Application of Undervoltage Protection to Critical Motors

Transcription

1 Application of Undervoltage Protection to Critical Motors Matt Proctor GE Grid Solutions 2018 Texas A&M Protective Relaying Conference

2 Agenda Undervoltage Scenarios (Transient, Sag & Complete Loss) Motor Response to Undervoltage Motor Control Types (Contactor, Breaker, VFD) How to Make PHASE UV (ANSI 27) Secure Against VT Failures

3 Transient Voltage Dip U U U2 A C U1 U3 U4 B D Fault 1 Bus Voltage (pu) Time (s) This fa ult is clea red in 0.2 seconds. Different buses on the system can have different restoration characteristics. Motor 1 Plant Load

4 Voltage Sag Example U U U2 U1 U3 U4 Fault 2 Fault occurs on utility & voltages begin to sag. A C B D Local generation props up voltage but only to approx. 35% of nominal. Motor 1 Plant Load Local generation begins to export Watts and Vars.

5 Complete Loss of Source U U2 U1 U3 U U4 The motor is completely decoupled from the source. A C B D If reclosing, beware transient torques that could damage the load, the motor or the motor coupling. Motor 1 Plant Load

6 Re-Acceleration Requirement Xx Xx Xx % Rated Current % Rated Torque

7 Re-Acceleration Requirement 85% Rated RPM = 510 Stall RPM = 0.85 x 510 = 434 RPM ΔRPM = 66 RPM How long does this deacceleration take? It depends. Inertia WR 2 Motor torque as voltage decays. Regeneration/Source Z interaction. Voltage recovery/dynamic Load Modeling analysis required.

8 Electrically Held Contactor fuse disconnect M M M OL OL OL Motor Motor Has an inherent but not very sensitive undervoltage pickup (60-70%). OL OL OL START STOP M M

9 Electrically Held with Ride-Through Solid state Rectifier (-) Energy storage devices built into a electrically held contactor can provide ride-through. Energy Storage In a digital overload relay, this contact can be programmed to stay open until a restart timer elapses. Beware the scenario where contactor drops out and recloses prior to ride-through timer elapsing.

10 Breaker Applications BUS 52 MotorMotor VT IED Breakers don t rely on a CPT like contactors do. Undervoltage must be measured by IED. + IED Close IED Trip UPS 52b 52a - CC Close Coil TC Trip Coil

11 Variable Frequency Drive Applications Variable Frequency Drive BUS disconnect fuse L C Aɸ Bɸ Cɸ Motor Rectifier DC Link Inverter VT IED #2 IED #2 Trip IED #1 Trip IED #1 + UPS - Drives can incur charging current when voltage dips & recovers. If a single IED cannot adjust its sampling frequency for both drive output and bus voltage, separate IED s must be used.

12 VT Failures BUS 52 MotorMotor VT IED VT Failures can & do happen. That s why fuses (usually) are used. UPS + - IED Close 52b CC Close Coil IED Trip 52a TC Trip Coil If the relay can t tell the difference, it will nuisance trip!

13 Methods of Making UV Secure V < 27 Pickup pu AND 27 Operate V > V 0 min A simple minimum voltage threshold prevents the UV element from operating when voltage reads 0. This solution does not allow the motor to trip when voltage is 0.

14 Methods of Making UV Secure BUS 52 MotorMotor VT IED V < 27 Pickup MCCB Input ON AND pu 0 27 Operate UPS + - IED Close 52b CC Close Coil TC IED Trip 52a Trip Coil IED IN Input Undervoltage not allowed to operate when MCCB s are open. Obviously, this solution requires MCCB s. MCCB 52a This solution doesn t address disconnection from the primary bus.

15 Methods of Making UV Secure V < 27 Pickup ΔI rms > threshold OR AND pu 0 27 Operate ΔI angle > threshold Relay algorithms are applied to check for change in current to allow undervoltage to operate. Reciprocating compressors have constantly changing load.

16 Methods of Making UV Secure V < 27(1) Pickup Watts < 0 (rev) Watts > 1.15 pu 0 do pu 0 OR AND 27(1) Operate Use a reverse power or a high forward power element to supervise undervoltage. If power reverses, UV can operate. If power surges, indicating a reacceleration, UV is allowed to operate. Using power measurements verifies that UV is not due to VT fa ilure. This element alone will not operate for total loss of source and zero backfeed, a typical totally dead bus scenario.

17 Methods of Making UV Secure V < 27(1) Pickup pu 0 Watts < 0 (rev) 0 do OR AND 27(1) Operate Watts > 1.15 pu V < 27(2) Pickup I rms = 0 AND pu 0 27(2) Operate Use a second (backup) undervoltage element that is allowed to operate when current = 0. There is no load to be lost, even if the undervoltage is due to a VT failure.

18 Conclusions For good reason, undervoltage is a common protection element to use on motors. Undervoltage tripping may need to be performed by an IED, especia lly if the motor is brea ker-fed. IED s are very smart and can help prevent nuisance trips for VT fa ilure situa tions. Varying load can trick those smart relays, but the relays flexible logic can be applied to address the concern.

19 Thank You Questions?