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Engineering challenges with distributed generation Schaefer, Madeleine 2012

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ENGINEERING CHALLENGES WITH DISTRIBUTED GENERATION   Madeleine Schaefer  April 10, 2012 University of British Columbia  EECE 492    ii  TABLE OF CONTENTS Glossary ........................................................................................................................................... iv 1 Introduction ............................................................................................................................ 1 2 Impacts of Distributed Generation ......................................................................................... 3 2.1 Power Flow Direction and Magnitude ............................................................................. 3 2.2 Voltage Regulation ........................................................................................................... 4 2.2.1 Reactive Power Compensation ................................................................................. 5 2.2.2 Tap Changing Transformers ...................................................................................... 6 2.3 Protection ......................................................................................................................... 7 2.4 Power Quality ................................................................................................................... 9 3 Level of Impact ...................................................................................................................... 11 3.1 Generator Characteristics .............................................................................................. 11 3.1.1 Size of Generation Unit ........................................................................................... 11 3.1.2 Type of Generation ................................................................................................. 12 3.1.3 Interconnection Design ........................................................................................... 12 3.2 System Characteristics ................................................................................................... 13 3.2.1 System Configuration and Impedance .................................................................... 13 3.2.2 Integration with Other Generation ......................................................................... 14 iii  4 Conclusion ............................................................................................................................. 15 Bibliography .................................................................................................................................. 16   iv  GLOSSARY Centralized Generation – Electricity generation which is typically connected to the transmission system through a generator station. Centralized generation is capable of producing large amounts of power usually by thermal, nuclear or hydro systems. Cold Load Pickup – Loss of load diversity after an extended outage results in a load which is significantly (two to five times) higher than the normal load of a feeder. [1] Distributed Energy Resources – Electricity generation which is sufficiently small so as to allow connection to the power system at almost any point. Usually 10 MW or smaller. [2]  Distribution System – “The portion of an electric system that transfers electric energy from the bulk electric system to the customers.” [3] Downstream – Refers to a device located at a distance further from the substation than the reference point. Feeder – All circuit conductors between the main distribution centre and the service equipment. [3] Inrush Current – When a transformer is energized, current flows on the primary winding as the flux develops. This current is much higher than the load current.[4] v  Reactive Power - The alternating exchange of stored energy between the capacitive and inductive elements of a system. Measured in Volt-Amperes reactive (VAr).  Service Transformers – The step down transformers connecting the distribution feeder to the customer.  Transmission System – Interconnected electric transmission lines and associated equipment which moves or transfers electric energy in bulk between points of supply and points for delivery. [3]  1  1 INTRODUCTION Distributed generation describes the connection of electricity generation close to the point of use, often directly to the distribution system [5]. Many electrical utilities are facilitating the connection of more distributed generation to replace or supplement centralized generation as a result of political, environmental, or economic pressures. There can be significant environmental and economic advantages related to connecting distributed energy resources; however, successful connections require that utilities overcome several engineering challenges.  Distributed generation can impact electrical power systems in multiple areas including power flow direction and magnitude, voltage regulation, protection, and power quality. These impacts vary depending on the characteristics of the generator and the system it is connecting to. Despite these impacts, coordinated connection of distributed generation has the potential to improve electric system reliability and power quality, and reduce peak power requirements and land-use [6].  Centralized generation is often connected to long transmission networks which incur considerable line losses. These transmission networks require significant investment to build and maintain. Connecting generation closer to the customer can reduce line losses. Allowing customers to connect generation can reduce the financial burden on utilities to build and maintain these facilities.  2  This report will focus on the engineering challenges in connecting generation to radial distribution feeders, and is organized as follows: Impacts of Distributed Generation, Level of Impact by Generator and System Characteristics, and Conclusions.  3  2 IMPACTS OF DISTRIBUTED GENERATION Traditionally, the distribution system was designed to deliver power from the substation to the customer. The complete system must maintain the voltage within a 10% range of the nominal value [7], limit interruptions to the customer, and minimize total harmonic distortion. Connecting distributed generation can impact the ability of utilities to provide reliable service to the customer as the distribution system was not designed to accommodate generation.  2.1 Power Flow Direction and Magnitude When generation is connected to a distribution feeder, it injects power onto the distribution system. In doing so, the generator reduces the amount of power that is required to flow from the distribution station. The generator may supply loads located up and downstream of the generator connection. Equipment close to the distribution station will be less heavily loaded because a portion of the demand is being supplied by the generator; however, equipment close to the generator will be more heavily loaded.  As the equipment closer to the generator experiences heavier loading, the operating conditions must remain within the distribution equipment ratings. This will not affect service transformers, but can affect feeder conductors. The main feeder is connected directly to the substation, and lateral feeders branch off from the main feeder to supply load. The main feeder is sized to carry the full current for the expected peak load, but as lateral feeders are not expected to carry the full load, they are often a smaller size. If a generator is planning to connect to a lateral feeder, 4  the conductor current carrying capabilities must be confirmed. If the generator capacity exceeds the current limit of the conductor, the generator is required to limit the production capacity. The conductor can be upgraded; however, this is time consuming and can disrupt the service of other connected customers.  If possible, it is preferable that the voltage of the feeder be increased.  2.2 Voltage Regulation The voltage along a feeder decreases because of losses in the line. These losses can be defined as shown below, where I is the current in the feeder, and R is the resistance of the feeder.           EQUATION 1: FEEDER VOLTAGE DROP The voltage drop is related to the length of the feeder and the load connected. Higher loads require more current, therefore the voltage drop along the line will be larger during times of peak demand. The voltage from the substation to the end of the feeder must be maintained at ±5% of the nominal voltage [7]. Figure 1 shows the effect of voltage drop along a feeder when the substation voltage is maintained at 1.0pu. The bus numbers represent points along the feeder.  5   FIGURE 1: VOLTAGE PROFILE [8] 2.2.1 Reactive Power Compensation One form of voltage control is through reactive power compensation. To boost the voltage, a utility may install shunt capacitor banks along the feeder or at the substation. These capacitor banks generate reactive power. To reduce the voltage, shunt reactors are installed at the substation to consume reactive power. Overhead transmission lines can generate or absorb reactive power depending on the load supplied by the lines [9]. While loads are mainly resistive, there is an inductive component, and the load will consume some reactive power. Considering the conditions at the substation and the load composition, the voltage may require inductive or capacitive reactive support.   6  2.2.2 Tap Changing Transformers A second form of voltage control is through tap changing transformers. The secondary winding of the step down transformer at the substation will have taps which allow the voltage ratio to be changed. Tap changing transformers allow a typical range of ±15% of the nominal voltage. On very long feeders, there may be series voltage regulators installed. Voltage regulators are a tap changing transformer with a voltage ratio of 1:1. The taps allow the voltage to be boosted. Figure 2 illustrates the tap changing transformer.  FIGURE 2: TAP CHANGING TRANSFORMER When a generator is connected to a feeder with series voltage regulators installed, there is potential for reverse power flow through the voltage regulators. Additional sensing equipment is required to for reverse power flow tap changing. The reference voltage for the voltage regulators is always the utility substation which ensures that proper operation of the voltage regulator [10].  7  Reactive support and tap changing transformers are set for the typical operating conditions of the feeder, taking into account the peak and base load levels, and voltage drop along the line. Device settings can be automatically or manually changed, however, the devices do not operate rapidly to adjust the voltage of the feeder. If manually set, connected generators will raise the voltage along the line and change the expected operating conditions. 2.3 Protection Protection devices are set to trip for expected fault current levels, while avoiding unnecessary tripping during transformer in rush current, cold load pick up, and under normal load conditions. Protection is coordinated so that downstream devices operate before upstream devices to limit the number of customers impacted by an interruption. Distributed generation can impact the coordination of protection devices through relay desensitization, islanding, and incorrect tripping.  Under normal conditions an over current relay is set to the maximum fault current seen by the next downstream device. Over current relay settings are calculated using the impedance to the fault location, and system voltage. Figure 3 shows the coordination of the operating times of the over current relays on a feeder.  8   FIGURE 3: OPERATING TIME OF OVER CURRENT RELAYS [11] When a generator is connected, it will supply a portion of the fault current. This causes the relay to experience a reduced fault current. This is called relay desensitization. If the distributed generator is supplying enough current to the fault, the amount of current seen by the relay may be below the relay settings, and the relay will not trip the circuit breaker to clear the fault.  Once a fault is cleared, the distributed generator must be disconnected from the distribution system. The generator will not be able to supply the disconnected load reliably. In addition, islanding presents a safety risk to utility field crews working on the line. Crews must be aware that the line is still energized, however, it is expected that a line is de-energized after a fault.  Distribution feeders most often use over current and instantaneous protection. These devices do not detect the directionality of the power flow. If a fault occurs on an adjacent feeder, the distributed generator may supply current. This can trip over current protection at the station and isolate the two feeders impacted. The desired operation only isolates the faulted feeder, allowing all adjacent feeders to continue to operate.  9  With the addition of distributed generation, protection devices may need to be re-coordinated. The redesign of the feeder protection scheme can incur labour costs for engineering and field support. 2.4 Power Quality Power quality is determined by voltage stability, continuity of power supply, and the voltage waveform [12]. Poor voltage stability is a result of under and over voltages, voltage sags and swells, phase shift, flicker, and frequency. The continuity of power supply is affected by momentary and sustained interruptions. The voltage waveform is affected by transients, phase unbalance, and harmonic content.  The injected power from a generator causes the feeder voltage to rise. Similarly, disconnection of the generator or a decrease in the power produced will then cause the voltage to drop. Rapid fluctuations in voltage are referred to as voltage sags and swells. If the voltage rises above 105% of the nominal voltage it is referred to as over voltage, and voltage levels below 95% of the nominal voltage refer to under voltage. Rapid changes in voltage can cause flicker which impacts the power quality of other connected customers.  Utilities measure the effect of outages on reliability of supply based on the number of customers affected, the duration of the outage, and the frequency of outages. As detailed in Section 2.3 Protection, incorrect tripping can impact the reliability of supply, and increase the number of momentary and sustained interruptions.  10  Once generators are disconnected, they must be synchronized to the bulk power system before connecting. Synchronizing the generator is required to maintain frequency stability; however, at the moment of connection, a transient may be produced on the system. This transient deforms the voltage waveform.  Some generation systems may require power electronics, such as inverters, to connect to the grid. These power electronics use high frequency switches which add to the total harmonic distortion of the feeder. Figure 4 shows how added harmonics can distort the sinusoidal signal.   FIGURE 4: TOTAL HARMONIC DISTORTION [13] Poor power quality can affect the operation of connected equipment, and the reliability of a utility system.    11  3 LEVEL OF IMPACT Successful connection of distributed generation must be coordinated on a case by case basis as there are multiple factors in determining how a generator will impact the distribution system.  3.1 Generator Characteristics Generators can cause an increase in voltage, and negatively impact the power quality of the distribution feeder. Variations in the size of the unit, type of generation, and interconnection design will result in different impacts. 3.1.1 Size of Generation Unit Large generation units will increase the voltage more as they inject a higher level of power onto the distribution system. Some utility customers choose to connect small generating units, often in the form of photovoltaic systems, to offset their own consumption and do not feed any power back into the electrical grid. This application simply reduces the load that must be served by the utility, and has limited impacts on feeder voltage because of the small size. With a minimized impact on voltage, the existing protection schemes will not require modification. Other customers install larger generation units, or multiple generating units with the objective of selling power back to the utility. In this application, the customer will try to connect the maximum amount of generation possible. This can significantly raise the feeder voltage. Modifications may be required on the existing protection and voltage regulation equipment.  12  3.1.2 Type of Generation Variable generation, such as wind and solar, cannot be dispatched or controlled without some form of energy storage. Despite this, many installations are connected directly to the distribution system. These forms of generation can ramp up and down production very quickly as the wind or sun exposure change. Rapid changes in the power injected can impact power quality through flickering. The distribution system must be able to accommodate the range in production, however, and the generator must minimize the negative impact on flicker.  Other forms of generation, such as biomass fuelled thermal plants, have consistent power production, and are controllable. The distribution system is only required to accommodate fluctuations in the load connected. It should also be noted, the efficiency of thermal plants decreases when the production output changes frequently.  Some forms of distributed generation can be controlled to provide reactive power support to aid in voltage control. Distributed generation for reactive power support has an advantage over capacitor banks as the generator may be able to finely adjust the amount of reactive power produced. Capacitor banks are adjusted by large step increments and induce switching transients [14].  3.1.3 Interconnection Design The interconnection of the generator to the distribution system must include protection, frequency synchronization devices, and isolation equipment. The design must consider all potential impacts to limit the negative effects of the generator on other customers connected 13  to the feeder. The generator protection considers internal and external faults. Detection of islanding conditions is required to ensure that the generator is isolated. In addition, grounding for the generator station must comply by the Canadian Electrical Code [10]. 3.2 System Characteristics The existing feeder design and distribution equipment capacities can limit the amount of generation that can be connected to the distribution system at each point. To connect some generation, system modifications may be required.  3.2.1 System Configuration and Impedance Most distribution feeders are connected in a radial configuration from the distribution system. In urban environments, feeders are densely loaded and relatively short. Most areas are served by a three phase system, with alternating single phase loads connected in order to minimize imbalance. For short feeders, the system impedance is lower and protection relays can be set sensitively as there is a significant difference between fault currents and load currents.  Rural feeders tend to be longer and lightly loaded. Rural loads may be fed from single phase lateral feeders which can increase imbalance. Because of long feeder lengths and the high associated feeder impedance, achieving sensitive protection relay settings for faults at the end of the line is challenging. Adding distributed generation to this environment can be limited by the sensitivity of the feeder relays.  14  In addition, heavily loaded lines like those in urban environments may operate at the lower end of the acceptable voltage range, therefore, when generation is connected the feeder voltage will be boosted but will still remain within the acceptable range. Lightly loaded rural feeders, often operate at the higher end of the acceptable voltage range and therefore may not be able to accommodate connection of large generation.  3.2.2 Integration with Other Generation Consideration must be made for other generation connected to the feeder. This relates to Section 3.1.1 Size of Generation Unit, but also to the location of where the generation is connected. If generation is concentrated near the substation, the amount connected will be limited by the impact on the voltage. More generation can be connected further from the substation, as the voltage at the end of the feeder will be the lowest.   15  4 CONCLUSION This report focused on the engineering challenges related to the connection of distributed generation. Despite challenges, many electrical utilities are facilitating the connection of more distributed generation to replace or supplement centralized generation.  Distributed generation can impact the electrical power systems in multiple areas including power flow direction and magnitude, voltage regulation, protection, and power quality. The impacts of generation on the distribution system depend on characteristics of the generator to be connected and the existing design of the distribution system. With coordinated design, distributed generation can achieve environmental and financial objectives.    16  BIBLIOGRAPHY  [1]  J. J. Wakileh and A. Pahwa, "Optimization of Distribution System Design to Accomodate Cold Load Pickup," Department of Electrical and Computer Engineering, Kansas State University, Manhattan, 1997. [2]  H. Zaralpour, K. Bhattacharya and C. A. Cantzares, "Distributed Generation: Current Status and Challenges," Institute of Electrical and Electronics Engineers, Inc.. [3]  J. Radatz, "The IEEE Standard Dictionary of Electrical and Electronics Terms," Institute of Electrical and Electronics Engineers, New York, 1997. [4]  M. Nagpal and C. Henville, Transformer Protection, Vancouver, 2012.  [5]  R. H. Salim, O. M and R. A. Ramos, "Power Quality of Distributed Generation Systems as Affected by Electromechanical Oscillations," IET Generation, Transmission & Distribution, Sao Carlos, 2011. [6]  U.S. Department of Energy, "The Potential Benefits of Distributed Generation and Rate- Related Issues That May Impede Their Expansion," U.S. Department of Energy, Washington, 2007. 17  [7]  Canadian Standards Association, "CAN-3-C235-83 Preferred Voltage Levels for AC Systems, 0 to 50 000 V," Canadian Standards Association, Ottawa, 2006. [8]  L. B. Perera, G. Ledwich and A. Ghosh, "Distribution Feeder Voltage Support and Power Factor Control by Distributed Multiple Inverters," Institute of Electrical and Electronics Engineers, 2011. [9]  B. I, A. S. Attia, T. S. Abdel Salam and M. A. R. Badr, "The Effect of Dispersed Generation Units with and Without Capacitor Banks on the System Losses and Voltage Regulation," CIRED, Turin, 2005. [10]  BC Hydro, "Interconnection Requirements for Power Generators," BC Hydro, 2010. [11]  M. Nagpal, Protection Accessories and Inverse Time Overcurrent Protection, Vancouver, 2012.  [12]  T. Ise, Y. Hayashi and K. Tsuji, "Definitions of Power Quality Levels and the Simplest Approach for Unbundled Power Quality Services," Department of Electrical Engineering, Osaka University, Osaka, 2000. [13]  Aim Energy Inc., "Harmonics," Aim Energy Inc. , 2004. [Online]. Available: http://www.cpe- aim.ca/html/aimenergy/Harmonics/harmonics.htm. [Accessed 9 April 2012]. [14]  H. Khan and M. A. Choudhry, "Performance Improvement In Distribution Feeders By 18  Installing Distributed Generation At Strategic Locations," Institute of Electrical and Electronics Engineers, Peshawar, 2006. [15]  A. Foss and K. Leppik, "Protection Challenges Facing Distributed Generation on Rural Feeders," Institute of Electrical and Electronics Engineers, Ottawa, 2010.   

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