Terdeteksi vibrasi abnormal pada Generator GT 4.1 di PT PLN Nusantara Power UP Muara Tawar. Penelitian ini bertujuan untuk menganalisis penyebabnya menggunakan Root Cause Analysis (RCA), dengan fokus pada fenomena sensitivitas termal. Pengujian dilakukan melalui Thermal Sensitivity Test dalam tiga tahap. Pada tahap pertama, peningkatan daya aktif (MW) dengan arus eksitasi tetap tidak menyebabkan peningkatan vibrasi. Pada tahap kedua, peningkatan daya reaktif (MVAR) dengan daya aktif tetap menyebabkan peningkatan amplitudo vibrasi dari 4,41 mm menjadi 6,09 mm pada bearing casing turbine end, serta dari 3,12 mm menjadi 4,46 mm pada rel shaft exciter end. Pada tahap ketiga, penurunan daya reaktif mengakibatkan amplitude vibrasi menurun, mengonfirmasi adanya pengaruh daya reaktif terhadap getaran generator. Pengujian tahanan isolasi (Insulation Resistance Test) menunjukkan Polarization Index (PI) stator 4,5–5,25 dan rotor 4,476, sesuai dengan standar IEEE 43-2013. Pengujian impedansi rotor menunjukkan nilai rata-rata 11,92 Ω, menandakan karakteristik linear tanpa indikasi hubungan pendek antarbelitan. Analisis ventilasi rotor menemukan 24 lubang pendingin tersumbat, menyebabkan distribusi panas yang tidak merata dan memicu rotor bowing. Berdasarkan Root Cause Analysis, vibrasi abnormal disebabkan oleh distribusi panas yang tidak merata pada rotor, yang memicu ekspansi termal tidak seimbang dan menyebabkan deformasi rotor. Penyumbatan lubang pendingin memperburuk kondisi ini, meningkatkan risiko misalignment serta ketidakseimbangan rotor. Setelah penggantian rotor, perawatan sistem pendinginan, dan penyesuaian arus medan, generator diuji kembali pada beban 138 MW dengan daya reaktif ±40 MVAR. Hasilnya menunjukkan amplitudo vibrasi berada di bawah batas ISO 20816-1 untuk operasi tanpa pembatasan (<7,1 mm/s RMS) dengan penurunan vibrasi ±20 mm. Penelitian inimerekomendasikan pemantauan kondisi generator secara berkala, perawatan sistem pendinginan, serta optimalisasi daya reaktif guna mencegah fluktuasi vibrasi berlebihan pada Generator GT 4.1.

Abnormal vibration was detected in Generator GT 4.1 at PT PLN Nusantara Power UP Muara Tawar. This study aims to analyze the root cause using Root Cause Analysis (RCA), focusing on the phenomenon of thermal sensitivity. Testing was conducted through a Thermal Sensitivity Test in three stages. In the first stage, an increase in active power (MW) with a constant excitation current did not cause an increase in vibration. In the second stage, an increase in reactive power (MVAR) with constant active power caused the vibration amplitude to rise from 4.41 mm to 6.09 mm at the bearing casing turbine end and from 3.12 mm to 4.46 mm at the shaft exciter end. In the third stage, reducing reactive power resulted in a decrease in vibration amplitude, confirming the influence of reactive power on generator vibrations. The Insulation Resistance Test showed Polarization Index (PI) values of 4.5–5.25 for the stator and 4.476 for the rotor, complying with IEEE 43-2013 standards. The rotor impedance test indicated an average value of 11.92 Ω, signifying a linear characteristic with no short circuits between windings. A rotor ventilation analysis revealed 24 blocked cooling holes, leading to uneven heat distribution and causing rotor bowing. Based on Root Cause Analysis, the abnormal vibration was caused by uneven heat distribution in the rotor, triggering unbalanced thermal expansion and rotor deformation. The blockage of cooling holes further worsened this condition, increasing the risk of misalignment and rotor imbalance. After rotor replacement, cooling system maintenance, and field current adjustment, the generator was retested at 138 MW load with ±40 MVAR reactive power. The results showed that vibration amplitude was below the limit of ISO 20816-1 for unrestricted operation (<7.1 mm/s RMS), with a vibration reduction of approximately 20 mm. This study recommends regular generator condition monitoring, cooling system maintenance, and reactive power optimization to prevent excessive vibration fluctuations in Generator GT 4.1.