A Review of Ancillary Services and Solar PV Inverters

REE517 – TERM PAPER ASSIGNMENT – LONG PHAM Overview of Ancillary Services and Solar Inverters Long Pham, Student Member, IEEE. Abstract—While the increasing penetration of solar energy into the low voltage distribution grid helps with meeting the rising demand of energy in a most efficient and sustainable way, it creates anxieties that the large-scale integration of solar distributed generation would destabilizes the grid. This paper reviews the ancillary services that keep the grid stable. Then solar PV inverter is reviewed before the discussion of the next generation of solar inverter known as smart inverter. The smart inverter incorporates advanced grid functions and networking interfaces in the inverter itself, allowing the utilities to dynamically modify the inverter’s autonomous behavior in real time. The technology not only allows high penetration of solar energy into the existing grid at a far more competitive Levelized Cost of Energy (LCOE), but also increases the stability, efficiency and storm-resiliency of the grid. Index Terms— Solar Inverter, Review, Central Inverter, String Inverter, Micro Inverter, Power Optimizer, Ancillary Services, Smart Inverter. I. INTRODUCTION The Governor of California, Jerry Brown, has called for 12,000 MW of renewable power generated “within the local power distribution grid”. The request is a part of the state’s goal under the Renewables Portfolio Standard (RPS) of attaining 33 percent of its energy from eligible renewable resources by 2020 [1-4, 64]. The bold move has the potential to bring substantial environmental and financial benefits to California. At the same time, it creates a tough technical challenge to the state’s distribution grids, which were originally designed for one-way power flow [3-4, 64]. In order to overcome this challenge, it is required a fundamental change in the inverter design as well as grid connection requirement standards [5]. Solar PV inverter design has been an attractive research topic over the last decade. The early literature largely focused on the discussion about high level solar system architecture, such as centralized inverter, string inverter, and distributed microinverter. The main driven force was to improve the overall energy yield [47-51]. Later, the research interest was about the reliability of inverter [52-59]. Recently, the topic of low cost and high power density has become an appealing area of research [60-63]. With the penetration of solar energy becoming higher, PV inverters with smart micro-grid functionality, or smart inverter, are becoming the major trend [64-79]. The purpose of this paper is to provide a comprehensive study of the literature on some of the topics that are most concern to the design of the future smart solar inverter. It serves as a starting point for further research or development on the smart solar inverter. First, an overview of traditional ancillary services is discussed to illustrate how they keep the grid stable. Then, solar PV inverters are classified and reviewed. Finally, the discussion on the smart inverter that has advance gridbalancing and communication functions is provided. II. OVERVIEW OF ANCILLARY SERVICES IN CONVENTIONAL POWER PLANTS The universal definition, descriptions and terminologies of ancillary services are not found in the literature [12-19]. Carvajal et al. [16] discussed four types of ancillary services: frequency regulation, voltage regulation, black-start service, and special protection systems. González et al. [12] gives a short terminologies comparison between different countries used for frequency regulation. This section discuss the main and most interesting ancillary services concerning the design of solar inverter. The terminologies use in this paper follow González et al. [12]. A. Frequency regulation (or active power control). In power systems, frequency is an indicator of the balance between power production and consumption. By changing the delivered active power, power plants balance their electricity production exactly for consumption levels and bring the frequency back to the set point. This control response is called frequency regulation. Power plants need some kind of power reserve to be able to ramp their output power up and down in order to bring the frequency back to the nominal value. Ideally, the response of power plants to a changing in load should be instantaneous and unlimited in the amount of power that it can change. In practice, implementing such a response in a single stage is too expensive or not feasible. Therefore, the total response of the grid has been divided into stages. Three stages of response have been described in the literature are: primary response, secondary response, and tertiary response [12-18]. The three stages of response are associated with the three types of power reserve: primary reserve, secondary reserve, and tertiary reserve. The three types of power reserve are different in the response time, the amount of power reserve, and the price to implement such reserves [12-18]. Fig. 1 depicts an example of deployment sequence of power reserves for frequency regulation as recommended by ENTSO-E [13]. 1) Primary power reserve In conventional power plants, primary power reserve is implemented by a droop controller inside the governor. When Page 1 of 9 REE517 – TERM PAPER ASSIGNMENT – LONG PHAM Fig. 1. Three-tiered approach of frequency control in Continental Europe; indicated time-scales are standard values which may differ between countries [12]. (An email has been sent to ask for permission to use this picture.) the governor detects an abnormal frequency, it controls the power output of the generator (by controlling fuel valve, gate, etc.) in the opposite direction of the change in frequency. For example, if the frequency is lower, the governor will increase the power output. The governor’s droop controller is a proportional controller. It cannot bring the frequency back to the pre-upset value (i.e. 50Hz in Europe, 60Hz in U.S.). Returning the frequency to set point is the job of Automatic Generation Control system (secondary response). The droop setting is the ratio between the amounts of power output percentage, to the amount of divergence from the center frequency. The response time of the primary power reserve should be fast. As recommended by ENTSO-E, the primary response should be activated after few seconds (0s - 30s) from a frequency deviation event of 20 mHz (mili Hertz). And it is not required to last for more than 15 minutes [13, 14]. 2) Secondary power reserve Secondary power reserve involves adjusting power output of multiple generators at different power plants. The control of secondary power reserve is done by the Automatic Generation Control (AGC). Although human can manually intervene the operation of AGC, the AGC runs automatically most of the time [12]. Secondary power reserve can be in the form of spinning reserve or non-spinning reserve. Spinning reserve is the extra generating capacity from generators that are already running and connected to the power system. Non-spinning reserve (or supplemental reserve) is the extra generating capacity from generators that are not running and not connected to the power system but can be brought online quickly and automatically by the AGC. As recommended by ENTSO-E, secondary power reserve should be activated quickly (30s – 15 mins) and should last as long as required [13]. Secondary response is not as fast as primary response, but it can bring the frequency back to set point. Other purposes of having a secondary response are: 1) to release the primary power reserve so that the power system can quickly response to another frequency change; 2) to optimize power flow in order to minimize the total operating cost, with transmission line congestion [13, 14] (i.e. economic dispatch, optimal power flow). 3) Tertiary power reserve Unlike the primary and secondary power reserve which are activated automatically, the tertiary power reserve is activated manually. Tertiary power reserve is usually come from generators that have a long start uptime (15 minutes to 60 minutes). It is used to cope with major systematic imbalances and to release secondary power reserve (spinning and nonspinning reserve) for more optimal power dispatch (lower cost) [12]. Tertiary power reserve is also called replacement reserve. Fig. 2 shows typical prices for different types of power reserve [20]. Generally, tertiary power reserve (replacement reserve) has the lowest price. B. Voltage regulation (or reactive power control). The resistance of transmission lines in power systems are not zero. Voltage drop from the generating plants to the substation depends on the amount of power being transmitted. It doesn’t Page 2 of 9 REE517 – TERM PAPER ASSIGNMENT – LONG PHAM Fig. 2. Prices for operating reserves in New York from April 2000 through November 2001 [20]. (An email has been sent to ask for permission to use this picture.) matter the power is reactive or active. Both active and reactive power contribute equally on the voltage drop, heat loss and burden on the transmission lines. While active power is generated at power plants, reactive power can be generated easily by using capacitors, STATCOM, etc. It is preferable to generate reactive power locally at the distribution level so that the transmission lines only have to be sized for active power transferring requirements. (Power should be generated right where it is consumed whenever possible.) In fact, the transmission lines is designed to transmit active power only. When a large inductive load goes online in the distribution network. The voltage at the substation is reduced because, at this moment, the required active and reactive power supplying the load are transferring in the transmission lines from the power plants. In order to bring the voltage of the substation back to the normal set point, the substation have to generate the required reactive power locally. This is usually done by connecting into the distribution network a capacitor. As highlighted in the previous paragraph, reactive power control is used to regulate the voltage of distribution network. Therefore it is called voltage regulation. The reactive power control in the distribution network is also considered one of the ancillary services. Of course, the distribution voltage can also be regulated using transformer taps and other methods. However transformer taps and other method are used less often. Only when reactive power compensation could not bring the voltage to the set point, these methods may be employed. Fig. 3. Low Voltage Ride-through requirements comparisons [22]. (An email has been sent to ask for permission to use this picture.) stay connected to the grid and provide voltage recovery support by injecting reactive power into the grid and ramping up the active power after the fault clearance with a limited gradient to harmonize with the ‘natural’ recovery of the grid [21, 22]. For example, in US GCs, power plants must stay connected even when the voltage at the point of common coupling (PCC) is zero up to 150ms (150ms accounts for the typical operating time of protection devices). Fig. 3 compares Low Voltage ride-through requirements of typical GCs. 2) Reactive current injection During grid fault, power plants are typically required to inject reactive current into the grid to support voltage recovery. Fig. 4 C. Other services helping in grid stabilizing Besides voltage and frequency regulations, the traditional grid have other functionalities to support the transient stability, such as inertial response, resynchronizing torque, reactive power injection, and fault ride-through (FRT) capability. 1) Fault ride-through (FRT) When a fault occurs at a point in the electrical grid, the grid voltage drops to a low level until protection devices isolate the fault area from the rest of the grid. The amount of voltage drop depends on many factors, such as, fault location relative to WPP, type of fault, etc. Generally, Grid Codes (GC) requires that power plants must Fig. 4. Minimum reactive current injection required by German GC [23]. (An email has been sent to ask for permission to use this picture.) Page 3 of 9 REE517 – TERM PAPER ASSIGNMENT – LONG PHAM Fig. 5. Minimum reactive current injection required by Spanish GC [23]. (An email has been sent to ask for permission to use this picture.) depicts minimum reactive current injection required by German’s GC. The active current can be reduced in order to fulfill the reactive current requirements. Fig. 5 shows the reactive current injection as required by Spanish GC. 3) Inertia response (IR) Unlike frequency regulation, which regulate the grid frequency, and therefore balances electricity generation and consumption, inertia response (IR) stabilizes the oscillation of the grid frequency Fig. 6. Traditional power plants inherently possess the inertia required to damp the grid frequency vibration due to the large inertia of synchronous generator’s prime mover. In renewable energy power electronics converter, various methods have been used to emulate the inertia response. Issued U.S. Patent 7345373 disclosed a method for stabilizing the swing in grid frequency by implementing an integrator that emulates a virtual frequency inertia with a custom magnitude m. The idea of electrical frequency inertia is exactly what we see in mechanical world: inertia of a body measured by its mass m. “By implementing an ability to sluggishly response to change we can attenuate the oscillation. The bigger the inertia magnitude m, the higher the ability it can absorb oscillations.” Choosing inertia magnitude is a trade-off between dynamic response and ability to absorb oscillation [24]. While a simple IR emulator, discussed previously, is a good and intuitive way to stabilize grid frequency, there are some situations and requirements in real world that the simple IR is not enough to do well. Patent application publication U.S. 20120313593 discloses some improvements to make power electronic converters behave better in stabilizing real-world grid frequency disturbances. In an embodiment of the invention, the virtual IR is implemented by controlling the amount of active power P injected into the grid in such a way that the frequency oscillation is reduced. In power generation systems, grid frequency reflects the balance between generation and consumption of active power. So if frequency is lower than the nominal frequency, the injected active power is reduced to bring the frequency back to nominal value and vice versa [25]. Besides, the IR may be combined with a frequency recovery function, which makes sure that activation of the IR function does not subsequently lead to over frequency events. If the total frequency at the point of interconnection above a certain threshold, the soft recovery function is activated and power output is adjusted according to a specified droop curve [25]. This improvement is necessary because in some situations, the simple IR may create over frequency event. In another embodiment of the invention, the IR function is only activated when the error in grid frequency is above a specified threshold. So most of the time when grid frequency is within a specified range of the nominal frequency, the IR function is disable. The thresholds for activating and deactivating the IR function are different. This hysteresis in the operation of IR function is needed to keep the overall system more stable [25]. Although synthetic inertia is not yet required, many countries have strongly recommended its use. It’s possible for synthetic inertia to be a grid requirement in the near future [26-34]. Fig. 6. Frequency oscillation is damped by synthetic inertia response, according to the simple method (401) in [24] compare to a better method (402) in [25]. (An email has been sent to ask for permission to use this picture.) III. OVERVIEW OF SOLAR PV INVERTERS In solar power systems, a solar inverter is a critical component. It converts the variable direct current (DC) output of the Photovoltaic (PV) modules into a clean sinusoidal alternating current (AC) that is used popularly by residential, non-residential and utility [7]. If the AC power output is fed into the utility grid, the inverter is categorized as grid-tie inverter. On the other hand, if the output power is used without connecting to the grid, the inverter is categorized as off-grid inverter [6]. Due to the intermittent of sun-light, off-grid solar system needs battery to balance between electricity generation and consumption. The added battery significantly increases the price-per-watt and complexity of off-grid solar system compare to grid-tie one [39]. In recent years, grid-tie solar PV installations has been increased exponentially, whereas the off-grid application of Page 4 of 9 REE517 – TERM PAPER ASSIGNMENT – LONG PHAM Fig. 8. Grid-type inverters: a) Central Inverter, b) String Inverter, c) Multi-string Inverter, d) Micro inverter [12]. (An email has been sent to ask for permission to use this picture.) solar PV has not gained adoption as quickly. Fig. 7 shows market share of grid-tie compare to off-grid PV system installations. However, off-grid application is expected to increase significantly in the next few years due to the growth of the emerging markets in underdeveloped countries and the reduction of solar system cost [2]. In the future, the two types of solar PV system may be partly merged together with the introduction of the smart inverter, which uses battery more efficiently and can operate in both off-grid and grid-tie modes, realizing the smart micro-grid. Unlike grid-tie inverters, grid connection and synchronization functions are not required in off-grid inverters. The popular classification of off-grid inverters is based on the output voltage waveform of the inverter, which can be: square wave, modified sine wave, or pure sine wave. The square wave and modified sine wave contain a large amount of high order harmonic which may generates heat, torque ripple, audible noise in inductive loads such as: long transmission lines, motor (electric fan, compressor, etc.) [85]. Pure sine wave inverters are usually more expensive but are required in standalone solar systems which power large, high power appliances. On the other hand, the modified sine wave or square wave inverters are suitable for low cost standalone solar systems, where power requirements are lower and the consistency of energy supply is not stringent. B. Grid-tie Inverter (Grid-connected or On-grid Inverter) Fig. 7. Share of grid-tied and off-grid PV system installation [39]. (Courtesy IMS Research) A. Off-Grid Inverter (Standalone inverter) The modern grid-tie solar industry was born through off-grid solar, a market which continues to be an important part of the industry. Off-grid, or standalone, solar power systems are used in remote, rural areas where the electrical grid is not available. They are also used in street/roadway lighting, remote vehicle (RV), leisure applications (lighting, communication and cooling for mobile homes, sailboats, vacation homes, mountain lodges, etc.) or as emergency backup power in homes and business in places prone to storms and power outages [83, 84]. In off-grid solar system, both the electricity generation and consumption are uncontrollable. So battery is employed as an energy buffer to do power balancing, which eventually produce a stable AC voltage output from the intermittent solar power. Grid-tie inverter is used in areas where the electrical grid is available. The main function of grid-tie inverter is to harvest as much power as possible from the PV modules into the grid. The most popular classification of grid-tie inverter is based on the connection type of PV modules into the inverter, which divides grid-tie inverters into three type: central inverter, string inverter, multi-string inverter, and micro-inverter [6-11, 40-43]. Central inverter, as its name implied, is based on centralized concept (a small number of very large inverters). In this configuration, PV modules are connected in series to create strings that have sufficiently high-voltage output. These strings of PV modules are then connected in parallel with a diode (to protect the reversed current which may damage the PV modules). These parallel strings are connected to a large inverter (Fig. 8a). The string and multi-string inverter are based on a more decentralized concept. Instead of connecting all strings to a single large inverter, each string is connected to a smaller Page 5 of 9 REE517 – TERM PAPER ASSIGNMENT – LONG PHAM inverter or DC/DC optimizer. Fig. 8b and Fig. 8c show the connection of string and multi-string inverter. The micro-inverter (or module inverter) is based on the most distributed concept. Instead of connecting PV modules into a string and then connect that string to a string inverter, each PV module has its own micro-inverter. The control and monitoring are done at module level. An apparent advantages of distributed concept is that a failure of a module will not take the entire PV system or PV string offline; power will continue to be sent to the grid. Moreover, module level inverter offer a modular design which may help reduce cost of installation. Fig. 2d show the connection of module inverter or micro-inverter, the most decentralized concept (a large number of smaller inverters). In central inverter, the maximum power point tracking (MPPT) is done at the central inverter. Centralized MPPT is not efficient because the optimal operating point (voltage and current), at which the central inverter trying to track, is not always the optimal operating point of individual PV modules. In other words, MPPT points of individual PV modules are not always the same with one another. Centralized inverter cannot track the possible highest power point if there is any mismatch between modules which may happen due to: 1) modules from different manufacturer having different specification and I/V curve, 2) different operating condition: light exposure/intensity (shaded, unshaded, partly shaded), temperature, etc. MPPT in string and multi-string inverter is done in each string of PV modules. So the MPPT performance is better compare to central inverter. The micro-inverter concept does MPPT at each PV module to attain even higher MPPT performance [46]. Elasser et al. [44] gives a comparative study of the annual energy yield between different configurations: centralized, string, multi-string, and module level MPPT. The paper concluded that an energy gain in the range of 4% to 12% can be achieved from distributed configuration over centralized configuration. The price per kW usually drops as the power output per inverter increase. In other words, it is cheaper to make a single 100 kW inverter compare to making 100 inverters with rating 1kW. On the other hand, the central inverter configuration has a single point of failure, while the micro-inverter concept has redundancy. So the centralized concept is less reliable and has higher system downtime compared to the decentralized concept. In addition, more energy is harvested in decentralized configuration due to better MPPT. While choosing larger inverter seems to be a more economical solution, the initial cost may take on a minor role when considering the total cost of ownership. Trade-offs are needed to be considered for overall installation and maintenance costs in contrast with the energy yield of the system over its operational lifetime. Central inverter are usually used in large solar farms which are connected to the medium voltage grid and operated by utilities. The string and multi-string inverters are more popular with lower power applications such as commercial buildings, factories, etc. On the other hand, the micro-inverter is more suitable for low power residential applications. Fig. 9 shows the annual installed grid-tie PV capacity by sector. Fig. 10W shows the solar inverter installation market forecast of grid-tie inverter as predicted by IMS Research. It could be inferred from the charts that the strong growth of central inverter is driven mainly by the increasing installation of the utility-scale PV projects. Fig. 9. Annual Installed Grid-Connected PV Capacity by Sector. (From IREC’s “2013 Annual Update & Trends Report”) Fig. 10. Solar inverter installation market forecasts. (Courtesy IMS Research) IV. SMART INVERTERS WITH ADVANCE GRID AND COMMUNICATIONS FUNCTIONS Traditionally, grid-tie inverters do not provide ancillary services to the grid [35]. They simply try to harvest as much solar energy as possible and put the energy into the grid. Since solar energy is intermittent and non-dispatchable, so is the energy that is being put onto the grid. While the behavior cause no harm when the penetration of solar energy is low, it could seriously destabilize the grid if the solar energy penetration is high [45, 67]. In off-grid solar systems, the inverter automatically regulates the output as required by load with the help of battery as an energy buffer. The battery suffers from very high charge/discharge cycle; and it has to be large enough to be able to do the energy buffer task. As a result, the system has very high up-front cost and maintenance cost. The smart inverter, as it is envisioned, employs both the grid and energy storage to solve the energy balancing problem intelligently. In fact, it combines the best of both off-grid and Page 6 of 9 REE517 – TERM PAPER ASSIGNMENT – LONG PHAM grid-tie inverter. Due to the smart usage, battery can has very small capacity and much longer life-time, which lead to lower cost [64, 67, 73]. The future of both grid-tie and off-grid solar PV inverter will be the smart inverter [76]. Smart inverters have batteries and capable of operating with and without the grid, creating a so called smart micro-grid. They can autonomously provide some important ancillary services that greatly improve grid balancing. In addition, standardized communication interface featuring in smart inverters enables utilities to easily monitor, control or change the behavior of smart inverter depending on the requirements of the local grid. Operating data are shared securely with the owner, utility and other stakeholders. Such inverters also allow installer and services technicians quickly diagnose issues, predicting possible problems and remotely upgrade certain parameters. Smart inverter is a crucial component of the future smart micro-grid [64, 67, 73, 76]. While the official definition and detail descriptions for smart inverter does not exist, a large number of researchers, working groups, organizations have been working on different aspect of the future smart inverter, such as: prototyping, testing, upgrade planning, recommendations technical requirements, etc. [6482]. The following is a quick review of some significant works. In January 2014, the Smart Inverter Working Group (SIWG) released “Recommendations for Updating the Technical Requirements for Inverters in Distributed Energy Resources” [64]. The proposal is the result of a year of joint effort between the California Public Utilities Commission (CPUC), California Energy Commission (CEC) and the Smart Inverter Working Group (SIWG). The main goal of the collaboration is “to set out the technical steps for the paradigm shift that is needed as California approaches greater numbers of installed DER systems, higher penetrations on certain circuits, and the implementation of a smart distribution system that optimizes interconnected resources” [64]. The working group proposed three major steps with detail milestones and timeframes for the inverter updating plan. According to the plan, first, some basic grid balancing tasks will be done autonomously in Distributed Energy Resources (DER); second, a communication standards for DER systems will be adopted; and third, advanced grid functions with communications capabilities will be defined and deployed [64]. In February 2014, The Electric Power Research Institute (EPRI) published “Common Functions for Smart Inverters, Version 3” as a recommendation for the future smart inverters’ functionalities [82]. The effort began in 2009 with the participation of more than 500 individuals representing inverter manufacturers, system & solution providers, utilities, universities all over the world. A large number of volunteers have met weekly/biweekly over the last four years to define a common set of functions and the way that each functions should be implemented (in the future smart inverter). The work pointed out that the next-phase of effort are implementation initiative and field experiments. It should be noticed that the recommendations are about common grid functions for smart inverter only. Communication standards are not proposed. As the document concluded, “until a common set of functions are identified and adopted, it is not possible to create open communication standards. And without communication standards, there is no interoperability” [82]. V. CONCLUSION AND RESEARCH DIRECTION This paper provided a review on some of the topics that are most interested for the design of the next generation smart solar PV inverters. First, the key ancillary services of the traditional grid were reviewed. In frequency regulation, different kinds of power reserves are available to response to any changes in power consumption. The power reserves are different in how quickly they can response, the amount of power, price, and the length that response can last. In voltage regulation, reactive power is generated locally in the distribution grid to regulate the voltage. Services that are required to recover the grid from a fault are: fault-ride through, active current injection. Inertia response is also imperative to attenuate frequency vibration that may happens. Second, solar PV inverters were reviewed. Solar inverters can be either off-grid or grid-tied. Off-grid inverters require a battery to balance the intermittent of solar energy. They can be classified according to the voltage output: square wave, modified sine wave, and pure sine wave. The grid-tie inverter is less expensive as it doesn’t need batteries. They can be classified corresponding to how the PV modules connected to the inverter: central inverter, string inverter, multi-string inverter, and micro-inverter. Finally, the concept of smart inverter was introduced. The smart inverter combines the best in both off-grid and grid-tie inverter. It has advanced communication and grid balancing functions. While the grid balancing functions are being done autonomously by the inverter itself, the autonomous behavior of these functions can be monitored and modified in real-time by the utilities through a uniform communication interface. The advanced communication, grid functions and standardization of the smart inverter will help reduce the LCOE further by reducing O&M cost, installation cost, manufacturing & design cost. The manufacturing & design cost will only be lower with standardizations. Studies are underway to standardize the smart inverter. Debates are going on to determine a common set of functions, and the best way to implement these functions. It is identified that demonstrations, design improvements, and field test could be of significant interest at this time. ACKNOWLEDGMENT The author gratefully acknowledges Professor Corsair for her mentoring on the original version of this document. REFERENCES [1] [2] California Office of the Governor, “Renewable Energy Statement,” Oct. 2011. The Governor’s Conference on Local Renewable Energy Resources. Luskin Center for Innovation, University of California at Los Angeles. 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