中压电网中性点接地方式及消除单相接地故障技术
摘 要
中压电网中性点接地方式的选择是一个综合性的技术问题，它涉及到供电可靠性、人身及设备安全、绝缘、过电压、继电保护、通信（电磁环境）、接地装置等各个方面。它具有理论研究与实际经验密切结合的特点，是电力系统实现安全与经济运行的技术基础和系统工程。
论文介绍了我国6~66kV中压电网中性点主要的接地方式及其各自的优缺点，论述了一种应用于中性点不接地系统消除单相接地故障及选线、消除铁磁性谐振的新方法以及实现这种方法的智能综合保护装置。
论文的主要创新之处：1）采用新的电压消弧法：用电抗器钳制故障相的电压，有效地防止接地电弧的重燃，消除弧光接地过电压，保证供电可靠性；2）归纳不接地系统单相接地故障的规律，确立其接地电阻与相电压、中性点位移电压关系的数学模型，提出判断故障相的依据、创造性地设计了相应的判断和保护程序，不论接地电阻值如何变化，都能够准确对高阻接地故障提供有效的保护，将以往难以察觉故障消灭在萌芽状态。3）采用新的消谐方法：当谐振发生时，直接在一相母线与地之间并接电抗器，使回路远离谐振点，并限制谐振过电压。
论文所述的保护装置通过了国家相关检验机构的全面、严格的电气性能和环境型式试验、电磁兼容试验、10kV电网的现场接地故障试验、并在多个变电站10kV电网系统挂网运行等，证实了这种新方法及装置的可靠性与实用性。该装置已获国家级新产品鉴定, 鉴定结论为：“装置的设计思路新颖，保护性能良好，其消弧、消谐的方法、对高阻接地故障的判别和保护处于国内领先水平”。
关键词：中性点接地方式、弧光接地 高阻接地 选线 铁磁谐振 智能综合保护装置
Neutral-poin grounding mode in medium voltage network
and Technique to eliminate single-phase ground fault
Abstract
It is comprehensive technological issue for medium voltage network to choose its neutral-point grounding mode. This issue will involve reliability of power supply; the safety of person and equipment; insulation; overvoltage; protective relay; communication (mangnetic environment); earthing device and so on. It has characteristic of combining theory with practice, meanwhile it is also technological basis and system engineering for power system to operate economically and safely.
This paper introduces main neutral-point grounding modes in 6-66KV medium voltage network in our country and its advantages and disadvantages in some cases. And present a kind of new method of eliminating single-phase ground fault; ferro-resonance and checking up line troubled in neutral-point non-ground system. At same time ,in this paper also describe a comprehensive intelligence device that implements these techniques.
The major innovation in this paper includes:1) developing a new voltage method that eliminates electric arc by a reactor that limit overvoltage between faulty phase and the ground. This reactor can effectually prevent electric arc from reigniting; and eliminate arc-grounding overvoltage in order to accomplish reliability of power supply.2) find a single-phase faulty rules in non-grounded system and a mathematical mode that relate to relationship between earthing resistance and phase voltage, neutral-point displacement voltage. As a result, we got a criterion that can judge faulty phase, according to this criterion we design protective program that can effectually protect network from high-resistance ground faulty(HRGF)no matter how resistance change, as a result we can destroy the single-phase faulty while it just right form..3)using a new method to suppress resonance: when resonance just happened, directly adding a reactor in parallel between a phase bus and ground in order to apart circuit from resonance point so as to limit resonance overvoltage.
Our protective device as described above has passed a series of tests conducted by relevant government inspection agencies. These tests include an extensive and rigorous electrical performance test; an environmental type test; an electromagnetic compatibility test; and operation in many 10KV substations. This new technique and protective device are shown to be reliable and practical. A national-level new product appraisal was completed and a certification was granted. It is stated in the certificate that:” the design of this device is new style, protective performance good, the method for eliminating electric are, resonance; criterion and protection identifying HRGF are first class in China.”
Key words: neutral-point grounding mode, electric arc grounding, high-resistance grounding, checking up line faulty, Ferro-resonance, comprehensive intelligence protective device
①发生单相接地故障时（例如A相），故障相的对地电容C0被短接；①When the single-phase earth trouble occurs (such as A phase), earth capacitance C0of fault phase is shorted;
②非故障线路3I01的大小等于本线路的接地电容电流，其电容性无功功率的方向为由母线流向线路；
②The size of non-fault line 3I01 is equal to the earth capacitive current of this line. The direction of inactive power of the capacitive character is to flow to the line from the bus bar;
③故障线路3I02的大小等于所有非故障线路的3I01之和，也就是所有非故障线路的接地电容电流之和；其电容性无功功率的方向为由线路流向母线；
③The size of non-fault line 3I02 is equal to the sum of all non-fault line 3I01, that is to say the sum of the earth capacitive current of all non-fault line; the direction of inactive power of the capacitive character is to flow to the line from the bus bar;
④若零序电流互感器的极性是以变电所母线流向线路为正方向，那么非故障线路的零序电流超前零序电压90°，故障线路的零序电流滞后零序电压90°，故障线路的零序电流与非故障线路的零序电流在相位上相差180°。
④If the polarity of the mutual inductor of zero sequence current calculates the positive direction by the flowing direction of the bus bar of transformer station, the advanced zero sequence voltage of zero sequence of non-fault line is about 90°. The delayed zero sequence current of zero sequence of fault line is about 90°. The phase difference of the phrase of the zero sequence of fault line and zero sequence of non-fault line is about 180°.
ZN05A装置采用了零序电流群体比幅比相加零序电压相位的选线原理：当3U0大于整定值时，启动对所有馈线的零序电流幅值排序，取幅值大的前2~4个电流比相，若某电流与其它电流方向相反，并滞后零序电压相位90º，则判定该线路接地，否则为母线接地。
ZN05A device adopts the line selection principle of summing up the amplitude and phase-comparison of zero sequence current colony rate and phrase of zero sequence voltages: When 3U0 is greater than the setting value, start the amplitude arrangement of the zero sequence current of all the feeders, and take the first 2-4 greater amplitude of current phase-comparison. If the current is conversed with the other current direction and the phrase of the delayed zero sequence voltage is about 90º, it may be determined that it shall be line grounding, otherwise shall be bus bar grounding.
此种方法为多重判据（又称3C方案），多重判据即为用二种及以上原理为判据，增加可靠性和抗干扰性能力，减少受系统运行方式、长短线、接地电阻的影响。排序避免了设定值，具有设定值随动的“水涨船高”的优点。它既可以避免单一判据带来的局限性，也可以相对缩短选线的时间，是较理想的方式。
This method shall be multiple criteria decision (is also called 3C scheme). The multiple criteria decision applies two types and above principle as the criteria. Increases the dependability and interference resistance, and reduces the effect of operation mode, line size and earth resistance of system. The arrangement process avoids the setting value, which has the advantage of “the rising tide raising all boats”. It can avoid the limitation that the single criterion brought, and may shorten the time to select the line relatively as well. It is a better ideal mode.
装置具有高速采样、高速数据处理和快速傅氏变换及谐波分析能力，在故障发生40ms内，就完成了32路零序电流选线，此时采集的故障电流信息未受到任何保护动作的影响，保证了以零序电流基波为基础的比幅比相选线原理应用的准确性，当馈线二次侧零序电流不小于20mA时，装置就能准确采样和选线。
The device is featured by high speed sampling, high-speed data processing, high speed fourier transform and harmonic analysis. In 40ms when fault occurred, this device is capable to finish 32 zero sequence lines selection. At this moment, the collected information of fault current does not effect by any protective movement, therefore, guarantee the accuracy of the application of line selection which based on the amplitude and phase-comparison of the fundamental wave of zero sequence current. When the side zero sequence current of the No.2 feeder is smaller than 20mA（the primary side shall be 0.8A）, the device may sample and select the line accurately.
4消谐新方法及原理
4 New method and principle of harmonic elimination
4.1常见的消除铁磁谐振产品的原理及缺陷
4.1Principle and defect of common products of ferromagnetic resonance elimination
铁磁谐振是由于系统中的电压互感器、变压器、消弧线圈等非线性电感受激而产生磁路饱和，引发持续的振荡。在正常运行条件下，电路的初始感抗大于容抗（ωL 0 > 1/ωC），电路不具备谐振条件，当电路受到扰动（如，电网的突然合闸、单相接地故障的发生和消除）使电感两端电压升高而导致铁芯饱和时，感抗随之减小，当ωL = 1/ωC时即满足谐振条件，在电感、电容两端形成过电压，并产生励磁涌流，形成持久不衰减的分频、工频或倍频谐振，其破坏力很大，轻则烧坏元件，重则烧毁设备。
Ferromagnetic resonance is the consistent oscillation due to the saturation of the stimulated nonlinear inductance such as potential transformer, transformer and arc-suppression coil, etc. Under the normal operating condition, the initial induction reactance of the line shall be greater than the capacitive reactance （ωL 0 > 1/ωC）. The line does not possess the resonance condition. When the lines are disturbed (for instance, the occurrence and elimination of sudden gate closing and the fault of single-phase grounding) and made the voltage of both ends of the inductance rise and cause the saturation of iron core, and the induction reactance is reduced as well. When ωL = 1/ωC meet resonance condition, both ends of inductance and electric capacity shall form the excess voltage, produce the magnetizing inrush current, and form the lasting frequency division, work frequency or frequency doubling, which shall burn out component or destroy the equipment when it is serious.
图9 串联谐振电路
Fig. 9 Series resonant circuit
对于电网中性点不接地系统，铁磁谐振的产生回路可简化为最简单的串联谐振电路（见图9）。电阻在回路中起限流作用，消耗谐振能量，减小谐振范围。通常国内的一次或二次消谐产品，就是在电压互感器的一次中性点或二次开口三角处加电阻，以电阻的耗能作用，阻尼和消除谐振。但是所有这些消谐产品均有消谐死区，消谐效果不理想，并且它们只能针对加装消谐电阻的电压互感器产生的谐振起作用，对系统母线上其它设备或操作等产生的谐振无能为力，以至系统中因谐振引起的事故时有发生。上述电阻消谐存在的缺陷原因可从能量的观点来阐明，见图10。
As to the ungrounding system of neutral-point of electric network, the circuit produced by the ferromagnetic resonance can be simplified for the simplest series connection resonance circuit (see Fig. 9). The resistance plays a current limiting role in the circuit. It may consume resonance energy and reduce the resonance range. Usually the domestic primary or secondary harmonic elimination product is to add increase the resistance at the primary neutral-points or secondary triangular area of the open vent of potential transformer. Apply the dissipation energy of the resistance to damp and eliminate the resonance. But all these harmonic elimination products all have the dead space of harmonic elimination. It is unsatisfactory to achieve the idea result of harmonic elimination. And they can only operate to the resonance produced by the potential transformer with additional installed harmonic elimination resistance. It is powerless to operate to the resonance of the bus bar of system or other operating equipments. Therefore, the accident caused by resonance is occurred occasionally in the system. The reason of the defect of above-mentioned harmonic elimination resistance may be illustrated from the view of energy. See Fig. 10.
图中纵坐标W代表能量，横坐标A代表谐振频率的幅值，WR为阻尼电阻的损耗能量，WL为电源通过非线性电感所引进回路的能量。在图10（a）中只要有稍微轻微的“扰动”就可以产生一个微小的振荡幅值，由于引进能量大于损耗能量，幅值就可以自发的持续上升，直至稳定点A0为止，这就是自激现象，亦称为“软激发”。在图10（b）中则不同，在0~A1的幅值范围内，WL不足以抵偿损耗WR，故起始的振荡就会衰减消失；若初始幅值超过A1，振荡就可以一直发展到幅值A2，建立其稳定的谐振，这是外激而产生的，又称硬激发。由此可见，电阻的耗能范围是一定的，它们的消谐范围也就是有限的。而系统的运行状态是变化的，电容、电感参数也随之改变，固定的电阻根本不能满足这些变化的需要，也就是说特定阻值的电阻只对特定范围的谐振有作用，对于超出这个范围谐振无能为力。所以电阻消谐效果不理想，存在消谐死区。
Ordinate W represents the energy; abscissa a represents the amplitude of resonance frequency; WR is the loss energy of the damping resistance; WL is the power switch-in from the circuit of the non-linear inductance introduces. In Fig. 10 (a) the slight “Disturb” may produce small amplitude of vibration. Because the inducing energy is greater than the losing energy, the amplitude raises spontaneously until reach a steady point A0. This is self-excitation phenomena, and is also called “Soft exciting”. It is quite different in Fig. 10 (b), within the amplitude range of 0-A1, WL is not sufficient to compensate the loss of WR, so the initial vibration shall decay and disappear; if the initial amplitude exceeds A1, the oscillation may reach to amplitude A2, and sets up its steady resonance. This is defied and produced from outside and also called hard exciting. Therefore, dissipation energy of resistance range is constant. And their harmony range is limited. The systematic operation state is changed constantly, and the electric capacity and inductance parameter is changed as well. The regular resistance can't meet demands of these changes at all. That is to say, the resistance of the resistance with specific value is to only operate to the specific range of resonance. As fro the resonance beyond this range shall be powerless. Therefore, the result of harmonic elimination of resistance is unsatisfactory and has the dead space of harmonic elimination.
软激发soft exciting 硬激发：hard exciting
图10 激发铁磁谐振的能量曲线
Fig. 10 Energy curves of excited ferromagnetic resonance
4.2 ZN05A装置消谐方法及原理
4.2 Method and principle of harmonic elimination of ZN05A device
从以上的分析可知，当谐振发生时ωL = 1/ωC，此时只要改变电感L的参数，使ωL ≠ 1/ωC，就能破坏谐振条件，消除铁磁谐振。
The analysis of the above shows, when the resonance occurred, ωL = 1/ωC. At this moment, changing the parameter of inductance L and make ωL ≠ 1/ωC may destroy the resonance condition and eliminate the ferromagnetic resonance.
装置消谐原理：harmonic elimination principle
等效电路：equivalent circuit
图11 消谐新方法及原理
Fig. 11 New method and principle of harmonic elimination
ZN05A装置消谐方法及原理如图11所示，当系统中发生谐振时，闭合开关K’,将电感L’并联到产生磁路饱和的那相母线和地之间(见图11(a))，这就改变回路电感参数，使ωL ≠ 1/ωC破坏了谐振条件，使系统自然脱谐(见图11(b))；同时旁路了产生了饱和的那相电感，提供了泄放励磁涌流的通路，并且钳制谐振过电压小于线电压，保护系统的元件和设备免遭谐振的摧毁。该消谐方法能满足系统各种运行方式的变化，对系统任何条件下产生的谐振都能够消谐。
The method and principle of harmonic elimination of ZN05A device see Fig. 11. When the resonance is occurred in the system, close switch K’and connect the inductance L’the circuit between saturated bus bar and ground (see Fig. 11 (a )). And it may change the inductance parameter of the circuit, and make ωL ≠ 1/ωC so as to destroy resonance condition. Therefore, the system is in a state of out-of-resonance naturally (see Fig. 11 (b)); at the same time the bypass produced the saturated inductance, which may offer the discharged passage of magnetizing inrush current, and the limited resonance voltage is smaller than the voltage of circuit. So as to protect systematic component and equipment exempt from the destroying of resonance. This method of harmonic elimination may meet the changes of different operation mode of system. And may eliminate the resonance produced to the system under all conditions.
通过三相10kV实验室的试验完全证实了上述论点：LXQ消谐器是目前国内广泛使用的一次消谐产品之一，它加在电压互感器（PT）一次中性点与地之间如图11（a）虚框所示，图12是使用LXQ消谐器，发生谐振时的录波图。
The test of three phases 10kV laboratory have totally proved above-mentioned arguments: LXQ harmonic elimination device is one of the primary harmonic elimination product used extensively in our country at present. It is installed between the location neutral-point and ground of potential transformer (PT). See Fig. 11 (a). Fig. 12 is the wave-recorded diagram when the resonance occurred for the application of the harmonic elimination device of LXQ.
图12消谐器LXQ无法消除的谐振
Fig. 12 The resonance that can't be eliminated by the LXQ device
波形表明：a时刻，单相接地发生；c时刻，接地消失，激发谐振；c~d时段，谐振持续稳定存在，即该消谐器对此种谐振无能为力。
The wave form shows: at a moment, single-phase grounding may occur; at c moment, the grounding may disappear and excite the resonance; at c-d moment, the resonance exists continually and steadily. It means that this harmonic elimination device is powerless for the resonance of this type.
图13 ZN05A装置可靠消除LXQ无法消除的谐振
The Fig. 13 The resonance that can be eliminated by the ZN05A device while can't be eliminated by the LXQ
图13是在与图5-4完全相同的条件下，使用消谐新方法的录波图，b~d时段：谐振发生区；d~e时段：装置消谐保护动作区；e时刻后：消谐完毕，系统恢复正常。该图证明：ZN05A装置能够消除前述LXQ消谐器不能消除的谐振，对系统提供无死区的消谐保护。
Fig. 13 is the application of new method for wave-recorded diagram of harmonic elimination under the same condition of Fig.5-4. At b-d moment: the area of the resonance occurs; at d-e period: operating area of the harmonic elimination protection of the device; at e moment: After the harmonic elimination process is finished, the system returns to normal. This picture proves: ZN05A device can eliminate resonance that can be eliminated by the ZN05A device while can't be eliminated by the above-mentioned LXQ, and provide harmonic elimination protection without dead area.
5.ZN05A智能型综合保护装置性能验证
5. Performance verification of ZN05A comprehensive intelligence grounding protection device
不接地电网系统发生单相接地是一个复杂的动态过渡过程，国内外的历史实践经验证实：计算机仿真不能替代现场试验，只利用低电压在产品的二次端进行单相接地的模拟试验，完全没有意义，不仅不利于提高产品制造质量，而且对电网的安全运行也十分有害。为了真实的、客观的验证智能综合保护装置的技术、安全性能等各项指标，广州宁志电力科技公司建立了三相10kV系统接地故障试验室，配备了具有国际领先水平的多通道示波记录仪，对装置进行了大量反复的弧光接地、高阻接地、金属接地、铁磁谐振等试验；在国家继电器检测中心对控制器通过了型式试验和电磁兼容试验；在广东省电力行业高低压电工产品检测中心对装置整体性能进行了型式试验；在南方电网佛山供电局110kV禄堂站用一条10kV馈线ZN05A装置进行了现场（弧光、高阻、金属）接地试验；在广东佛山、清远、肇庆等地区电网ZN05A装置挂网运行多年。通过这一系列的型式试验、现场试验和挂网运行我们取得了许多珍贵的、客观的、全面的单相接地故障数据和运行经验，并对这些数据和经验加以理论上的分析总结，设计出了符合实际的，高效安全的智能型接地综合保护装置。该装置通过了中国机械联合会和南方电网公司的新产品鉴定，鉴定结论为：装置的设计思路新颖、保护性能良好，其消弧、消蝎的方法，对高阻接地故障判断和保护水平处于国内领先水平。
The single-phase connection of the ungrounding network system is a complicated dynamic transition process. The historical experience and practice both at home and abroad proves: the emulation of the computer can't substitute the field test. It only utilizes low-voltage to carry on the single-phase grounding simulation test on the secondary terminal of products, which is totally meanness. It is not only unfavorable to improving the quality of products, but also very harmful to the safety operation of network system. For achieving the indexes such as technology of the intellectual comprehensive protector of verification and security performance, etc. in fairness and objectiveness, the Guangzhou Ningzhi Electric Science and Technology Co., Ltd has established three-phase 10kV systematic ground trouble testing laboratory. These labs equips with the multi-channel electrographic recording device, which takes a lot of repeated trial such as electric arc grounding, high-resistance grounding, metal grounding and ferromagnetic resonance, etc.; the National Relay Testing Center has carried out the modeling test and electromagnetic compatibility test for the controller. The high voltage and low voltage electric products testing center of Power Industry of Guangdong Province has carried out the modeling test and electromagnetic compatibility test for the complete unit; The Lutang station of Southern Network Foshan Power Supply Bureau has carried out the field grounding test of the 10kV feeder (electric arc, high and metal grounding) ; verifies the protective performance of 110kV ZN05A device; the ZN05A device has applied in the network of Foshan, Qingyuan and Zhaoqing of Guangdong ,etc. for many years. We has obtained a lot of precious, objective and overall fault data and operating experience of single-phase grounding through a series modeling test, field test and operation in network. We also analysis and summarize these data and experience, therefore, designed the practical, high-efficient and safe comprehensive protecting device of intelligent grounding. This device is approved by Mechanical Federation and Southern Network Corporation of China。 The conclusion is: The device has a novel design and good protective performance. The method of arc suppressing, arc extinguishing, judgment of the fault condition of high-resistance grounding and protective competence takes a leading state in our country.
6.结束语
6. Conclusion
通过对电网中性点各种接地方式的综合比较，在科学研究和大量反复的严谨试验的基础上，广州宁志电力科技公司独创性地取得了对高阻接地故障保护动作的判据，使ZN05A智能型接地综合保护装置能够捕捉微弱的故障信息，对馈线断线、绝缘老化等高阻故障提供准确的保护；同时验证了在故障相母线上投电抗器接地的保护方式是安全可靠的，它既保留了中性点不接地方式的优点：接地电流小，供电可靠性高，又弥补了不接地方式的单相接地电弧易重燃，内部过电压倍数高等缺点。当谐振发生时，直接在一相母线与地之间并接电抗器，能够使系统自然脱离谐振，并限制谐振过电压。在选线方面采用多重判据的选线原理和高速DSP能够保证小电流接地选线的高准确率。总之本论文论述的接地综合保护方法和装置比中性点直接接地，小电阻接地的大电流接地方式利用变电站的接地网更为安全，其消弧性能、以及对弧光过电压和谐振过电压的限制比消弧线圈更为有效，并且结构简单可靠，占地面积小，综合成本低，是一种值得推广应用的接地保护方法和装置。
Through the comprehensive comparing of the various grounding method of neutral-point of network, on the basis of scientific research and a large number of repeated rigorous tests, the Guangzhou Ningzhi Electric Science and Technology Co., Ltd. has obtained the criterion of the protecting movement of the fault of the high-resistance grounding originally, which enable ZN05A comprehensive intelligence grounding protection device of intelligent grounding may catch the faint fault information and provide the accurate protection for the high-resistance fault of broken feeder and aged circuit, etc. At the same time, it may verify that the protecting method of applying the reactor grounding on the fault bus bar is safety and relied. It kept the advantage of the ungrounding neutral-point type : the ground current is small, reliability of power supply is high, and made up the defect of easy restriking of the arc of single-phase grounding and high multiple of internal over-voltage of ungrounding device. When the resonance occurred, connecting the reactor between a phase bus bar and ground directly can isolate the system from the resonance naturally, and limit the over-voltage of resonance as well. The multiple criterions of circuit selection and high speed DSP in circuit selection principle can guarantee high rate of accuracy of circuit selection of small current. In general, this thesis illustrate the comprehensive protective method of ground and direct ground device of neutral-point and large current grounding of low resistance grounding which use the grounding network of transformer substation is safer. The arc performance, the restriction of over-voltage of arc and resonance of over-voltage is more effective than arc coil. The unit has a feature of simple construction, reliable operation, small floor coverage and low comprehensive cost, which is worthy of promotion and application.
2012.10.26