Guided reading

It is well known that there are many ways to enhance the short circuit withstand capability of a transformer, such as material improvement, design optimization and improvement, and good maintenance during operation, etc. However, the methods outlined in this article from Shanxi Hanzhong china transformer manufacturer are aimed at those transformers that are difficult to change or very costly to change.

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01 Installation of neutral point electric reactor

In general, the possibility of a single-phase short-circuit accident in the power system is much higher than that of a three-phase short-circuit accident in the power system.

Tips: enhancing the short-circuit withstand capability of the transformer can greatly reduce the damage of short-circuit accidents.

The single-phase short-circuit current is affected by the positive sequence impedance and the zero sequence impedance. An effective way to change the zero sequence impedance is to change the grounding method of the neutral point of the transformer, or install a neutral grounding electric reactor.

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Neutral grounding reactor

Neutral grounding reactors are used for low-impedance grounding of the neutral point of a three-phase network to limit fault currents during short-circuits between phases. (The fault current will be limited to the level of the phase-to-phase short-circuit current).

One reactor terminal is connected to the neutral of the network and the other terminal is grounded.

During normal operation of the power system, the current flowing through the reactor is almost zero because it is driven only by the unbalance of the three-phase network.

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Neutral point reactor for three-winding transformer

A common installation location for neutral point reactors is the compensation device called a high-voltage shunt reactor.

In China, star connection is usually used for high-voltage shunt reactors, and then a reactor is connected in series at the neutral point of the star connection.

Tips: This is the so-called "high voltage reactor is grounded through the small reactance of the neutral point".

The function of the reactor is to compensate the phase-to-phase and ground capacitors, speed up the termination of the secondary arc current, and make it easier to use a single-phase recloser.

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Grounding transformer earthing types – voltage at ground fault

To give an application example in 2004, located in Ningbo City, Zhejiang Province, China, engineers installed a small 15Ω reactor as a neutral grounding reactor for a 500kV transformer in Lanting substation, reducing the short-circuit current.

Details are shown in Table 1: Table 1 – Short Circuit Current Affected by Neutral Reactor in Lanting Substation

When the neutral point is connected to the reactor, the zero sequence impedance will change.

"The double-phase ground short-circuit current may be greater than the single-phase ground short-circuit current. Therefore, after installing the neutral reactor, it is necessary to check the single-phase and double-phase short-circuit."

02 Installation of the current limiting series reactor

A series reactor is a kind of high-voltage electrical equipment, which is mainly used to limit the short-circuit current and maintain sufficient voltage on the busbar of the distribution switchgear when a short-circuit occurs in the network.

It consists of an inductive coil, and this reactor is also used to compensate for reactive power, thereby improving the transmission capacity of the power line.

The use of reactors is a traditional and commonly used method of limiting short-circuit current, usually installed in areas where short-circuit faults may occur, and in series in circuits that need to limit short-circuit current.

Tips: Its principle is to reduce short-circuit current by increasing the impedance of the circuit. The advantage of this is that it is easier to install and operate in a safe and reliable way. The disadvantage is that the reactor will increase the power loss, so it may affect the stability of the power system.

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Core and Coil Assemblies for 11-MVAr Series Reactors (35 kV) with Step Switches (Siemens)

Current-limiting series reactors are usually used at the outlet on the low-voltage side, but can also be used on the 35 kv transformer medium-voltage side in a 220-kV transformer.

This method is suitable for three-phase short circuits and single-phase short circuits.

Take an example of ABB's Metro Grid project: they built a series reactor to transmit electricity from Sydney South to the Haymarket substation in Australia, which is the largest reactor ABB has ever produced.

03 Installation of large capacity quick switch

Typical representatives of high-capacity quick switches are Is-limiter (manufactured by ABB), Pyristor (manufactured by Ferraz) and C-Lip (manufactured by G&W).

Tips: this switch can protect electrical equipment from larger short-circuit current and prevent large-scale power outages caused by overcurrent damage to the main equipment.

It has several advantages in the technical field:

1) Fast cut-off capability (less than 2ms); 2) Quickly limit large short-circuit current: Is-limiter can detect and limit short-circuit current at the first rise (ie, within 1ms) 3) Small space usage 4) Easy to install and maintain

Additionally, it is a fault current limiting device that uses chemical charge and current limiting fuses to interrupt the fault current from the first quarter to the second half cycle (i.e. before the first peak).

In a typical Is-limiter design, the device consists of two current paths connected together in parallel. One path is a component with a full load current rating (with a high continuous current rating, say 3000 A) and the other path is a current limiting function via a current limiting fuse (usually 300 A with a continuous current rating < 15 kV ).

How it works can be described as follows: when a short circuit occurs, the current transformer module detects this signal and transmits it to the control module, which is then triggered and opens the disconnect switch to immediately "move" the short circuit current to in the fuse module. Here, the current is cut off.

Interestingly, the device uses explosives to obtain a fast shut-off capability that can limit both single- and three-phase short-circuit currents.

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Is-limiter insert holder for 12 kV, 2000 A inserts

The working process can be described as follows: 

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Is-limiter current limiting process

04 Installation of controllable fault current limiter

A fault current limiter (FCL) is also known as a short circuit current limiter (SCCL). There are several different types: superconducting fault current limiters (SFCL) and controllable fault current limiters based on electronic technology etc.

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Superconducting FCL made in China (35 kV/90 MVA)

The disadvantages of superconducting fault current limiters are:

1) The working environment is very harsh: liquid nitrogen (N2) is required for high temperature superconductors. The critical temperature is 77 K (about -196 °C). And cryogenic superconductors require liquid nitrogen and helium (He). The critical temperature is 4 K (about 269.15 °C). Once the operating temperature exceeds the critical temperature, the SFCL cannot maintain superconducting properties.

2) The technology is not mature enough and the number of SFCLs operating in Switzerland, Germany, UK and USA is less than 20.

Therefore, this is not suitable for modifying a running transformer.

However, electronically based controllable fault current limiters are more mature than SFCLs.

Tips: there are two types: series and parallel. How it works: use electronics to break or connect circuits at high speed. Then, capacitors, resistors or inductors in series or parallel at the bypass act immediately to increase the impedance of the circuit, thereby limiting short circuit current.

See the example below: Schematic of series controllable FCL

The controllable FCL shown in the figure above is a series type. It consists of a capacitor (C), an inductor (L) and a bypass switch (K).

Typically, the switch is inactive and open, and L and C operate at series resonance. This impedance can be considered zero compared to the total impedance. So the impact of FCL is completely acceptable.

Tips: when an accident is detected, K receives the command and shuts down to "take away" C immediately. The inductor (L) starts to limit the current at this point, thus limiting the fault current.

The advantage is:

1) No effect on the protection solution used 2) No effect on the stability of the current 3) Small space usage. Disadvantages are high cost and maintenance.

05 Contrast

The following table is a comparison of the above 4 methods:

Table 2 – Comparison of Methods for Enhancing the Short-circuit Withstand Capability of Transformers

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06 Write at the end

In practice, the risk of a three-phase short-circuit accident on the medium-voltage side of 500 kV and 220 kV transformers is very small.

The main risk is a single-phase accident. Enhancing the short-circuit withstand capability of a single-phase or limiting the short-circuit current of a single-phase can significantly reduce the occurrence of transformer short-circuit accidents.

Tips: In Table 2 above, we can find that new equipment can work more efficiently and respond faster, but they are beaten by traditional equipment in terms of economy, reliability, maturity, and maintenance experience.

So the best option is to install a neutral point reactor. If three-phase protection is really required, we can install a quick switch or FCL.

For low-voltage side accidents, you can choose to install a current-limiting series reactor, and if space is limited, you can choose a quick switch.

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