Various secondary circuit diagrams and their explanations

1. Figure E-103 is the circuit diagram of the DC bus voltage monitoring device. Please explain its function.

Answer: The DC bus voltage monitoring device mainly reflects the level of DC power supply voltage. KV1 is a low-voltage monitoring relay. KV1 is excited at normal voltage and its normally closed contact opens. When the voltage drops to the set value, KV1 loses excitation, its normally closed contact closes, and the HP1 light plate lights up and sends out an audio signal. KV2 is an overvoltage relay. When the voltage is normal, KV2 loses magnetism and its normally open contact is in the off position. When the voltage is too high and exceeds the set value, KV2 is excited, its normally open contact is closed, and the HP2 light plate lights up and emits an audio signal. .

2. Explain the functions of each component in Figure E-104 DC insulation monitoring device wiring diagram.

Answer: Figure E-108 is a commonly used insulation monitoring device wiring diagram. When normal, the voltmeter 1PV is open circuit, causing contacts 5-7 and 9-11 of ST1 (1-3 and 2-4 of ST1 to be disconnected) from Contacts 9-11 of ST2 are connected and the grounding relay KA is put into operation. When the insulation of the positive or negative pole drops to a certain value, the unbalanced bridge causes KA to operate and a signal is sent via KM (if the insulation resistance of the positive and negative poles to ground is equal, no matter how much the insulation drops, KA cannot operate and cannot send out a signal) signal, which is its disadvantage). At this time, 2PV can be used to check to determine which pole's insulation is declining (when measuring "+" to ground, 2-1 and 6-5 of ST2 are connected; when measuring "-" to ground, 1-4 of ST2 , 5-8 are connected. Normally, 2-1, 5-8, and 9-11 of the bus voltmeter transfer switch ST2 are connected, and the voltmeter 2PV can measure the voltage between the positive and negative bus bars, and the indication is 220V.),

If the insulation between the positive pole and the ground decreases, turn ST1 to I level, and its contacts 1-3 and 13-14 are connected. Adjust R3 until the bridge balance voltmeter 1PV indicates zero volts; then turn ST1 to II level. At this time, When its contacts 2-4 and 14-15 are connected, the total insulation resistance value of the DC system to ground can be read from 1PV. If the insulation between the negative pole and the ground is reduced, first put ST1 in the II position, adjust 3R to balance the bridge, then put ST1 in the I position, and read the total insulation resistance value of the DC system to the ground. If the positive electrode is grounded, the voltage between the positive electrode and ground is zero. The negative pole indicates 220V to ground. On the contrary, when the negative pole is grounded, the situation is opposite. Voltmeter 1PV is used to measure the total insulation resistance of the DC system, with a resistance scale painted on the disk.

Since there is an artificial grounding point in this insulation monitoring device, in order to prevent other relays from malfunctioning, the current relay KA is required to have a sufficiently large resistance value, generally 30kΩ, and its starting current is 1.4mA. When the insulation resistance of any pole When it drops to 20 kΩ, a signal can be sent out. There are two situations: degradation of insulation to ground and occurrence of ground fault.

3. According to Figure E-105, explain the hazards when point A and point C; point B and point C; point A and point B or point A and point D are grounded at the same time.

Answer: The DC system plays an important role in the substation. To ensure the long-term safe operation of a substation, there are many factors, among which the insulation problem of the DC system cannot be ignored. The DC system of the substation is relatively complex. It is connected to the terminal strip, terminal box, operating mechanism box, etc. of the outdoor power distribution device through cable trenches. Grounding is more likely to occur due to cable damage, insulation aging, moisture, etc., and a one-pole event occurs. When grounded, since there is no short-circuit current, the fuse will not blow and can still continue to operate, but it must be discovered and eliminated in time. Usually, it is required that the insulation resistance value of various small busbars, terminal loops, and secondary cables of the DC system to the ground should not be less than 0.5MΩ when measured with a 500V megger. The quality of DC link insulation must be monitored frequently. Otherwise, it will bring many unsafe factors to the operation.

Now take Figure E-105 as an example to illustrate the hazards of DC grounding. When points A and C in the figure are grounded at the same time, +WC and -WC form a short circuit through the earth, which may cause the fuses FU1 and FU2 to melt and lose the protective power supply; when points B and C are grounded at the same time, When, it is equivalent to short-circuiting the trip coil. Even if the protection operates normally, the YT trip coil is short-circuited. Even if the protection operates normally, the YT trip coil will not start, and the circuit breaker will not trip. Therefore, in the event of a fault, it will override the trip; When point A and point B or point A and point D are grounded at the same time, the protection will malfunction and cause the circuit breaker to trip. The hazards of DC grounding are not only the ones discussed above, but there are many more, so I won’t introduce them one by one here.

Because DC grounding will cause many harmful effects, a device is specially designed to monitor the insulation condition of the DC system, so that it can promptly prompt the fault of the DC system to the personnel on duty for rapid inspection and processing.

4. According to Figure E-106, the circuit breaker control circuit diagram with light monitoring (electromagnetic operating mechanism) explains the name of each component and the action process.

Answer: In the picture: +WC, -WC - control bus; FU1, FU2 - fuse, R1-10/6 type, 250V; SA - control switch, LW2-1a.4.6a.40.20.20/F8 type; HG — Green signal lamp, XD2 type, with 2500Ω resistor; HR — Red signal lamp, XD2 type, with 2500Ω resistor;

KL - intermediate relay, DZB-115/220V type; KMC - contactor; KOM - protective outlet relay; QF - circuit breaker auxiliary switch; WCL - closing small bus; WSA - accident tripping small bus; WS - signal small bus; YT—circuit breaker tripping coil; YC—circuit breaker closing coil, FU1, FU2—fuse, RM10-60/25 250V; R1—additional resistance, ZG11-25 type, 1Ω; R2—additional resistance, ZG11-25 type , 1000Ω; (+)WTW—flash small busbar.

(1) "After tripping" position

When the handle of SA is in the "after trip" position and the circuit breaker is in the trip position, its normally closed contact is closed, +WC passes through FU1 SA11-10 HG and additional resistor QF (normally closed) KMC coil FU2 -WC. At this time, the green signal light circuit is connected and the green light is on, which means that the circuit breaker is in the post-trip position. It also means that the power supply, fuse, auxiliary contact and closing circuit are intact and can be closed. But KMC will not act because the voltage mainly drops on HG and the additional resistor.

(2) "Ready to close" position

When the handle of SA rotates 90o clockwise to the "preparatory closing" position, SA9-10 is turned on, and the green light HG circuit is (+)WTW SA9-10 HG QF (normally closed) KMC

FU2 -WC is turned on, the green light flashes, and a preparatory closing signal is sent, but the KMC still will not start because there are HG and R in series in the circuit.

(3) "Close" position

When the SA handle rotates 45o clockwise to the "closed" position, the SA5-8 contact is connected, and the contactor KMC circuit is +WC SA5-8 KL2 (normally closed) QF (normally closed)

The KMC coil-WC is turned on and started, closing its contacts in the closing coil circuit to close the circuit breaker. After the circuit breaker is closed, the QF normally closed contact opens and the normally open contact closes.

(4) "After closing" position

After letting go, the SA handle automatically rotates 45o counterclockwise and returns to the vertical (ie "after closing") position, and the SA16-13 contact is connected. At this time, the red light HR loop is connected by FU1

SA16-13 HR KL coil QF (normally open) YT coil FU2

-WC

On, the red light is on, indicating that the circuit breaker is in the closed position, and also indicates that the trip circuit is intact and can be tripped.

(5) "Preparatory trip" position