Transformer

DESCRIPTION

The present invention concerns a device comprising a control- lable grounding transformer arranged to compensate for a residual operating current in a grounding of an alternating-current network having a power transformer according to the preamble of claim 1. The invention further relates to a method of producing a zero-point voltage in an alternating-current network by means of a power transformer according to the preamble of claim 7.

THE BACKGROUND OF THE INVENTION AND PRIOR ART

Today, transfer of electric power is mainly performed by means of a three phase voltage system. To fulfil the basic demands of the system - production and consumption of power must be in balance at every instance - large national transmission networks have been created, where all producers and consumers of electric power are connected. Then, connecting the national transmission networks to transnational transmission networks provides additional advantages with respect to the basic demand. One such transmission network is for example the Scandinavian NORDEL network.

To reduce the transmission losses in the transmission networks, the transport of energy is effected at high voltage, preferably 400 kV. The energy output to the consumers from the transmission network is carried out via distribution transformers, which, in turn, each supplies a limited geographical area. The distribution is mainly effected at 10-30 kV voltage. Large industry customers may here be coupled directly to the distribution network, while smaller consumers and households are supplied over yet another transformation to 400/230 V. Due to its mesh structure, the transmission network has a high availability. The distribution networks, however, that have a radial network structure, constitute the weak link in the transmission system. Errors occurring in an individual distribution line may lead to disconnection of large groups of consumers. The error protection technology therefore aims at improving the availability of the distribution networks.

In this context, it is important to note that the majority of the electric faults occur as the result of isolation break-through between one phase and earth - a so-called grounding error, when the actual transmission of energy is limited to the voltage system between the phases. From a protection point-of-view it would be desirable to treat the grounding errors separately, and if possible to limit the residual operating current to such an extent that line disconnection can be avoided.

The most successful protection concept is based on this basic principle developed already in 1917 by Waldemar Petersen. By connecting the zero point of the three-phase system to ground over an inductance having a matched network capacitance - the so-called Petersen coil - the residual operating current can be reduced by a factor ten to fifty. The current limitation is normally sufficient to guarantee the self-quenching of single-phase arcs, which represent the main part of the disturbances in air-line networks.

Today, Petersen's resonance grounding dominates in the Scandinavian and other European distribution networks. These networks have an overall availability that is superior to other comparable distribution networks having alternative concepts for system grounding.

In the ongoing conversion of the distribution networks from airline networks to underground cable networks the residual operating currents increase by a factor 30 to 50 due to the higher capacitance of the cable, which affects Petersen's resonance grounding twofold: on one hand, the self-quenching effect de- creases in the remaining air-line part as the residual operating currents increase, to eventually stop completely; and on the other hand, the self-quenching effect does not work at all in a cable network due to the short distance between the live conductor and ground (the shield). Thus, the problem is the non- compensated for residual current.

This problem was discovered and solved in the begi nning of the 90's. The device for residual current compensation developed at that point in time is used today as a complement to the Petersen coil, however has also been applied in distribution networks which up until then were designed based on completely different grounding concepts.

In contrast to the Petersen coil, which exclusively increases the source impedance in the grounding circuit, the residual current compensation eliminates the driving voltage in connection with ground faults by superimposing a counter voltage. This is not a completely trivial task, since the driving voltage is only partly known initially.

According to Thevenin's theorem, the fault current is determined by the driving voltage at the location of the fault, the fault impe- dance and the source impedance. Conversely: to achieve the complete elimination of the driving voltage at the location of the fault, it is required that the driving voltage be eliminated at the location of the fault, while the fault impedance is unknown and the source impedance can only be partly influenced via the Petersen coil.

The driving voltage at the location of the fault is represented by the phase voltage of the feeding distribution transformer in the phase which is impaired by the grounding fault (known and measurable respectively), plus the load-current depending volta- ge drop between the feeding transformer and the actual location of the fault, which may be far out in the (unknown) network.

Determining the latter became possible by development of a new measurement procedure, which was first published in the beginning of the 90's (see: Winter, K. , "Swedish Distribution Networks - A New Method for Earthfault Protection in Cable- and Overhead Systems", 5 ^th International Conference on Power System Protection. IEE conference publication No. 368, York/UK 1993).

The remaining problem, to generate a counter voltage with a possibility to control amplitude and phase angle in relation to the phase voltage of the distribution transformer, was solved by means of power electronics (pulse width modulated inverter). Today, there are reference plants for residual current compens- ation in power networks of voltage levels between 6 kV and 1 10 kV.

The power demand for residual current compensation depends on the voltage level, the size of the network and attenuation relationships. The distribution networks in the dense inhabited areas of the world may attain capacitive groundi ng currents in excess of 1000 A with uncompensated residual operating currents i n the order of 100 A and more. The power demand for compensating the residual operating current in such a network may be far more than 1000 kVA.

Inverters of this magnitude are relatively costly. The present invention proposes a simpler device for complete compensation of the residual operating current in order to lower the costs and improve the reliability.

SUMMARY OF THE INVENTION

The object of the present invention is to solve the above problem and offer a simpler device for compensating for the residual operating current. Another object is to eliminate the residual operating current completely. Yet another object is to improve the reliability of the compensation device and thereby the reliability of actual power transmission. Yet another object is to lower the costs for the residual operating current compensation.

The objects are attained by means of a device defined in the preamble of claim 1 , which is characterized in that a primary winding of the grounding transformer is coupled to the power network or a power source synchronized to the power network and a secondary winding of the grounding transformer is coupled between a zero point of the power network and ground, wherein the grounding transformer comprises two or more tap- changers and the device includes a unit configured to control the tap-changers for controlling secondary voltages of the grounding transformer with respect to amplitude and phase angle in relation to the voltage system of the feeding power transformer.

The advantage of the invention is that the number of line disconnections decreases or can be avoided completely. Thus, by means of the new device, the availability of the distribution network improves. The new device can be manufactured at low cost and is relatively simple to install in existing distribution networks.

According to one embodiment of the invention, the grounding transformer operates in three phases.

The device according to the invention may be used in different power networks having a single or three phase system.

According to another embodiment of the invention, said unit is a control and measurement unit coupled to the grounding transformer in order to control said secondary voltages between the zero point of the power network and ground. The device according to the i nvention may thereby be used together with known control and measurement units for compensation of the residual operating current.

According to yet another embodiment of the invention, the control and measurement unit is arranged to decide which of the se- condary voltages that is to be coupled between the zero point of the power network and ground. According to one embodiment of the invention, the control and measurement unit is arranged to control the secondary voltage of the grounding transformer until a condition for a full compensation for the residual operating current has been fulfilled. The device according to the invention may thereby be used together with known control and measurement units for complete compensation for the residual operating current.

According to another embodiment of the invention, the device is arranged to be used in parallel with a Petersen coil in order to eliminate a non-compensated for residual operating current from the Petersen coil.

The device according to the invention may improve the effect of an existing compensation device by being installed in parallel with a Petersen coil device. The objects are also attained by means of a method defined in the preamble of claim 7, which is characterized by using a device comprising a grounding transformer a primary windi ng of which is coupled to the power network or a power source synchronized to the power network and a secondary windi ng of which is coupled to between a zero point of the power network and ground, wherein the grounding transformer comprises two or more tap-changers and the device includes a unit configured to control the tap-changers, and controlling the secondary voltages of the grounding transformer in respect of amplitude and phase angle in relation to the voltage system of the feeding power transformer.

The method according to the invention improves the reliability of the compensation device. The availability is likewise improved.

According to one embodiment, said unit is a control and measu- rement unit coupled to the grounding transformer in order to control said secondary voltages between a zero-point of the power network and ground. The objects are also attained by means of a method of producing a zero-point voltage in an alternating current power network by means of a power transformer, characterized by a device comprising a grounding transformer with a primary winding coupled to the power network or a power source synchronized to the power network and a secondary winding coupled between a zero point of the power network and ground, wherein the grounding transformer comprises two or more tap-changers and a unit configured to control the tap-changers, and said unit is a control and measurement unit coupled to said grounding transformer to control a secondary voltage of the grounding transformer between the zero-point of the power network and ground, wherein

- the control and measurement unit regulates the secondary voltage with respect to amplitude and phase angle in relation to the voltage system of the feeding power transformer,

- the control and measurement unit decides which of the secondary voltages that is to be coupled between the zero point of the power network and ground, and

- the control and measurement unit regulates the secondary vol- tage until a condition for a full compensation for the residual operating current has been fulfilled.

According to one embodiment of the invention, the grounding transformer operates in three phases.

The objects are also achieved by using the above device for compensating for a residual operating current.

The objects are likewise achieved by using the device according to the above, or the method according to the above, in parallel with a Petersen coil to eliminate a non-compensated for residual operating current from the Petersen coil. BRI EF DESCRI PTION OF TH E DRAWI NGS

Figure 1 shows a single-line diagram over a distribution network havi ng a known device for compensating for the residual operating current.

Figure 2 shows the same distribution network having a device for compensating for the residual operati ng current according to the i nvention.

Figure 3 shows a vector diagram for the driving voltage dependi ng on the grounding error location in the distribution network.

Figures 4a, b show the circuit diagram and vector diagram

transformer according to the invention.

DETAI LED DESCRI PTION THE I NVENTION

Figures 1 and 2 show a distribution network with a control and measurement unit 2 for detection of grounding errors and control of a compensation device. Further, a feedi ng transformer 3, a collector rail 4 and a number of output distribution lines Li , l_2 - LN with a respective line switch 9 at the collector rail 4 are shown. Electric power transmission is usually effected by means of a three phase system , however also one and two phase systems (railway operation) exist. The common denominator here is that the transmission of the payload energy (to the consumers) is driven exclusively by the voltages between the phases. If an error occurs in the form of an isolation break-through between these phases, the line i n question is disconnected and consumers connected thereto via the line switch 9.

However, the majority of electric faults occur between a si ngle phase and ground (so-called grounding errors). Thereby, the entire system is shifted in relation to ground. Nevertheless, the voltages between the phases - which drive the payload energy - is not affected. The error current at the actual location of the fault is determined by the other currents in the network (mainly capacitive, however also resistive discharge currents). The sum of these discharge currents to ground - i ncludi ng the current at the location of the fault - is always zero (Kirchhoff's current law). This follows from the fact that the current at the location of the fault may only be zero if the sum of all other currents is zero.

The purpose of the compensation device i n both Figure 1 and 2 is to create this balance by producing a corresponding current between the zero point of the power network and ground.

Figure 1 thereby shows a known device for compensating for the residual operating current consisting of a zero-point reactor 5 with an auxiliary power winding 6 and an inverter 7 coupled thereto for compensation for the residual operating current. One spare means for disconnecting the line 8, e.g. in case of a fault in the compensation device, is also shown .

Figure 2 shows a device for compensating for the entire residual operating current by means of a controllable grounding transformer 1 according to the invention. The control and measurement unit 2 continuously measures zero sequence admittance YO on the output lines Li _-N and in the groundi ng connection of the compensation device E. The latest measures are stored i n the memory of the measurement unit. If a grounding error is detected, the measurement is repeated. Thereafter, YO values before and after the ground error instant are compared, line for line.

The ground error exists on the line which shows a deviation ΔΥ0 from its error-free measure. The deviation is used to control the compensation device. I n Figure 1 it is the known device with an inverter 7 and in Figure 2 it is the controllable grounding transformer 1 according to the invention . In both cases, a voltage UEN is generated between the zero point N of the power network and ground E. When the voltage UEN corresponds to the driving voltage at the location of the fault the deviation ΔΥ0 is reduced and thereby the residual operating current to zero (Thevenin's theorem).

Figure 3 shows how the driving voltage depends on in which phase Uu , Ui_2 or UL3 the fault has occurred and the location of the fault A, B or C in the distribution network. The driving voltage is represented by the phase voltage of the power transformer (in Figure 3 exemplified by the phase voltage Uu ) and the voltage drop over the line (l _x x Z _x ) between the power transformer and the location of the fault. The voltage drop over the line, in turn, is determined by the line's own impedance Z _x and current load l _x , which as a rule has the same magnitude in all phases.

As is shown in Figure 3, the compensation device - in order to cancel the driving voltage at the location of the fault - must create a zero-point voltage which can be adjusted sufficiently with respect to amplitude and phase angle in relation to the voltage system of the power transformer.

Figure 4 shows a controllable three phase grounding transfor- mer 1 according to the invention, which can generate such a zero-point voltage.

The transformer 1 has two tap-changers 10a, 10b by means of which three secondary voltages can be controlled with respect to amplitude and phase angle a in relation to the voltage system of the feeding power transformer 3. The transformer may have three, four or more tap-changers. When a control and measurement unit 2 is used, the control and measurement unit 2 decides, in case of a ground fault, which of the secondary voltages that is to be coupled between the zero point of the grounding transformer and ground. Thereafter, the secondary voltage of the transformer is controlled until a condition for a full compensation for the residual operating current (ΔΥ0 = 0) is fulfilled.

Figure 4b shows a vector diagram for the grounding transformer in Figure 4a. The delta-coupled primary winding generates three voltages shifted 120 ^° to one another in the nine secondary windings. By serial coupling, via two three-phase on-load tap- changers, each of three secondary windings 1 , 2 and 3 can be connected to one another, such that three resulting voltages are created, which, in turn, can be controlled with respect to amplitude and phase in relation to the voltage system of the feeding power transformer.

The device may be used for complete compensation for the entire residual operating current. Alternatively, the device may advantageously be used also exclusively for residual current compensation in networks with existing Petersen-coil grounding.

The invention is not restricted to the described embodiments in the figures, but may be modified within the scope of the claims.