Hao Zhu Dept. of Electrical & Computer Engineering University of Illinois, Urbana-Champaign ECE 498HZ: Power Distribution System Analysis Chapter 7: Voltage Regulation February 18, 2016

Voltage More Crucial in Distribution Systems!  A variety of automatic control mechanisms enforced to main voltageD 2 LTC Transformers Step-voltage regulators Shunt capacitors

Voltage Standards  ANSI (American National Standards Institute) 3 System voltage: RMS phasor of every portion of the system - Nominal system voltage - Max system voltage Service voltage: at connected points with users Utilization voltage: at connected points with equipment - Nominal utili. voltage Service V System V Utili. V

3-wire 120/240V Service  Favorable zone (Range A) –Nominal utilization voltage = 115 V –Maximum utilization and service voltage = 126 V –Minimum service voltage = 114 V –Minimum utilization voltage = 110 V  Tolerable zone(Range B) –Nominal utilization voltage = 115 V –Maximum utilization and service voltage = 127 V –Minimum service voltage = 110 V –Minimum utilization voltage = 107 V 4

Voltage Unbalance  For the metrics (all defined for volt. magnitudes)  For normal operations, V unbalance <= 3% 5

Step-Voltage Regulators  1-phase or 3-phase  1-phase can be used to form Y- or Δ- connection  Basically autotransformer + LTC –Varying tap (number of turns) of the series winding 6

Two-winding Transformer 7 Equivalent Circuit

General Constants (Same to Line Models) 8

To Do Ladder Iterations 9

“Step-up” Autotransformer  Connecting H1 to X2  Source: H1 and H2  Load: X1 and H2 10

General Constants 11

Step-down Autotransformer 12

For Both Types 13

Autotransformer Ratings 14

Step-voltage Regulators 15

Tap Changing Details 16 switching sequence Preventive autotransformer presents high impedance to circulating current and low impedance to parallel currents A on B off C on A on B on C off A off B on C on

It’s a Local Control 17

Control Settings  Voltage level: a desired voltage (say 120V) at a “load center”  Bandwidth: allowed deviation from the set voltage level (+/-1V)  Time delay: to stabilize the changing operations  Line drop compensator (LDC): to account for the line voltage drop between the regulator and the load center (detailed later) 18

Single-phase Step-voltage Regulators 19 “Raise” mode “Lower” mode

Generalized Constants 20

Line Drop Compensator (LDC) 21

LDC Design 22 Typically 120V Typically a few Amps

Example

Examples 7.5 and 7.6  Load center: 2500kVA at 4.16kV and 0.9 PF (lagging)  Use the compensator settings from Ex. 7.4  Voltage level = 120V with bandwidth = 2V  To solve: the tap position  Based on this tap position, solve for the actual load center voltage 24

Three-phase Voltage Regulators  Use the abbreviated circuit of Type-B single-phase voltage regulator 25

Wye-Connected Regulators 26

Example 7.7  Recall the ladder power flow Example 6.5  Generalized matrices for the line  Power flow solution  To solve: compensator designs? tap positions? Ladder with Vreg? 27

Ladder Iterations  Now we have two components: the Vreg and the line  Forward sweep: Vreg -> Line  Backward sweep: Line -> Vreg 28