NETA Protection | Energy Central – GWC Mag

INTRODUCTION

An out-of-phase synchronization (OOPS) event results in a torque transient on the generator and prime mover. It exposes the generator, generator step-up (GSU) transformer, and system elements to a current transient that puts mechanical and thermal stresses on windings and other conductors. These transients can be significant and result in through current magnitude equivalent to a three-phase fault located at the terminals of the generator. Since damage due to mechanical stresses tends to be cumulative in nature, it is important to detect these occurrences and clear the condition as quickly as possible. This article explores the phenomena and demonstrates how to program two generator protection relays to provide OOPS protection.

OOPS Protection Scheme Logic

The scheme requirements for an OOPS event are not onerous, and, in the interest of accurate targeting, a dedicated scheme is warranted. Traditional generator protection cannot typically detect OOPS events. Generally, most elements that might be responsive to the high currents during an OOPS event are time delayed, and high-speed differential elements are blind to this event. The inadvertent energization element is typically disarmed by the presence of voltage, thus disabling the element during an OOPS event. Furthermore, an OOPS event can continue to occur every time a generator is synchronized until the underlying cause is addressed.

The protection scheme logic is simple and given as follows:

FIGURE 1.  OOPS Scheme Logic

The 50 element should not be enabled while machines are online due to the inability to coordinate the element with other protective systems. An OOPS event, however, results in a high current magnitude at the instant of breaker closure. Use a 52A status signal, in conjunction with a timer set for a 15-cycle dropout delay, to disable the overcurrent element 15 cycles after the breaker closes. This condition must be cleared immediately because zero crossings may stop if the condition persists. Hence, there is no additional time delay provided and the element should trip instantly. This scheme also detects and trips high-speed for the slow breaker close OOPS scenario.

The maximum ac current that occurs during an out-of-phase sync is calculated as follows:

I_AC ≈ V1- Ðδ₀X_Total  = 2VX_Totalsinδ₀2             [1]

Where:

I_AC ≡               maximum ac current magnitude

V ≡             generator and system voltage magnitudes (typically one per unit)

X_Total ≡        sum of X_d”, X_T and X_S

d₀ ≡            synchronizing angle across the breaker

Typically, an angle of 60 degrees is chosen to determine the current magnitude. The pickup setpoint for this protective element should be sensitive enough to detect an OOPS event but also be high enough that it does not inadvertently operate on an acceptable synchronization. Equation [1] can be expressed as follows assuming an angle of 60 degrees and one per unit voltage magnitudes:

I_AC ≈ 1X_Total                                                    [2]

Real Life Example for a Combustion Turbine Generator (CTG)

An electric utility experienced an out-of-phase sync (OOPS) event this year for a CTG. Here is the 50 phase overcurrent protection function and logic need to configure the generator protection relays for the CTG, which is where the event occurred.

CTG AC Current Calculations (Generator Base):

X_d” = 0.212 per unit (unsaturated)

X_T = 0.15 per unit

X_S = 0.008 per unit

I_base = 2.798 amps secondary

I_AC = 1X_Total  = 10.212+0.15+0.008=  = 10.37  = 2.7027 per unit

I_AC = (2.7027 per unit)•I_base = 7.56 amps secondary (50 pickup)

M-3425A

Phase Overcurrent Pickup

IPSlogic

SEL-700G

Phase Overcurrent Pickup

50PX2P = 7.56 Amps secondary

50PX2D = 0

SELogic Control Equations

TR1 := 21C1T OR 21C2T OR SV08T OR 64G1T OR 87R OR 87U OR SV02T

# TRIP LOCKOUT 4C86G-1

TR2 := 24C2T OR (3PWRX1T AND NOT LOPX) OR 40Z1T OR 40Z2T OR 46Q2T OR

(59PPX1T OR 59PPX2T) AND 52AX OR OOST OR 81X4T OR SV02T

Program the SEL-700G to display an alarm when an OOPS trip has occurred as follows:

SET01 := SV02T

RST01 := TRGTR

Latch bit LT01 remains asserted until personnel presses the Target Reset pushbutton. This latch is programmed as a display point as follows:

Oscillography

Oscillography captured by one of the generator protection relays for the event described in the introduction is presented here.

FIGURE 2.  Stator Current during OOPS Event

The stator phase current grew extremely high in magnitude when the generator circuit breaker (GCB) was closed out-of-phase. The filtered current on C-phase recorded by the generator protection relay rose as high in magnitude as 29,650 Amps. A three-phase fault at the terminals of the CTG would produce 28,335 Amps based upon the saturated subtransient reactance (Xd”) of 0.158 per unit and a nominal current of 4,477 Amps. Based upon IEEE C50.13, IEEE Standard for Cylindrical-Rotor 50 Hz and 60 Hz Synchronous Generators Rated 10 MVA and Above, generators are built to withstand a three-phase fault right at the terminals. The current magnitude produced during the out-of-phase close exceeded the three-phase fault current. The generator bracing was inspected to verify it was still intact.

CONCLUSION

Consider the plot below, for a 60 degrees OOPS, which plots both current and torque as a function of the synchronizing angle, there is 5 per unit torque applied to the shaft. Therefore, it pertinent to inspect the machine. The OOPS protection scheme trips the 86 lockout and shuts down the unit. The root cause that led to the OOPS must be determined and properly investigated prior to another attempt to sync the machine.

FIGURE 3.  Torque and current for an OOPS event versus synchronizing angle