POWER QUALITY IMPACTS AND MITIGATION OF DISTRIBUTED SOLAR POWER

Transcription

1 POWER QUALITY IMPACTS AND MITIGATION OF DISTRIBUTED SOLAR POWER Presented by Ric Austria, Principal at Pterra Consulting to the IEEE San Francisco Chapter Feb 17, 2016 California Public Utilities Commission, 505 Van Ness Ave., San Francisco CA

2 Growing number of installations of solar photovoltaic (PV) Largest growth is in interconnections to distribution circuits (or feeders) Feeders originally designed to deliver electric power from the bulk system to the loads High penetration levels of PV introduce power quality concerns that feeders were not designed for BACKGROUND

3 HOW MUCH PV CAN A DISTRIBUTION FEEDER HANDLE There are several electrical aspects: Thermal Capacity seldom an issue Steady-State Voltage Range and Control occasionally an issue Stability and Frequency Control only an issue for microgrids or smaller grids, not Western interconnection yet Transient Overvoltage significant issue This talk focuses on the Power Quality aspect Both traditional PQ issues such as flicker, harmonics and voltage sag But also new issues such tap cycling and grounding

4 ABOUT PTERRA Pterra is an analytical consulting firm that provides services to the electric power industry Established in 2004 Have conducted numerous studies and training courses in various electric power fields including transmission, distribution, generation and energy markets, applicable to design, planning, research, operations, regulation and failure assessment Broad range of services power flow, short circuit, stability, voltage control, power quality, transients, production simulation Team-oriented approach to engagements focused on quality and timeliness More info at

5 RANGE OF RESPONSES TO PV PENETRATION None allowed The feeder isn t suitable for PV connections. Up to 15% of total load on the feeder this was apparently considered a safe level Typical feeders are fed from say a 5 MVA transformer that may be up to 75% loaded. So 15% would be about 375 kw. This is about 10 houses with rooftop PV. Up to 50% of total load on the feeder the new standard that entities such as the National Renewable Energy Lab are proposing No limit PVs as a renewable resource are the preferred resource. Fix the grid to support.

6 POWER QUALITY ISSUES RELATING TO PV CONNECTIONS Flicker technically, visible change in brightness of a lamp due to rapid fluctuations in the voltage of the power supply Harmonics distortions on the pure sine wave form of AC currents and voltages Voltage Sags response of PV to low voltage conditions that may lead to equipment damage Grounding some PV installations may move ground potential away from the range considered safe Transformer Tap Cycling duty cycles for switching equipment such as transformer tap-switching may exceed rating due to PV installations Voltage Imbalance asymmetrical distribution of single-phase units can cause imbalance beyond 3% of nominal voltage

7 FLICKER Definition Rapid fluctuations in voltage of power supply Affects human perception, as in flickering in lights Standards IEEE Std : Adopts the flickermeter IEC/TR :2008, Electromagnetic compatibility (EMC)-- Limits--Assessment of emission limits for the connection of fluctuating installations to MV, HV and EHV power systems Replaces the old GE Curve PV Impact Conversion from DC produced by the PV cells to AC needed to connect to grid has flicker This flicker is minimized by conditioning equipment Most PV flicker comes from changes in power levels i.e., changes in irradiation levels

8 Irradiation Curve min max simulated Value FLICKER FROM PV Gen(PU) 0.6 How much flicker is too much? time(hr) Measured using flickermeter POI Pst - Perception of light flicker in the short term. Short term is defined as a 10-minute interval. A B C One day change in solar irradiation Flicker from PV comes from power changes on the inverter Solar irradiation changes power as the sun rises and falls Additional flicker may occur during cloud cover events; i.e., clouds moving across the PV cells. More details in this article: V(PU) time(hr) One day Voltage Flicker

9 FLICKER LIMITS P lt. Perception of light flicker in the long term. Long term is usually defined as a 2-hour interval. P st. Perception of light flicker in the short term. Short term is defined as a 10-minute interval. P st 0.35 and P lt 0.25 Penetration Limits depends on the feeder stiffness, types of load, and on solar fluctuations

10 LIMITS TO PENETRATION DUE TO FLICKER Flicker is one of those electrical phenomena that is hard to evaluate The standard is based on the perception and sensitivity of a typical person The best way to measure is when the PV is already installed, but then how to limit penetration levels? Simulations using transient software such as PSCAD or EMTP-type products seems to indicate that flicker is not an issue even at 50% penetration However, the jury is still out, since it s the user public that will eventually provide the feedback headaches, epileptic episodes and other human indicators

11 HARMONICS Definition A harmonic is a component of a periodic wave having a frequency that is an integral multiple of the fundamental power line frequency Total harmonic distortion is the contribution of all the harmonic frequency currents to the fundamental Like flicker, harmonics are also distortions on the alternating current and voltage waves. But harmonics are more cyclical and prolonged Standards IEEE Std IEEE Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems Specifies acceptable limits for individual and total harmonics PV Impact The inversion process from DC produced by the photovoltaic cells to AC needed to connect with the grid inherently has harmonics in both the current and voltage output The harmonics from individual PV inverters are mostly innocuous as long as there are no resonance modes within the grid. Newer inverter designs have less harmonic content Sample harmonics from a PV inverter

12 HARMONICS Circuit resonant frequencies are primarily due to the interaction between power factor correction capacitors and overhead line inductance Potential issues come from inverters injecting harmonics at resonant frequencies Sample frequency scan of feeder circuit

13 Like flicker, it s easier to measure harmonic impact after the PV is installed LIMITS TO PV PENETRATION DUE TO HARMONICS Multiple PV installations aggregate their harmonics and thus increase potential for resonance These get added on to harmonics that are already on the feeder from other devices Typical mitigation is to add harmonic filters Something to watch out for: increasing use of inverter-type devices such as air conditioners and washing machines Practically, the limit is the point when harmonic distortion gets high enough from PV penetration that mitigation is required. Without mitigation, penetration limits of 50% peak load have been reported With mitigation, such as harmonic filters, penetration limits beyond 100% are possible

14 VOLTAGE SAGS Conditions Voltages can dip on a feeder, perhaps due to events upstream, on the transmission system, or loss of power conditioning equipment In the extreme, feeders can be operating at the Voltage Ledge, described in ltage-stability-the-voltageledge-seen-close-up/ Standards IEEE Standard 1547 Interconnecting Distributed Resources with Electric Power Systems Specifies voltage clearing times for PV Voltage in p.u. Clearing time in sec < to to > PV Impact In the voltage range of 0.5 to 0.88, PVs may clear quickly, within a few millisec, or as long as 2.0 sec This relatively wide range of response time can prolong voltage sags

15 LOW VOLTAGE ON A FEEDER Voltage Types PV Effects under Voltage Ledge Voltages following event in transmission system Type 1 Overshoot due to excess in reactive supply Type 2 Recover to preevent voltage, just enough reactive supply Type 3 Recover to low voltage, not enough reactive supply The Voltage Ledge Some PVs may drop out fast, causing local voltage to further drop Motor loads such as air conditioners or heaters, may trip off due to the low voltage causing voltages to rise Rising voltage may delay the clearing of other PV The low voltage condition stays for an extended period causing damage to equipment from heating

16 Phase A Distribution. GROUNDING Phase B Single Phase Distribution Transformer 1 Phase v Phase C Neutral Neutral Grounding at Substaiton For safety reasons, a distribution feeder needs to be effectively grounded as specified by the National Electrical Code Typical Single Family House Residential Customers Service ground at customer 3 Phase 480 Neutral Building 1 Service ground at customer Feeders may be operated as a threewire or four-wire system Four-Wire, Multi-Grounded-Neutral Distribution System

17 UNGROUNDED PV INVERTERS Three-phase PV inverters tend to be supplied as ungrounded systems Islanding Feeder Utility Open Breaker Island. When the feeder is islanded, a single-line-ground-fault can lead to over voltages, a big safety issue Manufacturers will certify tripping within 10 cycles for V>1.2 pu but for 3-phase faults The typical mitigation is to add grounding transformers to the feeder DG DG source Ungrounded Va = 1.73 PU Vb = 1.73 PU Vc = 0 Line-toground fault Open Open Open

18 PENETRATION LIMITS DUE TO GROUNDING ISSUES The magnitude of over voltage from the previously described phenomena Decreases if there is more load on the feeder Increases if there is more ungrounded PV connected to feeder Hence, mitigation is only required up to a certain level of PV penetration on the circuit Grounding issues have appeared for penetration levels as low as 25% light load. Utility approaches: Require all new ungrounded PV above a certain size to include a grounding bank Ask all PV on the feeder to share in cost of mitigation Deny any new ungrounded PV beyond 15% penetration

19 TRANSFORMER TAP CYCLING Description LTC load tap changing transformers: Regulate voltage on the feeder in response to changes in load Have a duty rating based on the number of tap changes over the life of the transformer Standards IEEE Std C General Requirements for Dry-Type Distribution and Power Transformers Allows for reduced ratings for unusual frequency of use PV Impact PVs introduce voltage fluctuations In response to changes in insolation, including cloud movement The voltage fluctuations may disrupt the daily cycle of LTC that are rated for serving load

20 TAP CYCLING Sample simulation for an LTC installed on 12.5 MVA, 46 KV/12.47 KV transformer Addition of PV increases the number of LTC movements

21 TAP CYCLING PENETRATION LIMITS At higher penetration levels, the range of voltage fluctuations from PV is broader This results in increases in the number of tap movements by LTCs connected to the same feeder Transformer maintenance outages, life cycles are impacted No known measures of maximum penetration levels yet, but the argument can be made that: Incoming PV should pay for higher maintenance costs And include an upgrade of the LTCs

22 IN SUMMARY Penetration limits due to Power Quality issues no mitigation Voltage Flicker up to 50% of feeder load, depending on solar irradiation, types of loads, types of inverters Harmonics up to 50% penetration possible without mitigation, subject to preexisting levels of harmonics and assuming feeder has capacitor banks Grounding issues appear for ungrounded PV as low as 25% of light load Tap Cycling no measures yet available Voltage Sags unknown Voltage Imbalance theoretically not limiting if utility is able to implement balancing With mitigation, high level of penetration are possible

23 THANK YOU For follow-up discussion and/or questions, please contact me at: