7. eaton dc balance of system and protection

© 2013 Eaton. All rights reserved.

DC Balance of System and Protection

Suraj Vernekar

Product Manager


Solar System Basics

3© 2013 Eaton. All rights reserved.

Basic overview of solar power generation

• Photovoltaic cells utilise the power of sunlight to convert photons to clean DC (Direct Current) electricity.

• DC electricity fed into a Power Inverter (PV Inverter) that converts and regulates the DC source into usable AC power.

• AC power can be used for local consumption – (£ saving)

• Domestic• Commercial

• AC power can be exported to the grid – (£ generation)

• Domestic and commercial excess un-used power• Larger PV farms


What is a PV cell?

• One cell – semiconductor device

• 6” square

• Sensitive to fault conditions

• Costly to replace

• On its own very fragile

Low output: 0.646V dc, 7.18A = 4.64W


What is a PV module/panel?

• Cells connected in series

• One panel

• Typically made of 54 cells• 1.5 x 0.6 metre

Symbol

Higher output: 34.9V dc, 7.18A = 250Wpeak


What is a PV string?

• A number of PV modules/panels in series is a string

• Typically made of 20 modules depending on type of module

• (Domestic systems typically 8 modules = 280V dc)

Higher output: 698V dc, 7.18A = 5kWpeak

34.9V 34.9V 34.9V 34.9V+ + = 105V dc

Symbol

n1 n1+n2+n


What is a PV array?

• A number of strings in parallel is an array

• Enables the current to be increased

• Help to overcome cable losses• Increased efficiency

• For example 16 strings in parallel

• 698V dc x (16 strings x 7.18A) = 80kWpeak

Higher output: 698V dc, 115A = 80kWpeak


Summary

1 Cell

0.646V DC x 7.18A= 4.64W

1 Module(54 cells in series) 34.9V DC x 7.18A

= 250W

1 String(20 modules in series)

698V DC x 7.18A= 5KW

1 Array(16 strings in parallel)

698V DC x 115A= 80KW


System topology


String inverter system designTypically commercial up to 1MW


Central inverter system – DC sideTypically large ground mount over 1MW


Central inverter system – AC side


Considerations for DC PV protection


Issues around solar systems

• Why is PV unique?• AC vs DC power• Fault conditions (overcurrent)

• Array fault scenario• Fault conditions (overvoltage)

• Direct lightning strikes• Indirect lightning strikes

• Impact on Efficiency


Why is PV unique?

• High DC voltage not AC voltage

• Many points of failure

• Relatively low short-circuit fault current

• Sensitive to overvoltage damage and generally susceptible to overvoltage conditions

• Installed in challenging environmental conditions:

• High ambient temperatures• Higher humidity than normal• Dust and sand• Animal interference

Remote solar farm Easily accessed power station


PV system Vs power station

Power Station

PV Array

Voltage generated 25kV AC 1000V DC

Power measured in: Giga Watts Kilo or Mega Watts

Short-circuit currents: Very HighTen of thousands of times normal current

Very Low1.3 times normal current

Circuit protection Traditional circuit protection

Specialist circuit protection

Installation Highly controlled, locally sited

Remote harsh environment

Operational Control Established control systems

Less easy to control


Fault scenarios

• Many connection points throughout the DC system

• Many points of possible failure

• Damage to panels

• Poor connectors

• Damaged cable

• Failure modes

• +/- ve to ground

• +ve to -ve


Overcurrent fault conditions

• System is live during daylight

• Cannot simply switch off the DC system• Difficult to disconnect PV panels from system for maintenance

• Easy to create short-circuit, loose cable, weather, animal interference

• Short-circuit fault currents relatevely low very difficult for non-gPV fuses to clear

• gPV fuse specially designed to clear 1.3 x fuse rating at 1000V dc

• Typical gG fuse only designed to clear 1.6 x fuse rating at 690V ac• For example a 10A gPV fuse (eg PV-10A10F) will clear a 13A fault current at

1000Vdc!

A gPV fuse MUST be used


Basic array fault scenario

Normal operationcurrent flows normally to system load

• Cable current /string = 1.56 x Isc = 1.56 x 7.74A = 12A• Cable current rating = 20A

System load

String current Isc = 7.74A

FuseS/C Current = (Np-1)x 125% x Isc

(16-1) x 125% x 7.74 = 145A

Short-circuitCurrent flows is diverted into fault string

System load

VS

• Short-circuit current > max cable current rating• Therefore individual string fusing is needed• A Cooper Bussmann PV-12A10F can be used


Overvoltage fault conditions

• PV Arrays are made of many semiconductor devices

• Susceptible to Lightning strikes due to open air locations

• Huge electromagnetic energy releases cause damaging voltage spikes onto output of PV arrays. Causing irreparable damage to PV panels.

Overvoltage = Module unusable = Costly repairs


Direct lightning strikes

20 kV

230 V

IT systemPower supply

Rst

110 kV

20 kV

1

1b

380 kV

110 kV

1 Direct / close lightning strike:1 Lightning strike into the internal

power grid or frame work

1a Voltage drop at the impulse earthing resistance Rst

Induced voltages in powergrid or external metal work

1b

1a

Lightning discharges in the external framework


Indirect lightning strikes

20 kV

IT systemPower supply

380 kV

110 kV

110 kV

230 V

20 kV

2c

2a

2 Distant lightning strike:

2c Fields of the lightning effect spreads

2aLightning strike at overhead line system

2bOvervoltage transient waves at the overhead line due to cloud-to-cloud flashes

2b

Rst

Lightning discharges in the power supply system


Types of overvoltage protection

Direct lightning strike Galvanic couplingInductive / capacitive coupling

Indirect lightning strike Conducted partial lightning currentsInductive / capacitive coupling

Type 1 Protection

Type 2 Protection

System protected from overvoltage damage

SPDs allow damaging overvoltages to pass harmlessly to Earth


Impact on Efficiency

• Due to the nature of PV systems output power can easily be affected by:-

• Shading• Dirt• Dust

• The actual physical size of large commercial and PV farms makes identifying faults very difficult

• Often left unattended for periods of time

• How do you know your system is working ok?

Reduced output = Reduced income


3 Ps of PV Protection

• People: protect people from injury and death

• Property: protect property from fire and damage

• Payback: protection of investment

Critical to have adequate system protection


One Combined Solution


• Combine output of many PV strings into one output

• Provide isolation of supply for maintenance

• Provide overcurrent protection for each string

• Provide overvoltage protection from damaging • voltage spikes

• Monitor complete system for output and health

• Provide all of the above in robust weatherproof enclosure

Combiner Box Function


Anatomy of a combiner box


• Securely houses and protects the internal components• IP65 rated• Include breather drains, suitable for tropical and sub-

tropical environments

• Choice of enclosure types

• UV stabilised Glass Reinforced Plastic (GRP)• Polyester or Polycarbonate

• Polycarbonate window optional

• Painted or stainless steel enclosure

Enclosure


• Primary function of combiner box is to group together incoming PV strings to achieve higher output currents.

• Typically from 3 to 24 input strings

• Glands or MC4 connectors

• MC4 connectors save time at point of installation

Number of strings


• Allows safe disconnection of panels

• 1000V DC specific switch disconnect

• Safely disconnect PV array from downstream equipment

• Critical for safe system maintenance

DC switch


• Low level fault protection

• Superior cycling withstand

• Fuse links available from 1A to 630A, 1000V DC

• 10X38mm typically used for string protection

• Housed in Eaton Bussmann gPV fuse holders

• Correct gPV fuse can protect individual strings and allow system to generate power without interruption in the event of a single string fault

Overcurrent protection

Eaton Bussmann leading gPV fuse specially designed for PV applications


• Eaton Bussmann recommends negative and positive string fusing possible depending on application

• Ensures maximum system protection• PV panel protection and downstream cable• Protects against reverse polarity and short-circuits

Overcurrent protection configuration


• Surge protection is required to protect the PV strings and array against overvoltage.

• Protects sensitive semiconductor PV panels from static discharges, voltage surges and spikes

• Special DC surge devices required for PV applications• Two ‘Classes’ Type 1 and Type 2• Type 2 surge protection used in most PV applications

(indirect lightning strikes)• Important to protect PV DC lines, AC lines and

communication lines

Surge protection


• A quick and effective method of remaining constantly updated on the performance and health of your system

• Individual String current • Total System Voltage• Combiner box temperature• SPD status (PV SPD & 240VAC SPD)• Main DC Switch status

• Industry standard RS485 Modbus with optional external service port for ease of integration and deployment

• Power supply options

Monitoring options


Summary

• Ratings

• Fuse Range: 1 - 32A• Volts 600 – 1000V DC• Strings 3 to 32• Sizes: 300x600 / 400x600 / 600x800/ 1050x850mm• Full overvoltage surge protection options• Monitoring and Wireless solutions• IP65 with IP65 breather vents as standard

• Features & Benefits

• Designed for harsh environments & high ambient temperatures• Fully customisable configurable design• Global manufacturing capabilities• Fully IEC compliant• Designed for long term reliability


Questions

7. eaton dc balance of system and protection

Engineering

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