I prepared this training presentation for the Business Development Team so that they could explain our systems design strategy for widespread grid stabilization through the use of power factor control attributes of PV inverters.

1. Reactive Power and Voltage Stabilization
David F. Taggart
http://www.linkedin.com/in/davidtaggart/

2. Outline
• The Basics
– Electricity
– Power
• Impact of Reactive Power
• Reactive Power, PF Control, Voltage Stabilization
• Implementing PF Control/Voltage Stabilization
NOTE: like with the financial guys, the power folks use different words for
identical meaning, which leads to the majority of the confusion
surrounding these topics. I have tried to pull the most salient points,
using the most common words, and tie them to the relevant aspects of
the PV power generation industry.

3. Electricity Basics
Electricity and Power

4. Electricity: the water analogy
• Wire is a pipe for water to flow in
• Current is the flow rate of water through the pipe
• Voltage is the pressure in the pipe, can be viewed as the delta in height
between the top and bottom of the pipe (i.e. the potential energy)
• Battery/generator is a pump that increases the pressure of the water
• Load/Resistance is a water
wheel taking energy from
the water
– the water is still there, it just
has less energy in it
Potential Energy
– the pipe resisting the flow of
water is another type of
resistance
• Electrons are the water
that gets recycled over and
over
• Circuit is the complete loop
of motion for the water

5. Electricity: flow of electrons
• Electrons move through a circuit from a negative pole to a positive
one, but current is said to flow from positive to negative (thanks Ben
Franklin!)
• Electrons don’t get “used up” but continue through a load until they
return to the source propelled forward by the voltage, thus being
recycled continually. They do however give up their energy once they
pass through the load
• The phrase “path of least resistance” is actually a little misleading.
More accurately, electricity takes the path of least resistance back to
their source
• For DC, the flow is continuous in one direction, for AC, it goes back
and forth the same amount, with no net displacement. Think of the
water moving back and forth through a paddle wheel

6. Electricity: voltage is not constant
• From the water analogy, you can see that voltage cannot be constant,
whereas current must be if there is indeed a circuit (closed loop)
• Because there are losses (resistance) and loads (water wheel) in a
circuit, voltage will drop as the electrons move through the circuit
• Factors affecting voltage include overall demand/load, utilization of
T&D lines, manipulation of system controls by the control centers, and
emergency situations occurring in the system
• The end users are the ones that require voltage stability within a
narrow range for their devices to work properly and have a useful
lifetime
• It is the control centers’ responsibility to control the voltage so that it
can satisfy this requirement

7. Power: the different types
• The term “power” typically refers to active/real power representing
the energy-related quantities in a grid, and is the product of voltage
(E) and current (I)
• For AC systems, voltage and current follow a sinusoidal wave form,
changing polarity every 180 degrees in time
• A complete cycle of 360 degrees occurs 60 times a second (60 hz)
• When E and I are not “in phase” (i.e. when they don’t cross the
median at the same point) a second component of power shows up
1. Active or real power (as we have been discussing)
2. Reactive or imaginary (unreal) power (the new one)
• The combination of the two is total or apparent power
• Lets agree on: +
– Total power: symbol is S, and units are
volt●amperes (VA)
– Real power: symbol is P, and units are
watts (W) Time →
– Reactive power: symbol is Q, and units -
are volt●amperes reactive (VAR)

8. Power: relationships
•
Q: Reactive Power
φ
P: Real Power

9. Power factor: unity (1)
• When PF is unity (1), the angle between real and total power (φ) is
zero
• This means that voltage and current are in-phase and all the power
is consumed by the load: none is returned (dark blue line)
• This is what happens for 100% resistive loads like lights, heaters,
ovens, etc., i.e. real power equals total power
S: Total Power
P: Real Power

10. Power factor: zero (0)
• When PF is zero (0), the angle between real and total power (φ) is 90o
• This means that voltage and current are exactly 90o out of phase and
all the power is returned to the grid unused
• The dark blue line shows that all the power is stored temporarily in
the load during the first quarter cycle and returned to the grid during
the second quarter cycle, so no real power is consumed
• This is what happens for 100%
reactive loads like capacitors, i.e.
reactive power equals total power
Q: Reactive Power
S: Total Power

11. Power factor: 0 < PF < 1
• When PF is between 0 and 1, φ is between 0 and 90 degrees
• This means that voltage and current are out of phase by φo, with
some power consumed, and some returned to grid (φ, dark blue line)
• This is what happens for combination resistive & reactive loads like
motors, blow dryers, drills, basically the majority of the loads
• The phase shift can be either
leading or lagging in reference
to whether the current is ahead
or behind voltage in time
Q: Reactive Power
Φ=45o
P: Real Power

12. Power factor: operational PF
• Different devices/loads operate at different power factors
• These devices are all inductive in nature (lagging) and thus need
capacitive (leading) reactive power to ensure their safe and efficient
operation as well as stabilize the voltage of the circuit they run on
• Compensating for their inherent operational power factor yields
significant benefits

13. The basics: summary
•

14. The basics: summary
• Q can be leading or lagging, in reference to current arriving before or
after the voltage in time, respectively
• Q is generated or consumed in almost every component of an AC
system
• Q does not travel far because reactive power dissipates up to 30%
faster than real power. Independent Generators and ISOs must
produce and transmit greater amounts of reactive power to
consumers, so it is best generated close to the load. This is why DG
PV has a particularly strong advantage as a Q generator
• Where lagging Q (VARs) are consumed, leading Q (VARs) must be
supplied
• Inductors are said to be lagging devices and consume reactive power
and capacitors are said to be leading devices and generate reactive
power
• The PF in a circuit can be measured with the wattmeter-ammeter-
voltmeter method, where the power in watts is divided by the product
of measured voltage and current
• The power factor of a balanced polyphase circuit is the same as that
of any phase

15. The basics: mnemonics
•
Current lags behind voltage in Current leads voltage in an
an inductive circuit capacitive circuit

16. Impact of Reactive Power

17. Reactive power: impact to the grid
• For the majority of components, loads and devices comprising the
grid, power is temporarily stored in them as it passes through them,
distorting its waveform before returning energy to the grid
• This distortion, or shifting of current in time with respect to voltage,
causes the total power to be greater than the real power (because the
shift causes the presence of reactive power)
• Inductive loads constitute a major portion of the power consumed in
industrial complexes, and due to their low PF, require much higher
currents than their real power needs would imply
• These higher currents require larger wires and other equipment to
transport, and increase the energy lost in the T&D system
• Due to the costs of larger equipment required and wasted energy,
electrical utilities will often charge a higher price to industrial or
commercial customers if their operations function at low power factor
• So to have an efficient system, whether you are a load, a generator, or
the grid itself, PF should be as close to 1 as possible

18. Reactive power: impact to the grid
• Reactive power (Q) is required to maintain the voltage in a T/D line to
enable the delivery of active power. Not enough Q and the voltage is
not high enough to push the current through the wire, and voltage
will sag and underserve the load. Too much and voltage can increase
to the point that infrastructure and loads can be damaged
• Reactive power must balance in the grid to prevent voltage problems
• The farther the transmission of power, the higher the voltage needs to
be raised to overcome the resistance to current flow
• Voltage drops related to reactive power contributed to blackouts in
the West (1996,) France (1978,) near failures in the PJM system (1999)
and significant voltage swings in the Midwest and Northeast in 2003

19. Reactive power: impact to the grid
• New York Times, 9/26/03

20. Reactive power: summary
• Reactive power (Q) is an essential component of the
current running through the grid
• While it provides no energy, it is required to stabilize the
voltage and balance the grid
• Every T&D line must allow room in the wires for reactive
power, to successfully serve the wide variety of loads
• The majority of loads on the grid are inductive (lagging PF)
and consume reactive power
• Reactive power must be generated (leading PF) to feed the
inductive loads (lagging PF) to maintain balance and keep
overall PF as near unity as possible
• Reactive power and voltage are interdependent

21. Reactive Power and Power Factor
Control

22. The grid: current situation
• Distributed renewable energy generators (DGs) inject
power at various network points in a controlled manner
dependant upon available “fuel”
• Consumers (de facto distributed) use electricity at different times
• Result: Varying grid voltages with the potential for
exceeding allowable ranges
Distributed
generators
Central
Large-scale
power
power
generation
plant
Consumers

23. The grid: extreme scenarios
• Grid voltage exceeds established corridor (at end user)
1. too much distributed generation, too little consumption
2. Too much consumption, too little generation
110%
= 253V Voltage
Central outside
power 100% the
generation = 230V
guideline
90%
value
= 207V
Voltage corridor
(according EN 50160 (UN ± 10%)

24. The grid: renewable stability
• Distributed generation from advanced PV power plants
can continually maintain this balance, in real-time, by
phase shifting of voltage and current (distributed
generation of reactive power) in the regional distribution
network
Distributed
generators
Central
Large-scale
power
power
generation
plant
Consumers

25. The grid: power factor control
• The results: The grid voltage can be corrected short-term
through the distributed generation of reactive power
– Compensation for minor regional network fluctuations
– Adjustment of the supply so that it remains within the required
voltage corridor (voltage regulation-grid stability)
Reduction in the
Central voltage excursion
power through reactive
generation
power control of
the supply

26. Result: stabilized grid voltage, day or night
• The integration of grid-stabilizing PV power plants in the
distribution network can stabilize the utility network with
regard to voltage which can:
– save grid expansion costs in the distribution and transfer network
level
– Provide for acceptance of additional renewable/intermittent
energy in the public grid without upgrades
– Generate new revenue streams for asset owners of distributed
utility and net-metered facilities
“It stands to reason, we must firstly increase the actual usable
grid capacity, be it through conventional grid expansion or
through other methods, such as an intelligent voltage stability
or intensified reactive power management.”
(German Federal Network Agency)

27. Implementing Power Factor Control and
Voltage Stabilization

28. Implementation: voltage stabilization
• Control effects tend to be localized to the region
of the grid where the injection of Q occurs
• Regulate to control voltage to a desired nominal
value
• Regulate to control voltage dynamically to keep
within desired range

29. Implementation: control
• Under a regulated environment, most utilities
own/control G&T&D in their own control area
– They provide reactive power just as they have had to
provide sufficient generation and voltage
• Restructuring has changed this and is causing
problems dealing with reactive power
– Merchant (non-utility) generation and related financial
incentives
– Transmitting power over longer distances with multiple
transactions

30. Implementation: problems
• Regulated electricity electric rates are based on kWh and
kVA, giving incentive for PF correction
• Restructuring and separation of G&T&D businesses:
– Generation: More likely kW based removing incentive for PF
correction
– Distribution: may not have significant incentive and strict budget
for installation of capacitors (to generate Q)
– Transmission: who will own and operate, and thus no incentive for
improvement
• Electricity is transmitted between control areas
– Communication required to properly operate the system, including
adjustments to reactive power
– ISOs (i.e., CAISO) have not yet defined any system rules concerning
reactive power

31. Implementation: ideas
• Generators receive “lost opportunity” revenue
payments when they must provide additional
reactive power
• Include specific VAR obligations and penalties for
non-compliance in each new interconnection
service agreement with generators
• ??

32. Thank You!
David F. Taggart
http://www.linkedin.com/in/davidtaggart/