1 Distributed Generation and Power Quality

2 Relaying considerations DG infeed may reduce the reach of overcurrent relays –DG feeds fault, so utility current is fault current minus DG contribution –Sympathetic tripping of feeder breakers –Defeat fuse saving

3 115 kV kV Put recloser here DISTRIBUTION Radial Line DG Only one DG: obvious solution to several problems

4 115 kV kV DG “Sympathetic” tripping of this circuit breaker (not desired) due to backfeed from DG Fault Solution is to use directional overcurrent relays at substation

5 DG Since DG feeds the fault (backfeed), it will likely defeat fuse saving Fault on tap

6 DG V distance Voltage Regulation

7 DG V distance Feeder trips and recloses DG disconnects Low voltage

8 DG V distance Feeder trips and recloses DG disconnects VR steps

9 DG V distance Feeder trips and recloses DG disconnects VR steps DG reconnects

10 Sequence –First we see interruption and reclosure with a voltage sag due to DG disconnect –Then voltage returns to normal (station step regulator or LTC) –Then DG reconnects, and we see a voltage swell –Then voltage returns to normal (station step regulator or LTC) Obviously needs to have coordinated voltage control –Limits to how much DG one feeder can stand

11 Power flow reversal in voltage regulators If excess DG during low load causes power to flow in reverse direction through voltage regulators misoperation is possible –Modern controls recognize this and change to reverse power mode (regulate in opposite direction so that controls operate correctly)

12 Wind generation case study 32 step V Reg switched capacitor a b cuntransposed line construction (typical) wind farm singly-fed induction gen

13 Detailed 3-phase study shows: –as wind generation increases, one outside phase voltage rises while the other drops –feeder has problems with frequent capacitor switching and/or votlage regulator stepping events –Cases like this may need dedicated feeder or doubly-fed generator to avoid power quality problems for other customers –If DG can generate reactive power, capacitor controls needs coordination with reactive generation

14 DG Transformer Connections Order used here is HV : LV feeding DG 1. Y gnd : Y gnd interconnects HV and LV grounds 2.  : Y gnd isolates LV ground from HV ground 3.  :  used on existing installations 4.Y :  used on existing installations 5.Y gnd :  similar to utility units

15 DG Transformer Connections 1.Y gnd : Y gnd interconnects HV and LV grounds. DG may need a neutral reactor to limit 1 phase to ground short circuit currents. DG may need 2/3 pitch winding to avoid large third harmonic votlages (which can cause third harmonic currents on HV and LV sides). - No phase shift between primary and secondary voltage.

Generator voltage harmonics Windings are distributed and often short-pitched (or chorded): –each coil spans a pitch of less than 2  /p where p is the number of poles, to reduce the time harmonic voltage induced

3-phase 6-pole 36-slot full-pitch stator winding

Development of stator winding

One coil of the winding: conductors in top of slot 1 return in bottom of slot 7, coil pitch = pole pitch, so a full-pitch winding

One coil of the winding: conductors in top of slot 1 return in bottom of slot 6, coil pitch = 5/6 pole pitch, so a 5/6-pitch winding

What’s so great about a 2/3-pitch coil anyway? Third harmonic flux linking a full-pitch (blue), 5/6-pitch (green), and 2/3-pitch (red) coils. Notice that the flux linkage (net area enclosed) is zero at the 2/3-pitch coil.

Third harmonics The third harmonic flux will not link a stator winding with a 2/3 pitch If third harmonic voltages are present, third harmonic circulating currents can be quite large if generator is solidly grounded

23 DG Transformer Connections  : Y gnd isolates LV ground from HV ground. DG may need 2/3 pitch winding to avoid third harmonic voltages, but third harmonic currents are contained on LV side. Some utility faults hard to detect due to phase shift.

24 DG Transformer Connections  :  used on existing installations. 4.Y :  used on existing installations. Ungrounded, unless DG provides ground. Some utility faults hard to detect due to phase shift.

25 DG Transformer Connections 5.Y gnd :  similar to utility units, isolated grounds, avoids problem with third harmonic currents.

26 Expected single-phase to ground fault currents on a radial distribution feeder

27 DG transformer showing possible zero-sequence currents