WO2014121438A1

WO2014121438A1 - Method and apparatus for current differential protection for uhvdc transmission line

Info

Publication number: WO2014121438A1
Application number: PCT/CN2013/071391
Authority: WO
Inventors: Xiaodong ZHENG; Nengling TAI; Guangliang Yang
Original assignee: Alstom Technology Ltd.
Priority date: 2013-02-05
Filing date: 2013-02-05
Publication date: 2014-08-14

Abstract

Methods and apparatus for current differential protection for UHVDC transmission lines are disclosed. A current differential protection system for a DC transmission line may comprises: a first measurement unit, configured to measure a first current flowing through a first terminal of the DC transmission line; a second measurement unit, configured to measure a second current flowing through a second terminal of the DC transmission line; and a protection unit, configured to remove first and second equivalent shunt branch currents of the DC transmission line respectively from the first and second currents to generate corrected first and second currents; to calculate a differential current based on the corrected first and second currents; and to compare the differential current with a predetermined current setting value to determine whether an internal fault occurs on the DC transmission line.

Description

METHOD AND APPARATUS FOR CURRENT DIFFERENTIAL PROTECTION

FOR UHVDC TRANSMISSION LINE

FIELD OF THE INVENTION

[0001] The present invention generally relates to High Voltage Direct Current (HVDC) transmission line protection, and more particularly, to current differential protection for UHV/EHV (Ultra High Voltage/Extra High Voltage) DC transmission lines.

BACKGROUND

[0002] A HVDC power transmission system uses direct current for the bulk transmission of electrical power, in contrast with the common High Voltage Alternating Current (HVAC). For long-distance transmissions, HVDC systems may be less expensive and suffer lower electrical loss. For underwater power cables, HVDC avoids the heavy currents required to charge and discharge the cable capacitors each cycle. For shorter distances, the higher cost of DC conversion equipments compared to an AC system may still be warranted, due to other benefits of direct current links.

[0003] Recently, the number of HVDC systems is increasing rapidly throughout the world, due to their advantages for long distance and large capacity power transmission, asynchronous interconnections, and their ability to prevent inadvertent loop flows in an interconnected AC system. These definite technical and environmental advantages make HVDC transmission systems more attractive than HVAC systems in power system projects. Over the last two decades, HVDC transmission systems developed more rapidly than HVAC transmission systems around the world.

[0004] Since the Zhou Shan HVDC Project was inaugurated in 1987, many HVDC projects have been put into operation in China. Most of them have ±500 kV voltage levels, exemplified by the Three Gorges-Changzhou HVDC power project, a typical conventional HVDC system in China. The HVDC line provides a transmission passage for the delivery of the green power from the Three Gorges Dam to the East China- Shanghai area. Nowadays, HVDC plays an important role in "West-East Power Transmission" and "Nationwide Power Grid Interconnection" in China. [0005] HVDC transmission lines are usually employed for interconnecting regional power networks, and thus their security and reliability not only contribute to the stability of the local system, but also to the stable operation of the connected regional power networks and even the whole power network. HVDC transmission lines are significant elements in the DC transmission system, and typically have a high faulty probability due to long distances. Consequently, it makes sense to the security and reliability of the DC transmission system if the performance of the relay protection for the DC transmission lines can be improved.

[0006] Current differential protection is one of the most important protections for a HVDC transmission line, and has been widely used in the practical projects. Theoretically, in the case of normal operation and external faults (i.e., faults occurring outside of the transmission line), the current differential protection for the DC transmission line will not be triggered. But for a long distance HVDC transmission system, currents caused by distributed capacitance are generated in the DC transmission line, and might lead to mal-operation of the current differential protection.

[0007] In fact, mal-operation of the current differential protection stems from the undesired interfering signals at two terminals of the DC transmission line. With such undesired signals, the instantaneous voltages and currents cannot reflect the real-time system conditions. So the current differential protection based on the instantaneous voltages and/or currents may be inadvertently triggered under external faults. To avoid the mal-operation of current differential protection, a more accurate measurement scheme is desired.

[0008] A Chinese Patent Application Publication CN101577417B discloses a current differential protection method for a HVDC transmission line. This current differential protection (CDP) principle for a HVDC transmission line is proposed by analyzing the fault characteristics of the DC transmission line. Based on the distributed parameter model, the current at any point of the transmission line can be calculated with the voltages and currents measured at local and remote conversion stations. When a fault occurs on the HVDC line, the sum of the currents flowing into any point on the HVDC line as calculated with each station's measurement data depends on the fault current and can reach maximum at the fault point. When no fault occurs or a fault occurs beyond the HVDC line, the sum of the currents flowing into any point on the HVDC line is small (near to zero in theory). According to this feature, CDP can be implemented. Compared with the known traveling wave protection, CDP can effectively identify internal or external fault of the transmission line and can work in all fault situations, and it features high reliability, low sampling frequency and simple algorithm. Compared with the traditional CDP used in a HVDC line, it is independent of the distributed capacitance over the long transmission line, and thus it can operate well in the transient situation and has a high speed. The simulation results show that this method can identify internal or external faults reliably and rapidly.

[0009] In the prior art CDP as mentioned above, a mode transformation matrix must be available for the decoupling of HVDC lines. This algorithm is very complex and could not remove the effects of transient harmonic currents and shunt branch currents in case of a long distance HVDC transmission system. Under external faults, the unbalanced current will lead to the mal-operation of the current differential protection and reduce the reliability of the current differential protection. To avoid this problem, the current setting value needs to be high, which may in turn result in a CDP failure under an internal fault with a high resistance. Thus, the sensitivity of current differential protection is compromised.

SUMMARY

[0010] With respect to the disadvantages of the existing current differential protection for DC transmission lines, this invention addresses the above mentioned problems by removing the effects of shunt branch currents and transient harmonic components in long distance UHVDC transmission lines.

[0011] In one embodiment, a method for current differential protection for a DC transmission line is disclosed, which may comprise: measuring a first current flowing through a first terminal of the DC transmission line; measuring a second current flowing through a second terminal of the DC transmission line; removing first and second equivalent shunt branch currents of the DC transmission line respectively from the first and second currents, to generate corrected first and second currents; calculating a differential current based on the corrected first and second currents; and comparing the differential current with a predetermined current setting value to determine whether an internal fault occurs on the DC transmission line. [0012] In an embodiment, it is determined that an internal fault occurs on the DC transmission line if the differential current is larger than the predetermined current setting value. The method may further comprise: triggering a current differential protection if it is determined that an internal fault occurs on the DC transmission line.

[0013] In an embodiment, the method may further comprise: measuring a first voltage at the first terminal of the DC transmission line; measuring a second voltage at the second terminal of the DC transmission line; and calculating the first and second equivalent shunt branch currents based on the first and second voltages and an equivalent shunt admittance of the DC transmission line.

[0014] In an embodiment, the first equivalent shunt branch current is calculated by multiplying a first fraction (1/a) of the equivalent shunt admittance with the first voltage, and the second equivalent shunt branch current is calculated by multiplying a second fraction (1/b) of the equivalent shunt admittance with the second voltage, wherein a > 1 and b > 1. For example, a =2, and b = 2.

[0015] In an embodiment, calculating the differential current further comprises: averaging the corrected first current in a first fundamental period of the first current to generate a first average current; averaging the corrected second current in a second fundamental period of the second current to generate a second average current; and calculating the differential current based on the first and second average currents.

[0016] In an embodiment, averaging the corrected first current further comprises integrating the corrected first current in the first fundamental period and dividing the integrated first current by the first fundamental period, and averaging the corrected second current further comprises integrating the corrected second current in the second fundamental period and dividing the integrated second current by the second fundamental period.

[0017] In an embodiment, the method may further comprises: calculating a restraining current based on the first and second average currents; and triggering a current differential protection to stop a power transmission over the DC transmission line if the differential current is larger than the predetermined current setting value and if a ratio of the differential current to the restraining current is higher than a predetermined threshold ratio. [0018] In another embodiment, a current differential protection system for a DC transmission line is disclosed, which comprises: a first measurement unit, configured to measure a first current flowing through a first terminal of the DC transmission line; a second measurement unit, configured to measure a second current flowing through a second terminal of the DC transmission line; and a protection unit, configured to remove first and second equivalent shunt branch currents of the DC transmission line respectively from the first and second currents to generate corrected first and second currents; to calculate a differential current based on the corrected first and second currents; and to compare the differential current with a predetermined current setting value to determine whether an internal fault occurs on the DC transmission line.

[0019] In yet another embodiment, an apparatus for current differential protection for a DC transmission line is disclosed, which may comprise: means for measuring a first current flowing through a first terminal of the DC transmission line; means for measuring a second current flowing through a second terminal of the DC transmission line; means for removing first and second equivalent shunt branch currents of the DC transmission line respectively from the first and second currents, to generate corrected first and second currents; means for calculating a differential current based on the corrected first and second currents; and means for comparing the differential current with a predetermined current setting value to determine whether an internal fault occurs on the DC transmission line.

[0020] The invention can eliminate the effects of both transient harmonic currents and shunt branch currents in a long distance UHV/EHV DC transmission line, and thus provides high accuracy and reliability for current differential protection.

BRIEF DESCRIPTION OF THE DRAWING(S)

[0021] FIG. 1 is a schematic diagram of an exemplary two-pole UHVDC transmission system according to an embodiment of the invention.

[0022] FIG. 2A is a schematic diagram of a distributed parameter model of a uniformly distributed DC transmission line according to an embodiment of the invention.

[0023] FIG. 2B is a schematic diagram of an equivalent pi (π) model of a uniformly distributed DC transmission line according to an embodiment of the invention. [0024] FIG. 3A is a diagram of differential current responses with/without a shunt branch under external AC system fault at rectifier side.

[0025] FIG. 3B is a diagram of differential current responses with/without a shunt branch under external AC system fault at inverter side.

[0026] FIG. 4 is a diagram of total harmonic currents under three typical faults of a DC transmission line.

[0027] FIG. 5 is a schematic flowchart of a method for current differential protection for DC transmission lines according to an embodiment of the invention.

DETAILED DESCRIPTION

[0028] The technical schemes of the present invention will be illustrated in further details below in conjunction with the drawings and embodiments.

[0029] FIG. 1 is a schematic diagram of an exemplary two-pole UHVDC transmission system 100 according to an embodiment of the invention. In an embodiment, the UHVDC transmission system 100 includes a rectifier side 1 10 for converting AC power supply to DC power, a pair of transmission lines 120a and 120b for DC power transmission, and an inverter side 130 for receiving the DC power from the transmission lines 120a and 120b and converting it to AC power. In the configuration as shown in FIG. 1, the transmission line 120a may be a positive DC transmission line, while the transmission line 120b may be a negative DC transmission line. For purpose of explanation, the rectifier side 110 is coupled to the transmission lines 120a and 120b at terminals M, and the transmission lines 120a and 120b are coupled to the inverter side 130 at terminals N. The rectifier side 110 may include rectifiers 111 to convert AC power supply to DC power, and DC filters 112 to eliminate the characteristic frequency harmonics, e.g., 12th, 24th and 36th harmonics. Similarly, the inverter side 130 may include DC filters 132 to eliminate the characteristic frequency harmonics and converters 131 to convert the received DC power to AC power. The rectifier side 110 and inverter side 130 may have other components as desired (e.g., converter transformers and smoothing reactors), and may be alternatively configured as known to those skilled in the art. [0030] The UHVDC transmission system 100 further includes a plurality of measurement units 121a, 121b, 121c and 121d, wherein each measurement unit is capable of measuring an instantaneous voltage and current at a respective point of a transmission line. For example, the measurement unit 121a can measure a voltage U_Mp at terminal M of the transmission line 120a and a current I_Mp flowing through terminal M of the transmission line 120a, and the measurement unit 121b can similarly measure a voltage U_Np and a current I _p at terminal N of the transmission line 120a. Voltages and currents at terminal M (U_MII and Ι_ΜΠ) and terminal N (U_NII and Ι_ΝΠ) of the transmission line 120b can be similarly measured by the measurement units 121c and 12 Id, respectively.

[0031] Protection units 122a and 122b at two sides of the DC transmission lines 120a and 120b can monitor in real-time the respective currents and voltages of the DC transmission lines 120a and 120b as measured by the respective measurement units 12 la- 12 Id, and communicate their measurements to each other, e.g., in wire or wireless connections. Each of the protection units 122a and 122b determines if the measured currents and/or voltages indicate abnormal, and if so, triggers a protection action at the corresponding rectifier side 110 and/or inverter side 130, for example, the associated circuit breakers is triggered to stop the power transmission.

[0032] Although FIG. 1 shows a two-pole UHVDC transmission system, those skilled in the art can understand that, one-pole UHVDC transmission system can be similarly implemented, with one transmission line (e.g., 120a) being used for power transmission. In this case, voltages and currents at two terminals of the transmission line can be measured (e.g., by measurement units 121a and 121b), and the protection units (e.g., 122a and 122b) at two sides of the DC transmission line can act in response to the measurements similarly. Other alternatives and modifications to such UHVDC transmission system are apparent to those skilled in the art. Further, a UHVDC transmission system according to the present invention may be applicable for UHVDC voltages higher than ±600 kV, as well as HVDC voltages lower than ±600 kV.

[0033] In one embodiment, the protection units 122a and 122b may operate in the form of current differential protection. Current differential protection works on the basic theory that the sum of the currents entering and exiting a node will equal to zero. Conventionally, a current protection relay (or a similar protection unit) compares the currents at two ends of a power line (vectors I_M and I_N, respectively) to calculate a difference between the two currents as a differential current I_DCL for DC differential protection:

I 1

[0034] In a normal condition, the current entering the transmission line (I_M) is equal to the current exiting the power line (I_N, with a sign opposite to that of I_M), thus the difference I_DCL equals to zero. If a path to earth or ground develops on the transmission line, the two currents I_M and I_N no longer cancel each other completely, but give a non-zero difference I_DCL- If the difference I_DCL exceeds a predetermined current setting value, the relay will trigger the associated circuit breakers to cut off the power transmission.

Shunt Branch Current of DC Line

[0035] FIG. 2A is a schematic diagram of a distributed parameter model of a uniformly distributed transmission line according to an embodiment of the invention. In particular, FIG. 2A illustrates an equivalent long distance transmission line model of a UHVDC transmission system. The transmission line model assumes a uniformly distributed line and comprises a plurality of infinitesimal sections, where an exemplary infinitesimal section 210 (for example, lkm of the transmission line) is particularly shown, where Ro represents the series resistance (Ω/km), L₀ represents the series inductance (H/km), Go represents the shunt leakage conductance (S/km), and Co represents the shunt capacitance (F/km). U_M and U_N are voltages at two terminals of the DC transmission line while I_M and I_N are currents at two terminals of the DC transmission line. The transmission line equations are: du . di

= K₀i + ₀—

dx dt = Lr₀U + C₀

dx dt

Where, u represents a voltage, x represents the length of an infinitesimal section, i represents a current flowing through the infinitesimal section, and t represents a time. [0036] FIG. 2B is a schematic diagram of an equivalent pi (π) model of the transmission line model as shown in FIG. 2A, where the DC line is substituted by a centralized parameter model taking shunt branches into consideration, it follows: sinh rl

Z = Z, sinh rl = Z

rl

, rl 7 tanh r/ / 2

tanh— =

2 2 r/ / 2 where,

c is the characteristic impedance of the DC line;

r is the propagation constant;

/ is the length of the DC line;

Z is the total series impedance of the DC line;

Y is the total shunt admittance of the DC line;

Z is the series impedance of the equivalent π model of the DC line;

Y is the shunt admittance of the equivalent π model of the DC line;

Igi and I_g2 are shunt branch currents of the equivalent π model at terminals M and N of the DC line, respectively;

I_cl and I_C2 are series branch currents of the equivalent π model at terminals M and N of the DC line, respectively.

[0037] Due to the long transmission distance of a UHVDC transmission line, the influence by shunt branches can't be neglected. So the shunt currents I_gX and I_g2 are significant and may lead to the mal-operation of differential protection under an external fault (for example, a fault at the rectifier side and/or at the inverter side).

[0038] FIG. 3 A is a diagram of differential current responses with/without a shunt branch under an external AC system fault at rectifier side, wherein the fault is assumed at time 0.5s. As shown in this figure, the plot 301 without a shunt branch is almost zero under external fault, while the plot 302 with a shunt branch is fluctuating after the external fault. As mentioned above, if the differential current exceeds a predetermined setting threshold, the protection unit will trigger the associated circuit breakers. Consequently, this fluctuation under external faults may lead to a current differential protection mal-operation.

[0039] FIG. 3B is a diagram of differential current responses with/without a shunt branch under an external AC system fault at inverter side. Similarly, the plot 303 without shunt branch is almost zero under external fault, while the plot 304 with a shunt branch is fluctuating after the external fault. This fluctuation may similarly lead to the current differential protection mal-operation under external faults.

[0040] In view of above potential current differential protection mal-operation, a novel current differential protection solution for a UHVDC transmission line is proposed in this invention, in which the influence by the shunt branches of the DC transmission line is removed. Thus, referring to FIG. 2B again, the differential current is calculated using the serial branch of the DC transmission line as below.

[0041] With the shunt branch currents being removed from the current differential protection, the unbalanced current due to external faults is small, which effectively avoids mal-operation of the protection unit under external faults.

Transient Harmonics in UHVDC Transmission System

[0042] In another embodiment, effects of transient harmonic components in a long distance UHVDC transmission line are taken into consideration. FIG. 4 is a diagram of total harmonic currents under three typical faults in a DC transmission system, wherein plot 401 denotes a total harmonic current on the DC line under a DC line fault, plot 402 denotes a total harmonic current on the DC line under an inverter fault, and plot 403 denotes a total harmonic current on the DC line under a rectifier fault. As shown in FIG. 4, the three typical faults each is assumed at time 0.5s, and the response of a total harmonic current in the UHVDC transmission line for each fault is illustrated separately. After a system fault, a number of harmonic currents appear in the DC transmission line, and the peak value of a total harmonic current reaches even up to 0.4p.u. The transient harmonic currents can affect the operation of current differential protection. For example, to differentiate the transient harmonic currents due to a DC line fault and a system (inverter and/or rectifier) fault so as to facilitate a current differential protection, the current setting value for differential protection may be carefully selected in a small range between the peaks of the plot 401 and other plots (402, 403). However, such small range for a current setting value may cause operation failures and instability.

[0043] As mentioned previously, to overcome the effects of transient characteristic harmonic components in a UHVDC transmission system, DC filters and smoothing reactors are generally coupled in a UHVDC system to absorb these harmonic currents. Such DC filters are used to eliminate the characteristic frequency harmonics, e.g., 12th, 24th and 36th harmonics. However, there are lots of non-characteristic harmonics (e.g., 1st, 2nd, 3rd, etc.) in the DC transmission system under external faults, so the measured currents and voltages at two terminals are not accurate under fault conditions. These transient harmonics can be expressed as follow:

n=\ u sum = / i B n s i Vnco 0J + ψ T n )

n=\ where,

n is the harmonic order and may be a positive integer, e.g., 1 , 2, 3...

O⁾ ₀ is the fundamental radian frequency; i_sum is the total non-characteristic harmonic current under a fault condition;

u_sum is the total non-characteristic harmonic voltage under a fault condition;

A_n is the amplitude of a non-characteristic harmonic current under a fault condition;

B_n is the amplitude of a non-characteristic harmonic voltage under a fault condition;

Φ„ is the initial phase angle of a non-characteristic harmonic current under a fault condition;

ψ_η is the initial phase angle of a non-characteristic harmonic voltage under a fault condition. [0044] To overcome the effects of transient non-characteristic harmonics, the average value Ψ(ζ^') is employed here for substituting the measured current value z.

where, is the average value of a current in a fundamental period To, wherein T₀ = ;

G¾ i(t) is the instantaneous current value.

[0045] For a sinusoidal excitation i{t) , in a period To, Ψ(ζ) = 0 , it follows:

Ψ(Α_η 5ίη(ηω₀ί + φ_η)) = 0

_ 2π

Where, ^ωο ⁼ and n is the harmonic order.

[0046] In other words, Ψ(ζ^') rejects all sinusoidal harmonic components. Thus, a current differential protection scheme implemented with Ψ(ζ^') instead of a measured current will not pick up on the transient harmonic components.

[0047] By analyzing the characteristics of a long distance UHVDC transmission line, this invention proposes an improved current differential protection solution for a UHVDC transmission line. Based on this invention, a more accurate instantaneous value can be obtained, and the effects of shunt branch currents and/or transient harmonic components can be removed. Therefore, not only the measurement precision can be improved, but also the sensitivity and reliability of current differential protection can be reinforced.

[0048] FIG. 5 is a schematic flowchart 500 of a method for current differential protection for a DC transmission line according to an embodiment of the invention. This method 500 can be implemented within a UHVDC transmission system, e.g., as that in FIG. 1.

[0049] At step 502, the currents flowing through the two terminals of the DC transmission line may be measured. Additionally, the voltages at the two terminals of the DC transmission line may be measured. For example, these currents and voltages may be sampled by suitable measurement units and the measured data may be saved into a data buffer in a differential protection unit.

[0050] At step 504, equivalent shunt branch currents of the DC transmission line can be removed from the measured currents respectively, to generate corrected currents. In one embodiment, the equivalent shunt branch currents may be calculated based on the measured voltages and an equivalent shunt admittance of the transmission line, e.g., by following equations:

I DCL ^~ I CI ⁺ ^C2 (!)

7^'

I CI ^~ U M

² (2) Ϋ

² (3)

where:

1_M and I_N are measured currents flowing through the two terminals of the DC line, respectively;

U_M and U_N are measured voltages at the two terminals of the DC line, respectively; Y is the shunt admittance of an equivalent π model of the DC line;

Id and Ic₂ are corrected currents flowing through the two terminals of DC line, respectively.

[0051] It shall be noted that, although the admittance parameter Y72 is used for the shunt branches based on the exemplary equivalent pi ( ) model as shown in FIG. 2, other admittance parameters for the shunt branches may be appropriately configured. For example, a first fraction (1/a) of the equivalent shunt admittance (Υ') can be multiplied by U_M to generate an equivalent shunt branch current at the terminal M, and a second fraction (1/b) of the equivalent shunt admittance (Υ') can be multiplied by U_N to generate an equivalent shunt branch current at the terminal N, wherein a > 1 and b > 1. According to the characteristics of shunt admittance, (1/a + 1/b) may approximately equal to 1. [0052] At step 506, the corrected currents each is averaged in a fundamental period of the corresponding measured current. For example, each corrected current may be integrated in its fundamental period, and the integral current may be divided by the fundamental period. The integration can remove transient harmonic currents in the corrected currents as follow:

^Γο ^r° (4)

[0053] At step 508, the differential current Id can be calculated based on the averaged currents, e.g., by the following equation:

/„ = |ψ(ΐ_β1) + ψ(ΐ_β2 )| ₍₅₎

[0054] Further, at step 508, the differential current Id is compared with a current setting value Isetting and thus the fault type can be identified. Under external fault or normal operation, the differential current is smaller than the current setting value (510):

^ d ^ ^setting

[0055] Under internal fault, the differential current is higher than the current setting value (512):

^ d ^ ^setting

[0056] Consequently, the comparison of the differential current with the current setting value can indicates whether an internal fault occurs on the DC transmission line.

[0057] At step 514, a current differential protection is triggered if it is determined that an internal fault occurs on the DC transmission line. For example, a circuit breaker associated with either or each side of the DC transmission system is triggered to stop a power transmission over the DC transmission line.

[0058] Optionally, a restraining current I_R can be calculated by the following equation:

2 (8)

[0059] The restraining current I_R can be used to avoid the DC transmission line from current differential protection mal-operation caused by unbalanced current. For example, when the differential current is higher than the current setting value, it further determines a ratio Kr of the differential current I_d to the restraining current I_R as a trigger signal for current differential protection. If the ratio Kr is higher than a predetermined threshold ratio Ks, the differential protection can be triggered; otherwise, the differential protection may not be triggered. In this situation, the restraining current can avoid mal-operation of current differential protection in case the differential current is higher than the current setting value under an external fault.

[0060] Utilizing the distributed parameters of a DC transmission line, the invention can remove shunt branch currents and transient harmonic currents of the DC transmission line. The algorithm is simple. Based on traditional measured currents and voltages, the shunt branch correction algorithm and averaging algorithm are employed to remove the shunt branch currents and transient harmonic currents. In a long distance DC transmission line, the measured currents at two terminals include lots of transient harmonic components and shunt branch currents, so the traditional differential protection is prone to mal-operation under external fault. To avoid the unbalanced current, the current setting value is usually raised, so the traditional current differential protection may fail to operate under internal fault with high fault resistance. This invention can remove shunt branch currents and transient harmonic components of the DC transmission line. So the reliability of the novel differential protection is higher than traditional differential protection. In PSCAD and MATLAB, the proposed protection scheme is tested over 50 times, and the results are prefect. With 30dB noise in the system, the proposed protection scheme is tested over 100 times, and the protection scheme's correct trigger rate is up to 99%.

[0061] The proposed current differential protection scheme for a UHVDC transmission line is simple and reliable since it does not need complex mode transformation. The invention can eliminate the effects of both transient harmonic components and shunt branch currents in a long distance UHV/EHV DC transmission line.

[0062] Based on the novel current differential protection scheme, the unbalanced current due to external faults is small, thus the range of differential current setting values can be larger. It helps to reduce the probability of the operation failures and enhance the reliability of this novel differential protection. [0063] Based on the novel current differential protection scheme, the current setting value can be set more accurately. It also helps to reduce the probability of operation failures under internal faults with high fault resistance. So it can greatly enhance the sensitivity of this novel differential protection.

[0064] The current differential protection described herein may be embodied in hardware, software, firmware, or any combination thereof. Any features described as units or components may be implemented together in an integrated device or separately as discrete devices. Software modules may be embodied in a non-transient computer readable tangible medium as instructions and can be executed by a processor to cause the processor to perform the operations as illustrated herein.

[0065] The previous description for embodiments is provided to illustrate, not limit, the technical schemes of the present invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the principles defined herein may be applied to other embodiments without departing from the scope of the disclosure. All such modifications and equivalents shall be within the scope of the present invention. Thus, the present disclosure is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope as defined by the following claims.

Claims

CLAIMS What is claimed is:

1. A method for current differential protection for a DC transmission line, comprising: measuring a first current flowing through a first terminal of the DC transmission line; measuring a second current flowing through a second terminal of the DC transmission line;

removing first and second equivalent shunt branch currents of the DC transmission line respectively from the first and second currents, to generate corrected first and second currents;

calculating a differential current based on the corrected first and second currents; and comparing the differential current with a predetermined current setting value to determine whether an internal fault occurs on the DC transmission line.

2. The method of claim 1, wherein it is determined that an internal fault occurs on the DC transmission line if the differential current is larger than the predetermined current setting value, wherein the method further comprises:

triggering a current differential protection if it is determined that an internal fault occurs on the DC transmission line.

3. The method of claim 1, further comprising:

measuring a first voltage at the first terminal of the DC transmission line;

measuring a second voltage at the second terminal of the DC transmission line; and calculating the first and second equivalent shunt branch currents based on the first and second voltages and an equivalent shunt admittance of the DC transmission line.

4. The method of claim 3, wherein the first equivalent shunt branch current is calculated by multiplying a first fraction (1/a) of the equivalent shunt admittance with the first voltage, and the second equivalent shunt branch current is calculated by multiplying a second fraction (1/b) of the equivalent shunt admittance with the second voltage, wherein a > 1 and b > 1.

5. The method of claim 4, wherein a =2, and b = 2.

6. The method of one of claims 1 to 5, wherein calculating the differential current further comprises:

averaging the corrected first current in a first fundamental period of the first current to generate a first average current;

averaging the corrected second current in a second fundamental period of the second current to generate a second average current; and

calculating the differential current based on the first and second average currents.

7. The method of claim 6, wherein averaging the corrected first current further comprises integrating the corrected first current in the first fundamental period and dividing the integrated first current by the first fundamental period, and averaging the corrected second current further comprises integrating the corrected second current in the second fundamental period and dividing the integrated second current by the second fundamental period.

8. The method of claim 6, further comprising:

calculating a restraining current based on the first and second average currents; and triggering a current differential protection to stop a power transmission over the DC transmission line if the differential current is larger than the predetermined current setting value and if a ratio of the differential current to the restraining current is higher than a predetermined threshold ratio.

9. A current differential protection system for a DC transmission line, comprising: a first measurement unit, configured to measure a first current flowing through a first terminal of the DC transmission line;

a second measurement unit, configured to measure a second current flowing through a second terminal of the DC transmission line; and

a protection unit, configured to remove first and second equivalent shunt branch currents of the DC transmission line respectively from the first and second currents to generate corrected first and second currents; to calculate a differential current based on the corrected first and second currents; and to compare the differential current with a predetermined current setting value to determine whether an internal fault occurs on the DC transmission line.

10. The system of claim 9, further comprising:

a circuit breaker coupled to the DC transmission line, wherein the protection unit is further configured to:

determine that an internal fault occurs on the DC transmission line if the differential current is larger than the predetermined current setting value; and

trigger the circuit breaker to stop a power transmission over the DC transmission line if it is determined that an internal fault occurs on the DC transmission line.

11. The system of claim 9, wherein the first measurement unit is further configured to measure a first voltage at the first terminal of the DC transmission line, the second measurement unit is further configured to measure a second voltage at the second terminal of the DC transmission line; and

wherein the protection unit is further configured to calculate the first and second equivalent shunt branch currents based on the first and second voltages and an equivalent shunt admittance of the DC transmission line.

12. The system of claim 11, wherein the protection unit is configured to calculate the first equivalent shunt branch current by multiplying a first fraction (1/a) of the equivalent shunt admittance with the first voltage, and to calculate the second equivalent shunt branch current by multiplying a second fraction (1/b) of the equivalent shunt admittance with the second voltage, wherein a > 1 and b > 1.

13. The system of claim 12, wherein a =2, and b = 2.

14. The system of one of claims 9 to 13, wherein the protection unit is configured to calculate the differential current by: averaging the corrected first current in a first fundamental period of the first current to generate a first average current; averaging the corrected second current in a second fundamental period of the second current to generate a second average current; and calculating the differential current based on the first and second average currents.

15. The system of claim 14, wherein the protection unit is configured to average the corrected first current by integrating the corrected first current in the first fundamental period and dividing the integrated first current by the first fundamental period, and to average the corrected second current by integrating the corrected second current in the second fundamental period and dividing the integrated second current by the second fundamental period.

16. The system of claim 14, further comprising:

calculate a restraining current based on the first and second average currents; and trigger the circuit breaker to stop a power transmission over the DC transmission line if the differential current is larger than the predetermined current setting value and if a ratio of the differential current to the restraining current is higher than a predetermined threshold ratio.

17. An apparatus for current differential protection for a DC transmission line, comprising:

means for measuring a first current flowing through a first terminal of the DC transmission line;

means for measuring a second current flowing through a second terminal of the DC transmission line;

means for removing first and second equivalent shunt branch currents of the DC transmission line respectively from the first and second currents, to generate corrected first and second currents;

means for calculating a differential current based on the corrected first and second currents; and means for comparing the differential current with a predetermined current setting value to determine whether an internal fault occurs on the DC transmission line.

18. The apparatus of claim 17, wherein it is determined that an internal fault occurs on the DC transmission line if the differential current is larger than the predetermined current setting value, wherein the apparatus further comprises:

means for triggering a current differential protection if it is determined that an internal fault occurs on the DC transmission line.

19. The apparatus of claim 17, further comprising:

means for measuring a first voltage at the first terminal of the DC transmission line; means for measuring a second voltage at the second terminal of the DC transmission line; and

means for calculating the first and second equivalent shunt branch currents based on the first and second voltages and an equivalent shunt admittance of the DC transmission line.

20. The apparatus of claim 19, wherein the first equivalent shunt branch current is calculated by multiplying a first fraction (1/a) of the equivalent shunt admittance with the first voltage, and the second equivalent shunt branch current is calculated by multiplying a second fraction (1/b) of the equivalent shunt admittance with the second voltage, wherein a > 1 and b > 1.

21. The apparatus of claim 20, wherein a =2, and b = 2.

22. The apparatus of one of claims 17 to 21, wherein the means for calculating the differential current further comprises:

means for averaging the corrected first current in a first fundamental period of the first current to generate a first average current;

means for averaging the corrected second current in a second fundamental period of the second current to generate a second average current; and

means for calculating the differential current based on the first and second average currents.

23. The apparatus of claim 22, wherein the means for averaging the corrected first current further comprises means for integrating the corrected first current in the first fundamental period and means for dividing the integrated first current by the first fundamental period, and the means for averaging the corrected second current further comprises means for integrating the corrected second current in the second fundamental period and means for dividing the integrated second current by the second fundamental period.

24. The apparatus of claim 22, further comprising:

means for calculating a restraining current based on the first and second average currents; and

means for triggering a current differential protection to stop a power transmission over the DC transmission line if the differential current is larger than the predetermined current setting value and if a ratio of the differential current to the restraining current is higher than a predetermined threshold ratio.