AUTOMATIC DIVIDING DEVICE IN THE ELECTRICAL CONNECTION SCHEME OF A POWER STATION

Postgraduate Student V. I. STAROSTIN

The severe consequences for a power system caused by swings and asynchronous operation, when active losses increase due to equalizing currents and, as a result, frequency and voltage decrease, have forced a number of power stations—especially in the last decade—to install dividing devices (protections).

Such dividing devices were created on the basis of the current or frequency principle, but their schemes have not been discussed in the literature to a sufficient extent. Having substantial shortcomings, current-based dividing devices have not justified themselves in operation.

The underestimation by some authors of articles of dividing devices operating on the frequency principle [1, p. 31] consists in the fact that they regard such devices as an emergency-prevention measure ensuring the normal operation of station auxiliaries when the frequency is lowered owing to a power deficit, but without disruption of parallel operation. In that case, indeed, there is no special need for dividing devices when other effective measures are available (automatic frequency load shedding). Statistics of recent years show that cases of loss of static stability have become a frequent occurrence in power systems with weak ties, and this leads to disruption of the operation of station auxiliaries; automatic frequency load shedding is powerless here to restore the system to its initial stable state. In addition, the lack of attention to such dividing devices was also explained by the fact that for them it is necessary to calculate settings by frequency, voltage, and time, and neither the emergency-prevention circular of the Technical Administration of the Ministry of Electric Power Stations (1954) nor the directive materials of the Technical Administration of the Ministry of Electric Power Stations (1950) gave an answer to this question. A generator or several generators with the auxiliary load were disconnected manually, and the measure of the disconnection time and the degree of disturbance of the regime was the condition when the personnel saw the beginning disruption of the technological process. Now, with the present state of power stations, the widespread introduction of high and superhigh steam parameters, large capacities, and unit schemes, where thermomechanical processes proceed rapidly and the auxiliaries are very sensitive to changes in frequency, it is necessary to return again to dividing devices, but already automatic ones, and to consider their purpose from different positions.

The skeleton diagram of the automatic dividing device (ADD) represents the following operating sequence of interrelated elements, both of the electrical connection scheme and of the automation equipment:

1. Under the existing operating mode of the power station, the scheme for separating the station auxiliaries and individual consumer units from the rest of the electrical scheme is set on the program device by means of jumpers.

The number of generators to be separated must be such as to ensure reliable supply to the auxiliary mechanisms, while the remaining part of the system does not experience an excessive shortage of active power.

1. When the normal operating mode is disturbed, caused by a decrease in frequency and voltage, the starting element of the ADD responds to this with its first and second cascade, or with the second alone, delivering a command pulse to the program device, which is started and acts, with specified time delays through intermediate relays, on the control circuits of the circuit breakers in several stages, creating the scheme that had been fixed on the jumpers of the program device.

In order to assess the possible variants of separating the auxiliaries and consumer units, it is necessary to dwell briefly on the characteristics of the electrical connection scheme of the thermal power station under study.

Characteristics of the electrical connection scheme of the first stage of development of the thermal power station

The electrical connection scheme shown in Fig. 1, used for many standard thermal power stations, requires no special explanation. The positions of the disconnectors and circuit breakers are fixed for the “normal” scheme. The elements of the scheme not involved in the division during operation of the ADD are not shown in Fig. 1. The thermal power station is connected to the power system by six 110 kV transmission lines, some of which simultaneously supply substations of the electrified railway. Especially important consumers receive power from the generator switchgear, GSW-6 kV, through reactored lines. All generators operating on the 6 kV GSW buses have a capacity of 30 MW each. According to the conditions of short-circuit currents, the joint operation of two or more generators on sections not separated by reactors is not permissible for long periods. The 110 and 35 kV buses have differential protection, and, under normal conditions, the bus-coupler circuit breakers are switched on, on the basis of the conditions for the fixed connection of elements. The 35 kV buses also supply especially important consumers (petrochemical plants and the water intake). As can be seen from the scheme, the auxiliaries have two backup supply lines, 10 sh and 20 sh, which apply voltage to the backup assemblies connected to one another in a loop arrangement. The auxiliary scheme has automatic transfer switching.

The carrying capacity of lines 1 sh and 2 sh is 750 A each (6.4 MW), and that of the remaining auxiliary lines is 1000 A each (8.7 MW), including the cable jumpers 10–20 sh together with the sectionalizing switch.

The generators of the first stage, connected according to a unit scheme, have the following capacities: No. 1—30 MW, No. 5—25 MW, No. 9—50 MW.

The second stage consists of four units of 50 MW each (the electrical scheme of the second stage is not shown).

Fig. 1. Electrical connection diagram of the power station.

In terms of the operation of the thermomechanical equipment, the second stage is not connected with the first; however, in the electrical scheme, at voltages of 110 and 35 kV and at the auxiliary-services voltage of 6 kV, both stages operate jointly. Each of the 4 units has a tap to its own auxiliary-services section, the load of which, at the maximum output of the unit, taking into account the part of the load falling on the two general-station sections, is 5 MW. Units 10 and 11 have three-winding transformers 110/35/6 kV; units 12 and 13 deliver power to the 110 kV buses. From the 5 reserve auxiliary-services assemblies of the first stage, loops enter the reserve assemblies 6, 7, 8 of the second stage and are commutated according to the same principle as for the first stage.

Electrical loads

First stage:
generating capacity: $7 \times 30 + 1 \times 25 + 1 \times 50 = 285$ MW
consumption at a voltage of 6 kV: I-II-III sec. GRU-6 kV — 11 MW
IV-V-VI sec. GRU-6 kV — 15 MW
consumption at a voltage of 35 kV — 50 MW
auxiliary-services consumption: 21 MW
losses in transformers: 2.3 MW
power delivered to the system: 185 MW

Second stage:
generating capacity: $3 \times 50 = 150$ MW
auxiliary-services consumption: 15 MW
losses in transformers: 1.5 MW
power delivered to the system: 133.5 MW

Note: the load distribution is taken according to the actual schedule of autumn 1963, but in the absence of unit 4, which is expected to be introduced by the end of 1964.

Variants of schemes for separating auxiliary-services and consumer units

There is a large variety of variants of separation schemes. Without being able to characterize each of the most suitable variants, we shall dwell on the choice of the best one for the given scheme of electrical connections, and in doing so we shall take into account a number of factors, such as: reliability of supply, the magnitude of the power lost by the system, the number of switching operations, the structure of the new scheme, the availability of reserve, etc.

In this respect, variant No. 1 most fully satisfies the requirements; in accordance with it, the operation of the ADS scheme will be described.

VARIANT No. 1 is a switching system in which an electrically connected unit is formed, combining the auxiliary-services load of the first and second stages, the consumer load of all six sections of GRU-6 kV, and the consumer load supplied from the 35 kV buses, with all operating generators on the 6 and 35 kV buses.

When an emergency condition arises, the ADS acts to:

1. Close bus-coupler circuit breakers 1 shk and 4 shk, or 2 shk and 5 shk (the first stage of preliminary separation).
2. Close sectionalizing circuit breaker 10—20 shr.

1. Switching on reserve busbars 20 to sections 11, 12, 13, 14 (sections of the second-stage units).

2. Disconnecting the working auxiliary-service lines of the units, both by isolators and by circuit breakers, to their section: reserve busbars 9, 11, 12, 13 (second stage of preliminary division).

3. Disconnecting transformers 11, 12, 13, 14 from the 110 kV side (third stage—final division).

This variant provides for supplying the auxiliary services of the first stage and two common-station sections of the second stage by the working lines, as in the normal scheme.

The unit sections fed from taps of their own unit are switched over to supply from reserve lines, reserve busbars 10 and 20. The need to preserve the supply of two common-station auxiliary-service sections of the second stage from working lines, reserve busbars 7 and 8, is dictated by the circumstance that these working lines receive supply from sections 5 and 6 of the 6 kV main switchgear, which in this variant have no connection with the 110 kV voltage.

ADVANTAGES. 1. Reliable provision of power for the auxiliary services of the first and second stages by all operating generators of the 6 kV main switchgear.

1. Preservation of automatic reserve switching on the auxiliary-service buses.

2. Reliable provision of power for especially responsible consumers over 35 and 6 kV lines.

3. Fewer operations with circuit breakers than in the other variants.

4. Low probability of asynchronous switching operations (circuit breakers of transformers 11, 12, 13, 14 from the 110 kV side and circuit breakers of the working auxiliary-service lines of the second stage).

DISADVANTAGES. 1. The need to reduce the delivery of power to the system, which places it under still more severe conditions.

In this case, with a total generating capacity of the first stage of 285 MW (hydrogen-cooled generators), the delivery of power to the system will amount to 75 MW.

The total power consumption by the auxiliary services of the first stage and by the consumer load fed from the 6 and 35 kV lines in normal operation is 100 MW; then the power deficit in the system will amount to 110 MW.

1. Increased, in comparison with the nominal value, power flow through the cable jumper from reserve busbar 5 to 8. This flow, with the total power consumed by the auxiliary services of the second stage at its full development, less the load of two common-station sections fed according to the normal scheme, amounts to 1500 A. The weighted average \(\cos \varphi\) on the auxiliary services of the second stage is higher than that of the first and is 0.85. In this case the overload of the cables of auxiliary-service reserve line, reserve busbar 20, will be 150%. However, taking into account the possibility of overloading 6–10 kV cables by up to 30%, and considering the short duration of such a condition and its exceptional rarity, such an overload may be allowed. Moreover, with the existing high-pressure boiler units TP-82, the inevitability of comparatively frequent repair or cleaning of heating surfaces becomes apparent; therefore, in practice, of the four existing units, at full development of the second stage one of them must be shut down, and then the auxiliary-service load will decrease by 25%, which will lead to a reduction of the flow to 1100 A.

As for the loss of power by the power system during separation, the following important factor must be taken into account. Other variants, apart from the one under consideration, do not make it possible to keep the second stage auxiliaries in operation under emergency conditions, and for this reason the power system loses 133.5 MW, which exceeds the power loss from the first stage (133.5 MW as against 110 MW). The essence of another variant is that, for supplying the auxiliaries, four sections of the 6 kV main switchgear are isolated together with generator No. 6 and the 5 and 20 shunt lines. In this case, by switching on the circuit breakers from the reserve auxiliary busbars and disconnecting the operating lines, except for the 5 shunt line, the auxiliaries are transferred to supply from generator No. 6, and by disconnecting 13 t and 45 sh—a separation from the rest of the scheme is carried out. A variant is possible in which the scheme of reserve lines 10 sh and 20 sh, with several of the generators of the left or right wing of the 6 kV main switchgear, is assembled in advance onto the reserve busbar system. When the circuit breakers from the reserve auxiliary busbars are switched on and the operating lines are disconnected, the auxiliaries are transferred to supply from the isolated generators, and by disconnecting the bus-coupler circuit breaker of the section on which the generator is located, the auxiliaries are separated from the rest of the scheme. Both variants provide for isolating the 35 kV bus load into an independent block. For this it is sufficient to disconnect one of transformers 11 t or 12 t on the 35 kV side, and the other on the 110 kV side, with simultaneous separation of the 6 kV main switchgear sections 12 sh or 23 sh.

Using the example of the cogeneration plant under study, one variant of the separation scheme has been considered, and a brief description of two other variants has been given. For power stations having a different scheme, a similar analysis should be carried out and the best variant selected, to which the automatic dividing device should then be applied.

Scheme of the automatic dividing device

The great variety of electrical connection schemes of power stations must, to a significant degree, determine the concepts and design features of the ADD.

Not having any information about the ADD schemes of other power systems, we shall dwell on the description of a scheme with a motor-driven time relay in the program device (the author’s scheme).

The device may be divided into three main elements:

1. A starting element, responding to the frequency in the system and to the pressure in the supply main.
2. A program device, ensuring selectivity of the switched-on and switched-off objects of the electrical connection scheme.
3. Signaling circuits, indicating the presence of abnormal operating conditions and faults in the main circuits of the ADD.

Figure 2 presents an expanded connection scheme of the ADD as applied to the first variant, although it can be applied to any of them. In this case it is necessary to add several output intermediate relays and jumpers to the scheme.

The starting element consists of two cascades of frequency relays set to different pickup values. Their values are determined by calculation. The underfrequency relays are supplied from the secondary circuits of the voltage transformers of busbar systems I or II of the 110 kV outdoor switchgear. Selection of the voltage from the busbar systems is performed by switch PU.

Fig. 2. Circuit diagram of the automatic dividing device.

The IVCh-OPA units used in the scheme must be adjusted in such a way that relay reset occurs at a frequency greater than the operating frequency by 0.3–0.5 Hz.

An increase in the reset-frequency value is achieved by reducing the active resistance of the second circuit [2, p. 79]. Then, when the frequency oscillates upward by 0.3–0.5 Hz, the contact system will be reliably closed. If, however, the frequency increases by an amount greater than the reset frequency, then the release of the contacts of the first-stage relay and its return to the initial position will be backed up by the selective operation of the second stage, with a longer time delay, whose frequency relay will hold a steady pulse.

To prevent false operations of the IVCh-OPA relay when it is energized by a surge or during sharp voltage fluctuations in the circuit of contacts 1RCh and 2RCh, an intermediate relay of type RP-251 is used, having an adjustable operating time delay within the range 0.07–0.12 sec.

The starting element can also receive an operating pulse from the contact pressure gauge KM, which responds to a reduction of pressure in the feed main below 120 ati. The main element of the program device is a direct-current motor time relay E-513 with a setting range of 6–60 sec [3, p. 96]. One normally closed contact serves to disconnect the motor after all contact circuits of the relay have operated. According to the kinematic conditions, the time delay of this contact must be greater than the time of the maximum relay setting by 0.5 sec. As is known, relays operating to open contacts are in more difficult conditions from the point of view of the contact system, and circuits with such contacts are unreliable. Therefore, the four normally closed contacts of the relay with a time delay must be remade as normally open.

In the circuits of the time-relay contacts, intermediate relays of type RP-23 with four normally open contacts and one normally closed contact are used for multiplying the output circuits. With a required relay power of 6 W and a contact capacity of E-513 and RP-23 relays of 50 W, up to eight such relays may, if necessary, be connected in parallel.

The output circuits of the program device include the contacts of the output intermediate relays and jumpers mounted on the panel. By means of the jumpers, the separation scheme for station service auxiliaries and individual consumer blocks is specified. These circuits are connected with the breaker-control circuits.

The ADU scheme is switched on by switch ShB, which has two positions: “automatic start” and “off.” For emergency manual starting of the scheme with a minimum time delay not exceeding 4 sec, a time relay 2RV is provided; in the circuit of its sliding contact, intermediate relay 5RP is connected, with a reset delay of up to 1 sec (relay of type RP-252). Manual starting of the ADU is carried out by turning the handle of switch 2PB to the “manual start” position.

The signaling monitors the serviceability of the alternating-current circuits from the 110 kV ZRU voltage transformers to the frequency relays and of the direct-current circuits (1LS), signals the starting of the ADU from the first stage (2LS), the second stage (3LS), and a reduction of pressure in the feed main below 120 ati (4LS).

OPERATION OF THE CIRCUIT. When the frequency drops below the settings of the starting elements, one or both frequency relays are energized, closing their contacts 2RCh or 2RCh and 1RCh. In this case the intermediate relay 2RPCh operates, or 1RPCh and 2RPCh operate simultaneously. The circuit of the time relay is closed: +ShUP, contacts 1PB, contacts 1RPCh (2RPCh), — ShUP. When voltage is applied to the electric motor and to the electromagnet, the time relay comes into operation and closes contacts with different time delays. Contact 1RV1 closes with the minimum possible delay—6 sec.; contact 1RV2—after 16 sec. (the value of this setting will be discussed below); contacts 1RV3 and 1RV4 close respectively after 18 sec. and 60 sec. (the time settings of stages I and II, determined by calculation). When relay 1RP operates, an impulse is applied through its contacts to switch on any two bus-section circuit breakers of the left and right wings of the 6 kV switchgear. The preliminary switching-on of 2 ShSMV is caused by the following circumstance: by connecting the sections of the left and right wings through the reserve bus system, their synchronous operation is achieved when, according to the splitting condition, the circuit breakers of transformers 11 t, 12 t, 13 t, 14 t on the 110 kV side are disconnected at the subsequent stage. This is why, despite the undesirability of “rigid” operation of the 6 kV switchgear generators, ShSMV should be switched on for a short time, thereby ensuring normal operations for splitting the circuit. When contact 1RV2 closes, the output intermediate relays 2RP1, 2RP2, 2RP3, 2RP4 operate, and the 10–20 shr bus-section circuit breaker is switched on, the circuit breakers of the sections of the second-stage TPP units are switched over to reserve supply from 10 shr and 20 shr, and the operating supply (soldering-off) of the units is disconnected. To ensure completion of the process, one of the contacts of relay (2RP1) provides self-holding. If, however, time relay 1RV drops out as a result of a rise in frequency, the impulse will not be applied to the intermediate relay; and the time-relay setting of 16 sec., close to the time setting of stage I, helps the preliminary splitting be completed as close as possible in time to the final splitting of the circuit. Indeed, if the time setting were smaller, say 10 sec., then when the frequency rises and 1RV drops out the impulse would be applied and splitting would occur at the second stage, although there is no need for this, since the disturbance of the operating condition will be restored by the frequency rise.

Upon the closing of contact 1RV3, after a time determined by the setting of stage I (18 sec.), the intermediate relays 3RP and 4RP are actuated. One of them (3RP) applies an impulse to disconnect the circuit breakers of transformers 11 t, 12 t, 13 t, 14 t on the 110 kV side, while the other—by closing contact 4RP—provides self-holding so that the process is completed. The presence of blocking contacts in this circuit, 1RPCh and RPD, is necessary to prohibit operation of stage I when the frequency drop calls for operation of only the second stage. When contact 1RV4 closes, the process proceeds analogously to the preceding one, with the sole difference that the final splitting of the circuit will occur 60 sec. after the start of the command impulse. The sequentially connected contact 2RPCh has the same meaning as 1RPCh, but only when there is a stage III. If it is necessary to start the ADU circuit manually, it is sufficient to place switch 2PB in the “manual start” position; then, upon instantaneous operation of 1RP, the first stage of preliminary splitting will take place. Operation of time relay 2RV energizes the intermediate relay

5RP through the sliding contact 2RV2, which, having a return time delay, will send a pulse to the intermediate relays 2RG of longer duration than the operating time of 2RP.

By the main contact 2RV1, with a time delay of 3 sec, the circuit of the intermediate relay 3RP is closed, and thereby the process of final division of the circuit is carried out. To prevent repeated starting of the ADS circuit in the event of sustained closure of the contacts of the frequency relays, paired, normally closed contacts of the intermediate relay 4RP are connected in series in the circuit of the electric motor of the time relay, for reliability. Preparation of the ADS circuit for further operation is carried out by opening the circuit with switch 1PB and then setting it to the “automatic start” position.

In addition to frequency starting, the circuit provides monitoring of the pressure in the supply main. When the pressure drops below 120 ati, the contact of the contact pressure gauge KM closes and the intermediate relay RPD operates; its contacts, on the one hand, shunt contact 1RPCh, sending a pulse to the intermediate relay 3RP provided that the time relay is put into operation by the closure of contact 2RPCh, and, on the other hand, switch on the annunciator panel, warning the operating personnel of the pressure drop.

With such a circuit, when the time relay is started from the second stage, final division can occur at any time delay from 18 to 60 sec, which depends on the degree of pressure reduction in the supply main. False operation of the ADS in the event of clogging of the impulse tubes of the contact pressure gauge, or if they are shut off by a valve, is thereby precluded, since a pulse can be given only when contact 1RV3 is closed, and for this the operation of the time relay due to the frequency reduction is required.

The division of the operation into three stages is caused not only by the technological necessity of the circuit’s operation, but also creates conditions for normal operation of the storage battery.

Advantages of the ADS Circuit

1. Reliability. The circuit eliminates a large number of circuits and contacts and, in addition, permits reserving the 1st stage by selective operation of the 2nd stage and by a pulse from pressure reduction. The presence of circuit elements operating on direct current also makes it reliable with respect to the supply condition.

2. Simplicity and compactness. The circuit uses the minimum possible amount of apparatus and relatively inexpensive apparatus. The installation fits on one standard protection panel or can be carried out in free spaces on other panels. To reduce cable jumpers to the output circuits of the switches, it is advisable to install the panel near the auxiliary-service panels.

3. Universality. The circuit is practically suitable for any division variant with minor additions and has manual starting. The output circuits of the ADS can be made for all switches that may participate in the division.

Disadvantages of the ADS Circuit

1. The circuit cannot be started automatically when the frequency drops to the setting of the second stage, 47.5 c/s (presence of a dead zone). In this case manual starting is required, if the regime is not restored for a long time.

is restored, and this is not always possible or feasible in a timely manner.

CONCLUSIONS

Considering the reasons that prompt the use of ADF at thermal power stations and analyzing the scheme, the following conclusions may be drawn:

1. ADF, by providing automatic starting, prevents disruption of the station auxiliaries both when frequency decreases because of an active-power deficit and as a result of a loss of stability in the parallel operation of power systems, and therefore is an effective emergency-control measure.

2. ADF should be used at high-pressure power stations, especially base-load stations that are part of a power system with weak intersystem ties. Power stations with unit schemes make the use of ADF especially necessary, since the processes occurring in the thermal-mechanical equipment there are more sensitive to decreases in frequency and voltage.

3. The operation of ADF is effective only when its starting is determined by frequency, voltage, and time settings obtained as a result of a joint calculation of the operating conditions of the electrical scheme and the thermal-mechanical equipment. In this case there can be no delay in ADF operation, and unjustified starts are eliminated.

4. The relative simplicity of the scheme makes it accessible, both in cost and in installation conditions, for use at any power station where it is needed.

5. A discussion in the press of ADF schemes is becoming desirable; such discussion will reveal the most suitable variants.

LITERATURE

1. Barkan Ya. D., Boyarevich V. Ya., On the application of dividing protections at power stations. Elektricheskie stantsii, No. 10, 1956.

2. Barzam A. B., System Automation, Gosenergoizdat, 1958.

3. Kukuevitskii L. I., Krupitskii A. Yu., Sakov A. D., Handbook of Relay Protection and Automation, Gosenergoizdat, 1962.