Coordinated Voltage Control Stephen Solis System Operations Improvement
Coordinated Voltage Control Stephen Solis System Operations Improvement Manager OTS 2018 Objectives Identify the difference between Transmission Operator (TO) and Qualified Scheduling Entity (QSE) roles required to maintain the voltage profile within established limits Identify the steps associated with the process of coordinating voltage (Voltage set point control) Identify the difference between Dynamic and Static reactive reserves and which is more important for transient stability Describe the necessity in maintaining adequate supply of reactive reserves 2 PUBLIC Terminology Voltage Profile Normally desired Voltage Set Points for those Generation Resources in ERCOT, as specified in the Protocols
Voltage Set Point The voltage that a Generation Resource (GR): maintains at its Point of Interconnection is initially communicated via the Voltage Profile may be modified by a Real-Time instruction from ERCOT or an ERCOT Transmission Operator through the GR or its QSE Established limits Range of voltages +/- 2% of the Voltage Set Point Not to be confused with system voltage limits here 3 PUBLIC Voltage Profile ERCOT coordinates seasonal studies with TSPs. Predetermined distribution of desired Voltage Set Points across the ERCOT region Based on sound engineering studies Uses the appropriate Network Operations model ERCOT or TOs may modify Voltage Set Points based on current system conditions 4 PUBLIC
Voltage Profiles ERCOT Responsibilities Post the Seasonal Voltage Profile on the MIS Coordinate with TOs and QSEs to maintain transmission voltage levels within normal and post-contingency limits Coordinate with TOs to deploy static Reactive Power reserves Posted Posted Voltage Voltage Profile Profile MIS>Grid>Transmission>Voltage MIS>Grid>Transmission>Voltage && Dynamic Dynamic Ratings Ratings r MVa 5 PUBLIC Voltage Profiles TO Responsibilities Monitor system voltages to stay within voltage limits
Maintain Dynamic Reactive Reserves (generating units) by operating static Reactive Resources (Caps/Reactors/LTC) Communicate directly with Generation Resources or QSEs to maintain Voltage Set Points Posted Posted Voltage Voltage Profile Profile MIS>Grid>Transmission>Voltage MIS>Grid>Transmission>Voltage && Dynamic Dynamic Ratings Ratings 6 PUBLIC Voltage Profiles Generation Resource Responsibilities Operate within 2% of the Voltage Set Point at the POI
Operate with Resources Automatic Voltage Regulator (AVR) in voltage control mode and notify its QSE and TO of any change in AVR status Voltage Set Point instructions shall be completed in no more than five minutes from receipt Posted Posted Voltage Voltage Profile Profile r MVa MIS>Grid>Transmission>Voltage MIS>Grid>Transmission>Voltage && Dynamic Dynamic Ratings Ratings 7 PUBLIC
Voltage Profiles QSE Responsibilities Ensure that Generation Resources maintain the desired Voltage Set Point and respond to Voltage Support Service (VSS) instructions Telemeter Real and Reactive Power, AVR and Power System Stabilizer (PSS) status to ERCOT and notify ERCOT of any change in a Resources AVR status Inform ERCOT if a Resource can not maintain its Voltage Set Point Posted Posted Voltage Voltage Profile Profile MIS>Grid>Transmission>Voltage MIS>Grid>Transmission>Voltage && Dynamic Dynamic Ratings Ratings r MVa 8 PUBLIC
NPRR 776 Provides transparency of both the actual measured value and target Voltage Set Point for the POI voltage between all affected entities (TO, ERCOT, QSE, and RE) Verbal Voltage Set Point instructions from the TO can continue however they currently occur today (QSE or GR) ERCOT will issue any Voltage Set Point instructions through the TO TO must update the telemetered as soon as practicable
9 PUBLIC Set Point vs Schedule vs System Voltage Limits Emer High Limit 1.1 pu -2% of VSP Nominal 1.0 pu Emergency Limits System Voltage Limits consist of Emergency High and Low Limits which must be operated within post contingency (.9/1.1 by default*). Normal Limits
Voltage Schedule/Range System Voltage Limits consist of Normal High Norm High Limit 1.05 pu and Low Limits which +2% of VSP must be operated within pre contingency Voltage Set Point (.95/1.05 by default*). Voltage Set Point Factors such as equipment limitations and UVLS set points must be considered Norm Low Limit .95 pu when establishing limits. * Unless specified differently by facility owners
Emer Low Limit .90 pu 10 PUBLIC Set Point vs Schedule vs System Voltage Limits Emer High Limit 1.1 pu If a Normal voltage limit is within 2% of the Voltage Set Point, the Normal Voltage limit marks the boundary of the Voltage schedule/range. -2% of VSP Nominal 1.0 pu Emergency Limits Voltage schedule or
range is the +/- 2% range around the Voltage Set Point Normal Limits Voltage Schedule/Range Voltage Set Point (either from Voltage Profile or Norm High Limit 1.05 pu real time instruction) +2% of VSP should be met within + or 2% Voltage Set Point Voltage Set Point Norm Low Limit .95 pu
Emer Low Limit .90 pu 11 PUBLIC Controlling POI Voltage Emer High Limit 1.1 pu Emergency Limits Generator operator may have to make additional adjustments to keep voltage at POI near Voltage Set Point. Normal Limits Voltage Schedule/Range Voltage at POI is set to
match the target Voltage Norm High Limit 1.05 pu Set Point with AVR in +2% of VSP service and in voltage control mode by the Voltage Set Point Generator operator. POI voltage -2% of VSP AVR will alter the generator reactive Nominal 1.0 pu power output to try and maintain voltage. Voltage Set Point Norm Low Limit .95 pu Emer Low Limit .90 pu 12 PUBLIC
Reactive Capability Generator is required to provide lagging or leading reactive capability up to the Resources Unit Reactive Limit (URL) at all times. When directed by ERCOT, Resource is compensated for operating above its normal URL if it has additional reactive capability than what is required. If generator cannot maintain voltage and has depleted its reactive capability, ERCOT and or the TOs are required to switch in available static reactive resources to restore the generators reactive reserves. Unit Reactive Limits (URL) .95 Power Factor The URL defines the maximum quantity of Reactive Power that a Generator Resource is capable of providing (Lagging) or absorbing (Leading) at .95 pf at its max real power capability.
13 PUBLIC Set Point vs Schedule vs System Voltage Limits Emergency Limits ERCOT and TOs should monitor generator reactive reserves to preserve the dynamic reactive capability of the generating units prior to depleting its reactive capability. Norm Low Limit .95 pu Normal Limits Voltage Schedule/Range
Emer High Limit 1.1 pu If Generator cannot maintain voltage at the POI and has depleted its Norm High Limit 1.05 pu reactive capability, ERCOT and or the TOs +2% of VSP will coordinate switching in static reactive devices Voltage Set Point to restore the generators reactive -2% of VSP reserves and support POI voltage POI voltage. Nominal 1.0 pu Voltage Set Point Emer Low Limit .90 pu 14
PUBLIC Dynamic vs Static Reactive Resources Dynamic Reactive Resources adjust reactive power output automatically in real-time over a continuous range within a specified voltage bandwidth in response to grid voltage changes operate to maintain a set point voltage or operate in a voltage droop mode Static Reactive Resources have fixed reactive power contribution of to the grid once connected capability varies according to voltage squared are switched (manual or automatic) in or out of service based on system conditions 15 PUBLIC Dynamic Reactive Resources Synchronous Generators Primary source of reactive power Limits on reactive power capability are dependent on
stator winding rating, field current rating, terminal voltage rating, and the active power output of the machine Generators consuming reactive power are in leading mode while generators producing reactive power are in lagging mode Synchronous Condensers synchronous machine whose shaft is not driven by a prime mover; rather, the shaft spins freely and the field voltage is controlled to produce or consume reactive power by adjusting set point voltage have several advantages as they contribute to system short-circuit capacity, have 10-20% overload capability for up to 30 minutes, and provide system inertia 16 PUBLIC Dynamic Reactive Resources Non-Synchronous Generators
Primary source of reactive power similar to Synchronous Generators Includes induction generators (e.g., Type I, II, and III wind plants) and electronically coupled resources (e.g., Type IV wind plants and solar) May have to be supplemented with other dynamic reactive resources to achieve reactive capability requirements Static Synchronous Compensators (STATCOM) Voltage source converter (VSC) device that consists of a DC voltage source behind a power electronic interface connected to the AC grid through a transformer Results in a controllable voltage source and hence reactive power output Approximately 22 in ERCOT based on naming 17 PUBLIC Dynamic Reactive Resources Static VAR Compensators (SVC)
Consists of thyristor-controlled reactors (TCRs), thyristor-switched capacitors (TSCs), and fixed capacitors acting as a harmonic filter The TCR consists of reactors in series with thyristor valves that continuously control the reactive power output by varying the current flow through the reactor A TSC consists of capacitors, reactors, and thyristor valves that simply switch the capacitor in and out of service The fixed capacitor is part of the filter that absorbs the harmonics generated by the thyristor switching, supplying a fixed reactive power to the grid Approximately 59 in ERCOT 18 PUBLIC Static Reactive Resources Fixed Shunt Devices include shunt capacitors and reactors fixed nominal rating and their reactive power production or consumption is dependent on terminal voltage Relatively inexpensive, but inability to manually switch in/out quickly enough to provide dynamic support limits their applicability to steady-state operation (no transient response capability)
Switched Shunt Devices fixed shunt devices that have the capability to automatically be switched in service based on control settings (AVR with set point and deviation range) or operator action 19 PUBLIC Reactive Reserves Dynamic Reactive Reserves Unused dynamic reactive capability (Qmax/Qmin) actual reactive output (Q) = Q lagging reserves, Q leading reserves adjust reactive power output in real-time over a continuous range automatically in response to changes in grid voltage Minimizes transient voltage dip to system disturbances and helps return voltage to within steady state voltage limits Not subject to the same voltage penalty as shunt capacitors ERCOT and the TOs have an obligation to monitor and ensure sufficient dynamic reactive reserves are available 20 PUBLIC
Static Reactive Resources Transmission Circuits Operating characteristic known as the surge impedance loading (SIL), which is the active power loading at which reactive power produced and consumed by the line are balanced Lines produce reactive power due to their natural capacitance, and the amount produced is dependent on the capacitive reactance (XC) of the line and the voltage Lines also consume reactive power due to their inductive reactance (XL). 345kV lines supply approximately 1 MVAr/mile of line charging while 138kV lines supply approximately 0.1 MVAr/mile) 21 PUBLIC Static Reactive Resources
Transmission Circuits (continued) 345kV lines supply approximately 1 MVAr/mile of line charging while 138kV lines supply approximately 0.1 MVAr/mile) 22 PUBLIC Other Voltage Control Devices Transformer tap changers do not produce reactive energy, but can pull and push VARs and alter system voltage Series Reactors Series reactors increase line impedance and limit short-circuit current. They are typically used to delay short-circuit current limited equipment upgrades or re-direct power flows in a network. Series Capacitors Used to increase power transfers of long transmission lines by compensating line inductive reactance. Mostly applied to
networks with generation remote from load centers, linked by long transmission lines. Series capacitors introduces the interaction of electrical and mechanical natural frequencies, resulting in SSR. 23 PUBLIC Reactive Reserves Static Reactive Reserves Available to adjust pre contingency voltages and reactive power needs Can be used to relieve reactive loading on dynamic reactive resources and restore dynamic reactive reserves Can vary with voltage Must have sufficient static reactive reserves to meet system needs throughout the operating day Adequate Reactive Reserves Adequate to maintain voltages within voltage limits both precontingency (normal limits) and post-contingency (emergency limits) Adequate to provide for acceptable transient response to system disturbance Static and Dynamic reactive devices should be coordinated such that static reactive devices are switched in or out to maintain maximum dynamic reactive reserves
24 PUBLIC Improved Statistics Everyone deserves to be commended in continuously improving voltage control. After synching voltage limits in 2014, still seeing an annual 15% reduction in voltage violations 25 PUBLIC Questions References: NERC Reactive Support and Control Whitepaper (May 18, 2009) NERC White Paper on FERC NOPR [Docket No. RM16-1-000] NERC Reliability Guideline: Reactive Power Planning and Operations (December 2016) ERCOT Protocols
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