Ground-fault Protection for DC Systems
直流（DC）系统有正负母线。 If either bus is intentionally grounded, then it is referred to as a grounded system. If neither bus is grounded, then it is referred to as an ungrounded DC system. A ground fault on a DC system may cause damage to the source as well as in the field. If the system is ungrounded, then it is possible to use a ground-fault relay by installing a ground-reference module between the two buses to establish a neutral point (see Figure 1). The ground-fault relay uses this reference to create a path for fault current to flow, which can then be measured. The ground-reference module is selected to limit fault currents to a very low value.
Figure 1.
Ground-fault Protection for AC Systems
Ungrounded AC systems, as shown in Figure 2, were used where continuity of power was critical. 如化工厂或炼油厂等可能安有未接地系统，因为加工环节中一旦断电，将导致重大经济损失和产品损失。 但经验表明，这类系统问题有潜在问题，正逐渐为电阻接地系统所取代。 未接地系统的两大问题：瞬态过压问题和接地故障难以定位。
Figure 2.
An ungrounded system has no point in the system that is intentionally the non-current-carrying metal parts to ground. 只有系统电容导至地面时才出现接地效果（如图2所示）。 Continuity of operation occurs because the system can operate with one phase faulted to ground. 间歇性故障或电弧故障会造成瞬态过压，对地放电。 These voltages are impressed on the phase conductors throughout the system until the insulation at the weakest breaks down. 这种击穿可能出现在电气系统的任意点，引起两相接地（相-地-相）故障。 Although a ground fault can be detected or alarmed on the system, it is difficult to determine the location of the fault.
There are two methods used to detect ground faults on ungrounded systems. 一种是监控相-地电压。 因为出现接地故障时，故障相电势将骤降至地电势，指示灯变暗。 无故障相的指示灯变亮。 A second method to detect a ground fault is to measure the insulation resistance. 当绝缘性下降时，持续监控绝缘电阻的继电器会发出分级警报，提供预测性维修提示。 A visual indicator or meter can also be used.
直接接地系统
由于非接地系统自身存在问题，故人们的观年产生了变化，大多设计都从非接地系统逐渐向接地转移。 多数情况下，接地系统类型为直接接地。 直接接地系统由一套导体组成，系统中至少一个导体或中性点被有意识地接地（通常为变压器或发电机绕组的中性点）。 The problem with the direct connection is that ground-fault current can be excessive, causing Arc-Flash hazards, extensive equipment damage, and possible injury to personnel. A solidly grounded system cannot continue to operate with a ground fault.
Figure 3.
在直接接地系统中，电源星形点（或中性点）被直接连至地面，确保系统和相-地电压的稳定。
The high ground-fault current is easy to detect with fuses, circuit breakers, or protection relays, or a combination thereof, allowing for selective tripping (tripping the faulted feeder and not the main feeder). 出现接地故障时，故障点损坏可能比较严重，因为系统阻抗（通常情况下很低）是接地故障中限制电能的唯一途径。 鉴于接地故障电流过大和弧闪危害，应将存在故障的馈线移除。 在接地故障期间，持续运行中不得这样操作。
Figure 4.
Figure 4 illustrates an example of the dangers associated with solidly grounded systems. In this example, a ground fault occurs and the overcurrent protection is set at 600 A.
假使该接地故障不是断路故障，而是因绝缘击穿或线路与地面部分间隙缩小而导致的弧闪故障。
Because of the arc resistance, fault current may be as low as 38% of the bolted-fault level. This can be in the range of a normal load or a slight overload. The fault current may be low enough that the overcurrent device (600-A circuit breaker) does not sense a fault, or may pick it up but not trip for a long time. 电源供能集中在弧闪处，会很快造成严重的设备损坏。
能量释放还会导致火灾，损坏厂房设施，甚至对人身造成极度危害。 除将直接接地改造为电阻接地外， 预防损害的最好办法就是在接地故障即将来临前对微弱的漏地电流进行探测。 为达到这样的目的，保护继电器必须能感知微弱的漏地电流，并且不会跳闸。
In modern facilities, equipment often generates noise or harmonics that can interfere with a protection relay's ability to function properly. 例如，噪音和谐频干扰可能高于接地保护继电器的预设值，可能造成继电器运行出错，即使没有系统故障也可能出现误报。 保护继电器必须能滤掉噪音或谐频，从而可以提供可靠保护。
Resistance-Grounded Systems
Resistance grounding solves the problems commonly associated with both ungrounded systems and solidly grounded systems. The name is derived from the addition of a resistor between the system neutral and ground (as shown in Figure 5). The specifications of the resistor are user determined to achieve a desired ground-fault current, which must be greater than the system capacitive charging current (explained later in this section).
Figure 5.
Transient overvoltages can be eliminated by correctly sizing the neutral-grounding resistor (NGR) to provide an adequate discharge path for the system capacitance. Continuity of operation with one ground fault is typically allowable when ground-fault current is <− 10 A. The NGR limits the available ground-fault current. This eliminates or minimizes point-of-fault damage (Arc-Flash Hazards) and controls the ground-fault voltage. Pulsing current can be used to locate ground faults when ground-fault current is <−10 A. Pulsing current is created by using a shorting contactor to short out half of the resistance, causing the ground-fault current to double (usually one cycle per second). 借助手持式零序电流仪探测波动接地故障电流，从而定位接地故障点。
电阻接地方式唯一缺点：电阻一旦出现故障，系统接地将失效。 建议安装电阻监控继电器对其进行保护。 A protection relay for resistance-grounded systems is used to detect a ground fault and to monitor the neutral-to-ground connection. It can be used to provide alarms or to trip the feeder from service upon the detection of a ground fault.
继电器中的脉冲发生电路可被用于定位接地故障。 继电器还能发出警报，在中性点至接地点的路径出现故障时还会跳闸。 For systems 5 kV and less, high resistance grounding can be used. 高电阻接地一般将电阻电流限制在10 A以内。 这样操作后，接地故障依然在系统中，只要电压变化在系统额定范围内。 For systems above 5 kV, neutral-grounding resistors are typically rated for 25 A or more, and ground-fault current is cleared within 10 s
系统电容充电电流
尽管与地面无物理连接，但是电气导体和所有元件的绕组均有电容性接地。 因此，每个相位上均有微弱电流流向地面。 这类电流不在特定位置产生，而是分布于整个系统，正如对地电容分布于整个系统中一样。 For analysis,it is convenient to consider the distributed capacitance as lumped capacitance, as shown in Figures 6 and 7.
Figure 6.
尽管分布电容不均衡，电流表示数也为零，因为流经CT窗口的全部电流还需经过CT窗口回流。 System charging current is the current that will flow into the grounding connection when one phase of an ungrounded system is faulted to ground (see Figure 9). 如采取适当预防措施，可以如下图一样测得该电流：
▪ 如果故障出现在CT供电侧，CT窗口中的总电流就不会为零。
▪ 电流表通常可以测出无故障相上的总电容电流。 该值为CT负载端所有设备的充电电流。
Figure 7.
A single-line diagram of a three-feeder, resistance-grounded system with a fault on feeder 3 is shown in Figure 8. A CT (A1 and A2) on unfaulted feeders will detect the charging current of that feeder. 故障馈线上的CT (A3)将检测无故障馈线上的总电阻电流（IR）和充电电流(I1 +I2) 。
Figure 8.