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.