Travelling wave pilot protection of a transmission line based on time synchronization

A mechanism for travelling wave pilot protection of a transmission line and method for receiving indications of a travelling wave from two terminals of a transmission line, wherein two terminals are time synchronized. The method includes making a trip decision based on the indications using a short trip window. An arrangement is configured to perform the disclosed method.

TECHNICAL FIELD

Embodiments presented herein relate to travelling wave pilot protection of a transmission line, and particularly to a method and an arrangement for travelling wave pilot protection of a transmission line.

BACKGROUND

Consider a regional power system having a strong internal transmission system transmitting power to another strong regional system on relatively weak Interties. Such a regional power system may experience issues with stability during disturbances, such as short circuits, loss of generation, loss of load, loss of one of the Interties, or any combination thereof. Prevalent practice to the solution of these issues is to include more Interties, increase the voltage to higher voltage levels (such as extra high voltage (EHV) levels or ultra high voltage (UHV) levels), or both. Another approach for better power system stability is to employ protection relays with high operation speed.

Travelling wave protection is one approach for super-high speed protection. There are different types of travelling wave protections, for example, travelling wave pilot protection based on directional comparison, travelling wave current differential protection, travelling wave protection based on distance measurements, etc.

Pilot protection is a practical and reliable mechanism for travelling wave protection. It only needs a small bandwidth channel to transmit binary information between terminals at end points of a transmission line.

One travelling wave protection mechanism is RALDA. Properties of such a travelling wave protection mechanism are, for example, disclosed in U.S. Pat. No. 3,878,460(A). U.S. Pat. No. 3,878,460(A) relates to an arrangement for detecting the direction of a fault from a measuring point. In short, in RALDA the polarities of the first wave fronts of local voltage and current are compared. If the polarities are the same, a backward fault is detected. If the polarities are each others reverse, a forward fault has occurred. Protection relays at a terminal will transmit the fault direction to other terminals. If both directions are forward directions, it means that an internal fault has occurred. Otherwise, it means that an external fault has occurred.

However, the security of travelling wave mechanisms such as RALDA may be influenced by harmonics. Under some conditions, the harmonics may lead to wrong detection of forward faults at both sides of the protected line, and thereby, it may finally lead to mal-trip according to the directional pilot protection principle. So, there is still a need for an improved protection of a transmission line.

SUMMARY

An object of embodiments herein is to provide efficient protection of a transmission line.

The inventor of there herein disclosed embodiments has discovered that communications in existing pilot protection schemes (including RALDA) is unsynchronized. In more detail, this means that unsynchronized binary information is exchanged between different terminals to implement logics for determining whether to block or unblock (also known as forward/backward) in pilot protection schemes. Because the data is unsynchronized and the transmitted signal delay is uncertain, the logics has to hold the blocking/unblocking signal (or forward/backward direction signal) for a long period of time. The long hold time leads to a long open window for making trip decisions. As a result thereof, the probability of mal-trip is increased.

According to a first aspect there is presented a method for travelling wave pilot protection of a transmission line. The method comprises receiving indications of a travelling wave from two terminals of a transmission line, wherein said two terminals are time synchronized. The method comprises making a trip decision based on the indications using a short trip window.

Advantageously this enables secure travelling wave pilot protection.

According to a second aspect there is presented an arrangement for travelling wave pilot protection of a transmission line. The arrangement comprises a processing unit. The processing unit is configured to cause the arrangement to receive indications of a travelling wave from two terminals of a transmission line, wherein said two terminals are time synchronized. The processing unit is configured to cause the arrangement to make a trip decision based on the indications using a short trip window.

According to a third aspect there is presented a computer program for travelling wave pilot protection of a transmission line, the computer program comprising computer program code which, when run on a processing unit of an arrangement, causes the arrangement to perform a method according to the first aspect.

According to a fourth aspect there is presented a computer program product comprising a computer program according to the third aspect and a computer readable means on which the computer program is stored.

It is to be noted that any feature of the first, second, third and fourth aspects may be applied to any other aspect, wherever appropriate. Likewise, any advantage of the first aspect may equally apply to the second, third, and/or fourth aspect, respectively, and vice versa. Other objectives, features and advantages of the enclosed embodiments will be apparent from the following detailed disclosure, from the attached dependent claims as well as from the drawings.

DETAILED DESCRIPTION

As noted above there are issues with existing mechanisms for travelling wave pilot protection.

As will be disclosed hereinafter, the proposed travelling wave pilot protection of a transmission line is based on time synchronization and a short trip window. This will enhance the security.

Hence, there is provided a method for travelling wave pilot protection of a transmission line. The method is performed by an arrangement10a,10b,10c. Reference is now made to the flowchart ofFIG. 5. Parallel reference is made to the power distribution system25ofFIG. 1disclosing an embodiment wherein the herein disclosed invention may be applied.

The power distribution system25comprises at least one arrangement10a,10bfor travelling wave pilot protection of the transmission line20. Reference symbols A and B symbolize the end-points (towards the transmission line20) of two terminals; terminal A, and terminal B, respective. The symbol F denotes a fault along the transmission line20.

Two or more arrangements10a,10bmay be operatively connected via a communications link23. Further, two or more arrangements10a,10bmay be part of a common arrangement10cfor travelling wave pilot protection of the transmission line20. The arrangement10a,10bmay be part of, or comprise, intelligent electronic devices (IEDs) operating as protection relays.

The power distribution system25further comprises power sources21a,21b, current and voltage transformers22a,22b, and circuit breakers23a,23b. Each terminal A, B, may comprise at least one power source, current and voltage transformer, and circuit breaker.

The method comprises, in a step S102, receiving indications of a travelling wave from two terminals A, B of a transmission line20. Each indication may be provided by binary information, such as synchronized binary information, representing travelling wave polarity, fault direction, blocking signal, unblocking signal, and/or trip signal for a fault (F) along the transmission line20. Alternatively, each indication may be the arrival time of the travelling wave at the respective terminal A, B. The two terminals A, B are time synchronized. The terminals A, B may have a combined time synchronization error less than 0.2 ms.

The method comprises, in a step S104, making a trip decision based on the indications using a short trip window. The short trip window may have a length in time corresponding to 2-6 ms, preferably 2-4 ms, which is shorter than the hold time, or trip window, in existing pilot protection mechanisms, for example, 20-50 ms.

Disturbance by harmonics or noise may lead to wrong detection of forward direction faults, which may even cause mal-trip in some cases. Assuming the mal-trip probability by noise or disturbance having a uniform distribution in time domain, a short (exposure) trip window will decrease the risk of mal-trips.

Embodiments relating to further details of the herein disclosed method and arrangement for travelling wave pilot protection of a transmission line will now be disclosed. General references are continued to the flowchart ofFIG. 5and the power distribution system25ofFIG. 1.

According to one embodiment, making the trip decision comprises a step S104bof confirming that the indications from the two terminals relate to the same travelling wave. This may be accomplished by confirming that travelling wave information detected from the two different terminals relate to the same fault disturbance by checking if they occur within the same short trip (time) window. Hence, that the indications from the two terminals relate to the same travelling wave is confirmed only if the indications are received within a certain time distance of each other. This certain time distance corresponds to the length (in time) of short trip window. Conversely, if the indications are received within a time distance from each other that is longer than the short trip window then it is not confirmed that the indications from the two terminals relate to the same travelling wave and hence represent a fault disturbance. The step S104bmay be preceded by a step S104aof time compensating at least one of the indications.

In order to implement the step S104bof confirming, a time counter may be triggered to start counting time once a first indication from one of the terminals A, B is received and triggered to stop counting time once a second indication from the other of the terminals A, B is received. If the time lapsed by the time counter is at most equal to the length of the short trip window then it may be determined that the two indications were caused by the same fault disturbance and indeed received within the short trip window.

There may be different ways to make the trip decision. According to an embodiment the trip decision relates to whether or not to cause a circuit breaker23a,23bto trip. As the skilled person understands, there may be different ways to determine whether or not to cause the circuit breaker23a,23bto trip. For example, with reference to step S104a, a trip decision to cause a circuit breaker23a,23bto trip is only made if the indications of the travelling wave from the two terminals A, B are received within the short trip window.

There may be different types of transmission line to which the travelling wave pilot protection may be applied. For example, as disclosed above, the transmission line20may be part of a power distribution system25. The transmission line20may either be an alternating current (AC) transmission line or a direct current (DC) transmission line.

The fault F may only be detected if the indications from the two terminals are received within the above-specified time interval.

If the fault F occurs at the middle of the transmission line, the arrival times of the travelling waves tAand tB, respectively, at the two terminals A and B are the same. Otherwise, the arrival times will be different.

Consider now the worst condition, namely that the fault F occurs at the beginning or end of the transmission line25to be protected. The largest time difference is:

In AC power systems, most of the transmission lines are shorter than 300 km (i.e., the line length is at most 300 km). This means that the time difference |tA−tB| is less than 1 ms since the speed of light (denoted by the light velocity in the above equation) is about 300000 km/s.

For example, assuming the synchronization error between the two terminals A and B to be less than 0.2 ms, a small trip window (such as 2-6 ms, depending on the line length and dispersion of fault detection time) can ensure indications of two ‘forward directions’ are coming from the same fault. This can decrease the possibility of the mal-trip at a low cost; if the travelling wave pilot protection works with line differential protection function, data communication for the travelling wave pilot protection may share the same time synchronization mechanism which is implemented for the communication of analogue data for differential protection, for example based on communications channels with echo based time synchronization.

A comparison between two trip window arrangements is shown along a time line inFIG. 2. At (a) is shown a segment26of length LDrepresenting delay as caused by uncertain delays of the channel, processing etc., and segment27of length LTW,arepresenting a long trip window for a typical existing pilot protection scheme. At (b) is shown a segment28of length LTW,brepresenting a short trip window as provided for by the present invention based on time synchronization. The segments in (a) and (b) are not necessarily drawn to scale, but in any case, LTW,b<LTW,a. For example, LTW,bmay be an order of magnitude smaller than LTW,a.

As shown inFIG. 2, for existing pilot protection schemes, since the data is unsymmetrical and the time delay is uncertain, a long hold time and long trip window, for example, say 50 ms, must be employed to ensure stable protection logic cooperation between the terminals A, B. For the herein proposed travelling wave pilot protection of a transmission line, which is based on time synchronization, the uncertain delay is removed by the use of time synchronization and assuming a small synchronization error (e.g. smaller than 0.2 ms). The arrival times of the travelling wave at the two terminals A, B are almost identical to each other. Thereby, the trip time window may be very short to ensure the detections of the travelling waves at different terminals are from the same event. For example, as noted above this short window may only be 2-6 ms long.

In existing pilot protection schemes based on unsymmetrical data, the hold time could be as long as 50 ms. Assume that the actual total delay (communication delay, processing delay etc.) is 10 ms. The actual trip window thus is 50−10=40 ms, whilst for the herein proposed travelling wave pilot protection of a transmission line based time synchronization, the trip window may be just 2 ms long, i.e., having a length being 1/20 of that of existing schemes. This means that the mal-trip risk in the proposed scheme also becomes 1/20 compared with existing pilot protection schemes, assuming the same algorithm is used for the two schemes and only the time synchronization aspect is different.

It should also be noted that harmonics and noise may also cause wrong external fault detection, which will block the trip for some time. It may even lead to failure of fault detection (decreased dependability). The synchronization and short trip window will improve the dependability of pilot protection schemes in addition to its contribution to the security of protection.

FIG. 3aschematically illustrates, in terms of a number of functional units, the components of an arrangement10a,10b,10cfor travelling wave pilot protection of a transmission line according to an embodiment. A processing unit31is provided using any combination of one or more of a suitable central processing unit (CPU), multiprocessor, microcontroller, digital signal processor (DSP), application specific integrated circuit (ASIC), field programmable gate arrays (FPGA) etc., capable of executing software instructions stored in a computer program product41(as inFIG. 4), e.g. in the form of a storage medium33. Thus the processing unit31is thereby arranged to execute methods as herein disclosed. The storage medium33may also comprise persistent storage, which, for example, can be any single one or combination of magnetic memory, optical memory, solid state memory or even remotely mounted memory.

The arrangement10a,10b,10cmay further comprise a communications interface32. As such the communications interface32may comprise one or more transmitters and receivers, comprising analogue and digital components for communications with at least one other arrangement10a,10b,10c, at least one of current and voltage transformer22a,22b, and at least one circuit breaker23a,23b.

The processing unit31controls the general operation of the arrangement10a,10b,10ce.g. by sending data and control signals to the communications interface32and the storage medium33, by receiving data and reports from the communications interface32, and by retrieving data and instructions from the storage medium33. Other components, as well as the related functionality, of the arrangement10a,10b,10care omitted in order not to obscure the concepts presented herein.

The arrangement10a,10b,10cmay be implemented in an electronic device. For example, at least the processing unit31may be part of an Intelligent Electronic Device (IED) such as a protective relay, comprised in the arrangement10a,10b,10c. Hence, such an electronic device may be configured to perform any step as herein disclosed. Hence, the indications of a travelling wave from two terminals may be acquired by a protective relay placed at one end of the transmission line20.

In this respect, it should be mentioned that a protective relay is different from a so-called fault locator. A fault locator is used to detect the fault location and is not a real-time device. Further, a fault locator outputs fault location with kilometers. It will never trip circuit breaker. In contrast, a protection relay is a device operating in real-time to detect whether the fault is inside or outside the protected zone. The protection relay is connected to a circuit breaker and will trip the circuit breaker if the fault is inside the protected zone.

FIG. 3bschematically illustrates, in terms of a number of functional modules, the components of an arrangement10a,10b,10cembodied as a travelling wave pilot protection mechanism according to an embodiment. The arrangement10a,10b,10cofFIG. 3bcomprises a number of functional modules; a measurement input module31a, a communications module31b, a fault decision module31c, a time counter module31d, and an output module31e.

The measurement input module31ais configured to receive indications of a travelling wave from two terminals A, B of the transmission line20. The communications module31bis configured to provide the data and information, such as the indications, to the fault decision module31c. The fault decision module31cis configured to, upon reception of the first of these indications, via the communications module31b, trigger the time counter module31dto start counting time. The fault decision module31cis further configured to, upon reception of the second of these indications, via the communications module31b, trigger the time counter module31dto stop counting time. The time counter module31dis configured to, via the communications module31b, provide a result of the counting of time to the fault decision module31c. The fault decision module31cis further configured to, based on the result received from the time counter module31dmake a trip decision. The output module31eis configured to send the trip decision to one or more circuit breakers23a,23b.

In general terms, each functional module31a-31emay be implemented in hardware or in software. Preferably, one or more or all functional modules31a-31emay be implemented by the processing unit31, possibly in cooperation with functional units32and/or33. The processing unit31may thus be arranged to from the storage medium33fetch instructions as provided by a functional module31a-31eand to execute these instructions, thereby performing any steps S102, S104, S104a, S104bas disclosed above.

FIG. 4shows one example of a computer program product41comprising computer readable means43. On this computer readable means43, a computer program42can be stored, which computer program42can cause the processing unit31and thereto operatively coupled entities and devices, such as the communications interface32and the storage medium33, to execute methods according to embodiments described herein. The computer program42and/or computer program product41may thus provide means for performing any steps as herein disclosed.