Rectification circuit design for high frequency switching power supply system

1 Composition, characteristics and types of low voltage power supply system

(1) The power supply system usually consists of two parts: AC subsystem and DC subsystem.

The AC subsystem usually consists of two parts: high voltage and low voltage.

The DC subsystem is usually composed of an AC-DC conversion part and a battery pack. The load part is composed of low-voltage AC load and DC load equipment. "Load" is generally referred to as "electrical equipment".

(2) Basic characteristics of low voltage power supply system

①Parallel redundancy is the main way to improve reliability, whether it is an AC power supply system or a DC power supply system.

② For low-voltage power supply systems, primary power supply is mainly mains power or power generation. It is the core of low-voltage power supply systems and the key to the reliability of power supply systems. Other voltage conversion power supplies are dependent on it. The DC power supply system relies on the AC power supply system to provide power. However, the DC power supply system can make appropriate supplements to the AC power supply system.

③ Uninterruptible power supply (UPS) is widely used and plays an extremely important role in the reliable power supply of loads.

④ Apply automatic switching (ATS) technology to control the load.

(3) Type of low-voltage power supply system for G-generation power supply

Common low-voltage AC (220/380V, 50Hz) power supply systems include: IT, TN-C, TN-S, TN-C-S, and TT power supply systems.

The safety of power supply means that it cannot harm people or damage equipment during power supply and distribution. Reliability refers to the ability to continuously supply power under certain conditions and time. This is a contradiction in the power supply system. When the safety of people and equipment is in danger, the power supply needs to be cut off; and cutting off the power supply will affect the continuous power supply of electrical equipment. The following introduces five commonly used AC power supply systems and grounding methods in the power supply system, and compares them in safety and reliability analysis.

2 IT power supply system and grounding method

The IT system is a three-phase three-wire power supply and grounding system. The neutral point of the transformer (or three-phase output of the generator set) is not grounded or grounded through high impedance. There is no neutral line (commonly known as zero line) N. There is only line voltage (380V), no phase voltage (220V), and the protective grounding wire (PE line) of the electrical equipment is independently grounded as shown in the figure. The capacitors C1, C2, and C3 in the figure are the power supply line to ground distribution capacitors.

When the power supply distance of the IT system is not long, the power supply reliability is high and the safety is good. The neutral point of the power supply side can also be grounded through high impedance.

When one phase of the IT system is grounded, the single-phase to ground leakage current is small and does not destroy the voltage balance of the power supply. It is generally used in places where power outages are not allowed or where continuous power supply is strictly required.

If a ground fault occurs in one phase, the phase can be cut off by fuse F, and the other two phases can be powered. In addition, the electrical equipment has grounding protection. When the single-phase insulation is damaged and touches the shell, making the metal shell live, people touching the live metal shell can avoid electric shock accidents. This is because the current flows through two parallel circuits, one through the grounding wire and the earth, and the other through the human body and the earth. Since the grounding resistance (required not to exceed 4Ω, the maximum is not more than 10Ω) is much smaller than the human body resistance (minimum 1000Ω), most of the current enters the ground through the grounding body, and only a small part of the current passes through the human body, that is, the current passing through the human body does not exceed the human body safety current, thereby protecting the safety of equipment and personnel.

At this time, the neutral point drifts, and the voltage of the other two phases to the ground will increase to 380V. That is to say, the voltage of the other two phases to the ground was 220V. When one phase grounding fault occurs, the voltage of the other two phases to the ground increases to 380V. However, the voltage between each phase (line voltage) is still symmetrical and balanced, so the three-phase electrical equipment can continue to operate. In order to prevent another phase of the non-grounded phase from being grounded, causing a two-phase short circuit, the regulations stipulate that the continuous operation time when a single phase is grounded shall not exceed 2 hours. If the fault is not eliminated in time, the insulation facilities will be subjected to excessive voltage for a long time, which will lead to accidents.

When the single-phase grounding current of a neutral point ungrounded system exceeds the specified value, the neutral point should be grounded through an arc extinguishing coil to avoid intermittent arcs, overvoltage or short circuits, reduce the grounding arc current and make the arc extinguishing easier. The arc extinguishing coil is actually a reactor coil.

Assume that phase C is short-circuited to ground. Due to the existence of neutral point grounding reactance, the inductive current lags behind by 90°, while the line distributed capacitance current leads by 90°, thus effectively reducing the arc of the short-circuit current, as shown in Figure 2.

The TT power supply system is not suitable for communication equipment with single-phase power because it is not equipped with a neutral line N. This type of equipment is only suitable for places with special requirements, such as electric steelmaking, important operating rooms, important laboratories, underground mines or tunnel command posts, and special equipment in important communication hubs. This power supply system has high voltage resistance requirements for electrical equipment.

In addition, what about the case where the neutral point is directly grounded?

When single-phase grounding occurs in a neutral point directly grounded system, a single-phase short circuit is formed through the grounded neutral point, generating a large short-circuit current. The protection unit will operate to cut off the faulty line, allowing the other parts of the system to operate normally.

Since the neutral point is directly grounded, when single-phase grounding occurs, the voltage between the neutral point and the ground is zero, and the voltage between the non-grounded points and the ground does not change.

3 TN-C power supply system and grounding method

The TN system has a three-phase four-wire system in which the neutral point of the power supply is directly grounded and has a neutral line N, a protective line PE or a protective neutral line PEN.

If the N line and PE line in the system are combined into a PEN line, the system is called a TN-C system.

If the N line and PE line are completely separated in the system, the system is called a TN-S system.

If the N line and PE line in the first part of the system are combined into the PEN line, and the N line and PE line in the second part are completely separated, it is called a TN-C-S system.

When a single-phase shell-touching leakage fault occurs in the TN system, a single-phase short-circuit loop will be formed. Because the loop does not contain any grounding resistance, the impedance of the entire loop is very small and the short-circuit current is large, which is enough to ensure that the fuse is blown in the shortest time, the protection device or automatic switch trips, thereby cutting off the power supply of the faulty equipment and ensuring the safety of people and equipment.

The TN-C power supply system is often called a three-phase four-wire power supply system. In this system, the neutral line N and the protective grounding line PE are combined into one, that is, its working neutral line also serves as a protective line, commonly known as the PEN line, as shown in Figure 3. It is extremely unstable, causing the neutral line grounding potential to drift. Not only does it make the equipment casing charged, which is unsafe for people, but also because this drifting potential is superimposed on the potential reference point, the electronic equipment that uses it as the reference potential is interfered by the noise voltage, increasing the noise level of the voice and making the equipment unstable. Therefore, the TN-C system should not be used as the power supply and grounding method for communication hubs.

4 TN-S power supply system and grounding method

The TN-S power supply system has five wires, namely three phase wires U, V, W, one neutral wire N and one protective grounding wire PE. The power system is grounded at only one point, and the exposed conductive parts of the electrical equipment (such as casing, frame, etc.) are connected to the PE wire, as shown in Figure 4.

This power supply system is highly sensitive to ground faults, and the lines are economical and simple. In general, as long as appropriate switch protection devices and sufficient conductor cross-sectional area are selected, safety requirements can be met. At present, this power supply system is widely used and is suitable for places where the three-phase load is relatively balanced and the single-phase load capacity is small.

When using this system, it is very dangerous not to use some equipment for zero protection and some equipment for ground protection. Because once the insulation of the phase line of the grounding equipment is damaged, the fuse will blow and the current will be large, and the faulty electrical appliances cannot be cut off in time, and the shell of the zero-connected equipment will carry dangerous voltage. Therefore, special attention should be paid to not mix grounding and zero-connection.

In the communication hub, it is difficult to achieve three-phase load balance due to the presence of a certain number of single-phase loads. The unbalanced current on the PEN line, plus the third harmonic current generated by the switching power supply or rectifier and the higher harmonic current caused by fluorescent lamps in the line, will be superimposed on the neutral line N under non-fault conditions, and the current will be large and small.

The characteristic of TN-S power supply system is that the neutral line N and the protective grounding line PE are no longer connected except that they are grounded together at the neutral point of the transformer. Current flows through the neutral line N when the three-phase load is unbalanced, while no current flows through the PN line under normal circumstances. This power supply system is completely safe and reliable after grounding. This system is often used when independent substations are set up in buildings or military facilities. It just adds an extra PE line, which increases the project investment cost. In addition, because no current flows through the PE line, the system has strong electromagnetic adaptability. TN-S system can be used as the preferred power supply and grounding system for communication hubs.

5 TN-CS power supply system and grounding method

The TN-C-S power supply system consists of two grounding systems. The front part has four wires, which is the TN-C power supply system; the rear part has five wires, which is the TN-S power supply system. The dividing point is at the connection point of the N line and the PR line. After separation, it is not allowed to be merged again.

This power supply system is generally used in places where the power supply of civil buildings is drawn from regional substations. The TN-C power supply system is used before the household is connected, and it becomes the NS power supply system after the household is connected. At present, it is also common in newly built communications and other facilities.

Since the PEN line of the system has current when it is working normally, the PE line of the system and the metal shell of the electrical equipment connected to the PE line have voltage to the ground, but the PEN line of the system is mostly the system trunk line with low impedance and low voltage to the ground. Therefore, this system grounding method is not suitable as the best power supply system and grounding method for communication hubs.

6 TT power supply system and grounding method

The TT power supply system is usually called a three-phase four-wire power supply grounding system. This system is often used in places where the equipment power supply comes from the public power grid, and is more common in civil suburbs.

Characteristics of TT power supply system: There is no electrical connection between the neutral line N and the protective ground line PE, that is, the neutral point grounding and the PE line grounding are separate, so the shell of the equipment has no direct connection with the grounding of the power supply. That is, the exposed conductive parts of the equipment are not related to the system grounding point, and their respective grounding devices are grounded separately.

The equipment casing is at ground potential and will not generate sparks or arcs, so it is relatively safe. However, when a ground fault occurs, the grounding current needs to flow through the equipment grounding resistance Re and the power supply neutral line grounding resistance Rn. The loop impedance is large, and the fault current is smaller than that of the TN power supply system, which reduces the action sensitivity of the line protection device.

When the system is operating normally, regardless of whether the three-phase load is balanced, when the neutral line N is energized, the PE line is not energized, as shown in Figure 6.

When one phase (line) of the equipment is damaged, voltage will be applied to the equipment housing. At this time, it is unsafe for personnel to touch the neutral point connection line or the equipment housing connected to the neutral line, and the potential of the other two phases relative to the ground will also rise by more than 300V. Therefore, this power supply system must pay special attention to the reasonable configuration of high-sensitivity overcurrent protection devices.

When the phase line touches the shell, because the line resistance is very small, the W-phase voltage is almost entirely added to the two grounding resistors (power supply neutral point grounding resistance Rn, protective grounding resistance Re). According to the grounding resistance regulations, these two resistances must not exceed 4Ω (some areas actually require no more than 10Ω), so the ground short-circuit current value can be obtained by the following formula

I1=U/(Re+Rn)=220/(4+4)=27.5(A)

I2=U/(Re+Rn)=220/(10+10)=11(A)

The corresponding single-phase power is P=Ulcosφ=220×11×0.8=1936(W)

A current of 27.5A can cause a fuse with a rated current of 10A to melt (a fuse can only melt quickly if the current passing through it is more than 3 times the rated current), thus cutting off the power supply. A current of IIA can cause a fuse with a rated current of 4A to melt and cut off the power supply, thus preventing electric shock accidents.

However, for electrical equipment with a fuse rated current greater than 10A, this short-circuit current cannot cause it to melt quickly, so that there is a voltage of 110V on Rn and Re, that is, the metal shell of all electrical equipment connected to the grounding device has a voltage of 110V to the ground. When the human body comes into contact with the metal shell of the equipment, electric shock will occur. Therefore, this system can be used in a small power range, such as when it does not exceed 1kW, it is reliable.

In addition, when the fault current of the system is small, it can be compensated by installing a leakage protection switch to improve the protective grounding function.

As can be seen from the above, protective grounding is applicable to electrical equipment in power supply systems with ungrounded neutral points. For power supply networks with grounded neutral points, protective grounding has limitations. In order to protect electrical equipment, enable fuses and other protective devices to operate reliably, and avoid the risk of electric shock, protective grounding is used when the neutral point is grounded, such as the TN power supply system.

It is worth noting that the same power supply system should be used in a region, and multiple power supply systems should not be mixed at the same time to ensure the safe and reliable operation of electrical equipment.