Improvement of relay protection for arc fault in GIS

1 Arc fault protection operation issues

1.1 The role and significance of arc fault protection

Take the 57-transformer 35kV system with double busbar as an example (primary wiring as shown in Figure 1), each of the 12 GIS switchgears produced by ABB is equipped with a circuit breaker gas chamber and two busbar gas chambers, which are independent gas chambers, and are equipped with a gas pressure sensor with a rated working pressure of 130kPa. When the gas chamber pressure drops to 120kPa, the low pressure alarm is activated; when the gas chamber pressure reaches 150kPa, the overpressure alarm is activated; when the gas chamber pressure suddenly reaches 190 kPa, it is judged as a rare internal arc fault, and the arc fault protection is activated, which quickly responds to the arc fault, trips the switch in the first time, and cuts off the short-circuit current. If the arc fault is not removed, the pressure release disk opens to release gas to protect the GIS gas chamber equipment when it reaches 200kPa.

Figure 1 35kV primary system wiring diagram of Wuqi substation

1.2 Arc Fault Protection Action Principle

After all the normally closed contacts of the pressure sensors of each SF6 gas chamber are connected in series, they are then connected in series with the coil of the arc fault start relay, and the normally closed contacts of the relay are connected to the switch fault trip circuit. During normal operation, all the normally closed contacts of the pressure sensors are closed, the arc fault start relay coil is energized, the normally closed contacts of the relay are disconnected, and the switch fault trip circuit is cut off; when an arc short circuit fault occurs in a gas chamber, when the detection pressure of the gas pressure sensor reaches 190kPa, its normally closed contacts are opened, the arc fault start relay coil loses power and is restored, and its normally closed contacts of the relay are closed, connecting the switch fault trip circuit, causing the switch to trip. As shown in Figure 2 Arc fault protection start and trip circuit.

Figure 2 Arc fault protection starting and tripping circuit

1.3 Original design of arc fault protection

All normally closed contacts of the pressure sensors of each SF6 gas chamber, such as K701.1, K701.2, and K701.3 (these 3 contacts are for the 3 gas chambers of cabinet 531), are connected in series, and then connected in series with the coil of the arc fault starting relay KC14 (KC15) (installed in 1 group each in the incoming cabinets 531 and 532). The normally closed contacts 21 and 22 of the relay are connected to the power incoming line (KC14) and the main coupling (KC15) switch fault tripping circuit.

During normal operation, the normally closed contacts of all pressure sensors are closed, the arc fault starting relay coil is energized, the normally closed contacts of the relay are disconnected, and the switch fault tripping circuit is cut off; when an arc short circuit fault occurs in a gas chamber, the detection pressure of the gas pressure sensor reaches 190kPa, its normally closed contacts are opened, the arc fault starting relay coil loses power and is restored, and its normally closed contacts of the relay are closed, connecting the 35kV incoming line 531, 532 and busbar 533 switch fault tripping circuit, causing the switch to trip.

1.4 Arc Fault Protection Operation Issues

Since arc protection takes the protection of switchgear as the priority, and the 35kV system is limited by the double busbar operation mode and the need to adjust the operation mode very flexibly, the following operation problems are caused:

(1) When the DC power supply of the incoming switch of the 35kV GIS switch cabinet is lost, the coils of the arc fault starting relays KC14 and KC15 lose power and are restored, the normally closed contacts of the relays are closed, and the switch fault tripping circuit is connected. Since the protection power supply is also lost at the same time, the switch will not operate. However, if the DC power supply of the switch cabinet is supplied at this time, the protection power supply is energized, and the normally closed contacts 21 and 22 of the arc fault starting relay have not yet operated to open, the switch fault tripping circuit is still connected, causing the switch to trip erroneously.

(2) For the purpose of protecting the equipment, when an arc short circuit occurs, all possible power supply lines and bus tie switches must be cut off. All normally closed contacts of the pressure sensors used to detect the gas pressure of each SF6 gas chamber are connected in series. When an arc short circuit occurs in one of the gas chambers of any switch, according to the design requirements, the 35kV incoming line 531, 532 and bus tie 533 switches will trip at the same time, causing the entire 35kV system of the 57 transformer to lose power, which often expands the scope of the accident.

(3) This protection is not combined with the specific operation mode of our power system. According to the consideration that the power supply of the 57-substation 35kV system is only provided by the 35kV side incoming line switch of the main transformer, when designing the arc fault protection, only the 35kV side incoming line (531, 532) of the main transformer and the bus tie switch are set. In fact, the 57-substation 35kV system may also be powered by the propylene transformer through the 535 and 539 circuits of the 535 and 539 interconnection lines. Our self-provided power station also often reverses power to the 57-substation 35kV system through the 2nd transformer 534 and 540 circuits. Therefore, under the original design, the GIS equipment will inevitably face the severe accident of arc fault for a long time. Therefore, it is necessary to add arc fault protection to these cabinets. See Figure 1 for details of the 35kV primary main wiring diagram of the 57-substation.

2 Analysis of Arc Fault Protection Improvement

2.1 Improved analysis

In order to eliminate the above hidden dangers and ensure the safe operation of our power system in the future, the design of arc fault protection of the 35kV system of the 57th transformer was improved.

Each arc fault protection is started by adding overcurrent protection. When the DC power supply is lost and then restored, the overcurrent action condition will not be reached because the current has not changed. The original arc fault protection power recovery false tripping can be avoided.

At the same time, the scope of the fault can also be determined by whether the overcurrent element of the power supply line switch is actuated or not: that is, the overcurrent element of the power supply line switch on the fault side is actuated, and the arc fault protection is tripped; the overcurrent element of the power supply line switch on the non-fault side is not actuated, and the arc fault protection is not actuated. Then the overcurrent non-delay action is used to ensure the requirement of fast protection.

Considering the possibility that the propylene substation supplies power to the 35kV system of the 57 substation through the 535 and 539 circuits, and also considering the possibility that our factory's self-contained power station supplies power to the 35kV system of the 57 substation through the 534 and 540 circuits, the above four circuits are also equipped with the same arc fault protection as the 35kV side incoming line switch of the 57 main transformer (531 and 532) (using the contacts K14 and K15 of the incoming line cabinet).

Since the operation mode of the double busbar is very flexible, when the arc fault protection of any gas chamber is activated, the power supply incoming switch supplying power to the faulty gas chamber can no longer be designed with a simple scheme to distinguish the fault range, so all switches that meet the conditions will trip. Therefore, the power outage of the faulty gas chamber will no longer trip the 35kV bus tie switch at the same time (the K15 originally used for the bus tie is changed to be used for other cabinets).

In order to ensure that the above requirements are met at the same time, the design of arc fault protection for the 35kV system of the 57th transformer is improved as follows: In the arc fault tripping circuit, an overcurrent protection without delay is added to start the arc fault protection. The schematic diagram is shown in Figure 3. The arc fault tripping circuit of the power supply line switch.

Figure 3 Arc fault tripping circuit of power supply incoming switch

The figure shows the improvement scheme of the arc fault tripping circuit of the power supply incoming line switch. Switches 531, 532, 535, 539, 534, and 540 are independently set in each secondary circuit. OUT in the figure is the overcurrent protection output contact without delay, which is provided by the existing integrated protection relay SEL of each line (535 and 539 are SEL-351 relays, 534 and 540 are SEL-311L relays, and 531 and 532 are SEL-351A relays). The power switch protection relay is closed when overcurrent is detected.

2.2 Improved reanalysis

After the above scheme was improved, the overcurrent protection without delay was added, which solved the problem that needed to be solved in the arc fault protection operation defect. However, after considering the actual situation of our factory, such as the transformer capacity is not large enough, full load operation often occurs, and load imbalance often occurs in the 35kV operation mode. If the DC power supply disappears and the power is re-supplied, a large motor is started at the same time, which will cause the overcurrent protection to operate, and the arc fault protection will still be mistakenly started. The overload current and short-circuit current in the power grid are sometimes not much different, so the overcurrent protection set according to the maximum load current cannot meet the sensitivity requirements in some cases, but because the load impedance and short-circuit impedance have different properties in the two cases of overload and short circuit, that is, the voltage amplitude will change. Therefore, taking advantage of the existing comprehensive protection SEL has voltage components, a low voltage lock is added to the arc fault tripping circuit, and it is allowed to operate only when there is a low voltage. These can be easily achieved in the existing comprehensive protection SEL products. Now OUT is a low voltage latching overcurrent protection outlet without delay, which is used to start arc fault protection. The wiring is the same as before, and the added logic equation is directly set in the SEL relay.

After analyzing the previous situation, we found that the protection after adding low-voltage lockout can continue to maintain speed and selectivity in various situations, while greatly improving sensitivity and reliability.

3. Improvement plan for arc fault protection of 35kV system in Wuqi Substation

3.1 Circuit composition

All 36 air chamber pressure sensors are connected in series, and then connected in series with the coils of arc fault starting relays KC14 and KC15 of cabinets 531 and 532 (4 of these relays are connected in parallel); the six possible power supply line comprehensive protection SELs are independently set with overcurrent protection, and their OUT ports are respectively connected to different contact groups of KC14 and KC15, and then connected to their respective tripping circuits.

3.2 Calculate and set the GIS arc protection overcurrent start value

3.3 Motion Analysis

When an arc short circuit fault occurs in a certain gas chamber, when the detection pressure of the gas pressure sensor reaches 190 kPa, its normally closed contact opens, all arc fault starting relay coils lose power and reset, and its normally closed contact closes, that is, the corresponding tripping circuit contacts of 35kV incoming lines 531, 532, 2 transformer 534, 540, propylene transformer 535, 539 are connected respectively, and then the respective overcurrents are used to determine whether the action conditions are met. That is, if the switch that supplies power to the gas chamber will start the fault tripping circuit to cause the switch to trip.

4 Conclusion

The "low voltage lockout without delay overcurrent" and arc fault protection are used in combination, making full use of site conditions. The scheme is reasonable, the wiring is simple, and it solves the problem of arc fault protection being used in dual-bus multi-power supply systems. It also solves the problem of switch mis-tripping when the arc fault protection control power supply is restored.