POWER QUALITY IMPACTS AND MITIGATION OF DISTRIBUTED SOLAR POWERPresented 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

BACKGROUND

• 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

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 cyclingand grounding

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 www.Pterra.com

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.”

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

FLICKER

Definition

• Rapid fluctuations in voltage of power supply

• Affects human perception, as in flickering in lights

Standards

• IEEE Std 1453.1-2012:

• Adopts the “flickermeter”

• IEC/TR 61000-3-7:2008, “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

FLICKER FROM PV

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: http://www.pterra.com/flicker-trouble-ahead-for-solar-pv-inverters/

How much flicker is too much?

One day Voltage Flicker

• Measured using flickermeter

• Pst - Perception of light flicker in the short term. Short term is defined as a 10-minute interval.

0 5 10 15 200

0.2

0.4

0.6

0.8

1

time(hr)

Gen(P

U)

Irradiation Curve

min

max

simulated

Value

0 5 10 15 20

1.004

1.005

1.006

1.007

1.008

1.009

1.01

time(hr)

V(P

U)

POI

A

B

C

FLICKER LIMITS

• Plt. Perception of light flicker in the long term. Long term is usually defined as a 2-hour interval.

• Pst. Perception of light flicker in the short term. Short term is defined as a 10-minute interval.

• Pst ≤ 0.35 and Plt ≤ 0.25

• Penetration Limits – depends on the feeder stiffness, types of load, and on solar fluctuations

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

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 519 - 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

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

LIMITS TO PV PENETRATION DUE TO HARMONICS

• Like flicker, it’s easier to measure harmonic impact after the PV is installed

• 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

• 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

Something to watch out for: increasing use

of inverter-type devices such as air

conditioners and washing machines

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 http://www.pterra.com/voltage-stability-the-voltage-ledge-seen-close-up/

Standards

• IEEE Standard 1547 –“Interconnecting Distributed Resources with Electric Power Systems”

• Specifies voltage clearing times for PV

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 sagsVoltage in p.u. Clearing time in

sec

<.5 0.16

.5 to .88 2.0

1.1 to 1.2 1.0

>1.2 0.16

LOW VOLTAGE ON A FEEDER

Voltages following event in transmission system

Voltage Types

• Type 1 – Overshoot due to excess in reactive supply

• Type 2 – Recover to pre-event voltage, just enough reactive supply

• Type 3 – Recover to low voltage, not enough reactive supply – The Voltage Ledge

PV Effects under 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

GROUNDING

• For safety reasons, a distribution feeder needs to be effectively grounded as specified by the National Electrical Code

• Feeders may be operated as a three-wire or four-wire system

Phase A

Phase B

Phase C

Single Phase

Distribution Transformer

1 Phase

120-240 v

Service ground at

customerTypical Single Family House

Residential Customers

Grounding at Substaiton

Neutral

3 Phase 480

Neutral

Neutral

Service ground at

customer

Four-Wire, Multi-Grounded-Neutral

Distribution System

Distribution.

Building 1

UNGROUNDED PV INVERTERS

• Three-phase PV inverters tend to be supplied as ungroundedsystems

• 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

Open

Vc = 0

Va = 1.73 PU

Vb = 1.73 PU

Line-to-

ground fault

Island.

Islanding

Feeder

Utility

Open Breaker

Open

Open

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

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 C57.12.01 -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

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

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

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 pre-existing 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

THANK YOUFor follow-up discussion and/or questions, please contact me at:

ricaustria@pterra.us