Impact of High PV Penetration on Grid Operation Yahia Baghzouz Professor of Electrical engineering University of Nevada Las Vegas
Overview Introduction/Background Effects of High PV Penetration on Distribution System Operation A the system level At the distribution level Experimental Setup for Testing a Smart Inverter Autonomous Functionalities Non-Unity power factor operation OV/UV and OF/UF ride through Dynamic Volt/Var control Islanding test Harmonic Analysis
Introduction Over the past decade, electric power generation by means of (PV) has experienced a rapid growth worldwide, and in some areas, electric distribution feeders have already reached a penetration level of over 25%. At the system level, high PV penetration requires more ramping and frequency regulation capabilities. At the distribution level, high PV penetration leads to several technical issues related to power quality, voltage sags and swells, excessive operations of voltage regulating equipment, reverse power flow, and over-voltages.
Higher ramp rates - California s net estimated load curve Source: CAISO 2013
Impact of Large-Scale PV Integration in Southern Nevada Case Study [1] The study evaluated the impact of large-scale photovoltaic (PV) and distributed generation output on NV Energy s electric grid system in southern Nevada. It analyzed the ability of NV Energy s generation to accommodate increasing amounts of utility-scale PV and DG, and the resulting cost of integrating renewable resources. It also quantified the impact of variable PV generation output on NV Energy s system operations, including balancing reserve requirements. [1] http://www.pnnl.gov/main/publications/external/technical_reports/pnnl-2067.pdf
Existing and proposed large PV installations Cases Studied:
Solar irradiance and PV Power Since ground-based irradiance data is not available at the PV plant sites, a method* was developed by SNL to simulate timesynchronized minute-by-minute irradiance at each of the PV sites using estimates of irradiance from geostationary satellite imagery. http://www.geos.noaa.gov (*)Simulation of One-Minute Power Output from Utility-Scale Photovoltaic Generation Systems, C. Hansen, J. Stein,A. Ellis, Sandia National Laboratories, August 2011
PV Output Profiles One year of AC power output, at one-minute intervals, were produced for each PV plant. PV output profiles for a typical clear day and for a cloudy day under a high PV penetration are illustrated below. Note: variability becomes smaller when PV plants are installed over a large area.
Study Results Integration of large-scale PV increases regulation and load following requirements that must be supplied by NV Energy s generating resources, mostly from combined cycle units. On average, 1 MW of additional thermal generating capacity must be reserved for regulation for each 25 MW of PV capacity installed in NV Energy s southern Nevada system. On average, 1 MW/min of additional ramping capability must be reserved for regulation for each 75 MW of PV in NV Energy s southern system.
Balancing Service Ramp Statistics More solar generation causes higher frequency of regulation movements. Base Case Case 5A
Voltage-Related PV Impacts High penetrations of PV can impact circuit voltage in a number of ways. Voltage rise and voltage variations caused by fluctuations in solar PV generation are two of the most prominent and potentially problematic impacts of high penetrations of PV. These effects are particularly pronounced when large amounts of solar PV are connected near the end of long and lightly loaded feeders.
Overvoltage Pockets of high voltage can occur on the distribution circuit during low-load conditions. Higher voltage can cause DG (including PV inverters) to trip off-line.
Substation Transformer Net Power Flow (25% PV Penetration under cloudy days) 16 14 Load with PV Load without PV PV Power 12 Load and PV Power (MW) 10 8 6 4 2 0 7/11/10 0:00 7/11/10 6:00 7/11/10 12:00 7/11/10 18:00 7/12/10 0:00 7/12/10 6:00 7/12/10 12:00 7/12/10 18:00 Time (mm/dd/yy hh:mm) 7/13/10 0:00 7/13/10 6:00 7/13/10 12:00 7/13/10 18:00 7/14/10 0:00
Distribution Voltage Regulation The under-load-tap-changing transformer(ultc), or load tap changer(ltc), allows the transformer taps to be changed while the transformer is energized. A typical range of regulation is ±10% with 32 steps - each corresponds to (5/8)%. Voltage control is also achieved by means of voltage regulators and/or switched capacitors along the feeder.
Concern Over Voltage Flicker One main concern over such fluctuations is the possibility of voltage flicker - a voltage quality problem that is annoying to the eyes. The borderline of visibility of flicker at 1 minute interval is 0.7% voltage dip.
Functions of Conventional Grid-Tied PV Inverters Maximum Power Tracking Grid synchronization Grid Monitoring - Disconnect
Advanced Inverters Newer gird codes have been developed in Europe which require advanced inverters that can provide additional functions to help mitigate or reduce the severity of these problems. In the USA, IEEE recently updated its Interconnection Standard 1547 through an amendment, i.e., IEEE Std. 1547a that allows many of the advanced inverter functionalities. At the regional level, the State of California which has the highest penetration of PV systems has already begun to adopt advanced inverter capabilities.
Required system response time to abnormal voltages (IEEE Std. 1547a)
Required system response time to abnormal frequencies (IEEE Std. 1547a)
Common Inverter Functionalities Advanced inverters are controlled by software applications; and many of their electrical characteristics can be modified through software settings and commands. With pre-established settings, many inverter-based distributed resources can operate autonomously by adjusting their output to local conditions: Common functions: Inverters have the capability of riding through minor disturbances to frequency or voltage. These functions are called under/over frequency ride-through and under/over voltage ride-through.
Common Inverter Functionalities Injecting or absorbing reactive power into or from the grid (i.e., dynamic reactive power control). These functions make system stability maintenance easier by keeping voltage and frequency within specified limits. Oversizing an inverter allows reactive power generation/absorption even during peak irradiance. Soft start involves staggering the timing of reconnection of inverters on a single distribution circuit, to avoid spikes in the active power being fed onto the grid as it returns to normal functioning, limiting the risk of triggering another grid disturbance.
Advanced Inverter Testing Methods Typically, PV and grid simulators are used to test the advanced functionalities of smart inverters. Instead, we used a real PV system and local generation to test five key autonomous functions of an advanced inverter: soft-reconnect, non-unity power factor operation, over/under-voltage ride through, over/underfrequency ride through, dynamic Volt/Var control.
Experimental Set-up 12 kw PV array, 12 kva, 480 V advanced inverter, 30 kva, 480V /208 V transformer 25 kw resistive load bank (adjustable) 9 kvar inductive load bank (adjustable) 1.8 kvar capacitive load (adjustable) 208 V local grid supply.
Adjusting inverter parameters The inverter has a controller that provides a gateway for system monitoring and recording. The controller is equipped with a communication protocol (Modbus) that is commonly used by the solar industry for communication in PV power plants. Changing the default values of various parameters embedded in software requires a special code that was obtained from the manufacturer. Communication with the inverter is achieved wirelessly by means of a control panel through a router.
Communication with SMA inverter
Generator voltage & frequency control Voltage Control: The AVR set-point control is biased externally by bucking or boosting the sensing voltage input to the built-in generator controller, and this was accomplished by using of an external variac. Frequency Control: The governor set-point control is externally triggered by means of a DC control signal that is asserted by a microcontroller circuit.
Under-Frequency Ride Through Advanced PV inverters can assist the grid with frequency regulation during load-generation mismatch.
Over-frequency ride through
Under-Voltage Ride Through Advanced PV inverters can improve power quality by remaining connected to the grid during temporary faults.
Over-voltage ride-through
Non-unity power factor operation Non-unity power factor operation help compensate for drop in the overall feeder power factor, and regulate the voltage.
Dynamic Volt/Var Control Advanced PV inverters can assist the grid with localized voltage regulation by absorbing/generating reactive power.
Concern over Islanding Concern has been raised over the interference of these advanced control functions with islanding detection schemes embedded in these inverters. Reason for concern: some of these grid support functions are intended to stabilize the voltage and frequency, while some active islanding detection methods are based on destabilizing these electrical quantities. Such concern is amplified under load matching and/or when multiple inverters from different manufacturers share the same distribution circuit.
Islanding Detection Several passive and active methods exist to prevent an islanding condition. Starting with the classical over-under voltage (OUV) and over-under frequency (OUF) detection of conventional inverters, consider the system below.
Let Islanding Detection P G /P DR = α, and Q G /Q DR = β In conventional inverters, it is fair to assume that P DR and Q DR are constant. P Q DR DR (1 ) (1 ) V 2 old R V, 2 old old L (1 2 old LC).
V V new old new old 1, 1 b b 2d 2 4d, 1 b (1 d), 1 d 2 old LC. Case of R-L load (with no capacitance): new old 1. 1 Islanding Detection Changes in voltage and frequency after utility outage:
Islanding Detection The deviations in voltage and frequency depend on the magnitude and direction of the net power flow into the grid. When local generation closely matches the local demand, the voltage and/or frequency deviations will be too small, and the thresholds for the OUV and OUF relays cannot be set arbitrarily small. Due to the this non-detection zone (NDZ), grid-tied inverters incorporate one or more active methods to detect and prevent islanding even when the net active and reactive power flow into the grid is negligible.
Islanding Detection These active schemes basically push against the grid to determine to what degree it resists voltage and/or frequency changes; either the grid is stiff (not islanded) or movable (islanded). Some published active methods for islanding prevention include voltage harmonic monitoring, phase jump detection; slide-mode frequency shift, impedance measurement, and active frequency drift. The means by which commercial inverters detect and prevent unintentional islands are not standardized, and manufacturers rarely if ever share such detail.
Islanding Detection The most popular grid support function available in advanced inverters is the Volt/Var control that is designed to assist the electric utility with local voltage regulation. Perhaps the second important function the Hz/Watt control that helps regulate the system frequency, but this requires the incorporation of an energy storage system. When these functions are activated, they respond to voltage and frequency deviations by modifying the inverter real and reactive powers, P DR and Q DR, in a manner that reduces the impact of abnormal grid conditions. The resulting voltage and frequency responses depend on the coefficients that define the F-P and V-Q curves and delay time associated with the control system.
Typical Q-V and P-f Curves Let Q DR = g(v) and P DR = h(f).
Islanding Detection After an outage, the new voltage and frequency will lead to new inverter powers Q DR,new = g(v new ) and P DR,new = h(f new ). This will in turn lead to newer values of voltage and frequency, and the process continues until steady state values are reached. Meanwhile, if the inverter is equipped with the active frequency drift method, it will continuously attempt to cause larger and larger deviations in frequency. The concern is clear as these grid support functions work against this active anti-islanding method if they operate during the same time period, the result can possibly lead to an unacceptable islanded operation.
UNLV Microgrid Facility
Smart Inverter Islanding Test Sampling rate of power recorder: 500 milliseconds, Transient recorder triggered after sensing an abnormality.
Unity power factor operation (Case 1) Case No. P DR /Q DR (kw/kvar) PF (%) P G /Q G (kw/kvar) α β t (ms) 1 4.70/-.0.26 99.8 0.00/0.00 0 0 1,020
Unity power factor operation (Case 2) Case No. P DR /Q DR (kw/kvar) PF (%) P G /Q G (kw/kvar) α β t (ms) 2 4.78/-0.28 99.8 0.00/-0.13 0 0.46 65
Unity power factor operation (Case 3) Case No. P DR /Q DR (kw/kvar) PF (%) P G /Q G (kw/kvar) α β t (ms) 3 4.65/-0.27 99.8-0.08/-.03-0.02 0.11 715
Case No. Non-Unity power factor (Case 4) P DR /Q DR (kw/kvar) PF (%) P G /Q G (kw/kvar) α β t (ms) 4 3.90/2.35 85.6 0.15/-0.02 0.04-0.01 200
Unity power factor operation (Case 5) Case No. P DR /Q DR (kw/kvar) PF (%) P G /Q G (kw/kvar) α β t (ms) 5 5.35/1.35 97.0-0.22/0.16-0.04 0.12 114
Dynamic Volt/Var Control (Case 6) Case No. P DR /Q DR (kw/kvar) PF (%) P G /Q G (kw/kvar) α β t (ms) 6 5.30/-1.72 95.0 0.12/-0.10 0.02 0.06 3,330
Analysis of Inverter Current Distortion A secondary concern is that PV inverters may add to power quality problems that are associated with the proliferation of nonlinear loads, i.e., the impact of noise and harmonics generated by these static power converters. This concern led to the development of industry standards that limit these disturbances. In the United States, PV inverters are required to meet UL 1741 Std. which is based IEEE 1547-2014 Std.. Herein, the total distortion of the current, in percent of the inverter rated current is limited to 5%. This quantity is sometimes referred to as Total Demand Distortion (or TDD) a term which was originally derived for nonlinear loads
TDD vs. THD TDD is often confused with the conventional Total Harmonic Distortion (THD) that is calculated in percent of the fundamental-frequency current component. TDD and THD are identical only under rated power operation. Otherwise, THD is always larger than TDD(sometimes by orders of magnitude). To satisfy the limit set IEEE 1547 Std., manufacturers install carefully-designed passive harmonic and EMI filters at the inverter terminals in order minimize current distortion and switching noise when operating at rated condition.
Effect of distortion in supply voltage Since the inverters are not at fault when the voltage supply already contains some distortion, the Standard further specifies that the harmonic current injections shall be exclusive of any harmonic currents due to harmonic voltage distortion present in the Area EPS without the DR connected. Although PV inverters rarely (if ever) operate at their specified power rating, interconnection standards do not address harmonic emissions when such systems operate below rated conditions. Next, a 14 kva inverter is tested for harmonic content under sub-rated conditions.
Trend of RMS value of supply voltage over a 24-hour period
Trend of low-order harmonics of supply voltage over 24 hr a period IEEE Std. 519-2014 allows supply voltages below 1 kv to contain up to 8% THD and 5% limit for each individual harmonic.
Current waveform variation with solar irradiance grid operation
Harmonic content of current waveforms (% of fundamental) under different levels of solar irradiance grid operation
Harmonic content of current waveforms (in Ampere) under different levels of solar irradiance grid operation.
Current waveform variation under different power factor operation
Harmonic content of current waveforms (% of fundamental) under different power factor operation.
Islanded Operation
Current waveform variation with solar irradiance islanded operation.
Harmonic content of voltage and current waveforms islanded operation
Variation of current THD with inverter power generation
Conclusion No firm conclusion could be drawn on the impact of such functions and the islanding detection methods embedded in the inverter as the inverter ride time varied erratically even under very similar (but not exact) conditions. Future work will concentrate on determining the MPPT and anti-islanding methods used in the inverter through rigorous field tests and the dynamic response of the Volt/VAr function, so that its ride time during a utility outage can be predicted with a high level of confidence. The lesson learned here is that events are difficult to exactly duplicate when using real-life systems.
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