Analysis of Causes for Grounding Transformer Protection Misoperation
In China's power system, the 6 kV, 10 kV, and 35 kV grids generally adopt a neutral-point ungrounded operation mode. The distribution voltage side of main transformers in the grid is usually connected in delta configuration, which provides no neutral point for connecting grounding resistors. When a single-phase ground fault occurs in a neutral-point ungrounded system, the line-to-line voltage triangle remains symmetrical, causing minimal disruption to user operations. Moreover, when the capacitive current is relatively small (less than 10 A), some transient ground faults can self-extinguish, which is highly effective in improving power supply reliability and reducing outage incidents.
However, with the continuous expansion and development of the power industry, this simple method no longer meets current demands. In modern urban power grids, the increasing use of cable circuits has led to significantly higher capacitive currents (exceeding 10 A). Under these conditions, the ground arc cannot be reliably extinguished, resulting in the following consequences:
Intermittent extinction and reignition of the single-phase ground arc can generate arc-ground overvoltages with amplitudes reaching up to 4U (where U is the peak phase voltage) or even higher, lasting for extended durations. This poses severe threats to the insulation of electrical equipment, potentially causing breakdowns at weak insulation points and leading to significant losses.
Sustained arcing ionizes the surrounding air, degrading its insulating properties and increasing the likelihood of phase-to-phase short circuits.
Ferroresonant overvoltages may occur, easily damaging voltage transformers and surge arresters—potentially even causing arrester explosions. These consequences severely endanger the insulation integrity of grid equipment and threaten the safe operation of the entire power system.
To prevent such incidents and provide sufficient zero-sequence current and voltage to ensure reliable operation of ground-fault protection, an artificial neutral point must be created so that a grounding resistor can be connected. This need led to the development of grounding transformers (commonly referred to as "grounding transformers" or "grounding units"). A grounding transformer artificially creates a neutral point with a grounding resistor, typically featuring very low resistance (usually less than 5 ohms).
Additionally, due to its electromagnetic characteristics, the grounding transformer presents high impedance to positive- and negative-sequence currents, allowing only a small excitation current to flow through its windings. On each core limb, two winding sections are wound in opposite directions. When equal zero-sequence currents flow through these windings, they exhibit low impedance, resulting in minimal voltage drop across the windings under zero-sequence conditions.
Specifically, during a ground fault, the winding carries positive-, negative-, and zero-sequence currents. It presents high impedance to positive- and negative-sequence currents but low impedance to zero-sequence current. This is because, within the same phase, the two windings are connected in series with opposite polarity; their induced electromotive forces are equal in magnitude but opposite in direction, effectively canceling each other out, thus presenting low impedance to zero-sequence current.
In many applications, grounding transformers are used solely to provide a neutral point with a small grounding resistor and do not supply any secondary load. Therefore, many grounding transformers are designed without a secondary winding. During normal grid operation, the grounding transformer operates essentially in a no-load state. However, during a fault, it carries fault current only for a short duration. In a low-resistance grounded system, when a single-phase ground fault occurs on the 10 kV side, highly sensitive zero-sequence protection quickly identifies and temporarily isolates the faulty feeder.
The grounding transformer is active only during the brief interval between fault occurrence and the operation of the feeder’s zero-sequence protection. During this time, zero-sequence current flows through the neutral grounding resistor and the grounding transformer, following the formula: I_R = U / (R₁ + R₂), where U is the system phase voltage, R₁ is the neutral grounding resistor, and R₂ is the additional resistance in the ground fault loop.
Based on the above analysis, the operational characteristics of a grounding transformer are: long-term no-load operation and short-term overload during faults.
In summary, a grounding transformer artificially creates a neutral point to connect a grounding resistor. During a ground fault, it exhibits high impedance to positive- and negative-sequence currents but low impedance to zero-sequence current, thereby ensuring reliable operation of ground-fault protection.
Currently, grounding transformers installed in substations serve two primary purposes:
Supplying low-voltage AC power for substation auxiliary use;
Creating an artificial neutral point on the 10 kV side, which, when combined with an arc suppression coil (Petersen coil), compensates for capacitive ground-fault current during 10 kV single-phase ground faults, thereby extinguishing the arc at the fault point. The principle is as follows:
Along the entire length of conductors in a three-phase power grid, capacitance exists both between phases and between each phase and ground. When the grid neutral is not solidly grounded, the capacitance to ground of the faulted phase becomes zero during a single-phase ground fault, while the voltages of the other two phases rise to √3 times the normal phase voltage. Although this increased voltage remains within the insulation design limits, it increases their capacitance to ground. The capacitive ground-fault current during a single-phase fault is approximately three times the normal per-phase capacitive current. When this current becomes large, it easily sustains intermittent arcs, exciting resonant oscillations in the grid’s inductive-capacitive circuit and generating overvoltages up to 2.5–3 times the phase voltage. The higher the grid voltage, the greater the risk from such overvoltages. Therefore, only systems below 60 kV may operate with an ungrounded neutral, as their single-phase capacitive ground-fault currents remain small. For higher-voltage systems, a grounding transformer must be used to connect the neutral point through impedance.
When one side of a substation’s main transformer (e.g., the 10 kV side) is connected in delta or wye without a neutral brought out, and the single-phase capacitive ground current is large, there is no available neutral point for grounding. In such cases, a grounding transformer is employed to create an artificial neutral point, enabling connection to an arc suppression coil. This artificial neutral allows the system to compensate capacitive current and extinguish ground arcs—this is the fundamental role of the grounding transformer.
During normal operation, the grounding transformer experiences balanced three-phase voltage and carries only a small excitation current, operating essentially unloaded. The neutral-to-ground potential difference is zero (neglecting minor neutral displacement voltage from the arc suppression coil), and no current flows through the coil. If, for example, phase C suffers a ground fault, the resulting zero-sequence voltage (derived from the asymmetry) flows through the arc suppression coil to ground. The coil generates an inductive current that compensates the capacitive ground-fault current, thereby eliminating the arc—functionally identical to a standalone arc suppression coil.
In recent years, multiple misoperations of grounding transformer protection have occurred in 110 kV substations in a certain region, severely affecting grid stability. To identify the root causes, analyses were conducted, corrective measures implemented, and lessons shared to prevent recurrence and guide other regions.
With the increasing use of cable feeders in 110 kV substation 10 kV networks, single-phase capacitive ground currents have risen substantially. To suppress overvoltage magnitudes during ground faults, many 110 kV substations now install grounding transformers to implement low-resistance grounding, establishing a zero-sequence current path. This enables selective zero-sequence protection to isolate ground faults based on location, preventing arc reignition and ensuring safe power supply.
Since 2008, a certain region has retrofitted its 110 kV substation 10 kV systems to low-resistance grounding by installing grounding transformers and associated protection devices. This allows rapid isolation of any 10 kV feeder ground fault, minimizing grid impact. However, recently, five 110 kV substations in the region experienced repeated misoperations of grounding transformer protection, causing outages and threatening grid stability. Thus, identifying causes and implementing solutions is essential.
1. Analysis of Causes for Grounding Transformer Protection Misoperation
When a 10 kV feeder suffers a ground fault, the feeder’s zero-sequence protection at the 110 kV substation should operate first to isolate the fault. If it fails, the grounding transformer’s backup zero-sequence protection trips the bus tie and main transformer breakers to contain the fault. Therefore, correct operation of 10 kV feeder protection and breakers is critical. Statistical analysis of misoperations in five substations shows that feeder protection failure is the primary cause.
The 10 kV feeder zero-sequence protection operates as follows: zero-sequence CT samples → protection initiates → breaker trips. Key components are the zero-sequence CT, protection relay, and breaker. Analysis focuses on these:
1.1 Zero-sequence CT errors causing misoperation
During a ground fault, the faulty feeder’s zero-sequence CT detects fault current, triggering its protection. Simultaneously, the grounding transformer’s zero-sequence CT also senses the current. To ensure selectivity, feeder protection settings (e.g., 60 A, 1.0 s) are lower than grounding transformer settings (e.g., 75 A, 1.5 s to trip bus tie, 2.5 s to trip main transformer). However, CT errors (e.g., -10% for grounding transformer CT, +10% for feeder CT) can make actual pickup currents nearly equal (67.5 A vs. 66 A), relying only on time delay. This increases the risk of grounding transformer overreach.
1.2 Incorrect cable shield grounding causing misoperation
10 kV feeders use shielded cables with shields grounded at both ends—a common EMI mitigation practice. Zero-sequence CTs are typically toroidal, installed around the cable at the switchgear outlet. During a ground fault, unbalanced current induces a signal in the CT. However, if the shield is grounded at both ends, circulating shield currents also pass through the CT, distorting measurement. Without proper installation (e.g., shield ground wire passing correctly through the CT), feeder protection may fail, causing grounding transformer overreach.
1.3 Feeder protection failure causing misoperation
Although microprocessor-based relays offer high performance, product quality varies. Common failures involve power, sampling, CPU, or trip output modules. If undetected, these can cause protection refusal, leading to grounding transformer misoperation.
1.4 Feeder breaker failure causing misoperation
Aging, frequent operations, or poor-quality breakers (especially older GG-1A types in rural areas) increase failure rates. Control circuit faults—particularly burnt trip coils—prevent breaker operation even when protection commands a trip, forcing grounding transformer backup to act.
1.5 High-impedance ground faults on one or two feeders causing misoperation
If two feeders experience simultaneous high-impedance ground faults on the same phase, individual zero-sequence currents (e.g., 40 A and 50 A) may stay below the feeder pickup (60 A), but their sum (90 A) exceeds the grounding transformer setting (75 A), causing overreach. Even a single severe high-impedance fault (e.g., 58 A) combined with normal capacitive current (e.g., 12–15 A) can approach 75 A. System disturbances may then trigger misoperation.
2. Countermeasures to Prevent Misoperation
2.1 Address CT errors
Use high-quality zero-sequence CTs; reject units with >5% error during commissioning; set protection thresholds based on primary values; verify settings via primary injection testing.
2.2 Correct cable shield grounding
Route shield ground wires downward through the zero-sequence CT and insulate from cable trays; avoid contact before the CT.
Leave exposed conductor ends for testing; insulate the rest.
If the shield ground point is below the CT, do not route it through the CT. Avoid placing the ground point within the CT window.
Train protection and cable personnel on proper installation.
Enforce joint acceptance inspections by relay, operations, and cable teams.
2.3 Prevent protection refusal
Use proven, reliable relays; replace aging or faulty units; enhance maintenance; install cooling/ventilation to prevent overheating.
2.4 Prevent breaker refusal
Use reliable, modern breakers (e.g., spring- or motor-charged sealed types); phase out old GG-1A cabinets; maintain control circuits; use high-quality trip coils.
2.5 Mitigate high-impedance fault risks
Promptly investigate and clear feeders when ground alarms occur; reduce feeder lengths; balance phase loads to minimize normal capacitive current.
3. Conclusion
While grounding transformers improve grid structure and stability, recurring misoperations highlight hidden risks. This paper analyzes key causes and proposes practical solutions to guide regions that have installed—or plan to install—grounding transformers.
Zigzag (Z-Type) Grounding Transformers
In 35 kV and 66 kV distribution networks, transformer windings are typically wye-connected with a neutral point available, eliminating the need for grounding transformers. However, in 6 kV and 10 kV networks, delta-connected transformers lack a neutral point, necessitating a grounding transformer to provide one—primarily for connecting arc suppression coils.
Grounding transformers use zigzag (Z-type) winding connections: each phase winding is split across two core limbs. Zero-sequence magnetic fluxes from the two windings cancel each other, resulting in very low zero-sequence impedance (typically <10 Ω), low no-load losses, and utilization of over 90% of rated capacity. In contrast, conventional transformers have much higher zero-sequence impedance, limiting arc suppression coil capacity to ≤20% of transformer rating. Thus, Z-type transformers are optimal for grounding applications.
When system unbalance voltage is large, balanced Z-type windings suffice for measurement. In low-unbalance systems (e.g., all-cable networks), the neutral is designed to produce 30–70 V unbalance voltage for measurement needs.
Grounding transformers can also supply secondary loads, serving as station service transformers. In such cases, the primary rating equals the sum of arc suppression coil capacity and secondary load capacity.
The primary function of a grounding transformer is to deliver ground-fault compensation current.
Figure 1 and Figure 2 show two common Z-type grounding transformer connections: ZNyn11 and ZNyn1. The principle behind low zero-sequence impedance is as follows: each core limb contains two identical windings connected to different phase voltages. Under positive- or negative-sequence voltage, the magnetomotive force (MMF) on each limb is the vector sum of two phase MMFs. The three limb MMFs are balanced and 120° apart, forming a closed magnetic path with low reluctance, high flux, high induced voltage, and thus high magnetizing impedance.
Under zero-sequence voltage, the two windings on each limb produce equal but opposite MMFs, resulting in zero net MMF per limb. No zero-sequence flux flows in the core; instead, it circulates through the tank and surrounding medium, encountering high reluctance. Consequently, zero-sequence flux and impedance are very low.
