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Technology | March 2012

SMPS vs thyristor rectifiers: Need and benefits

The new products will enhance the performance of supply systems and are modular and scalable when compared to currently deployed conventional thyristor-based DC power supply systems, says Murthy S.

Power electronics has a wide range of applications in the field of electrical power systems. DC power supplies form an essential part of many applications like power distribution, BTS in telecommunications, utility industries like oil, gas etc. The power range of these DC power supplies varies from a few VA/watts to several MVA/MW. The most common form of a DC power supply is AC-DC conversion system, in which, the required level of DC power to the load is supplied through an AC source at the input. Most of the AC-DC conversion systems have different elements like the input transformer, rectifier, smoothing, regulator, etc., but all of these may not be present in every design. One such application of DC power supplies is for charging batteries, which are used to provide power back-up for mission-critical equipment in utilities. The circuitry and systems involved are termed as DC power systems [DCPS].

Enter any power utility site today, and you will most likely come across large and bulky battery charger systems. These are usually based on conventional thyristor technologies. A typical thyristor-based DCPS utilises ferro-resonant techniques, a primary reason for massive size, large weight. Further these systems do not have the scope for modular expansion, due to which, frequent changing of the DCPS with each expansion of the switching/transmission system becomes a concern. And since these systems are installed in industrial mission-critical applications, as long as they are functional and meeting their original design-requirements, they are neither replaced nor upgraded.

The problems associated with thyristor-based systems namely huge space requirements, bulkiness, lower power factor and lower efficiency have largely been eliminated with the advent of switching mode power supplies technologies [commonly referred to as SMPS-based DCPS. By virtue of SMPS technology, in addition to being lighter, and less bulky than conventional thyristor technologies, there is the added advantage of self checking features with remote communication facility. As compared to conventional thyristor-controlled DCPS, the operating characteristics such as transient response, power factor, efficiency, total current harmonic distortion and ripple noise markedly improved in switch mode power supply (SMPS) based DCPS. Moreover, these systems have been widely adopted for various critical sectors like signalling in railways, mobile switching centre (MSC) in telecom services for serving mission critical equipment and in power plant equipment.

In most developed economies, where efficient and environment-friendly technologies take precedence both from a ROI as well as government legislation, most infrastructure verticals have switched over to SMPS-based DCPS. But in India, conventional DCPS systems are still very prevalent, even though the advantages are evident. Since conventional thyristor-based DCPS have been very sturdy with low-failures reported on the field, the economics of CapEx may have overtaken the need for transition to newer technologies. Also, any new equipment that has to be introduced in this industry has to go through extensive and long testing processes and procedures, sometimes as long as two years. This may have created a high-barrier of entry for a newer technology to be adopted quicker in the Indian context. But now with operating costs and power conservation becoming a serious consideration, the marketplace is very keen to adopt more efficient, cost saving, and scalable power systems.

Let us now see the technical merits of SMPS technology for DC power systems and most importantly how it fulfils the requirements of this industry, without any added risks.

Power Semiconductor device - history

Power electronics devices made headway in the converter application with the advent of silicon controlled rectifiers which replaced bulky mercury arc rectifiers to a great extent. Soon, bipolar transistors arrived as competition to SCR with superior turn-off capabilities. However these devices had their own drawbacks such as high base current requirements, incapability to block reverse voltages etc. Power MOSFET, modern integrated circuits and discrete power semiconductor manufacturing technologies burst onto the scene with higher gain, parallel operations and much higher operating frequencies.

While improvements were being tried out on the SCR regarding its turn off capability, the gate turn-off thyristor were proposed. But due to restrictive cost and requirement for an extremely high turn-off control prevented the adoption of these devices. The insulated gate bipolar transistor – basically a MOSFET-driven bipolar has been a successful proposition with its switching frequency and ease of connection. Presently there are few hybrid devices and intelligent power modules, being marketed by some manufacturers. However these devices must prove themselves before they are accepted by the industry.

SMPS Design

In these systems, the conversion of AC to DC is accomplished in two stages as given below.

1. First stage conversion: The input AC voltage is ‘rectified’ and converted to high-voltage DC.
2. Second stage Conversion:
a) Rectified high-voltage DC is stored in capacitors.
b) High-voltage DC is then converted in to a very high frequency AC (20 KHz and higher). Conversion of high voltage DC to high frequency AC is achieved by means of very powerful and fast switching semi-conductor devices.
c) High frequency AC is stepped down to the required level by means of a small high frequency transformer.
d) Stepped down AC is rectified to DC of desired voltage and filtered by means of high frequency filters.

Salient features of SMPS technology

• SMPS-based power plants use very high frequency switching, thereby reducing the size of the transformers and chokes by almost 15 per cent. This reduction in size reduces not just the size of the overall equipment, it also reduces the footprint of the equipment, thereby saving floor-space.
• Being small and light in weight, these systems are ideally suitable for modular designs. Modular designs allow for perfect compromise between CapEx and OpEx tradeoff. Being modular, they are scalable, which means new deployments can be installed with minimal configurations, i.e., lower CapEx. As and when the systems need to be scaled up [capacity increase], one can just buy additional modules, which come under operational expenses.
• These power plants have a very high conversion efficiency and consume less power resulting in low operational cost
• SMPS offers a very high power factor (near unity) making the system more efficient and make easy to comply with state electricity boards PF norms.
• These power plants have a very high reliability and are therefore less prone to faults which results in low operational costs.
The systems are solid-state systems which require much less routine maintenance and their compatibility with micro-processor based control systems enables remote supervision and control.
• All these power plants are provided with auto float/charge operation to recoup the battery lost capacity faster.

SMPS-based power plants are composed of:

• Control, alarm and monitoring part used remote supervision and control
• Distribution and switching for switching and interconnecting devices for batteries, load, and input.
• Float rectifiers-cum-float chargers (FR/FCs) or float rectifier-cum battery charger (FR/BC) modules: These units are modular, plug-in type, rack mountable supplied with rack enclosures, which are fully pre-wired for the ultimate capacity of DCPS. The modules supplied are hot swappable and designed to over N+N redundancy to ensure reliable and quality
power always.

Features of DCPS

Auto float/charge operation: Normally the power plant voltage remains at 54 V for VRLA batteries and 52.8 V for flooded lead acid conventional batteries. The float voltage can be set in the range from 48 V to -56 V. When the power plant is restored after any interruption, the master controller sets up the DCPS voltage to 55.2 V for faster recoup of the battery. When the battery is fully charged, the controller reverts the DCPS voltage to 54 V in case of VRLA batteries and 52.8 V in case of conventional flooded batteries respectively. These two modes of operation are called auto float/charge operation.
Deep discharge prevention: The deeper a battery gets discharged, the lower it reduces the life of the battery. Any discharge of the battery beyond 80 per cent of its rated capacity drastically reduces the life of the battery.

This can be understood by the fact that an exchange battery when discharged to depth of discharge (DOD) of 20 per cent may give up to 3,000 such cycles. Same battery at 50 per cent DOD may give about 2,000 such cycles and will give only 1,400 cycles to 80 per cent DOD. The same battery if discharged to its full rated capacity may give about 600 such cycles only.

Battery under-voltage protection: The SMPS continuously monitors the voltage of battery during charging. When the battery reaches the pre-set voltage, it is isolated from the load thus preventing further discharge of the battery. When the power plant starts to deliver the output, the battery gets automatically reconnected to the float bus to get recharged from the power plant and gets ready to take load in the case of any interruption in the AC power supply.

The voltage corresponding to 80 per cent shall be set to prevent the discharge of battery beyond 80 per cent DOD.
Battery health check: Master controllers in DCPS monitor each cell of the battery for voltage and temperature. It is also possible to monitor the current, trickle current and battery voltage at set periodicity (programmable).

The same parameter can also be monitored using centralised control and monitoring system through RS-485 port.

Control of the battery temperature: The importance of temperature control in RLA batteries can be very well understood by the well-known fact that the life of the battery is reduced to half when working temperature increases by 100 C above the specified ambient.

The two major reasons for rise in battery temperatures are increase in ambient temperature and increase in battery chemical reaction. Both these factors can be controlled by SMPS power plants.

Conclusion

There has been a limited usage of SMPS-based DCPS in this country in the last 10 years more so because of the limitations in the AC input voltage [i.e., the wide voltage swing range].

The latest version of SMPS based DCPS have got a wider AC input voltage variation i.e., about 90-300 V making it more suitable and rugged under different power system conditions. Our company in collaboration with RT has developed state-of the art technology based SMPS with improved power factor, efficiency, and modular design catering to various applications across different sectors.

The author is V-P (Marketing) at VMC Systems. Views are personal.
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