## 2 DTPPF Fault Identification Method  
### Analysis of Fault Causes in DTPPFs  
The structural topology of a DTPPF is shown in Figure 1.  
In Figure 1, Q01 (Table A1) is the circuit breaker, KM0 is the main contactor, FS1-FSn are fuses, KM1-KM_n_ are contactors, Cs is the equivalent capacitance of the filter capacitor bank(FCB), \(L\) is the equivalent inductance of the primary inductance winding of the electromagnetic coupling filter reactor(ECFR), \(I_{h}\) is the harmonics current generated by nonlinear loads, and \(I_{f}\) is the harmonics current absorbed by the DTPPF.  
The DTPPF consists of a main circuit and a control system. The main circuit is composed of an electromagnetic coupling filter reactor (ECFR) and an FCB, forming a series filtering branch. The control system consists of a controller, trigger board (PTB), harmonic acquisition module (WD), and touch screen (TS), which are used to control the DTPPF and dynamically adjust the inductance \(L\).  
The ECFR consists of a primary inductance winding and a secondary inductance control winding connected to the thyristor PEIC. The primary inductance winding of the ECFR is connected in series with the FCB to form a filtering branch. When a certain frequency resonates (referred to as the \"resonant frequency\"), the harmonics current absorbed by the DTPPF is maximal, indicating the best filtering effect. The resonant frequency is equal to \(h\) times the fundamental frequency, where \(h\) is the harmonic order, and the corresponding harmonic current is called the \(h\)-th harmonic current.  
Based on the \(h\)-th harmonics current and the total harmonic distortion (THD) as well as the designed absorption current, the controller determines the \(C_{s}\) and then switches the filter capacitors on or off (\(C_{1}\), \(C_{2}\),...\(C_{x}\)). At the same time, the controller calculates the control signal \(u_{k}\) through the control algorithm and outputs it to adjust \(L\) until the DTPPF resonates at the \(h\)-th frequency.  
When the DTPPF operates at the rated load, the temperature increase of the filter capacitors remains within allowable limits. Nevertheless, any increase in the harmonics current accelerates this rise. When the temperature rise exceeds acceptable levels, the temperature of the insulation material increases as well, resulting in thermal aging. This causes a gradual decline in the capacitance value of the filter capacitors, ultimately shortening their lifespan. Typically, for every 7-8 \({}^{\circ}\)C increase in temperature, the lifespan is halved. When the filter capacitors are damaged or their capacitance decreases, the resonant frequency shifts, causing the DTPPF to become detuned, which in extreme circumstances can result in hazardous operating conditions such as burned out main contactors and fuses.  
The presence of the harmonics current induces vibration in the core of the ECFR and deformation of its windings due to the resulting electromagnetic forces. In severe cases, the insulation material between windings may break down, resulting in short circuits between turns. An excessive harmonics current can cause the thyristor on the secondary side of the ECFR to break. Both phenomena impede continuous adjustment of the inductance of the primary side of the ECFR. When the thyristor on the secondary side of the ECFR fails, the resonant frequency shifts, causing the DTPPF to become detuned.  
When the filter capacitors are damaged or their capacitance decreases, or when the thyristor on the secondary side of the ECFR fails, the resonant frequency shifts, causing the DTPPF to become detuned. This leads to deterioration of the filtering effect, and potentially to unsafe operation in severe cases.  
Figure 1: Topology structure of DTPPF.