IBR Modeling Fundamentals Songzhe Zhu WECC MVS Workshop September 17, 2020 ISO Public Outline • Modeling guideline for all IBRs connecting to transmission and subtransmission – Power Flow Representation – Dynamic Models – Active power – frequency control – Reactive power – voltage control – Fault ride-through • Solar PV • BESS and Hybrid ISO Public Page 2 Basics of modeling IBR connecting to transmission and sub-transmission ISO Public Page 3 Power Flow Representation • transmission and sub-transmission connected IBR 1 Interconnection Transmission Line 2 Substation Transformer 3 Equivalent Collector System 4 Equivalent Pad-mounted Transformer 5 EQ Point of Interconnection Equivalent Generator Plant-level Reactive Compensation (if applicable) Typical Single-Generator Equivalent Power Flow Representation ISO Public Collector System & IBR Equivalencing • Equivalent impedance of the collector system shall be represented – Inverters respond to the terminal voltage – Voltage at POI and terminals are quite different • Multi-generator representation may be needed – Multiple main GSUs, with separate collectors behind them – Significantly diverse impedances behind different feeders – Inverters by different manufacturers are installed behind the feeders and these inverters have different control and protection settings ISO Public Page 5 Multi-Generator Representation 1 Interconnection Transmission Line 2 Substation Transformer 3 Equivalent Collector System 4 Equivalent Pad-mounted Transformer 5 G1 Equivalent Generator 1 Point of Interconnection 6 7 8 G2 Equivalent Generator 2 G3 Equivalent Generator 3 9 Illustrative Multi-Generator Equivalent Power Flow Representation * Although not illustrated by this example, all var devices should be modeled explicitly. ISO Public Page 6 Positive Sequence Dynamic Models • Generic models approved by WECC • Very flexible to model different control setups • Models are supported and benchmarked among different software platforms • Easy access to model documents and user guides • Generally applicable for systems with a short circuit ratio of 3 and higher at the point of interconnection ISO Public Page 7 Generic Dynamic Models Enhanced model approved or under development Description Model Name Applicability Notes Converter REGC_A REGC_B REGC_C All IBR: current source model All IBR: voltage source model All IBR: REGC_B plus PLL and inner current control loops REEC_A REEC_C REEC_D Type 3 and 4 WTG, solar PV Battery energy storage Enhanced model for all types of IBR REPC_A REPC_B REPC_C For controlling single device For controlling multiple devices Enhanced model for controlling single device LHVRT LHFRT Voltage ride-through Frequency ride-through Electrical control Plant controller Ride-through protection * Models specific to WTGs are discussed in wind plant modeling session. ISO Public Page 8 Generic Model Structure REPC Q/V reference P reference REEC iq command ip command ISO Public REGC Network Interface Page 9 Scaling for the Equivalent Generator Size • Pmax and MVA base in the power flow model and dynamic models are aggregated values • Power flow model – – MVA base is the sum of the individual MVA rating of the inverters – Pmax is the maximum active power output from the equivalent generator in accordance with the generation interconnection study and interconnection agreement • often lower than the sum of the individual rated MW of the inverters due to the practice of oversizing inverters • Dynamic models – – Model parameters are expressed in per unit of the MVA base for the model – Typically MVA base matches the MVA base in the power flow model ISO Public Page 10 Active Power – Frequency Control Options • Power flow model: base load flag (BL) – BL = 0: Pgen can be dispatched downward and upward – BL = 1: Pgen can be dispatched downward only – BL = 2: Pgen is fixed • Dynamic model: REPC – frqflag= 0/1: frequency response no/yes – ddn & dup: downward & upward regulation gain – fdbd1 (+) & fdbd2 (-): over- and under-frequency deadband for frequency response (pu) ISO Public Page 11 Active Power – Frequency Control Options (Cont’d) • With earlier version of the models (prior to Aug 2020) – base load flag is not used in the dynamic simulation – block frequency response through frqflag/ddn/dup Functionality BL frqflag ddn dup No response 2 0 - - Down regulation only 1 1 >0 0 Up and down regulation 0 1 >0 >0 • With model enhancement – Base load flag is used to block frequency response in reec and repc models except for repc_b. Functionality BL frqflag ddn dup No response 2 - - - Down regulation only 1 1 >0 - Up and down regulation 0 1 >0 >0 ISO Public Page 12 Active Power – Frequency Control Key Parameters • Other control parameters in REPC for frequency response – Kpg: proportional gain – Kpi: integral gain – Tlag: lag time constant Non-step deadband ISO Public Page 13 Reactive Capacity Requirement • Interconnection requirement for IBR reactive capacity has evolved, e.g. – No requirement – 0.95 power factor at POI – FERC Order No. 827: 0.95 power factor (dynamic var) at high side of the substation transformer • The modeling recommendation in this presentation focuses on IBR complying with FERC Order No. 827 ISO Public Page 14 Model IBR Reactive Capability • Inverter P-Q capability – Manufacturer provides P-Q capability curves under different ambient temperatures and DC voltages – Use the P-Q capability curves to verify if there is sufficient capability to meet the interconnection requirement • Generator reactive capability in the power flow model – Model the required reactive capability – Qmax and Qmin of the equivalent generator are reactive capability at Pmax, limited by the minimum amount to meet the interconnection requirement • Generator reactive capability in the dynamic models – The physical capability is modeled, not limited by the PF requirement ISO Public Page 15 Reactive Power – Voltage Control in Power Flow Assuming the only dynamic var sources are inverters – • If inverters regulating voltage at point of measuring (POM) – Voltage regulation bus is the high-side bus of the GSU – The Generator is set to cont_mode = 2 with pf = 0.95, i.e. the power flow solution will try to hold voltage at the regulated bus constant within Q limits specified by pf • If inverters regulating terminal voltage – The Generator is set to cont_mode = -2 with pf ≤ 0.95, i.e. the power flow solution will try to hold terminal voltage constant within Q limits specified by pf • Voltage regulation of LTC transformers • Controlled shunts – SVD ISO Public Page 16 Power Flow Modeling Limitation on Reactive Power / Voltage Control Coordination • Reactive power – voltage control is coordinated by the power plant controller (PPC) • Inverter reactive output is controlled along a voltage droop curve • Most power flow software do not currently model PPC and can’t do voltage droop control – PPC power flow model is under development. WECC MVS has published the model specification. • Discuss more on PPC control in the hybrid plant modeling session. ISO Public Page 17 Reactive Power – Voltage Control in Dynamic Models • Different voltage control options are modeled by the combination of pfflag, vflag and qflag in reec model and refflag in repc model Functionality PfFlag Vflag Qflag RefFlag Constant local PF control 1 N/A 0 N/A Constant local Q control 0 N/A 0 N/A Local V control 0 0 1 N/A 0/1 1 1 N/A Plant level Q control 0 N/A 0 0 Plant level V control 0 N/A 0 1 0 1 1 0 0 1 1 1 0 N/A 0 2 0 1 1 2 Local coordinated V/Q control Plant level Q control + local coordinated V/Q control Plant level V control + local coordinated V/Q control Plant level PF control* Plant level V control + local coordinated V/Q control* * repc_b only ISO Public Page 18 Plant Level Reactive Power – Voltage Control Options • Voltage Control: RefFlg=1 – Select the regulating bus (Vreg) – Set the monitored branch – Set VcmpFlg=1 if using line drop compensation (Rc and Xc) – Set VcmpFlg=0 if using reactive droop (Kc) • Constant Q Control: RefFlg=0 – Select the monitored branch ISO Public Page 19 Multiple Device Plant Control: REPC_B Multiple Device Control Plant PF Control ISO Public Page 20 Plant Level Reactive Power – Voltage Control Key Parameters • Key parameters – – – – – – Control deadband (dbd) Input (emax/emin) and output (qmax/qmin) limits Control gains (kp/ki) Intentional phase lead (Tft) Communication lag (Tfv) Voltage threshold to freeze plant voltage integral control (vfrz) ISO Public Page 21 Inverter Level Reactive Power – Voltage Control Options If no plant controller – PF Control Constant Q Control Local V Control Local Coordinated Q/V Control ISO Public Page 22 Inverter Level Reactive Power – Voltage Control Options (Cont.) If coordinated with plant controller – Plant level Q or V Control Plant level Q or V Control and Local Coordinated Q/V Control ISO Public Page 23 Inverter Level Reactive Power – Voltage Control during Voltage Dip • Voltage dip: Vt < Vdip or Vt > Vup • During voltage dip, local Q control and local V control freeze • K-factor control: proportional gain Kqv ISO Public Page 24 Coordinate Plant Level and Inverter Level Controls • Key factors to achieve desired control performance – – Choose control option: plant level control or plant level control and local coordinated control – At what voltage levels, freeze plant level Q/V control (vfrz) and local Q/V control (vdip), taking into account plant controller regulates POM bus voltage while the inverter controller regulates terminal bus voltage – At what voltage levels, k-factor control shall be activated – Control gains and time constant associated with each control mode – P/Q priority ISO Public Page 25 Inverter Current Limit iq control Ip and Iq control come together ip control ISO Public Page 26 Invert Current Limit (Cont.) • Define the maximum inverter current imax • REEC_A: voltagedependent current limits for ip and iq separately (VDL1 and VDL2) • Total current 𝑖𝑖𝑝𝑝2 + 𝑖𝑖𝑞𝑞2 is limited by imax • During low voltage, ipcmd or iqcmd may be reduced until the voltage recovers depending on P/Q priority ISO Public Page 27 P-Q Priority ISO Public Page 28 Example of Different Control Strategies Kvi = 40 & plant control Vdip=0.9, Kqv = 2.0 Vt at fault is below 0.25. Iqcmd rises quickly to 1.3. Vt at fault is below 0.25. Iqcmd rises quickly to 1.07. After fault, initial Vt is 1.2. After fault, initial Vt is 1.196. Slower plant control keeps voltage at 1.2 for about 0.13 sec. Plant control freezes for voltage outside [0.9,1.1]. Iqcmd reduces immediately post fault. Control Setup 1: slow plant control and no voltage dip and kqv control Control Setup 2: enable voltage dip and kqv control ISO Public Frequency Ride-through • Lhfrt model parameters should reflect the actual frequency protective relay settings • The settings should be PRC-024 compliant • Frequency calculation in positive sequence stability programs are not accurate during and immediately following the fault • Work-around of false frequency tripping for a close-by simulated fault – – Use lhfrt in “alarm only” mode and analyze each individual alarms – Disable frequency tripping under low voltage condition (dypar[0].v_f_inh in javaini.p) – Do not set instantaneous tripping and always include some delay for frequency tripping ISO Public Page 30 Voltage Ride-through • Lhvrt model parameters should match the actual voltage protective relay settings • The settings should be PRC-024 compliant – PRC-024 requirement is set with voltage at the high side of the substation transformer (POM) – The actual protection is set with terminal voltage – The voltage setpoints should take into account the difference between inverter terminals and POM ISO Public Page 31 Modeling solar PV plants connecting to transmission and sub-transmission ISO Public Page 32 Model Solar PV Momentary Cessation • Model structure: REGC, REEC_D, REPC • Modeling elements – Current reduction during cessation [REEC_D].VDLq and VDLp • set current limits to 0 for both ip and iq when the voltage is below Vmc-lv or above Vmc-hv – Disable low voltage power logic [REGC].lvplsw = 0 – Ramp control [REGC].rrpwr, iqrmax and iqrmin – P/Q priority during recovery [REEC_D].pqflag – Voltage dip logic [REEC_D].vblkl = Vmc-lv, vblkh = Vmc-hv – Current recovery delay [REEC_D].Tblk_delay ISO Public Page 33 Comparison of REEC_D with REEC_A for Modeling Momentary Cessation • REED_D has the full capability of modeling momentary cessation, while REEC_A does not. REEC_A MC low voltage threshold MC high voltage threshold Voltage-dependent reactive current limit* Voltage-dependent reactive current limit* Active current recovery delay Reactive current recovery delay REEC_D vdip vup VDL1 4 pairs of (vq, iq) VDL2 4 pairs of (vp,ip) Thld2 vblkl vblkh VDLq 10 pairs of (vq, iq) VDLp 10 pairs of (vp, ip) Tblk_delay Not modeled Tblk_delay ISO Public Page 34 REEC_D Model Enhancement ISO Public Page 35 Converting REEC_B to REEC_D Parameter Name • REEC_D is an expansion of REEC_B. If the solar PV inverters do not use momentary cessation, the previous REEC_B models can be easily converted to REEC_D by adding parameters in this table. Value rc 0 Xc 0 Tr1 0 Kc 0 Vcmpflag 0 Ke 0 Iqfrz 0 Thld 0 VDLq (-1.0, imax), (2, imax), (0,0) … VDLp (-1.0, imax), (2, imax), (0,0) … vblkl 0 vblkh 2 Tblk_delay 0 iqfrz 0 thld 0 thld2 0 vref1 0 pflag 0 ISO Public Page 36 Modeling BESS and hybrid power plants connecting to transmission and subtransmission ISO Public Page 37 Definition of Hybrid Power Plant • A generating resource that is comprised of multiple generation technologies that are controlled by a single entity and operated as a single resource behind a single point of interconnection (POI). • Single point control of multiple generators is the key that requires additional modeling capability. ISO Public Page 38 Two Types of Configuration DC-Coupled AC-Coupled ISO Public Page 39 BESS Plant and DC-Coupled Hybrid Plant – Power Flow Model • DC-coupled hybrid plant is modeled the same way as a BESS only plant • Batteries and solar PV arrays on the DC side are modeled in a single generator • Pmin in the power flow model represents the maximum charging power – For stand-alone BESS, pmin < 0; – For hybrid, pmin <0 if charging from the grid; pmin = 0 if DC-side charging only. ISO Public Page 40 BESS Plant and DC-Coupled Hybrid Plant – Dynamic Model • Use the second generation RES models: regc, reec_c or reec_d, repc • Reec_c includes simulation of the state of charge Common simulation set-up mistake: PGEN < 0 and SOCini = 1.0; PGEN > 0 and SOCini = 0 • Reec_d does not have the state of charge logic any more. ISO Public Page 41 AC-Coupled Hybrid Plant • Different technologies are modeled by separate generators • Single point control needs to be implemented in both the power flow model and the dynamic model – Power Plant Controller (PPC) power flow model is being developed – Repc_b has been enhanced for better hybrid frequency response control ISO Public Modeling Requirement for AC-Coupled Plant • Frequency / active power control – The total MW injection at the point of interconnection is limited by the contractual maximum – Different components have different frequency response • Voltage / reactive power control – Plant reactive output limit is typically 0.95 power factor at the high side of substation transformer – Power plant controller coordinates operation of the inverters, transformer tap changers, SVDs, and other var devices to maintain the regulated bus voltage within a deadband from the voltage schedule ISO Public Proposed PPC Power Flow Model • A PPC model is defined by: – Individual devices such as generators, SVDs, and other controllable reactive devices* – A regulated bus – QV characteristics at the regulated bus – Plant real power limits *Transformers that control tap will not be part of the PPC ISO Public Page 44 PPC QV Characteristics • The power flow solution represents an operating point such that the Mvar being injected at the Regulated Bus from the devices in the PPC will follow a QV characteristic with a deadband. – For example, Qdb is 0 or equal to the var losses on the gen-tie; qmax and qmin represents 0.95 lag/lead power factor at the regulated bus ISO Public Page 45 Real Power Monitoring • The PPC model monitors the real power injection at the monitored bus and generate warning messages if the injection is outside the plant real power limits. ISO Public Page 46 An Example of PPC Model 1 Interconnection Transmission Line 2 Substation Transformer 3 Equivalent Collector System 4 Equivalent Pad-mounted Transformer 5 PV Point of Interconnection T1 SVD “SD” 100 MW 7 6 8 BT T2 230kV Equivalent Generator for Solar PV Equivalent Generator for Battery 100 MW 34.5kV PPC: Solar-BESS Devices Device Type Bus 5 "PV" Generator Bus 8 "BT" Generator Bus 3 "SD" SVD Reactive Power Control Regulated Bus Bus 2 Qmax (Mvar) 34 Qmin (Mvar) -34 Real Power Monitor Monitored MW At Bus 1 from Bus 2 Pmax 100 Pmin -100 690V QV Curve at Bus 2 ISO Public Page 47 Dynamic Model for AC-Coupled Hybrid • Use regc, reec and repc_b modules. Module Grid interface BESS Electrical Component controls Non-BESS Component Plant controller Aux control Voltage/frequency protection PSLF modules regc_* reec_c or reec_d PSSE modules REGC* REECC1 or REECD1 Use appropriate modules for the gen type repc_b PLNTBU1 REAX4BU1 or REAX3BU1 lhvrt/lhfrt VRGTPA/FRQTP A ISO Public Page 48 More on REPC_B • Invocation notes – In PSLF implementation, REPC_B is invoked from one of generators in the plant. It is important to have REPC_B invoked from an online generator. – The regulated bus and the monitored branch must be specified for REPC_B. • Reactive control – Qmax and qmin are plant level reactive limits; on the system MVA base in PSLF implementation • Frequency control – Set frqflag to enable plant level frequency response – Use base load flag to enable or block individual component response Component Solar PV - Frequency response, down only regulation BESS - Frequency response, up and down Plant controller BaseLoad flag 1 0 Module reec_d reec_c or reec_d Repc_b with Frqflag=1, dup > 0, ddn > 0 ISO Public Page 49 Reference • WECC MVS, Solar PV Plant Modeling and Validation Guideline https://www.wecc.org/Reliability/Solar%20PV%20Plant%20Modeling%20and% 20Validation%20Guidline.pdf • Pouyan Pourbeik, Memo RES Modeling Updates 083120_Rev17 https://www.wecc.org/Administrative/Memo_RES_Modeling_Updates_083120_ Rev17_Clean.pdf • WECC MVS, Converting REEC_B to REEC_A/D https://www.wecc.org/Reliability/WECC%20White%20Paper%20on%20Convert ing%20REEC%20rev202008.pdf • WEC MVS, Hybrid Plant Modeling Enhancement https://www.wecc.org/_layouts/15/WopiFrame.aspx?sourcedoc=/Administrative/ WECC%20White%20Paper%20on%20modeling%20hybrid%20solarbattery.pdf ISO Public Page 50
