Technical Articles
Conventionally, power factor correction systems consist of power factor correction capacitors switched using capacitor duty contactors. The re-switching time is the sum of capacitor discharge time (within 75 seconds as per IEC 60831) and response time of contactor. Such arrangement is suitable for applications where few switching operations take place per day.
Many applications increasingly require real-time reactive compensation. With this, the demand for dynamic power factor correction arises and faster switching of capacitors becomes inevitable.
This article discusses the need for thyristor switching module and the care to be taken when designing systems with thyristor switching modules.
Faster response is needed when the demand for reactive power is rapidly fluctuating either due to rapidly changing load conditions or process requirements. For certain loads, like,
The demand for reactive power comes frequently and for short duration of time.
Large reactive power demand for very short durations has severe ill-effects, such as,
Contactors are electro-mechanicaI devices which switch capacitors after a lag. Also, due to discharge time of capacitors, the re-switching time of contactors becomes longer than 60 seconds. This makes contactor a slow device.
In-rush current in capacitor switching are reduced to 10 times the rated current if capacitor duty contactors are used. However, a certain pulse element is inevitable as can be seen in figure 1.
Inrush current occurences cause high eletromechanical forces within the capacitor. The dielectric inside the capacitor is highly stressed due to this high current. This could lead to reduction in the life of capacitors.
In order to obviate the above shortfalls, the thyristor switching modules are used.
A thyristor switching module (TSM) is a fast, electronically controlled thyristor switch for switching capacitor loads within a few milliseconds as often and as long as required.
First of all, being a thyristor-based switch, the thyristor switches the capacitor without delay. A thyristor switching module works on the principle of zero-voltage switching, i.e., the capacitor is switched ON only when the voltage waveform is at its zero crossing. The current through a capacitor is given by:
Ic = C dVc / dt
Where, lc is capacitor current, C is capacitance and Vc is voltage across capacitor. This current is directly proportional to the rate of change of voltage across the capacitor. Thus, when capacitors are switched ON at zero crossing of supply voltage, the voltage applied to the capacitor is almost zero and grows steadily following the sine wave. Thus, the inrush current becomes equal to the rated current.
Another point to be noted here is that the capacitors used with TSM should be fitted with quick discharge resistors (QDR). If the capacitors are used with the usual discharge resistors, then, the capacitor may not get fully discharged at the time of re-switching. In such a situation, there will still be a significant voltage across the capacitor resulting in high inrush current. Quick discharge resistors ensure that the capacitor is completely discharged before its re-switching. The typical discharge time of normal resistors is to reach less than 50 V within 60 seconds. However, for QDR, the discharge time is reduced significantly so that there is a higher profitability of switching at zero crossover, making the capacitors suitable to be used with thyristor switching modules.
The typical arrangement of thyristor switches in a thyristor switching module is given in figure 3. Two terminals of a delta connected-capacitor are connected to the line via TSM while the third is connected directly to the line (all three phases have high speed fuses for branch protection). This configuration obviates the need to use three switch-pairs while ensuring that the three-phase capacitor is connected only when both switch pairs are ON or the module is ON. Each switches pair is a combination of two thyristors connected in anti-parallel for operation with ac voltage.
Peak Inverse Voltage (PIV) rating of a thyristor switch is the maximum peak voltage that the thyristor can withstand in the reverse biased condition. It is the maximum voltage that it must block when it is in OFF condition. A module with thyristors of lower PIV, say 1600 V, is likely to fail in presence of high voltage surges. Hence, higher values of PIV are desirable. Typically, in capacitor switching applications, the PIV ratings are up to 2200 V.
For short circuit protection of a thyristor switching module, a faster protection device is needed so that it will act before the thyristor switch in case of a short circuit. Normally, an HRC fuse has a time characteristics based on the I2t characteristics of cables as per IEC 60269-2. In case of a short circuit, the thyristor will below before the fuse. However, a semiconductor back-up (or a high speed) fuse has a time characteristics based on I2t characteristics of a thyristor. Hence, semiconductor fuse or high speed fuse can protect the thyristor module.
Being electronic switches, the heat generated due to losses needs to be dissipated quickly. The thyristor switching modules are generally designed with fins for better heat dissipation. They should be mounted in a position so that maximum air flow is possible. Additionally, it is always recommended to use an adequately-sized fan in the compartment where TSMs are mounted in an APFC panel.
Some thyristor switching modules come with in-built fans for cooling. Practically, it is difficult to spot a fan failure in such a case and an undetected fan failure may lead to failure of TSM. The TSM module becomes unnecessarily bulky in such cases. Hence, it is best to use fan-less modules with an adequately-sized fan on panel ceiling.
A TSM is connected to the line via a semiconductor fuse and its output is given to the three-phase capacitor or a detuned filter (in case harmonics are present). Triggering of a TSM can be done by means of dynamic power factor controllers. Triggering can be done by a controller which has a transistor output.
Typically, a TSM has a 24 V or/and a 12 V input and the respective currents required for triggering are approximately 15 mA and 20 mA respectively. Sometimes, more than one TSM is required to be connected to the same controller output for making a larger step, say, 100 kVAr. In such a case, more than one TSM can be connected to one output of the controller. The maximum number of TSMs that can be connected to a controller is restricted by the dc supply available and the maximum current limit of a stage output of the controller output.
Another point to be noted here is that in case of TSM, there is no electrical isolation. Due to the switching principle of the thyristor-modules the PFC-capacitors are permanently loaded at the peak value of the grid voltage (DC current) even when they are disconnected. Even when the thyristor switches are off, no electrical isolation is given. Hence, even after switching off the incomer of the APFC system, parts should only be touched after the standard discharge time of the capacitors.
Thyristar switching modules being expensive is often a concern for industry. An economical solution without compromising the performance of power factor correction panel can still be devised in many applications. Typically, an industry consists of a variety of loads - some fixed loads that are always running, same varying motor loads that run for same part of the day and some highly fluctuating loads. The following solution is suggested:
The most economical solution can be to provide some fixed capacitors for the loads that are ON throughout the day, a contactor-based switching for capacitors providing compensation to varying motor loads and a thyristor-based switching for capacitors providing compensation to highly fluctuating loads.
Abhinav L Purkar,
Manager Product Management & Marketing (PMM)Abhinav Purkar is a Product Management, Marketing, and Application Engineering professional with over 10 years of experience in the Power Quality domain. He has extensive expertise in power quality analysis, harmonic mitigation, reactive power compensation, energy efficiency, and electrical system optimization, helping customers improve system reliability and operational performance. With proven experience in product lifecycle management, market development, portfolio growth, and go-to-market strategy, Abhinav has successfully driven initiatives that enhance customer value and strengthen market presence. His strong technical foundation, combined with strategic business acumen, enables him to bridge the gap between customer requirements and innovative product offerings.
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