1 Selection of protection mode
The typical connection between the small hydropower station and the system is shown in figure 1. There are usually two types of substation I: the first voltage level is1135/10 kV, and the wiring group is Y0/Y/δ-11; The second voltage level is 220/ 1 10/35 kV, and the connection group is Y0/Y0/δ- 1 1, as shown in the dotted box. For the protection configuration of 1 DL switch, the first connection is analyzed as an example.
Figure 1 Connection Diagram of Small Hydropower and System Interconnection
1. 1
The protection configuration is: directional current limiting, directional overcurrent (or three-stage distance protection), synchronous pressureless verification reclosing, and low-voltage and low-frequency disconnection.
(1) Time-limited current quick break
According to the matching with the quick break protection of outgoing current on the bus on the opposite side of this line, and avoiding the fault on the other side of the transformer connected to this bus, the setting is carried out. When avoiding the fault on the other side of the transformer connected to the opposite bus, the impedance of the small hydropower unit is several times larger than that of the protected line and transformer, and there is generally no protection range.
(2) Overcurrent
According to the setting of the maximum load current passing through the protection in the positive direction, this time limit matches the highest time limit of the next level protection. The maximum load current in the positive direction should be considered after load rejection fh in the second station, but the sensitivity of small hydropower stations is often insufficient when they operate in small mode.
(3) Distance protection
Small hydropower is a weak power supply, and the short-circuit current level is low, which makes the performance of distance protection device in an unstable area, with high investment and less use.
(4) At the same time, verify the reclosing without pressure.
Most of them are because it is not easy to synchronize after disconnection, and the power of small hydropower station and substation II is difficult to balance, so it is impossible to verify the coincidence of the same period, which eventually leads to the collapse of small power grid.
(5) Low voltage and low frequency disconnection
After the large power supply loses power, due to the small capacity of small water and the large power shortage, the frequency and voltage drop rapidly, and the low-frequency relay can't be exported, just disconnecting the device, which can't be used as line protection and should not be adopted.
1.2 current protection disconnection mode
The protection configuration is as follows: the current in the direction of switch 1 DL is connected to switch 2 DL in time delay, and the small hydropower station is disconnected when the connecting wires XL 1 and XL2 fail. It should cooperate with the quick-break (or speed-limit break) protection of outgoing line on the bus on the opposite side of the line, and meet the maximum load current passing in the positive direction of protection. When the contact wires XL 1 and XL2 fail, sufficient sensitivity is required.
The maximum operating current calculated according to the fault sensitivity of each tie line often cannot meet the maximum load current passing through in the mode of small hydropower station, which limits the operation of small hydropower station. At the same time of sudden load rejection fh in station ⅱ, it is easy to cause the disconnection of small hydropower stations.
1.3 voltage protection off mode
The protection configuration is as follows: the switch 1 DL acts to trip the switch 2 DL with low voltage delay, and the small hydropower station will be disconnected when the connecting wires XL 1 and XL2 fail. If it is an instantaneous fault, the power supply to the user can be restored by using the switch on the F side of XL 1 and XL2 line reclosing system. The voltage protection should match the first or second section of the next level protection. When the tie wires XL 1 and XL2 fail, sufficient sensitivity is required.
Voltage protection disconnection overcomes the shortcomings of current protection disconnection: first, the smaller the operation mode of small hydropower, the higher the sensitivity of voltage protection; Second, voltage protection does not limit the output of small hydropower generation. It can not only improve the reliability of power supply to users, but also adapt to various operation modes of small hydropower, which is an economical and effective protection method.
In order to prevent voltage circuit disconnection, voltage circuit disconnection locking device (such as LB- 1A relay of Xuji Factory) should be installed. In case of dead zone of directional element, protection or switch failure, in order to prevent non-synchronous coincidence, reclosing devices of switches on XL 1 and XL2 side of tie lines shall be equipped with voltage loss and synchronous verification devices. According to the actual situation of the power grid, I-section direction current-voltage interlock can be added, and a part of the line can be protected with a fast protection device, and a current locking circuit can be added to improve the reliability of protection. When the low voltage has a matching relationship with the reverse protection device, the directional element may not be set or stopped.
Voltage analysis of protection device under fault condition
In the simple power grid shown in Figure 2, let the positive sequence impedance equivalent to the system fault point be equal to the negative sequence impedance x, the zero sequence impedance equivalent to the system fault point be X0, the positive sequence impedance from the voltage calculation point on one side of the transformer to the fault point on the other side be δ x, and all values in the calculation take the modulus of their nominal values, so the positive and negative sequence components are as follows:
( 1)
Fig. 2 Schematic diagram of simple power grid
2. 1 three-phase short circuit
I= 1/X，U =δX/X(2)
2.2 two-phase (phase b and phase c) short circuit
(3)
(1) Y/Y- 12 transformer side is short-circuited.
The voltage vector diagram of the calculation point is shown in Figure 3, then:
Substituting formulas (1) and (3) into the above formulas, we get:
Since the above voltage is based on the nominal value of the phase voltage, it needs to be calculated according to the line voltage, so:
(4)
Fig. 3Y/Y- 12 Short circuit of BC phase on one side of transformer
(2)Y/δ- 1 1 transformer δ side two-phase short circuit
The voltage vector diagram of the calculation point is shown in Figure 4.
(5)
Figure 4Y/δ- 1 1 BC phase short circuit on δ side of transformer
(3)Y/δ- 1 1 transformer Y-side two-phase short circuit.
The voltage vector diagram of the calculation point is shown in Figure 5.
(6)
Figure 5Y/δ- 1 1 BC phase short circuit on Y side of transformer
2.3 Two phases (phase B and phase C) are short-circuited and grounded.
(1) Y0/Y- 12 Y0 side is short-circuited to ground.
The voltage vector diagram of the calculation point is shown in Figure 6.
(7)
Figure 6 Y0/Y- 12 Transformer Y0 side BC phase short circuit grounding
(2) Y0/δ- 1 1 Y0 side two-phase short circuit grounding.
The voltage vector diagram of the calculation point is shown in Figure 7.
(8)
Fig. 7 Y0/δ- 1 1 Side BC Phase Short Circuit Grounding
2.4 Single-phase (phase A) grounding
IA 1=IA2= 1/(2X+X0)
UA 1=(X+X0)/(2X+X0)
UA2=X/(2X+X0)
(1) Y0/Y- 12 Y0 side single-phase grounding.
The voltage vector diagram of the calculation point is shown in Figure 8.
(9)
Fig. 8 Y0/Y- 12 Transformer Y0 side A is grounded.
(2) Y0/δ- 1 1 Y0 side single-phase grounding
The voltage vector diagram of the calculation point is shown in Figure 9.
( 10)
Fig. 9 Y0/δ- 1 1 transformer Y0 side a is grounded.
3 setting calculation of voltage protection disconnection
3. 1 The operating voltage is set according to the protection sensitivity.
(1) When the fault is three-phase short circuit, two-phase short circuit or two-phase short circuit grounding.
From formulas (2), (4) and (7), we can get:
Where Udz.j—— is the working voltage of the low-voltage relay;
ZBH- impedance from the protection device to the end of the protected line;
Zxtf.max—— impedance of small hydropower system behind protection device under maximum operation mode;
KE—— the ratio of generator potential to primary rated voltage of PT, which is1.1;
KK—— Reliability coefficient, taking 1.3.
Because the impedance of small hydropower units is large, if Zxtf.max is three times that of UDZ Zbh. J = 35.75 V, and the minimum range of DY series low-voltage relay with rated voltage of 100 V is 40 V, which can completely meet the setting requirements. If the setting requirements are not high, static voltage relay can be used.
(2) When XL1line is grounded in single phase.
As can be seen from formula (9), the relay voltage at the protection installation place is greater than 50 V, so it is impossible to disconnect the undervoltage protection. However, when XL 1 line trips due to the single-phase grounding protection action of the switch on the F side of the system, for substation I: If the neutral point on the high voltage side of the main transformer is not grounded, the gap protection will remove the fault after a certain delay (generally 0.5 s); If the neutral point of the high voltage side of the main transformer is grounded, its zero sequence protection will have a certain delay to remove the fault. Therefore, the combination of main transformer gap protection and zero sequence protection and system F-side switch reclosing can make up for the shortage of under-voltage protection disconnection.
3.2 The cooperation between the action voltage and each outlet of the relative bus
Take the wiring shown in figure 1 as an example. Let the reactance of the protection extending to the lower line be X. When the working voltage of the low-voltage relay is UDZ. J, there is the following equation. After the extension range x is obtained, the matching relationship with the outlets of the opposite bus can be determined.
Where kz- is the increasing coefficient, which is equal to (xmf.min+xxl2+xnf.min)/xmf.min.
XNf.min—— In the minimum mode, small hydropower F drops to the reactance of bus N;
XMF.min—— In the minimum mode, the system F drops to the reactance of bus M;
Reactance of xxl 2-xl2 line.
3.3 The cooperation between the action voltage and the tertiary side of the opposite main transformer
In the wiring shown in figure 10, bus M is the installation place where the undervoltage protection is disconnected, the zero-sequence reactance of the line to which bus N belongs is n times of its positive-sequence reactance, and the unit value of low-voltage operating voltage is Udz. As long as we find the range X 1 that extends out of the bus N under various faults, we can easily check the matching relationship with each outlet protection of the bus N.