transformation. Short distribution loops – No transmission Loads were incandescent lamps and possibly dc motors (traction). “Eyewitness to dc history” Lobenstein, R.W. Sulzberger, C. Pearl Street Station: 6 “Jumbo” 100 kW, 110 V generators

4. 4 © A. Kwasinski, 2015 History Competing technologies for electrification in 1880s: Tesla: ac Large power plants (e.g. Niagara Falls) Voltage transformation. Transmission of electricity over relatively long distances Loads were incandescent lamps and induction motors. http://spiff.rit.edu/classes/phys213/lectures/niagara/niagara.html Niagara Falls historic power plant: 38 x 65,000 kVA, 23 kV, 3-phase generatods

5. 5 © A. Kwasinski, 2015 Power Grids Topology Divided among the following parts Generation Transmission Distribution / consumption

6. 6 © A. Kwasinski, 2015 Power Grids Topology Often standard grid configurations are used based on IEEE case studies. IEEE 14 bus case

7. 7 © A. Kwasinski, 2015 Generation Electric power is generated at power stations. Generated power = load + losses Electric power generators are classified between base and peaking units. Peaking units Base units Wind + PV Natural gas, liquid fuels Hydroelectric Coal Nuclear

8. 8 © A. Kwasinski, 2015 Generation Peaking units: The tend to be used to follow the load. Most of them are fueled by natural gas but some units using liquid fuels are also used. They tend to have a relatively faster response (their power output can be adjusted relatively fast) and they tend to come online and offline relatively often. Their power output is in the range of a few 100s MW to a very few 1000s MW.

9. 9 © A. Kwasinski, 2015 Generation Coal fired power plants: Equipped with base generation units They operate continuously at a constant output. Power stations rating could reach a few 1000s MW. They tend to have a slow response

10. 10 © A. Kwasinski, 2015 Generation Nuclear power plants: Equipped with base generation units They operate continuously at a constant output. Power stations rating is typically of a few 1000s MW. They tend to have a slow response

11. 11 © A. Kwasinski, 2015 Transmission High Voltage: from 69 kV to 800 kV ac and more. Few dc lines are also found. Power Capacity of each line: from 50 to 1,000 MW Carry power long distances from buses of a substation to buses in another substation. Low power losses Passive portion of a grid Generally in a mesh configuration with some redundancy

12. 12 © A. Kwasinski, 2015 Distribution Primary distribution (feeders and laterals) voltage levels: 12 – 34 kV ac (medium voltage) Secondary distribution: 480 V – 120 V ac (low voltage) Power capacity for each circuit 10 – 40 MW Passive grid. Spans shorter distances than the transmission portion. Distribution circuits have relatively higher losses than transmission lines. Generally in a radial configuration from a substation. Typically, no redundancy

13. 13 © A. Kwasinski, 2015 Consumers Generally classified among: Residential Industrial Commercial Each type of consumer group tend to have a different demand profile that may change depending on the seasons. Electric meters represent the border between electric utilities and consumers. Some consumers are now adding local electric power generation.

14. 14 © A. Kwasinski, 2015 Substations Divide transmission from distribution and generation (substations are highlighted in red in the figure on the left). Transformers are a main equipment found at substations. Circuit breakers are also another key component found at substations. Actuation and sensing equipment is also found at substations. Capacitors and inductors can also be found at substations.

15. 15 © A. Kwasinski, 2015 Layout examples: Substations

16. 16 © A. Kwasinski, 2015 Another layout example and transformers Substations

17. 17 © A. Kwasinski, 2015 Substations Circuit breakers.

18. 18 © A. Kwasinski, 2015 Substations Sensing and actuation components (a lot more about this throughout this course).

19. 19 © A. Kwasinski, 2015 Dispatch Center Centralized location. Responsible for monitoring power flow and coordinating operations so demand and generation are match in an economically optimal way. That is, from a stability perspective demand (plus losses) needs to equal generation but from an operational perspective, such match needs to be achieve in an economically optimal way. Source: Scientific American

20. 20 © A. Kwasinski, 2015 Introduction Control variables in dc power systems Voltage Control variables in ac power systems: Voltage amplitude Phase: (angular) frequency and angle Phasors Used to represent ac signals in single-frequency systems through a fixed vector in the complex plane.

21. 21 © A. Kwasinski, 2015 Introduction Power in ac systems Instantaneous power: Real power: related with irreversible energy exchanges (work or dissipated heat). That is, real power represents energy that leaves or enters the electrical circuit under analysis per unit of time, so the energy exchanges occur between the circuit and its environment. Constant part

22. 22 © A. Kwasinski, 2015 Introduction Power in ac systems Complex power Notice that and that So a magnitude called complex power S is defined as Power factor (in power systems with one frequency) is defined as It provides an idea of how efficient is the process of using (and generating) electrical power in ac circuits: Q = 0 (resistive load) Q > 0 (inductive load) Q < 0 (capacitive load)

23. 23 © A. Kwasinski, 2015 Introduction Power in ac systems Reactive power: related with reversible energy exchanges. That is, reactive power represents energy that is exchanged between the circuit and electric or magnetic fields in a cyclic way. During half of the cycle energy from the sources are used to build electric fields (charge capacitors) or magnetic fields (“charge” machines) and during the other half cycle exactly the same energy is returned to the source(s). e.g. in an inductor:

24. 24 © A. Kwasinski, 2015 Power flow along lines Consider the following example of a generator on the left connected to the rest of the grid through a line. Assumptions: The rest of the grid is “stiff” as a result of many large interconnected generators with a combined large inertia. When it is “stiff” the voltage at the rest of the grid side cannot be changed. Since the combined power output of the generators of the rest of the grid is much more than that of the generator on the left, then it is as if the rest of the grid acts as an ideal infinite power source. The generator on the left is considered to be a “real” one with an internal impedance Zs. The interconnecting line has an impedance Z L. Usually X L >>R L

25. 25 © A. Kwasinski, 2015 Power flow along lines Output real and reactive power of an inverter (or any source) equal In conventional power grids X L >>R L, and in generators X S >>R S. Also δ is also usually small, so –Hence, real power flow is dependent on the angle difference between the two ends and reactive power is dependent on the voltage differences between the two ends

26. 26 © A. Kwasinski, 2015 Consider now the following example of a small 4-bus grid. Important information: Parameters: Line impedances (includes transformer impedances). State: bus voltages, real and reactive powers provided at each bus (conventionally, a generated power at the bus is positive and a consumed power by loads at the bus is negative) Power flow along lines

27. 27 © A. Kwasinski, 2015 Notice that in reality generators output power and maximum power that can be transmitted through lines are constrained. Grid operators need to consider these constrains and economical aspects to decide how much power to generate at each generator. Using the previous relationships the power flow along lines and voltages in buses can be calculated from: where P k and Q k are the real and reactive powers provided at a bus k, and Power flow along lines (Upper case: admittance matrix elements Lower case: line admittance components) (Bus voltages)

28. 28 © A. Kwasinski, 2015 Considering once again X/R ratios much higher than 1, small angle differences, and per unit relative voltage magnitudes at each bus close to 1, the previous equations can be simplified to the dc power flow equations Then, the power flow along lines is given by Power flow along lines

29. 29 © A. Kwasinski, 2015 To find the power flow along lines we need to calculate: To calculate the above equation we need to solve This is an undetermined system of equations (the matrix is singular) then, the voltage (magnitude and angle) at a bus (called slack or swing bus) is set (usually a relative per unit voltage of 1 with an angle of 0). As a result, the equation for the slack bus replaced by this set voltage value and the real and reactive power at this bus are now unknown. Other knows and unknows are: In a PQ (load) bus: P and Q are known, voltage is unknown In a PV (generator) bus: P and V are known, reactive power and voltage angle are unknown. Power flow along lines

30. 30 © A. Kwasinski, 2015 Important conclusions: P relates to angle differences and Q relates to voltage differences. System operators need to balance demand and generation in an economically optimal way. To achieve their objective system operators need to know the state of the system (i.e. know voltages and power flow). To know voltages at buses and power flow along lines system operators need to compute an algorithm that solves a system of equation (notice that a real power systems may have thousands of buses). To solve such system of equations operators need to know system parameters (e.g. line impedances) and demand levels (i.e. need to measure voltages and currents). Notice that all basic components of a cyber-physical system are mentioned: algorithms, physical components, computation and sensing… However, power grids are cyber-physical system with basic capabilities and limited cyber-physical integration. Power flow along lines

31. 31 © A. Kwasinski, 2015 Electric Power Generation Concepts Field Excitation Q Synchronous generators Input: Mechanical power applied to the rotor shaft Field excitation to create a magnetic field constant in magnitude and that rotates with the rotor. Output: P and Q (electric signal with a given frequency for v and i)

32. 32 © A. Kwasinski, 2015 Synchronous generators Open circuit voltage: Magneto-motive force (mmf) Electric Power Generation Concepts

33. 33 © A. Kwasinski, 2015 Effect of varying field excitation in synchronous generators: When loaded there are two sources of excitation: ac current in armature (stator) dc current in field winding (rotor) If the field current is enough to generate the necessary mmf, then no magnetizing current is necessary in the armature and the generator operates at unity power factor (Q = 0). If the field current is not enough to generate the necessary mmf, then the armature needs to provide the additional mmf through a magnetizing current. Hence, it operates at an inductive power factor and it is said to be underexcited. If the field current is more than enough to generate the necessary mmf, then the armature needs to provide an opposing mmf through a magnetizing current of opposing phase. Hence, it operates at a capacitive power factor and it is said to be overexcited. Electric Power Generation Concepts

34. 34 © A. Kwasinski, 2015 Field Excitation Q Relationship between reactive power and field excitation The frequency depends on the rotor’s speed. So frequency is controlled through the mechanical power. Pmec is increased to increase f Pmec is decreased to decrease f http://baldevchaudhary.blogspot.co m/2009/11/what-are-v-and- inverted-v-curves.html Electric Power Generation Concepts

35. 35 © A. Kwasinski, 2015 As we saw before, the simplified equivalent circuit for a generator and its output equation is: Rest of the grid Assumption: the rest of the grid is “stiff” so during short circuits or load changes E is constant V is the output (terminal) voltage Electric power provided to the rest of the grid Electric Power Generation Concepts

36. 36 © A. Kwasinski, 2015 It can be found that Output frequency of a generator is proportional to its rotor angular velocity. Ideally, the electrical power equals the mechanical input power. The generator’s frequency depends dynamically on δ which, in turn, depends on the electrical power (ideally it equals the input mechanical power). So by changing the mechanical power, we can dynamically change the frequency. Likewise, the reactive power controls the output voltage of the generator. When the reactive power increases the output voltage decreases. Generator’s angular frequency (rotor’s speed) Grid’s angular frequency Electric Power Generation Concepts

37. 37 © A. Kwasinski, 2015 Voltage and frequency control Matching generation and demand (plus losses) is essential for power grid stability. If generation is less than demand, frequency drops. If demand is less than generation, frequency increases. Additionally, voltage needs to be controlled within a given range To achieve these goals in an economically optimal way, power grids control system is structured in mainly 3-levels. Primary control: Local control using locally measured variables. Main goal: adjust electrical power output (i.e. adjust mechanical power input) to compensate for mismatches between generation and demand. Secondary control: Local control using locally measured variables. Main goal: frequency (or voltage in some cases) regulation to a given nominal set point. Tertiary control: Performed at a central location. Main goal: determine operation nominal set points in order for optimal operation. Primary control is often implemented through droop controllers.

38. 38 © A. Kwasinski, 2015 Voltage and frequency control Droop control It is an autonomous approach for controlling frequency and voltage amplitude of the generator. It takes advantage that real power controls frequency and that reactive power controls voltage. A droop controller is based on the following static laws to adjust a new set point with respect to a nominal one Nominal frequency Drooped frequency

39. 39 © A. Kwasinski, 2015 Voltage and frequency control Droop control Then a simple (e.g. PI) controller can be implemented. It considers a reference voltage and a reference frequency: If the output voltage is different, the field excitation is changed (and, thus, changes Q and then V). If the frequency is different, the prime mover torque is changed (and thus, changes P and then f).

40. 40 © A. Kwasinski, 2015 Voltage and frequency control Operation of a generator connected to a large “stiff” grid: A large grid is seen as an infinite power bus. That is, it is like a generator in which changes in real power do not cause changes in frequency changes in reactive power do not originate changes in voltage its droop control curves are horizontal lines

41. 41 © A. Kwasinski, 2015 Voltage and frequency control Operator of a generator connected to a large “stiff” grid When connected to the grid, the voltage amplitude and frequency is set by the grid. In order to synchronize the oncoming generator, its frequency needs to be slightly higher than that of the grid, but all other variables need to be the same.

42. 42 © A. Kwasinski, 2015 Voltage and frequency control Real grids are not ideally “stiff” so frequency can change. In large grids with many generators with a high inertia and normal load changes frequency changes relatively slowly. However, frequency changes faster than changes in generators rotational speed (because of the relatively high inertia). So, when the load changes, the grid frequency changes, too. when the frequency changes, the primary droop controller adjusts the generator output to match generation and demand based on the new frequency. If there is a sufficiently large mismatch between generation and demand that cannot be compensated with a droop controller, stability can be lost and the system may collapse.

43. 43 © A. Kwasinski, 2015 Voltage and frequency control Real grids are not ideally “stiff” so frequency can change. Once the frequency deviates from its nominal value due to a mismatch between generation and demand, it is desirable to bring it to its nominal value. This is accomplished by the secondary controller:

44. 44 © A. Kwasinski, 2015 Voltage and frequency control Grid-connected operation of a generator After the generator is paralleled to the grid then its power output can be adjusted in order to minimize overall system cost for generating electricity. That is, it is desirable to generate more power from the least costly units. At a central location, the grid operator determines how much power to generate at each unit based on load estimates. The, droop lines are adjusted to change the generator power output Operating frequency Adjusted droop lines No load droop line Higher power output

45. 45 © A. Kwasinski, 2015 Additional comments In conventional ac grids, large machine inertia helps to maintain stability. Since frequency needs to be regulated at a precise value, imbalances between electric and mechanical power may make the frequency to change. In order to avoid this issue, mechanical power applied to the generator rotor must follow load changes. The same approach described with real power output can be used to adjust reactive power based on voltage regulation. Droop control is an effective decentralized controller. Optimal operation set points need to be transmitted from the central operations center to the generators.

46. 46 © A. Kwasinski, 2015 Additional comments Operation and monitoring of electric power grids is usually performed with a SCADA (supervisory control and data acquisition) system. At a basic level a SCADA system includes: Remote terminals Central processing unit Data acquisition (sensing) units Telemetry Human interfaces (usually computers). SCADA systems require communication links but, usually, these are dedicated links separate from the public communication networks used by people for their every day lives.

47. 47 © A. Kwasinski, 2015 Additional comments Operating a power grids involves dealing with a broad time scale range: Source: NERC