Finding «earth» in the DC network of the substation

"Ground" in the DC network is one of the emergency situations that often occur in distribution substations. The direct current in a substation is called operating current; it is intended for the operation of devices for relay protection and automation, as well as for the control of substation equipment.

The presence of "earth" in the DC network indicates that one of the poles is shorted to earth. This mode of operation of the permanent network of the substation is unacceptable and in the event of an emergency of the substation may lead to negative consequences. Therefore, in the event of this situation, it is necessary to immediately start looking for damage and repair it as soon as possible. In this article, we will look at the process of finding and removing a short circuit to ground in the DC network of the substation.

The occurrence of «earth» in the DC network is recorded on the central signal panel of the substation by light and sound alarms. The first thing to do is to make sure that there is indeed a ground on the DC mains.

The electrical panel of the substation usually contains a voltmeter to monitor the insulation and corresponding switching devices, by switching which you can measure the voltage of each of the poles to ground. In one position of this switch, the voltmeter for monitoring the insulation is connected to the circuit «ground» — «+», in the other position — respectively — «ground» — » -«. The presence of voltage in one of the positions indicates that there is a ground fault in the DC network.

If there are two separate sections of the DC board that are not electrically connected, it should be possible to check for voltage to ground for each section separately.

The presence of grounding in the permanent network indicates that the insulation of one of the cable lines is broken, which supplies the operating current to relay protection and automation devices or directly to equipment elements and other permanent consumers in the substation. Or the cause could be a broken wire that subsequently came into contact with the ground or grounded equipment.

This mode of operation is unacceptable, because in this case the device that receives power through this cable may not work properly or even be damaged (if one of the cores is interrupted). For example, one of the high voltage circuit breaker drive solenoids. If the cable that supplies DC power to this solenoid is damaged, then in the event of an emergency, such as a line short, this breaker will fail, potentially damaging other equipment.

Or, for example, protection devices based on microprocessors.As a rule, the microprocessor terminals of the substation equipment protection are supplied with direct current for control. These cabinets are powered by several cables coming out of the DC board. In most cases, one cable feeds several cabinets, for example six.

If this cable is damaged, then the terminals of the microprocessor for protection, automation and control of the equipment will be disconnected. Therefore, all six connections will remain unprotected, and in the event of an emergency, the equipment will not be disconnected and may be damaged (in the absence or damage of backup protections).

Therefore, it is necessary to detect the damage that led to the occurrence of grounding as soon as possible.

The search for grounding in the DC network is reduced to the subsequent disconnection of all outgoing lines that are powered by the DC cabinet of the substation. Let's give an example of finding the place of failure.

We turn off the circuit breakers that supply the electromagnetic ring of the 110 kV circuit breakers and check the insulation control. Normally, the electromagnetic ring is powered by two circuit breakers in different sections of the DC board to ensure high circuit reliability.

If there is no voltage on either pole with respect to ground, this indicates that the ground is on the solenoid ring of the 110 kV switches. Otherwise, that is, if there are no changes and the grounding remains, we turn on the previously turned off circuit breaker and proceed to further detect the fault. That is, we turn off the rest of the circuit breakers one by one, followed by checking the insulation control using a voltmeter.

So when a line is found, when it is disconnected, the ground disappears, you need to find and fix the fault. Consider the sequence of further actions to detect the malfunction in case of an earth fault in the solenoid ring.

After that, our goal is to locate the damage. The solenoid ring of 110 kV circuit breakers consists of several sections. The DC cable runs from the DC switchboard to the secondary switch cabinet of one of the 110 kV breakers. In this cabinet, the cable branches: one goes directly to the control circuit of this circuit breaker, and the other to the secondary switch cabinet of the next circuit breaker.

From the second cabinet, the working current cable passes to the third and so on, depending on the number of switches located in the 110 kV switchgear of the substation. From the last switch, the cable goes to the DC board, that is, all the solenoids of the switches are connected in a ring.

There are circuit breakers in every second switch cabinet. One of them supplies the operating current to the breaker, and the other to the next secondary switch cabinet. To locate the damaged area, we turn off the switch in the secondary switch cabinet that supplies voltage to the entire ring, for example, to the first cabinet to which the operating current is supplied from the first section of the DC panel.

Thus, by turning on the 110 kV solenoid ring breaker from the first section of the DCB, we apply voltage to the cable going to the secondary switching cabinet of the first breaker.

We turn on this switch and check the insulation control.If there is a "ground", the fault is definitely located in that section of the cable. If the insulation check is normal, proceed with further search of the damaged area.

We turn off the switch that supplies voltage to the secondary switch cabinet of the second switch and turn on the switch that supplies the operating current to the control circuit of the first 110 kV switch, check the insulation control. The appearance of «earth» indicates that the fault is in the secondary switching circuits of the circuit breaker. In this case, the switch must be taken in for repair to eliminate this malfunction.

It is also necessary to actuate the solenoid ring by leaving the link switch off where damage to the secondary circuits is found. The next step is to check the insulation control to make sure there is no more earth fault in the DC network.

If, after applying operating current to the first switch, the insulation control remains normal, proceed. We turn off the switches in the second cabinet that supply the operating current to the second switch and to the next, third secondary switch cabinet.

In the first cabinet, we turn on the switch that supplies voltage to the second cabinet, that is, we connect the cable from the first cabinet to the second cabinet of the secondary switching to the ring.

Likewise, if a "ground" occurs, that section of the cable is damaged. Otherwise, that is, when the insulation control is normal, we turn on the breaker in the second cabinet, which supplies voltage to the DC circuits of the second switch, we check the insulation control to make sure that there is or is not a «ground».

In the same way, we do phased inclusion of sections of the solenoid ring and check the insulation control. Initially, when checking the cable that goes from the first section of the DC switchboard to the first secondary switch cabinet of the breaker, it is necessary to check the second cable that feeds from the second section of the DC board and goes to the secondary switch cabinet of the breaker.

It is possible that the fault is located on the second cable, and in order not to do unnecessary work - do not check the switch circuits and cable lines placed between the secondary switch cabinets, it is necessary to check both cables at once.

It should be noted that when the circuit breaker is removed for repair, in the secondary switch cabinet where faults are found on the operating current circuits, it is not always possible to turn off this switch remotely or from an actuated location, since one of the conductors of the secondary switching circuits may be broken.

If the control circuits of the circuit breaker are defective and it is not possible to turn off the circuit breaker manually, from the location, then remove the load from the circuit breaker and disconnect it from both sides with disconnectors. If possible, it is necessary to remove not only the load, but also the voltage from the switch, because in the absence of load at the user, the line disconnector switches off the capacitive currents of the line, which is not recommended.

See also: The main operational errors of personnel when performing operational switches, their prevention