Faults in power transformers may lead to widespread consequences, both in the form of power failure for a large group of customers and in the form of that the faulty transformer has to be exchanged or at least repaired. Both consequences are troublesome and costly for the supplier of the electrical power. The consequences of unwanted disconnections of healthy equipment, such as power transformers, is also very costly. In the worst case, the unwanted disconnections can result in wider black-outs.
Protection devices such as differential relays normally safeguard power transformers. Differential relays typically have a minimum operating current level set to 30% of the protected power transformer current rating. The set value should be that high in order to prevent unwanted operation of the differential relay due to the On Load Tap Changer (OLTC) that typically is used in modern power transformers. When OLTC moves from one position to another, amplitude mismatch between power transformer winding currents will outcome causing a false differential current. Normally, the range of an OLTC might be around 15% of rated voltage so the contribution of around 15% to differential current may occur.
The set value should also be high enough in order to prevent unwanted operation of the differential relay due to Protection CT (current transformer) errors, or unequalities.
Known transformer differential relays are usually not sensitive enough for low-level internal faults, which may happen for example within a power transformer tank. Power transformer winding turn-to-turn faults belong to such type of internal faults. In the same time, according to available fault statistic, turn-to-turn faults are one of the most common internal faults inside a power transformer.
Traditional power transformer differential relays utilize individual phase currents from different windings of the transformer in order to form the phase-vise differential currents. In modern numerical differential relays these differential currents are usually formed by using mathematical equations, which are dependent on the vector group of the power transformer.
The most common weaknesses of a traditional power transformer differential protection are long operation delays in case of heavy internal faults followed by main CT saturation due to 2nd harmonic blocking feature, and unwanted operations for external faults. They also have bad sensitivity for low level internal faults, i.e. winding turn-to-turn faults, which are thus allowed to develop into more severe faults, involving the power transformer iron core.
Long delays for heavy internal faults, they can be in the order of several tens of milliseconds, are a consequence of the harmonic distortion of the fault currents as they are seen by the differential relay. The harmonic distortion is due to initial heavy saturation of the current transformers under fault condition. Harmonic restrain criterion prevents immediate operation of the differential protection.
Further, power transformer differential protections show a tendency to unwanted operations for faults external to the protected zone with the power transformer, particularly for external earth faults.
Within this area of technology several inventions try to deal with these kinds of problems and some patents have been granted.
As an example U.S. Pat. No. 5,514,978 is a patent that includes measuring of negative sequence impedance by using voltage measurements! The document describe an invention that determines the existence of a turn fault that comprises estimating a current differential by dividing the negative sequence voltage phasor by a characteristic negative sequence impedance and subtracting the result from the negative sequence current phasor, and comparing the estimated current differential with a threshold current differential.
In the present invention measurements of the voltage is not used at all. The present invention is instead based on comparison of negative sequence currents from the different sides of the protected power transformer.
The U.S. Pat. No. 6,507,184 concerns a method and apparatus for differential current measurement in a three-phase power system. This invention is arranged to measure the differential current between a first and a second terminal and to obtain, for each phase, a measure of these currents. This patent does not involve a negative sequence differential protection at all.
The invention according to the U.S. Pat. No. 6,518,767 concerns protection of power transmission lines and includes tripping of the circuit breaker. Similar circuitry is also used for negative sequence current quantities, with the negative sequence preselected values being set substantially lower to produce a more sensitive response to possible faults in the line.
The present invention concerns mainly the technical area of power transformer protection, but can as well be extended as power line protection or a combination of them, while the above patent concerns protection of a power transmission line only. The power transformer according to the present invention introduces phase shift and voltage level difference between power transformer sides. Therefore the negative sequence currents from different power transformer sides have to be first related to each other. After that the negative sequence current differential principle or method is used and a direction comparison is made to protect power transformers against internal short-circuit and ground faults. Above this the present invention as well protect the power transformer against turn-to-turn faults, which are series faults and not a shunt fault as in the patent application U.S. Pat. No. 6,518,767. This turn-to-turn fault protection capability is an important advantage of the present invention. This turn-to-turn fault is the most common, but in the same time the most difficult, fault to detect within a power transformer/autotransformer.
The invention according to U.S. Pat. No. 6,483,680 compares the phase angle difference between fundamental frequency (i.e. 50 Hz or 60 Hz) and second harmonic component (i.e. 100 Hz or 120 Hz) of the power transformer differential currents. More precisely, the invention compare complex ratio between these two phasors. This is done in order to determine if transformer is faulty during energizing (i.e. switch-on) of power transformer. This principle does not detect any internal or external fault during normal operating condition (i.e. through-load condition) of the power transformer, because it can only operate during power transformer energizing from one side. A typically power transformer is switched on just a couple of times per year (i.e. often just once for the yearly inspection). This means that the power transformer is energized and connected on at least two side for most of its life time (typically more than 95%). Therefore U.S. Pat. No. 6,483,680 has very limited use during this short time of power transformer energizing. Additionally this method typically can not detect small faults like turn-to-turn faults even during power transformer energizing.
The patent application WO02/33426 concerns a line differential protection system for a power transmission line. All three phase current values are obtained from both the local end and the remote end of a power transmission line. Comparison elements are arranged to compare the ratio and angel values against preselected values, which establish a restrain region in the current ratio plane. Current values, which result in a ratio outside of the region, result in a tripping of the circuit breaker. Similar circuitry is used for negative sequence current quantities, with the negative sequence preselected values being set substantially lower to produce a more sensitive response to possible faults in the line.
This invention also concerns a differential protection system for a power transmission line. The system is not usable for a power transformer protection system.