Voltage Stability and Reactive Power in the PV Industry


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I prepared this training presentation for the Business Development Team so that they could explain our systems design strategy for widespread grid stabilization through the use of power factor control attributes of PV inverters.

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Voltage Stability and Reactive Power in the PV Industry

  1. 1. Reactive Power and Voltage StabilizationDavid F. Taggarthttp://www.linkedin.com/in/davidtaggart/
  2. 2. Outline• The Basics – Electricity – Power• Impact of Reactive Power• Reactive Power, PF Control, Voltage Stabilization• Implementing PF Control/Voltage StabilizationNOTE: like with the financial guys, the power folks use different words for identical meaning, which leads to the majority of the confusion surrounding these topics. I have tried to pull the most salient points, using the most common words, and tie them to the relevant aspects of the PV power generation industry.
  3. 3. Electricity BasicsElectricity and Power
  4. 4. Electricity: the water analogy• Wire is a pipe for water to flow in• Current is the flow rate of water through the pipe• Voltage is the pressure in the pipe, can be viewed as the delta in height between the top and bottom of the pipe (i.e. the potential energy)• Battery/generator is a pump that increases the pressure of the water• Load/Resistance is a water wheel taking energy from the water – the water is still there, it just has less energy in it Potential Energy – the pipe resisting the flow of water is another type of resistance• Electrons are the water that gets recycled over and over• Circuit is the complete loop of motion for the water
  5. 5. Electricity: flow of electrons• Electrons move through a circuit from a negative pole to a positive one, but current is said to flow from positive to negative (thanks Ben Franklin!)• Electrons don’t get “used up” but continue through a load until they return to the source propelled forward by the voltage, thus being recycled continually. They do however give up their energy once they pass through the load• The phrase “path of least resistance” is actually a little misleading. More accurately, electricity takes the path of least resistance back to their source• For DC, the flow is continuous in one direction, for AC, it goes back and forth the same amount, with no net displacement. Think of the water moving back and forth through a paddle wheel
  6. 6. Electricity: voltage is not constant• From the water analogy, you can see that voltage cannot be constant, whereas current must be if there is indeed a circuit (closed loop)• Because there are losses (resistance) and loads (water wheel) in a circuit, voltage will drop as the electrons move through the circuit• Factors affecting voltage include overall demand/load, utilization of T&D lines, manipulation of system controls by the control centers, and emergency situations occurring in the system• The end users are the ones that require voltage stability within a narrow range for their devices to work properly and have a useful lifetime• It is the control centers’ responsibility to control the voltage so that it can satisfy this requirement
  7. 7. Power: the different types• The term “power” typically refers to active/real power representing the energy-related quantities in a grid, and is the product of voltage (E) and current (I)• For AC systems, voltage and current follow a sinusoidal wave form, changing polarity every 180 degrees in time• A complete cycle of 360 degrees occurs 60 times a second (60 hz)• When E and I are not “in phase” (i.e. when they don’t cross the median at the same point) a second component of power shows up 1. Active or real power (as we have been discussing) 2. Reactive or imaginary (unreal) power (the new one)• The combination of the two is total or apparent power• Lets agree on: + – Total power: symbol is S, and units are volt●amperes (VA) – Real power: symbol is P, and units are watts (W) Time → – Reactive power: symbol is Q, and units - are volt●amperes reactive (VAR)
  8. 8. Power: relationships• Q: Reactive Power φ P: Real Power
  9. 9. Power factor: unity (1)• When PF is unity (1), the angle between real and total power (φ) is zero• This means that voltage and current are in-phase and all the power is consumed by the load: none is returned (dark blue line)• This is what happens for 100% resistive loads like lights, heaters, ovens, etc., i.e. real power equals total power S: Total Power P: Real Power
  10. 10. Power factor: zero (0) • When PF is zero (0), the angle between real and total power (φ) is 90o • This means that voltage and current are exactly 90o out of phase and all the power is returned to the grid unused • The dark blue line shows that all the power is stored temporarily in the load during the first quarter cycle and returned to the grid during the second quarter cycle, so no real power is consumed • This is what happens for 100% reactive loads like capacitors, i.e. reactive power equals total power Q: Reactive PowerS: Total Power
  11. 11. Power factor: 0 < PF < 1• When PF is between 0 and 1, φ is between 0 and 90 degrees• This means that voltage and current are out of phase by φo, with some power consumed, and some returned to grid (φ, dark blue line)• This is what happens for combination resistive & reactive loads like motors, blow dryers, drills, basically the majority of the loads• The phase shift can be either leading or lagging in reference to whether the current is ahead or behind voltage in time Q: Reactive Power Φ=45o P: Real Power
  12. 12. Power factor: operational PF• Different devices/loads operate at different power factors• These devices are all inductive in nature (lagging) and thus need capacitive (leading) reactive power to ensure their safe and efficient operation as well as stabilize the voltage of the circuit they run on• Compensating for their inherent operational power factor yields significant benefits
  13. 13. The basics: summary•
  14. 14. The basics: summary• Q can be leading or lagging, in reference to current arriving before or after the voltage in time, respectively• Q is generated or consumed in almost every component of an AC system• Q does not travel far because reactive power dissipates up to 30% faster than real power. Independent Generators and ISOs must produce and transmit greater amounts of reactive power to consumers, so it is best generated close to the load. This is why DG PV has a particularly strong advantage as a Q generator• Where lagging Q (VARs) are consumed, leading Q (VARs) must be supplied• Inductors are said to be lagging devices and consume reactive power and capacitors are said to be leading devices and generate reactive power• The PF in a circuit can be measured with the wattmeter-ammeter- voltmeter method, where the power in watts is divided by the product of measured voltage and current• The power factor of a balanced polyphase circuit is the same as that of any phase
  15. 15. The basics: mnemonics• Current lags behind voltage in Current leads voltage in an an inductive circuit capacitive circuit
  16. 16. Impact of Reactive Power
  17. 17. Reactive power: impact to the grid• For the majority of components, loads and devices comprising the grid, power is temporarily stored in them as it passes through them, distorting its waveform before returning energy to the grid• This distortion, or shifting of current in time with respect to voltage, causes the total power to be greater than the real power (because the shift causes the presence of reactive power)• Inductive loads constitute a major portion of the power consumed in industrial complexes, and due to their low PF, require much higher currents than their real power needs would imply• These higher currents require larger wires and other equipment to transport, and increase the energy lost in the T&D system• Due to the costs of larger equipment required and wasted energy, electrical utilities will often charge a higher price to industrial or commercial customers if their operations function at low power factor• So to have an efficient system, whether you are a load, a generator, or the grid itself, PF should be as close to 1 as possible
  18. 18. Reactive power: impact to the grid• Reactive power (Q) is required to maintain the voltage in a T/D line to enable the delivery of active power. Not enough Q and the voltage is not high enough to push the current through the wire, and voltage will sag and underserve the load. Too much and voltage can increase to the point that infrastructure and loads can be damaged• Reactive power must balance in the grid to prevent voltage problems• The farther the transmission of power, the higher the voltage needs to be raised to overcome the resistance to current flow• Voltage drops related to reactive power contributed to blackouts in the West (1996,) France (1978,) near failures in the PJM system (1999) and significant voltage swings in the Midwest and Northeast in 2003
  19. 19. Reactive power: impact to the grid• New York Times, 9/26/03
  20. 20. Reactive power: summary• Reactive power (Q) is an essential component of the current running through the grid• While it provides no energy, it is required to stabilize the voltage and balance the grid• Every T&D line must allow room in the wires for reactive power, to successfully serve the wide variety of loads• The majority of loads on the grid are inductive (lagging PF) and consume reactive power• Reactive power must be generated (leading PF) to feed the inductive loads (lagging PF) to maintain balance and keep overall PF as near unity as possible• Reactive power and voltage are interdependent
  21. 21. Reactive Power and Power FactorControl
  22. 22. The grid: current situation • Distributed renewable energy generators (DGs) inject power at various network points in a controlled manner dependant upon available “fuel” • Consumers (de facto distributed) use electricity at different times • Result: Varying grid voltages with the potential for exceeding allowable ranges Distributed generators Central Large-scale power powergeneration plant Consumers
  23. 23. The grid: extreme scenarios • Grid voltage exceeds established corridor (at end user) 1. too much distributed generation, too little consumption 2. Too much consumption, too little generation 110% = 253V Voltage Central outside power 100% thegeneration = 230V guideline 90% value = 207V Voltage corridor (according EN 50160 (UN ± 10%)
  24. 24. The grid: renewable stability • Distributed generation from advanced PV power plants can continually maintain this balance, in real-time, by phase shifting of voltage and current (distributed generation of reactive power) in the regional distribution network Distributed generators Central Large-scale power powergeneration plant Consumers
  25. 25. The grid: power factor control • The results: The grid voltage can be corrected short-term through the distributed generation of reactive power – Compensation for minor regional network fluctuations – Adjustment of the supply so that it remains within the required voltage corridor (voltage regulation-grid stability) Reduction in the Central voltage excursion power through reactivegeneration power control of the supply
  26. 26. Result: stabilized grid voltage, day or night• The integration of grid-stabilizing PV power plants in the distribution network can stabilize the utility network with regard to voltage which can: – save grid expansion costs in the distribution and transfer network level – Provide for acceptance of additional renewable/intermittent energy in the public grid without upgrades – Generate new revenue streams for asset owners of distributed utility and net-metered facilities “It stands to reason, we must firstly increase the actual usable grid capacity, be it through conventional grid expansion or through other methods, such as an intelligent voltage stability or intensified reactive power management.” (German Federal Network Agency)
  27. 27. Implementing Power Factor Control andVoltage Stabilization
  28. 28. Implementation: voltage stabilization• Control effects tend to be localized to the region of the grid where the injection of Q occurs• Regulate to control voltage to a desired nominal value• Regulate to control voltage dynamically to keep within desired range
  29. 29. Implementation: control• Under a regulated environment, most utilities own/control G&T&D in their own control area – They provide reactive power just as they have had to provide sufficient generation and voltage• Restructuring has changed this and is causing problems dealing with reactive power – Merchant (non-utility) generation and related financial incentives – Transmitting power over longer distances with multiple transactions
  30. 30. Implementation: problems• Regulated electricity electric rates are based on kWh and kVA, giving incentive for PF correction• Restructuring and separation of G&T&D businesses: – Generation: More likely kW based removing incentive for PF correction – Distribution: may not have significant incentive and strict budget for installation of capacitors (to generate Q) – Transmission: who will own and operate, and thus no incentive for improvement• Electricity is transmitted between control areas – Communication required to properly operate the system, including adjustments to reactive power – ISOs (i.e., CAISO) have not yet defined any system rules concerning reactive power
  31. 31. Implementation: ideas• Generators receive “lost opportunity” revenue payments when they must provide additional reactive power• Include specific VAR obligations and penalties for non-compliance in each new interconnection service agreement with generators• ??
  32. 32. Thank You!David F. Taggarthttp://www.linkedin.com/in/davidtaggart/