net metering installation and commissioning, 2 ... · pdf filejob sheet 2 –...

© Festo Didactic Inc. 88752-20 19

Job Sheet 2 – Commissioning the Net Meter

Utility Grid Interaction

Grid-connected solar energy systems typically include a utility meter to monitor energy levels

into and out of the utility grid. These utility meters are most commonly connected between the

AC load center, also called an AC power distribution panel, and the utility grid service entrance

(Figure 2-1). The meters are usually calibrated in kilowatt-hours (kWh).

Figure 2-1. Grid-Tied System.

Many utility meters can measure energy in both forward and reverse directions. Net metering

and dual metering are two common methods for tracking imported and exported power (Figure

2-2).

Figure 2-2. Net Metering and Dual Metering.

Net metering uses one standard utility meter to monitor energy usage in both directions, so

the customer and utility company pay the same rates for the electricity. Dual metering uses

two individual meters, one for each direction. Each ratcheting utility meter can only spin in

Commissioning the Net Meter

20 © Festo Didactic Inc. 88752-20

supplied power while charging the customer higher rates for utility-supplied electricity. Digital

smart meters can display many different values (separate values for each direction) and can be

used in place of two meters. Service billing rates can change per season and are based upon

service type, such as industrial, commercial, or residential. The time of day and control of water

heating can affect electricity costs, as well.

The location where a utility-interactive power generating system is interconnected to the utility

grid is called the point of connection (Figure 2-3).

Figure 2-3. Utility Grid Point of Connection.

The AC power distribution panel (or the AC circuit breaker box in your trainer) houses the main

service disconnect, which is a circuit breaker or a fused switch. As shown in Figure 2-4, the side

of the breaker that connects to the utility meter is called the supply side of the power distribution

system. The opposite side of this breaker, where additional breakers protect branch circuits that

feed multiple AC loads, is called the load side of the power distribution system.



Figure 2-4. Supply-side and Load-side Inverter Connections.

Utility-interactive inverters can be connected to either side of the main service disconnect. For

load-side inverter connections, the inverter output requires a circuit breaker, called a back-feed

circuit breaker. For supply-side inverter connections, the inverter output also requires a circuit

breaker, or fused switch, in addition to the main service disconnect.

Meter Reading

Most old electric utility meters, or watt-hour meters, are the electromechanical induction type.

pointers to indicate the measured value in kilowatt-hours (kWh). Each pointer rotates either

clockwise or counter-clockwise, depending on its multiplier position. The rotating direction

is clockwise on the rightmost dial and the direction alternates for each dial toward the left.

side, back onto the utility grid. When interpreting the values indicated (in the normal forward

direction), the pointer must be on or past each number in order to be valid. For example, if the

pointer is between 8 and 9, the value indicated is 8. If the pointer is between 9 and 0, the value



indicated is 9. If the pointer is directly on 7, look at the dial to the right of the one that you are

reading. Use 7 if the dial on the right has passed 0; otherwise, use 6. Figure 2-5 shows a value

of 5,112 kWh.

NOTE: The dial pointer must be on or past each number in order to be valid.

Figure 2-5. Watt-Hour Meter Dials.

Other analog watt-hour meters have a numerical display to indicate the measured value in

kilowatt-hours (kWh). Both meter types incorporate a large disk that rotates faster with higher

power. By counting the number of full disk revolutions within an interval of one minute or longer,

you can estimate the power consumed in watts (W), as shown in the following equation.

Power (W) = Kh x revolutions x 3,600 / time (s)

NOTE: The value of Kh is printed on the face of the watt-hour meter.

Often, the rotating disk is calibrated from 0% to 100% of a revolution in 1% increments. For

smaller loads, such as in your trainer, you can count a fraction of one full disk revolution in

percentage (%) and use the following equation to determine the power consumption.

Power (W) = Kh x revolution (%) x 36 / time (s)

NOTE: The value of Kh is printed on the face of the watt-hour meter.

For example, if the disk only moved 3% in 60 seconds (on a meter with a Kh value of 7.2),

about 13 W was consumed.

For digital utility meters that display energy in watt-hours (Wh) directly and not as a percentage

of a rotating disk, use the following equation to determine power:

Power (W) = energy (Wh) x 3,600 / time (s)

The above equation assumes that the AC voltage is nominally 120 V AC. However, for more

accurate results, you can use the following equation to determine power:

Power (W) = energy (Wh) x 432,000 / voltage (V) / time (s)



For example, if the meter had taken 36 seconds to count 1 watt-hour, the power was 100 watts

at 120 volts.

100 W = 1 Wh x 432,000 / 120 V AC / 36 s

Many smart watt-hour meters use a digital display to indicate the measured value. Some smart

meters can also report power quality, outages, and other information. They typically support

automated meter reading (AMR) or remote meter reading (RMR), which permits the data to be

read remotely.

Net Meter Features

The digital utility meter on your training system (Figure 2-6) measures watt-hours (Wh), is UL/

Figure 2-6. Digital Utility Meter.

To complete this Job Sheet using the digital utility meter, record the digital count before and

determine the energy in watt-hours for a given time period.

provided on the utility meter. Only one counter operates at a time to provide net metering.

Figure 2-7. Digital Counters.



The counter on the left displays the energy exported (generated and placed on the utility grid),

and moving in the reverse direction. The counter on the right displays the energy imported

(consumed by AC loads in excess of all power generated) and moving in the forward direction

(Figure 2-8).

The digital meters are used in a manner similar to analog ones; however, the measured

resolution is improved.

Figure 2-8. Direction of Energy.

Net Meter Operation

The digital utility meter on your training system operates by using a toroidal current-sensing

transformer to remotely sense current (Figure 2-9).

Figure 2-9. Current Transformer (CT).

The current transformer (CT) sends a small signal voltage to an analog-to-digital converter

(ADC) inside the utility meter. The digital output of the ADC is processed by an on-board

microcontroller that keeps track of the power levels over a period of time. The microprocessor

sends a short pulse of electrical power to the appropriate counter to increment the displayed

value by 1 whenever one watt-hour (Wh) has passed in a particular direction (Figure 2-10).



Figure 2-10. Utility Meter Block Diagram.

NOTE: The utility meter requires about 2 watts to power its internal circuitry.

System Check Out

After a careful visual inspection of the wiring and assembly, power must be applied to the meter

to check for proper connections and operation. A red light-emitting diode (LED) lamp on the

meter should glow whenever the meter is powered (Figure 2-11). One of the two green LED

the respective counters is incremented by a value of 1.

Power

Indicator

Figure 2-11. Digital Utility Meter.



OBJECTIVES

In this job, you will become familiar with the interactive electrical functions of the

net meter on the grid-tie training system. You will apply power to the system, check

proper operation of the net meter, and record your results.

EQUIPMENT REQUIRED

Refer to the Equipment Utilization Chart in Appendix A to obtain the list of equipment

required for this job.

SAFETY PROCEDURES

Before proceeding with this job, complete the following checklist.

You are wearing safety glasses.

You are wearing safety shoes.

You are not wearing anything that might get caught such as a tie, jewelry, or

loose clothes.

If your hair is long, tie it out of the way.

The working area is clean and free of oil.

Your sleeves are rolled up.

Instructor initials: __________

PROCEDURE

NOTE: For information regarding the Lockout/Tagout (De-energizing) and

Energizing procedures, see Appendix D.

Lockout/Tagout Procedure

Perform the Energizing procedure.

Connection to the Power Source

CAUTION:The following procedure steps must be completed in the sequence

presented. Deviating from this order could cause damage to the inverter and void

the warranty.



Open the door on the AC distribution panel, and turn on the main and

inverter circuit breakers (Figure 2-12).

Main Inverter

Figure 2-12. AC Distribution Panel.

Move the power lever to the On (up) position on the AC disconnect

switch (Figure 2-13).

Power

Lever

Figure 2-13. AC Disconnect Switch.

NOTE: These steps apply the AC grid voltage from the wall outlet to the output of

the inverter.



Open the door on the combiner box (Figure 2-14) and ensure that there

are four fuses installed in the holders.

Fuse

Holders

Figure 2-14. Combiner Box Fuse Holders.

Move the power lever on the DC disconnect switch (Figure 2-15) to the

On (up) position.

Power

Lever

Figure 2-15. DC Disconnect Switch.

NOTE: These steps have made the necessary DC connections to the input of the

inverter. The inverter is installed with its factory default settings, which are a DC

input range of 125–250 V DC. The system is now ready to power up using the

solar array simulator as a substitute for solar panels.



Power-Up

The solar array simulator (Figure 2-16) is used to simulate sunrise and

sunset conditions without the physical solar panels installed.

Figure 2-16. Solar Array Simulator.

Make sure the Voltage Control knob is set to 0%.

Set the multimeter to read DC voltage (200-volt range).

Insert the test probes into the red and black test points on the solar

simulator, matching like colors.

At this point, the LED indicators and the LCD display are off. This

is too low for operation.

Turn on the power switch on the front of the solar simulator. The red

lamp to the left of the switch illuminates.

NOTE: If the lamp does not light, review the power connection to the source.

Slowly turn the control knob on the solar simulator from 0% to 100%

while watching the voltage on the meter, the LEDs, and the LCD on the

cover of the inverter.



After a short time, all of the LEDs on the front panel of the inverter light

up. Once this occurs, record the voltage on the multimeter in the space

provided.

All LEDs lit: V DC

Continue to turn up the control knob. Notice that the LCD lights up; but

it does not display anything. Once this occurs, record the voltage on the

multimeter in the space provided.

Display lit: V DC

NOTE:

enough power to feed the grid. The inverter is initializing.

Continue to turn the control knob. The LCD displays the status. Record

the voltage on the multimeter in the space provided.

Display status: V DC

The following messages are displayed during this process:

Sunny Boy 700U

WR7xxUxxE

BFR Version x.xx

SRR Version x.xx

Continue to turn up the control knob. The green LED begins to blink at

a rate of once per second. The red and yellow LEDs are no longer lit.

Record the voltage on the multimeter in the space provided.

Green LED blinks; red and yellow LEDs no longer lit: V DC

NOTE: While this is taking place, the display on the inverter reads, “Mode-

waiting.” The inverter now has enough power and is checking the condition of the

grid in preparation to make a connection. If the inverter fails to connect to the grid

three times in a row, it waits 10 minutes before its next attempt.

After the green light blinks for 10 seconds, a relay click is heard and the

LED remains steady.

NOTE: The inverter is now operating in normal MPP mode.

The display has a background illumination feature that can be activated

by knocking on the lid.



While in normal operating mode, the inverter displays three sets of data.

The current operating status appears beneath this as follows:

E-today: 5.35 kWh

Mode: MPP

The second display provides the current power. The PV voltage (in our

case, the solar array simulator voltage) appears beneath this as follows:

Pac: 700 W

Vpv: 200 V

The third display provides the accumulated yield of the inverter since its

installation. The total operating hours appears beneath this as follows:

E-total: 175.5 kWh

h-total: 512 h

At this point, the inverter is synchronized with the utility grid.

following data.

E-today: kWh

Mode:

Pac: W

Vpv: V

E-total: kWh

h-total: h

Simulate an AC disturbance by turning off the AC disconnect switch.

The yellow LED illuminates for 5 seconds, turns off for 3 seconds, and

blinks twice. This code repeats three times. If the disturbance continues,

the code repeats until the disturbance is corrected.



The display appears as follows:

Disturbance

Fac-Bfr

To correct the disturbance, turn on the AC disconnect switch.

The yellow LED goes out and the green LED begins to blink with the

display reading, “Mode-waiting.”

The green LED blinks. Once the AC power has been restored for 5

minutes, the inverter reconnects to the grid and the green LED becomes

steady.

NOTE: When servicing the inverter, always disconnect the DC before the AC.

Powering Up Load Outlets

Open the door on the AC distribution panel (Figure 2-17).

Turn on the load 1 and load 2 circuit breakers.

Load 1 Load 2

Figure 2-17. AC Distribution Panel.

Verify that there is power at the outlets by plugging in the outlet tester

(Figure 2-18) and making sure the proper phase lights are lit.



Load Outlet 1

Load Outlet 2

Outlet Tester

Figure 2-18. Outlet Tester.

Turn off the load breakers.

Locate the power/usage monitor (Figure 2-19) and plug it into load

outlet 1.

Power/Usage

Monitor

Figure 2-19. Power/Usage Monitor.

Locate the two light socket adapters and two incandescent lamps.

Thread the lamps into the adapters.

Plug one lamp assembly into the power/usage monitor in load outlet 1.

Turn on the load 1 circuit breaker and verify the light is lit.



Step through the functions of the power/usage monitor and record the

following data in the space provided.

Volts:

Amps:

Watts:

VA:

Hz:

PF:

kWh:

Hours:

NOTE: The kWh and hours are accumulative and may have a reading of 0 until

the unit has been on for some time.

Figure 2-20. Lamp Assemblies.

Plug the other lamp assembly into the load outlet 2 (Figure 2-20).

Turn on the load 2 circuit breaker and verify that the light is lit.

Power Consumed/Used

Double the power measured at load outlet 1 to determine the total

power of both loads.

Total load power: watts = load 1 power: watts x 2

NOTE: Incandescent light bulbs rated for 60 W at 130V AC consume about 50 W

at 120 V AC.



Power Generated/Produced

Record the Pac total on the inverter display. This is the total AC power

at the inverter’s output that is being generated from the DC power at the

inverter’s input.

Pac total:

Power Sold/Delivered/Provided

Subtract the total power of both loads from the Pac total. This is the total

power that is exported back to the utility grid.

Grid power: watts =

Pac total: watts – total load power: watts

Energy Exported

Turn off both load breakers.

Use the net meter to measure and record the total time (in seconds)

required for the inverter to provide 1 watt-hour of energy to the utility

grid.

Time (per Wh):

Use the following equation to calculate and record the approximate total

power (in watts) that is being exported to the utility grid.

Power (W) = 3,600 / time (s)

Total exported power (based on the net meter):


Energy Imported

Turn on both load breakers.

Did both lamps light up?

Yes

No



Disconnect the output of the inverter by turning off the AC disconnect

switch.

Did both lamps turn off?

Yes

No

Explain the behavior of the lamps.

least once.

Yes

No

Use the net meter to measure and record the total time (in seconds)

required for the two loads to consume 1 watt-hour of energy from the

utility grid.

Time (per Wh):

Use the following equation to calculate and record the approximate total

power (in watts) that is being imported from the utility grid.

Power (W) = 3,600 / time (s)

Total imported power (based on the net meter):


You just determined the total load power (P2) by using the net meter

and you also measured the total load power (P1) by using the power/

usage monitor. Use the equation below to determine the difference

between the two power values.

P(delta) = [(P2–P1)/P1] x 100%

P(delta) = %



Did your two values of total load power match each other within a

tolerance of 10%?

Yes

No

Lockout/Tagout Procedure

Perform the Lockout/Tagout procedure.

Ask your instructor to check and approve your work.

Procedure Questions

1. A utility meter is normally connected between the utility grid and the

a. branch circuit breaker.

b. main service panel or AC load center.

c. DC disconnect switch and GFPD.

d. inverter input.

2. In what electrical unit does a utility meter measure energy?

a. Milliampere (mA)

b. Kilowatt (kW)

c. Volt (V)

d. Kilowatt-hour (kWh)

3. What type of electrical component is used to remotely sense AC current?

a. Capacitor

b. Diode

c. Transistor

d. None of the above is correct.



Name: _________________________________ Date: ______________________

Instructor approval: ___________________________________________________

4. On the digital utility meter, what does the red LED lamp indicate?

a. Faulty circuit

b. Standby mode

c. Power on

d. Reset mode

5. On the digital utility meter, what do the green LED lamps indicate?

a. Incremental units of energy

b. Decremental units of energy

c.

d. All of the above are correct.

6. Can you use the net meter to determine electrical power in watts (W)?

a. Yes

b. No

net metering installation and commissioning, 2 ... · pdf filejob sheet 2 –...

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