PST Series Booster FRA: Detecting Lead Disconnection and Winding Integrity Issues
Phase-shifting transformers (PSTs) with series boosters—used for precise power flow control in interconnected transmission grids—have complex internal connections: series windings, exciting windings, regulating windings, and tap changers. Lead disconnection or winding shorts in the series booster circuit can cause incorrect phase angle shifts, leading to circulating currents and grid instability. A Transformer Frequency Response Analyzer, applied with PST-specific test configurations, reliably detects these hidden faults.
PST Series Booster Construction and Failure Modes
The series booster contains exposed leads and connections subject to:
Vibration-induced lead loosening (from normal operation or nearby faults)
Thermal cycling causing connector relaxation
Partial discharge erosion of lead insulation (from transient overvoltages)
Mechanical damage during transport or installation
These failures produce characteristic FRA patterns when analyzed with proper baseline comparison.
Detecting Lead Disconnection via High-Frequency FRA
Series booster lead disconnection typically affects high-frequency response:
Open lead (complete disconnection): The series winding section becomes floating. FRA shows a sudden drop to -80 dB above a certain frequency (capacitive coupling only). The cutoff frequency corresponds to the open cable resonance.
High-resistance connection (partial disconnection): A narrowband notch (2–5 kHz wide) appears at the frequency where the inductive reactance equals the contact resistance. Notch frequency shifts with load current (not observable offline).
Lead-to-ground short: Parallel resonance creates a sharp amplitude spike (+10 to +20 dB) at the resonant frequency of the lead inductance and stray capacitance.
Winding Shorts in Regulating and Series Windings
Shorted turns in PST windings produce:
Broadband amplitude reduction (3–15 dB) across mid and high frequencies: The shorted turn acts as a lossy coupled circuit, damping all resonances.
Disappearance of one or more resonant peaks: The short changes the modal structure; specific peaks vanish completely.
Temperature-dependent FRA: Shorted turns cause local heating; repeating FRA while warming the transformer may show progressive CC decline with temperature.
Case Example: Lead Disconnection in PST Series Booster
A 300 MVA phase-shifting transformer showed inconsistent angle readings (±2° variation). Oil DGA was normal. FRA testing was performed on the series winding (source on H1-X1, response on H2-X2). Compared to baseline:
Mid-band (10–100 kHz) CC = 0.86 (baseline 0.98)
A deep notch (-25 dB) appeared at 220 kHz, not present in baseline
Low and high bands outside notch were normal
This narrowband notch indicated a high-resistance connection (not a full open). Internal inspection found a bolted lead connection in the series booster with torque 40% below specification, causing intermittent contact. The connection was re-torqued, and FRA returned to baseline. Without FRA, the intermittent lead would have eventually failed open, causing loss of phase angle control.
Testing Protocol for Series Booster Diagnostics
Isolate the series booster from the main transformer for focused testing:
Open the series booster disconnect switches (typically located on the PST tank side).
Perform end-to-end FRA on the series winding only (e.g., H1 to X1, H2 to X2).
Compare to factory baseline of the series booster alone (not the complete PST).
If no standalone baseline exists, compare to the sister phase (assuming symmetrical design).
Transfer Function Measurements for Complete PST
When the series booster cannot be isolated, use transfer function modes:
HV series winding to neutral: Source on H1, response on H0. The signature includes series booster and main winding. Baseline comparison may show mid-band changes.
Exciting winding to series winding: Source on exciting winding (shunt) and response on series booster. This mode is highly sensitive to series booster impedance changes.
For phase-shifting transformers, the Transformer Frequency Response Analyzer is the only non-invasive diagnostic capable of detecting lead disconnection and winding shorts in the series booster. Regular FRA testing ensures angle accuracy and prevents uncontrolled power flow.
