Fault handling of DC voltage divider in UHV converter station

Author： GOZ Electric Time：2024-11-29 09:52:09 Read：13

During the power transmission process, the UHV converter station is equipped with a unique DC voltage divider. The DC voltage divider measures DC voltage, and its function is the same as that of AC voltage transformer. In the DC transmission currently in operation, the DC voltage divider is used to measure the DC voltage of the pole bus and the neutral bus. The measured data is transmitted to the converter station through the data transmission system, and then the UHV DC transmission system is protected according to the data. In the power grid system, the failure rate of the DC voltage divider is higher than that of the AC voltage divider, and it is more likely to cause charge loss after a failure.

1 DC voltage divider structure and principle

1.1 DC voltage divider structure The primary equipment of the pole bus DC voltage divider is divided into two parts: the voltage divider unit and the balancing unit. Its components are shown in Figure 1.

The voltage divider unit and the balancing unit are important components of the DC voltage divider. The balancing unit of the DC voltage divider is connected to the voltage divider board and the isolation amplifier. When a fault occurs, the fault is handled.

1.2 Principle of DC voltage divider

The DC voltage divider body protection is SF6 low pressure protection, and the density relay on the voltage divider has 3 contacts. The three different contacts of the relay correspond to the SF6 low pressure first stage alarm, SF6 low pressure second stage alarm, and SF6 low pressure trip. The alarm values of different stages are different, the first stage alarm value is 0.3 MPa, the second stage alarm value is 0.27 MPa, and the trip value is 0.22 MPa. Each DC voltage divider is equipped with 3 density relays, and the entire DC voltage divider has 3 trip contacts. When the SF6 pressure of the DC voltage divider is lower than the trip value, the "three out of two" trip outlet judgment is realized through the pole protection. The DC voltage divider of the UHV DC project adopts the form of a resistor-capacitor voltage divider, which consists of two parts: one is the high-voltage unit, that is, the high-voltage arm in the figure, which is composed of many levels of resistors and capacitors in series and parallel, represented by R1 (voltage dividing resistor) and C1 (voltage equalizing capacitor); the other is the low-voltage measurement system, that is, the low-voltage arm in the figure. The low-voltage unit is composed of resistor R2 and capacitor C2, which are connected in parallel. The low-voltage system also includes a secondary measurement system, which is composed of R3 and C3 in parallel. In the low-voltage bridge arm of the DC voltage divider, the role of resistor R2 is to form a voltage divider with R1 to reduce the voltage in the circuit. Capacitor C2 can compensate for the capacitance in the high-voltage arm. After installing the DC voltage divider, capacitor C2 needs to be adjusted to match C2 with the secondary measurement system C3 and cable capacitor C4 for system measurement.

2 Case Analysis

During the inspection of power lines, it was found that the back-end pole 1 protection system (A system) of a certain UHV converter station had a fault, which was manifested as the low pressure of the DC busbar DC divider SF6 locked. The bipolar was not locked, the power was not lost, and the power was 1000 MW. In view of this situation, analysis and treatment were carried out.

2.1 Preliminary Fault Analysis

On-site inspection, the pressure A/B/C of the SF6 meter of the DC divider of pole 1 was normal, and there was no obvious abnormality in the related relays in the protection panel cabinet; the PPR1A, PPR1B and PPR1C panels of the pole 1 protection were checked, and it was found that the low pressure signal of the SF6 gas of the DC divider of pole 1 was opened in the A set protection (the K339 relay was high level), and there was no input signal in the B set and C set. Read the data of the divider meter. Since the SF6 pressure of the pole bus is low, a level 2 alarm and a level 1 trip are set, and the alarm and trip signals are connected to the pole control and pole protection respectively. The OWS backend only reports "Pole 1 Pole Protection System A: DC Pole Bus DC Divider SF6 Pressure Low Lockout", and the pole control system has no alarm. The initial judgment is that the external contact of the input protection system is misconnected. The protection device PPR1A is connected to the pole 1 DC divider terminal box, which is connected to the root of the DC divider. Considering the weather conditions on the day of the fault, the "DC Pole 1 Bus DC Divider SF6 Pressure Low Trip" was opened on site and the PPR1A screen terminal X301:39 of the pole 1 protection was opened, and the OWS signal was restored. The DC pole bus DC divider SF6 pressure low protection changed from "three out of two" to "two out of two".

2.2 Fault handling

In order to find out the cause of the fault, the DC control protection system was checked and it was found that the relays in the PPR1B and PPR1C panels were working normally. There was no fault signal input in the non-electrical protection systems of B and C. The K339 relay indicator in the PPR1A panel of pole 1 was on. The voltage measured by a voltmeter at the input terminal X301:39 of the K339 relay was 113 V. It was confirmed that the external SF6 density relay contact was turned on and the positive voltage was turned on in the protection cabinet.

The SF6 density relay was installed on site in the U1 DC divider of the pole 1 busbar. When the gas pressure dropped below 0.22 MPa, the SF6 density relay contact signal was connected from the DC divider body to the protection panel cabinet. The protection panel realized the "three out of two" function of non-electrical protection through the control and protection device. When two sets of non-electrical protection issued a DC divider low pressure alarm, the DC divider was exported and the pole 1 was locked.

On-site investigation showed that the DC voltage divider was about 3.6 m from the ground, the busbar at pole 1 was in step-down operation (640 kV), and the safety distance was 9 m (reference 660 kV DC), which met the safety work requirements on site. An insulating ladder was set up and further inspection revealed that the reading of the SF6 density relay on site was about 0.34 MPa, which did not reach the low pressure alarm value, and the meter was judged to be normal. There was no gas leakage, and the signal should be a false alarm, which may be caused by a terminal failure.

The output voltage of the gas relay terminals was measured in sequence. The voltage of terminal 31 was +115 V, terminal 34 was +115 V, and terminal 32 was +50 V, which fluctuated up and down. It was initially judged that the insulation between terminal 32 and the +110 V DC voltage terminal decreased. The fault was analyzed and confirmed to be an insulation fault between the density relay terminals. The solution of replacing the relay was adopted. After replacement, the density relay wiring was restored. The SF6 leak detector was used for inspection, and no leakage was found. The terminal voltage was normal. After restoring the "DC Pole 1 Busbar DC Divider SF6 Pressure Low Trip" input to the Pole 1 Pole Protection PPR1A Panel Terminal X301:39, the OWS background was stable and no abnormal signal appeared.

3 Analysis of Operation and Maintenance Countermeasures

In the operation of the converter station, the DC divider is an important equipment that provides input for multiple protections. To ensure the normal operation of the DC divider, the operation and maintenance personnel should adopt appropriate operation and maintenance strategies, including the following:

(1) Strengthen the inspection of the divider terminal box to ensure good sealing of the terminal box to avoid moisture.

(2) When repairing the DC divider, the non-electrical secondary safety measures of the divider should be taken to avoid false operation.

(3) Pay close attention to the operation and contamination of the DC divider. When the hydrophobicity is found to be deteriorated, take measures such as spraying PRTV in time.

(4) Strengthen the monitoring of the DC divider density relay pressure, check the meter and the body sealing, and deal with SF6 leakage in time.

(5) Strengthen the rain and moisture protection management of the DC voltage divider, SF6 density relay meter and secondary junction box, and take measures such as installing rain covers, heaters, and placing desiccants. At the same time, regularly measure the circuit insulation to ensure that the secondary circuit insulation is good.