Renewable Energy plant controller model (REPC):
PV Plant:
A PV plant contains many large inverters which are connected in parallel, and at miles distance
by AC cables also called (collector system). Within the power plant supervisory control is
implemented to the invertors to achieve plant-level objectives. PV plants provide low-voltage
and high-voltage ride-through capabilities when the level of PV penetration increases, it also
provides ancillary services that contain reactive power support, oscillation damping etc. In the
figure below wind power plant is represented as single generator.
The output of a PV inverter is usually 208 V or 480 V and parallel combination of many
inverters represents a PV plant. The transformers adjacent to plant are connected to substation
transformers; however several of nodes are connected in a daisy-chain fashion and are then
connected to the substation transformer.
Interconnection of Plant controller:
The plant controller provides real and reactive power references which are Qref and Pref to the
electrical control. Qref corresponds to either reactive power or voltage. The electrical control
then translates the real and reactive power references into current commands for the converter.
WECC Renewable energy plant control model (repc_a).
The structure of renewable energy plant control model is depicted in the figure below, the model
can be studied separately by breaking down into two parts i.e. reactive and active control loops.
Reactive current loop is a bit complex comparing with active current loop which is straight
forward.
Reactive control loop:
Reactive power control loop is more flexible and can operate in unique modes. Reactive control
is configured either in plant-level voltage control using vcmpflg (flag parameter) or in reactive
power control using refflg (flag parameter). The control error from after passing through the flag
is passed through a deadband and limiter to the input of PI controller. At the end the reactive
power reference is provided to the electrical model. It is important to discuss the control
provided by the flags and how they behave during different operations.
Control of Voltage in Reactive control loop:
To study about voltage regulation there is voltage bus interconnected with the flag “VcompFlag”
provides options for different modes of operation. The remote bus is defined as the bus farther
from the terminal voltage. The electrical distance from the terminal voltage is represented by the
reactance Xbr, the current flowing through the branch is represented by Ibranch, and the reactive
power flowing in the branch is represented by Qbranch. The regulated voltage is located in a
remote bus from a monitored bus voltage Vreg. Two types of voltage compensation can be
activated in the REPC block diagram
Voltage control flag (Vcompflag):
When Vcompflag=1, the voltage to be controlled is measured from Vreg bus and it is
compensated by using voltage drop .
1. When Regulating v1 (high side of transformer) and the access is only to voltage Vt, so to
control V1, the branch current (Ibranch = Ixfrmr) and (Xbr = Xxfrmr); voltage (Vreg =
Vt); and the voltage drop is computed from “Ixfmr” “Xxfmr”.
2. When regulating V2 and monitoring only voltage V1, so to control V2, the branch current
(Ibranch = Ibranch1) and (Xbr = Xs); the voltage (Vreg = V1); and voltage drop is
computed from “Ibranch1” “Xs”
When Vcompflag=0, The voltage to be controlled is measured from the Vreg bus, and it is
compensated by estimating the voltage drop by Kc Qbranch.
1. Regulate V3 and having access only to the voltage V2, so to control V3, the (Qbranch =
Qbranch1) and voltage (Vreg = V2), and the voltage drop is approximated by Kc1
Qbranch1.
2. Regulate V1 and having access only to the voltage Vt, so to control voltage V1, the
(Qbranch = Qout) and voltage (Vreg = Vt), and the voltage drop is estimated by Kcxfmr
Qout.
Reactive Power Control in Reactive control loop:
In the reactive power control block diagram, there is a logical switch called RefFlag which
provides options to control the reactive power.
Reactive Power control flag (RefFlag):
1. When RefFlag = 1, it configure the controller to regulate the voltage based on the
measured voltage Vreg.
2. When RefFlag = 1, Configure the controller to regulate the reactive power flow in a
specified branch.
Active power Control loop:
The function of this control loop is to modulate the real power output of the plant to support
system frequency and/or maintain a constant real power output at the plant level. Active power
control loop consist of a single flag called FreqFlag similarly in reactive control model it also
provides two different modes of operations.
Frequence Flag:
When FregFlag =1, it configures the controller to operates the real power in a specific branch
(Pbranch) by using PI controller. It also includes the reference Plant_ref and frequency response
dP/dF.At this point, to disable the frequency response dP/dF into the power command mix is to
set the slope Ddn = Dup = 0.
When FreqFlag = 0 it configures the controller to operate the reactive power flow in a specific
branch (Pbranch) to follow the reference real power command Plant_Pref using the feedforward
linear path (bypassing the PI controller) without the influence of frequency response.
Frequency regulation.
The error ∆Freq which is obtained from reference frequency Freq_ref and measured frequency
Freq is passed through deadband block. The slope obtained is controlled through Ddn and Dup
and the output is limited by the limiters DdnMin and DupMax.