IEEE Power & Energy Magazine - May/June 2014 - 89

A typical PV solar generation plant is composed
of multiple individual "generators" connected to the
electrical network via power electronics.

may/june 2014

Automatic Voltage Regulation (AVR)
AVR Control Mode
On
Off
Mode

Current Setpoint New Setpoint
528.0 kV

Voltage Control
Power Factor Mode
Var Mode

528.0 kV

0.98

0.2 MVar

Confirm
Changes

Cancel
Changes

figure 2. An HMI screen on AVR modes.

1.000

0.995

-10

0.990

-20
-30

0.985

-40
-50
-4

-2

2
4
Time (s)

Power Factor

A key component of a grid-friendly pv power plant is a
plant-level controller. it is designed to regulate real and
reactive power output from the pv plant, such that it
behaves as a single large generator. While the plant is composed of individual small generators (or, more specifically,
inverters), with each generator performing its own energy
production based on local solar array conditions, the function of the plant controller is to coordinate the power output
to provide typical large power-plant features such as active
power control and voltage regulation (through reactive
power regulation).
the plant controller provides the following plant-level
control functions:
✔ dynamic voltage and/or power factor regulation of the
solar plant at the point of interconnection (poi)
✔ real power output curtailment of the solar plant
when required so that it does not exceed an operatorspecified limit
✔ ramp-rate controls to ensure that the plant output does
not ramp up or down faster than a specified ramp-rate
limit, to the extent possible
✔ frequency control to lower plant output in case of overfrequency situation or increase plant output (if possible) in case of under-frequency
✔ start-up and shut-down control.
the plant controller implements plant-level logic and
closed-loop control schemes with real-time commands to
the inverters to achieve fast and reliable regulation. it relies
on the ability of the inverters to provide a rapid response to
commands from the plant controller. typically there is one
controller per plant controlling the output at a single highvoltage bus (referred to as poi). the commands to the plant
controller can be provided through the supervisory control
and data acquisition system (ScAdA) human-machine
interface (hMi) or even through other interface equipment,
such as a substation remote terminal unit.
Figure 1 illustrates a block-diagram overview of the control system and its interfaces to other devices in the plant.
the power plant controller monitors system-level measurements and determines the desired operating conditions of
various plant devices to meet the specified targets. it manages capacitor banks and/or reactor banks, if present. it has
the critical responsibility of managing all the inverters in the
plant, continuously monitoring the conditions of the inverters and commanding them to ensure that they are producing

the real and reactive power necessary to meet the desired
settings at the poi.
the plant operator can set an active power curtailment
command to the controller. in this case, the controller calculates and distributes active power curtailment to individual
inverters. in general, the inverters can be throttled back only
to a certain specified level of active power and not any lower
without causing the dc voltage to rise beyond its operating
range. therefore, the plant controller dynamically stops and
starts inverters as needed to manage the specified active power

MVars

Power Plant Controller Architecture

0.980
10

Measured Power Factor
Commanded Power Factor
Total Vars

figure 3. Dynamic power-factor regulation.
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Table of Contents for the Digital Edition of IEEE Power & Energy Magazine - May/June 2014