Utilities have several options when they need to ground or earth the neutral point of their distribution network that largely depend on the characteristics and composition of the network. Each method, resistor, reactor or arc suppression coil has its merits and its disadvantages.

The Guangdong Provincial Electric Power Co. (GPEPC, Guangzhou, China) has a predominantly 10-kV overhead distribution system in the older cities and an increasing number of 10-kV underground cable feeders in the newer cities. The neutral point of the overhead distribution system traditionally has been grounded via controllable arc suppression coils (CASCs), while resistors are used on cable networks. However, both methods have operational disadvantages. To improve system-fault performance, the GPEPC's Power & Research Institute developed a new intellectualized CASC that is able to respond quickly to system faults, thus improving the reliability of the distribution network. This new CASC grounding method is expected to dominate the two existing methods without incurring their disadvantages.

Improvement of CASC

The GPEPC's Power & Research Institute set out to develop a CASC with the following key operational characteristics and system benefits:

• A linear voltage/current (V-A) characteristic curve for the complete voltage range zero to 1.1 Vph (where V = phase-voltage) to provide compensation for all values of neutral voltage (Vn) under especially for high impedance grounding and arc grounding faults.

• A fast response to suppress the grounding arc and to clear the series of grounds that occur at short time intervals (one second or shorter) that often occur during thunderstorm conditions.

• To easily adapt to the changes of the developing distribution network with maintenance-free design and high-reliability operations.

The majority of the CASCs in service are designed to enable excitation impedance adjustment, but the V-A characteristic curve is not linear for the complete voltage range (zero to 110% Vph) because of the magnetic hysteresis. They also have other disadvantages, including slow response times, narrow or stepped adjustable current output, complex construction or mechanically operated structures and contacts.

To meet the goals, the Research Institute developed a new type of CASC, called the KD-XH Intellectualized and Quick Responding Arc Suppressing Apparatus for Distribution Network Applications.

Principles and Features of KD-XH

The core part of the KD-XH system is the transformer with a high short-circuit impedance, which is controlled by thyristors on the secondary side. Figures 1 and 2 show the basic principle circuit and the equivalent circuit.

The primary high-voltage winding is the normal working winding (NW) connected to the power system. The control winding (CW) is the secondary low-voltage winding short-circuited by two contrary-paralleled thyristors (SCR). The key feature is the high short-circuit impedance of the transformer, which is about 100% — a value much higher than that of a normal transformer. Varying the firing angle of the thyristors controls the inductive current output from the NW. This means that as the conducting angle varies from zero to 180 degrees, the output current from the NW changes from zero amperes to the rated current without interruption. By adding a third winding to the transformer as the compensating winding (CmW) connected with filters that correspond to the third and fifth harmonics or even higher order, the harmonic current that results from operation of the thyristors reduces to permissible values.

The advanced design and operational principles of KD-XH offer excellent performances and unique characteristics, including:

• A straight-line V-A characteristic curve in full range of zero to 110% Vph.

• Fast response times (less than 5 msec) and the current output can be adjusted from zero to rated current continuously without any steps. In view of the KD-XH's quick-response characteristic, under normal conditions, this equipment does not operate near the system resonant point to restrict the possible dangerous overvoltages because of series resonance. Therefore, the additional resistor — a necessity for other types of arc suppression coils to restrict the resonance — is not required.

• The construction of this type of compensation coil is as simple as a conventional transformer without any on-load tap-change switches and no gaps in the magnetic path, which makes it easy to manufacture.

• An optional device of the KD-XH system can precisely locate the grounded line.

• The controller's other functions include:

  • Intellectualized auto-setting and processing
  • Suitable to control and operate in parallel with CASCs
  • Strong anti-interferences capability
  • Able to trip the grounded line.

Unlike other CASC equipment, the location of the grounded line and the compensation for the capacitive current of the KD-XH system can go simultaneously to maintain its quick response characteristic, thus rapidly extinguishing the grounding arc.

KD-XH Field Trials

The KD-XH arc suppression coil has been subjected to comprehensive simulated and field trials including the direct grounding, arc grounding and high impedance grounding faults on an industrial application unit (10.5 kV @ 500 kVA). Tests at the Research Institute's high-voltage facilities and at a network substation produced the same results, providing confirmation of the KD-XH's operational effectiveness. Recorded data for a KD-XH/10.5 kV unit in service on site for more than a year showed 106 operations, of which 17 grounding faults were permanent faults and the remaining 89 grounding faults were self-cleared after compensation with residual current through the grounding point of less than 5 A. On two occasions, the KD-XH compensation lasted for more than 60 min.

Table 3. KD-XH performance on Sept. 5, 2000.

Fault number	Time	Grounded line	Neutral voltage Vn (V)	Output current IL (A)	Capacitive current IC (A)
50	10:45:28	05	5160.6	57.2	54.3
51	10:45:37	05	5668.4	62.0	59.7
52	10:45:43	01	4682.4	49.2	49.3
53	10:45:46	01	5667.8	61.7	59.7
54	10:45:54	01	5674.2	61.9	59.7

Table 4. Comparison of the new CASC, traditional CASC and resistor methods of grounding.

	Resistor grounding	Traditional CASC grounding	KD-XH CASC grounding
Treatment of ground faults	Tripped for all ground faults	No Tripping for all ground faults	Tripped for permanent faults only
Required insulation level network	Low	High	Low
Reliability of power	Low	High	Highest
Effect on the environment	Serious	Slight	Very slight
Application	Cable line	Overhead line	All distribution networks

Table 1 shows the statistic data of neutral-voltage statistics. Table 2 details the time interval-based statistics data where it depicts 12 occasions when successive grounding faults occurred within a time interval of 3 to 10 seconds. Table 3 presents detailed data recorded on Sept. 5, 2000, during a period of intense thunderstorm activity throughout which a series of successive grounding faults occurred at very short time intervals. Furthermore, the data show that the compensations are reasonably accurate as the difference between the CASC's output current and the capacitive current is small.

The performance on the application effectiveness of the KD-XH based on field tests and operations in service can be summarized as follows:

• Accurate compensation and high efficiency for extinguishing grounding arcs.

• Adaptable for distribution networks, escalating capacitive current with gradual developed phases and free of maintenance.

• Enable to adopt a new type of neutral grounding so that instantaneous grounds can be self-cleared by the CASC and in the event of a permanent fault, the grounded lines can be identified and isolated quickly. GPEPC decided that this method was very promising for the neutral-point grounding of distribution networks and would endeavor to extend its application in the future.

Table 4 shows the superior performance of the new CASC compared with the two existing methods of neutral grounding.

The research and development of the KD-XH CASC including field-testing, organizing commercial manufacture and installation took GPEPC's Power & Research Institute two years to complete.

Distribution Network Applications

Since the first KD-XH unit was completed in April 2000, the commercial production of KD-XH units developed rapidly. A series of products are available for a range of system applications in China, including units for series applications with capacities ranging from 150 to 1210 kVA (dry or oil-immersed). For multi-busbar application in a substation, the CASCs can be controlled by individual controller Type KZ-IA or can be managed by only one controller Type KZ-IIA according to different customers' equipment. Manufacturing facilities are being considered to produce KD-XH units for export.

Disadvantages of grounding distribution systems via arc suppression coils include cross-country faults that can increase the problem of finding the grounded line. In the past 10 years, several devices able to detect the grounded line have been developed and installed in China. Because the best products have an accuracy of some 80% to 90%, GPEPC's Power & Research Institute recently developed a grounded-line locating device that, following field-testing, showed 100% accuracy. If proved successful, this device will resolve the problem of cross-country faults. The combination of this grounded-line locating device and the KD-XH CASC, having a linear V-A characteristic and fast response, will ensure instantaneous ground faults are self-cleared and permanent faults are located and instantaneously isolated.

Lu Guoqing graduated from Tsinghua University in 1968 and received the senior engineer degree in 1992. Guoqing has worked on high-voltage (HV) research in the fields on insulation techniques, insulation coordination and testing techniques for HV equipment for many years. Currently, she is leader of the development team for network equipment at Guangdong Provincial Power & Research Institute.

Jiang Xinyu was awarded the MSEE degree from Xi'an Jiaotong University and joined the Guangdong Provincial Power & Research Institute in 1998. In addition to network equipment development, he is responsible for research on HV and power electron technology.

Ouyang Xudong received the BSEE degree from Tsinghua University and joined Guangdong Provincial Power & Research Institute in 1995, having special responsibilities for HV technology research.

Zhou Liangcai graduated from Huabei Electrical Power University in 1963 and was awarded the senior engineer degree in 1988. He is head of the Guangdong Provincial Power & Research Institute and has responsibilities in the study of HV technology.