Instructions

Scenario 1
Perfect Conditions
Basic

Situation

You are at a 138 kV substation. The dispatcher has asked you to manually close breaker 1234 to tie Bus A to Bus B. The sync check relay has been bypassed for maintenance. You observe the synchroscope and instrument readings below.

Instrument Readings

Bus Voltage (A) 139.2 kV
Incoming Voltage (B) 138.8 kV
Voltage Difference 0.3%
Synchroscope Slow CW ~ 1 rev/15s
Slip Frequency ~0.067 Hz

Synchroscope

Slow clockwise rotation

What should you do?

Correct Action

Close the breaker as the needle approaches 12 o'clock. These are ideal synchronization conditions.

Analysis

Slip frequency = 0.067 Hz, meaning the phase angle changes at approximately 24°/sec. With a typical 5-cycle breaker (83 ms closing time), the angle changes only about 2° during closing — well within acceptable limits.

Voltage difference is 0.4 kV out of 138 kV (0.3%), far below the typical 5% limit. This small mismatch will cause negligible reactive power exchange upon closing.

Why not wait for the needle to stop? A perfectly stationary needle at 12 o'clock would require exactly matched frequencies, which is rare in practice and not necessary. The slow clockwise rotation means the incoming source is slightly faster, which is actually preferred — it ensures the generator will pick up load smoothly after paralleling.
Scenario 2
Fast Needle
Intermediate

Situation

Same substation setup. The dispatcher is pressuring you to close quickly — there is a system reliability concern and they need the tie closed as soon as possible. You observe the synchroscope needle spinning rapidly.

Instrument Readings

Bus Voltage 138.5 kV
Incoming Voltage 138.2 kV
Voltage Difference 0.2%
Synchroscope Fast CW ~ 1 rev/3s
Slip Frequency ~0.33 Hz

Synchroscope

Fast clockwise rotation

What should you do?

Correct Action

Do NOT close. The slip frequency (0.33 Hz) far exceeds the typical acceptable limit of 0.067–0.1 Hz. Request frequency adjustment.

Analysis

At 0.33 Hz slip, the phase angle is changing at approximately 120°/sec. With a 5-cycle breaker closing time (~83 ms), the angle changes by about 10° during the closing operation. Combined with human reaction time (~200 ms), the total angular uncertainty is approximately 34° — far too high for a safe closure.

Even if you timed the closure perfectly at 12 o'clock, the rapid rate of angle change means that any small timing error results in a large angular mismatch at the instant of contact. At 138 kV, each degree of mismatch produces roughly 2.4 kV across the breaker contacts.

Dispatcher pressure does not override physics. The dispatcher may not be aware of field conditions. Your responsibility as the operator at the breaker is to communicate the conditions and refuse to perform an unsafe operation. IEEE C50.12 and most utility operating procedures set a maximum slip frequency, typically 0.067 Hz (one revolution in 15 seconds) for manual synchronizing. Always communicate before acting.
Scenario 3
Wrong Direction
Intermediate

Situation

You are at a 230 kV substation performing a manual breaker close to parallel an incoming generator with the system. The synchroscope needle is rotating counter-clockwise at a slow, steady rate.

Instrument Readings

Bus Voltage 230.1 kV
Incoming Voltage 229.8 kV
Voltage Difference 0.13%
Synchroscope Slow CCW rotation
Rotation Rate ~1 rev / 12s CCW

Synchroscope

Slow counter-clockwise rotation

What should you do?

Correct Action

Do not close yet. Request frequency adjustment so the incoming source rotates clockwise (incoming slightly faster than bus).

Analysis

Counter-clockwise rotation indicates the incoming source frequency is slightly lower than the bus frequency. While it is technically possible to close at 12 o'clock during a CCW rotation, best practice strongly favors clockwise rotation for several reasons:

1. When the incoming generator is slightly faster (CW), it immediately picks up real power load upon closing, which is the normal desired operating condition.

2. With CCW rotation (incoming slower), the generator initially acts as a motor, absorbing power from the system. The governor must then respond to increase fuel/steam, which creates a transient operating condition.

3. Timing is more intuitive with CW rotation — the operator can anticipate the needle reaching 12 o'clock and account for breaker closing time.

Note: Closing at 6 o'clock (option D) would be closing at maximum voltage difference — 180° out of phase — which would be catastrophic. The direction of rotation does NOT change where 12 o'clock (0°) is on the synchroscope.
Scenario 4
Voltage Mismatch
Intermediate

Situation

You are at a 138 kV substation. A generating unit has just come online and you need to parallel it to the system. The synchroscope shows excellent phase angle and frequency conditions, but the voltage meters tell a different story.

Instrument Readings

Bus Voltage 142.5 kV
Incoming Voltage 131.0 kV
Voltage Difference 8.5%
Synchroscope Slow CW ~ 1 rev/15s
Slip Frequency ~0.067 Hz

Synchroscope

Slow CW — angle/frequency OK

What should you do?

Correct Action

Do NOT close. Request excitation adjustment on the incoming generator to reduce the voltage difference to within 5% (preferably 0–2%) before synchronizing.

Analysis

A synchroscope only indicates the phase angle difference and slip frequency between two sources. It does NOT indicate voltage magnitude difference. The synchroscope can show "perfect" conditions while voltage mismatch is dangerously high.

An 8.5% voltage difference (11.5 kV at 138 kV) will cause a large reactive power (MVAR) surge at the moment of closing. The lower-voltage source (incoming unit) will immediately absorb a large amount of reactive power from the system. This can:

• Cause voltage regulation problems across the system
• Overload generator excitation systems
• Stress transformer windings and insulation
• Trip protective relays (loss-of-field, over-excitation)

Industry standard: Most utility synchronization procedures limit voltage difference to 0–5%, with 0–2% preferred. The synchroscope is only one of three checks required: (1) phase angle near 0°, (2) slip frequency within limits, and (3) voltage magnitude within limits. All three must be satisfied.
Scenario 5
Dead Bus Closing
Basic

Situation

After a substation outage, you need to re-energize Bus B from Bus A through a tie breaker. Bus B is completely de-energized — confirmed by voltage indicators. No voltage on the incoming side. The synchroscope needle is not rotating and appears to sit limp.

Instrument Readings

Bus A Voltage 138.2 kV
Bus B Voltage 0 kV (dead)
Synchroscope Not rotating / Limp
Bus B Status De-energized confirmed
Grounds All removed per SWO

Synchroscope

Needle stationary / no signal

What should you do?

Correct Action

This is a dead bus closure. Verify the bus is dead, all grounds are removed, and the switching order is complete, then close the breaker. No synchronization required.

Analysis

Dead bus closing is fundamentally different from synchronizing two live sources. With no voltage on the receiving bus, there is no opposing voltage source and therefore no risk of out-of-phase closure. The synchroscope cannot function without voltage on both sides — it is not malfunctioning, it simply has no signal.

The key safety concerns for dead bus closing are entirely different from synchronization:

• Confirm the bus is truly de-energized (use approved voltage indicators/detectors)
• Verify all protective grounds have been removed
• Confirm the switching order authorizes the closure
• Ensure downstream loads and equipment are ready for energization
• Be aware of cold load pickup (inrush) if loads are connected

Key concept: Synchronization is only required when connecting two energized (live) AC sources. Dead bus closing is a simple energization operation. Understanding this distinction is critical for substation operations. Many operating procedures have separate sections for "live bus/live line" (sync required) vs. "dead bus" or "dead line" closing.
Scenario 6
180 Degrees Out — The Worst Case
Advanced

Situation

A new technician at a 345 kV substation is eager to close a breaker to restore a critical tie. The synchroscope needle is at the 6 o'clock position (180°) and barely moving. The tech reaches for the close button, noting that the voltages and frequencies look good and the needle is nearly stationary.

Instrument Readings

Bus Voltage 347.0 kV
Incoming Voltage 346.5 kV
Voltage Difference 0.14%
Synchroscope ~180° (6 o'clock)
Needle Movement Barely moving

Synchroscope

Needle near 180° — barely moving

What should you do?

Correct Action

ABSOLUTELY DO NOT CLOSE. 180° is the worst possible phase angle for synchronization. This closure would be catastrophic.

Analysis

At 180° phase difference, the voltage across the breaker contacts equals the sum of both source voltages:

V_across = V_bus + V_incoming = 347.0 + 346.5 = 693.5 kV

This is approximately twice the rated voltage appearing across the breaker contacts at the instant of closing. The resulting transient current would be:

• Potentially exceeding the breaker's rated short-circuit interrupting capability
• Limited only by the system impedance and source impedances
• Producing enormous electromagnetic forces on bus conductors and equipment

Physical consequences of a 180° closure at 345 kV:

• Generator shaft torsional failure (shaft can physically twist and break)
• Transformer winding deformation and insulation failure
• Bus structure physical damage from electromagnetic forces
• Fire and explosion risk at the breaker
• System-wide cascading failures from the resulting disturbance
• Potential injury or death to personnel in the substation

This scenario has caused real-world catastrophic failures. A 180° out-of-phase closure is one of the most destructive events that can occur on a power system. The fact that voltages are matched and frequencies are nearly identical (needle barely moving) makes this scenario deceptively dangerous — everything looks "good" except the needle position. This is exactly why synchroscope training emphasizes that the needle MUST be at or near 12 o'clock (0°), not simply "stationary." The position matters, not just the movement.
Scenario 7
Synchroscope Stuck
Advanced

Situation

You are preparing to close a tie breaker at a 138 kV substation. The synchroscope appears to be stuck — the needle is not moving at all and is sitting at about the 2 o'clock position (~60°). Both voltmeters confirm both sides are energized. Both frequency meters read 60.00 Hz.

Instrument Readings

Bus Voltage 138.4 kV
Incoming Voltage 138.1 kV
Bus Frequency 60.00 Hz
Incoming Frequency 60.00 Hz
Synchroscope Stuck at ~60° (2 o'clock)

Synchroscope

Needle frozen at ~60°

What should you do?

Correct Action

STOP and investigate. Never trust a single instrument, especially one that may be malfunctioning. Do not close until the synchroscope condition is resolved.

Analysis

A synchroscope needle that is completely stationary while both sides are energized has several possible causes, each with different implications:

1. Instrument failure (mechanical): The synchroscope motor or needle mechanism is broken. The displayed angle has no meaning. Closing could be at any actual angle.

2. Blown fuse or open circuit: One of the PT circuits feeding the synchroscope has lost its signal. The instrument is not receiving proper input. The displayed angle is meaningless.

3. PT wiring error: The synchroscope may be connected to incorrect phases, or a phase reversal may exist. The displayed angle could be offset by 120° or 240° from the true angle.

4. Frequencies truly identical: It is theoretically possible that both sources are at exactly the same frequency with a fixed 60° phase offset. However, maintaining exactly identical frequencies for any sustained period is extremely unlikely on a real power system. Slight governor variations continuously adjust frequency.

If the needle is truly stuck at 60° and this represents the actual phase angle, closing would apply approximately:

V_across = 2 x 138.25 x sin(30°) = 138.25 kV across the breaker contacts — essentially full line-to-ground voltage. This is unacceptable.

Never trust a single instrument. Cross-check with voltmeters (check voltage across the open breaker if possible), independent frequency meters, and phase angle meters. If the synchroscope is deemed unreliable, use an alternative synchronizing method: voltage matching across the breaker (zero voltage = in phase), or a portable phase angle meter, or check lamps (dark lamp = in phase). Do not proceed with a suspect instrument.
Scenario 8
Emergency Island Reconnection
Advanced

Situation

A system disturbance has islanded part of the transmission system. You are at the tie point substation. The dispatcher urgently needs you to reconnect the island. System conditions are unstable — voltages and frequencies are fluctuating on both sides. You observe the instruments and synchroscope.

Instrument Readings

Bus Voltage 135.2 kV (depressed)
Island Voltage 141.0 kV (elevated)
Voltage Difference ~4.3%
Synchroscope CW ~ 1 rev / 8s
Slip Frequency ~0.125 Hz
Conditions Fluctuating

Synchroscope

Moderate CW, slightly unsteady

What should you do?

Correct Action

Communicate conditions to the dispatcher. If authorized for emergency closing with widened parameters, carefully time the closure at 12 o'clock during a stable window. The emergency does not override physics.

Analysis

Island reconnection is one of the most challenging synchronization operations a field operator can face. The conditions here are not ideal but may be acceptable for an emergency operation:

Voltage difference (4.3%): Within the typical 5% limit, but on the high side. The reactive power surge upon closing will be significant but manageable.

Slip frequency (0.125 Hz): Above the normal 0.067 Hz limit but below the absolute maximum typically used in emergencies (0.2 Hz). Phase angle changes at ~45°/sec, giving about 3.7° change during a 5-cycle breaker close — still acceptable.

Fluctuating conditions: This is the most concerning factor. Varying voltage and frequency mean the needle speed and position are unpredictable. The operator must watch for a "window" of relatively stable conditions.

Why not refuse entirely (option B)? While conditions are non-ideal, an islanded system will continue to degrade over time. Frequency may diverge further, voltage collapse may occur, or generators may trip. Delaying indefinitely could make reconnection impossible.

Why not close immediately (option A)? Rushing without proper assessment and communication is never acceptable, even in emergencies. A bad closure creates a second emergency.

Emergency synchronization is a team effort. The field operator must communicate exact conditions to the dispatcher, who can then coordinate with generation operators to improve conditions from their end. Many utilities have specific "emergency synchronization" procedures with widened but still defined parameter limits. The operator at the breaker has the final say on whether conditions are safe to close — the emergency does not transfer responsibility for the closure to the dispatcher. A bad closure during an emergency turns one island into a wider blackout.

Performance Assessment

Complete all 8 scenarios to see your performance assessment.