CN104218549A - Inter-phase short circuit protecting method for inverter-based distributed generation (IBDG) power distribution network - Google Patents

Abstract

The invention relates to an inter-phase short circuit protecting method for an inverter-based distributed generation (IBDG) power distribution network. The method includes the steps of firstly, three-section type negative-sequence current protection is set on the power side of each circuit, and no protection is set on the opposite side; when a fault is judged to be a sample space fault, the breakers on two sides of the circuit are tripped so as to isolate the fault; secondly, two-section type positive-sequence current protection is used to reflect three-phase symmetrical short circuit, the two-section type positive-sequence current protection and the three-section type negative-sequence current protection jointly form the main inter-phase short circuit protection of the power distribution network, when system short circuit allows a starting element to actuate, if the ratio R of negative-sequence current to positive-sequence current is larger than a threshold epsilon, unsymmetrical short circuit occurs, the negative-sequence protection is opened, and the positive-sequence protection is locked; or else, the negative-sequence protection is locked; thirdly the two-section type positive-sequence current protection reflecting the three-phase short circuit is still disposed on the power side, and no protection is disposed on the opposite side. The inter-phase short circuit protecting method is simple in composition, easy to implement and is unaffected by IBDG access.

Description

The phase fault guard method of distribution network comprising inverse distributed power

Technical field

The present invention relates to generation of electricity by new energy and the relay protection field of electric power system, be specifically related to a kind of phase fault guard method be applied to containing inverse distributed power distribution system.

Background technology

Along with the development of global economy and improving constantly quality of power supply requirement, the drawback of the large-scale networked electric power system of tradition " large-sized unit, bulk power grid, high voltage " also reveals gradually.Distributed generation technology because it is efficient, environmental protection, feature flexibly of powering, obtain increasing concern, obtain development at full speed.Meanwhile; as a large amount of distributed power source (Distributed Generation; DG) after accessing power distribution network; the design feature of conventional electrical distribution net single-side power, radial power supply changes; the change of network configuration directly has influence on selectivity and the sensitivity of the protective relaying device of existing power distribution network; cause the generation of the problem such as tripping, malfunction, bring negative effect to the power supply reliability of power distribution network.Access the adverse effect that may cause the existing protection philosophy of power distribution network and automation equipment for DG, Chinese scholars proposes the improvement project of many existing protections and new protection philosophy, is mainly divided into following several thinking:

(1) fault current of distributed power source access point is limited ^[1-3].This scheme generally at DG branch road serial connection current limiting reactor, weakens the adverse effect of DG increasing-aid current to protection by rational optimum configurations.This method ensures the selectivity protect and sensitivity by the electrical link weakening distributed power source and power distribution network, is faced with the problem to power distribution network support deficiency under fault condition.Meanwhile, higher requirement is proposed to the output characteristic of the net side converter of inverse distributed power.

(2) existing distribution protection scheme is improved ^[4-5].Document [4] propose based within the voltage source Equivalent Model of resistive, according to the running status of fault type and current system, the short circuit current that when calculating short circuit, system and distributed power source provide separately.Its main thought be first attempt to reduce the impact of distributed power source access on protection seting value, then traditionally current protection method is adjusted.But the transient potential in its Equivalent Model and substitutional resistance cannot calculate from service conditions, the Practical Project that have impact on program Equivalent Model is worth.Document [5] is pointed out when system three phase short circuit fault; the fault current variable quantity direct proportionality that inverse distributed power power stage variable quantity is experienced with protection; therefore inverse distributed power is equivalent to invariable power model; propose the adaptive guard scheme according to DG power output adjustment protection definite value; but the invariable power model that it adopts is comparatively large at alternate unbalanced fault time error, also needs perfect further.

(3) based on the intelligent protection of Wide-area Measurement Information ^[6-8].The general principle of wide area protection is that distribution system is divided into multiple isolated area, Agent or LED in each region processes local fault message respectively, by the information interaction comprehensive descision position of fault between Agent or LED.Document [6] is using the positive sequence component of fault current and negative sequence component amplitude sum as fault characteristic information, the electric current comprehensive magnitude size of more each branch, with the direction of advance that the positive direction of maximum branch is fault search, wide area communications sytem is utilized to search for fault zone.Document [8] proposes on the basis of existing directional current protection, and the protection I section of every for DG upstream bar feeder line and its next stage protection for feed line I section are formed a communication unit, according to the protection act result comprehensive descision fault of new setting principle and two-stage.This kind of protection is very large to the dependence of communication system, and complex structure, construction cost is higher.

Above-mentioned document is not all discussed to the control strategy of DG, and the different control strategies of DG will directly affect the setting principle of fault characteristic and protection.Therefore, be necessary to propose a kind of protection new departure taking into full account DG control characteristic, improve the security of operation containing DG power distribution network and power supply quality.

List of references

1, Wu Gang, Lu Yuping, Hua Lidan etc. distributed power generation adopts fault current limiter on the impact of relaying protection performance, Jiangsu electrical engineering .2007,26 (2): 1-4.

2, Wang Xizhou, Chen Xin, distributed power generation and the research of distribution protection harmony. relay, 2006,34 (3): 15-19.

3、Tang?G，Iravani?M?R.Application?of?a?fault?current?limiter?to?minimize?distributed?generation?impact?on?coordinator?relay?protection//Proceedings?of2005International?Conference?on?Power?Systems?Transients?June19223，2005，Mont?real，Canada：1582163.

4, Sun Jing ruthenium, Li Yongli, Li Shengwei etc. distribution network comprising inverse distributed power self-adaptive current fast tripping protection, Automation of Electric Systems, 2009.33 (1): 81-84,89.

5, Yu Qiong, Yu Sheng, Li Xiaohui. containing distributed power source distribution self adaptation electric protection scheme, protecting electrical power system and control, 2012,40 (5): 110-115.

6, Lin Xia, Lu Yuping, Wang Lianhe. the many power failures zone location new method under distributed power generation condition, electrotechnics journal, 2008,23 (11): 139-145,165.

7, Ma Jing, Wang Xi, rice is superfine. containing the power distribution network adaptive guard new method of distributed power source, and electric power network technique, 2011,35 (11): 204-208.

8, Zhang Yanxia, Dai Fengxian. containing protection for feed line new departure of distributed power source power distribution network, Automation of Electric Systems, 2009,33 (12): 71-74.

Summary of the invention

The object of the invention is, overcome the above-mentioned deficiency of prior art, provide a kind of and form simply, be easy to realize, do not access the phase fault guard method affected by IBDG.Consider containing the power distribution network based on positive sequence component power limitation control IBDG when there is phase fault; IBDG can be equivalent to three-phase positive sequence fundamental frequency controlled current source; only be included in positive sequence network; therefore IBDG flows to the negative-sequence current of electrical network and distribution not impact; so; in the power distribution network containing IBDG, configurable syllogic unbalanced fault protection reacts asymmetric phase fault; both there is higher sensitivity; what can solve again that IBDG accesses that power distribution network brings loses selective problems, and the setting value of unbalanced fault protection does not need along with whether the access of IBDG changes.

Technical scheme of the present invention is as follows:

A phase fault guard method for distribution network comprising inverse distributed power, comprises several aspect below:

(1) arrange syllogic unbalanced fault protection at the mains side of every bar circuit, offside does not install protection.When being judged as troubles inside the sample space, by the circuit breaker of tripping circuit both sides while of remote signal, thus isolated fault.Wherein,

Negative-sequence current I section setting value I ^Ι _2.opand operation time limit the principle pressing the not malfunction of bar line outlet unbalanced fault is adjusted;

The setting value I of unbalanced fault protection II section ^iI _2.opwith operation time limit coordinate with lower bar circuit I section:

Unbalanced fault protection III section operation time limit follows ladder principle, starting current adjust by avoiding the maximum negative-sequence current of normal operation;

(2) adopt the short circuit of two-part positive sequence directional current protection reflection three-phase symmetrical, together constituted the phase fault main protection of power distribution network by two-part positive sequence directional current protection and syllogic unbalanced fault protection.After system short-circuit makes starting component action, if negative-sequence current is greater than threshold value ε with the ratio R of forward-order current, be then judged to be unsymmetrical short-circuit, open passive moods, locking positive sequence direction protection; Otherwise then locking passive moods, open positive sequence directional current protection; Threshold value ε gets 0.2 ~ 0.4; In addition, after starting component, add the time delay of certain hour, to ensure the accuracy that fault type judges, and escape transient process and the circuit breaker three-phase breakers asynchronous closing of IBDG;

(3) the two-part positive sequence directional current protection reacting three-phase shortcircuit is still installed in mains side, and offside does not install protection; I, II section of each positive sequence directional current protection all needs to form communication unit with the positive sequence directional current protection of lower bar circuit, and the actuating signal of following bar circuit positive sequence directional current protection I section or II section is as block signal; If this circuit I, II section action and do not receive block signal, be then judged as troubles inside the sample space, by remote signal tripping simultaneously bilateral circuit breaker, isolated fault; If this route protection receives block signal, be then shown to be outside protection zone and three-phase shortcircuit occurs, do not send trip signal, wherein,

Positive sequence directional current protection I section setting value I ^Ι _1.opadjust according to this line end three-phase fault action message; II segment value I ^iI _1.opadjusting by escaping maximum load current, adopting stairstepping operation time limit, upwards coordinating step by step by line end.

Preferably, negative-sequence current I section setting value I ^Ι _2.opand operation time limit adjust by following formula:

$\left\{\begin{array}{c}{I^I}_{2.\mathrm{op}}=K_{\mathrm{rel}}^I{I}_{2.\max }\\ t_{\mathrm{op}}^I=0\end{array}\right.$

In formula: I _2.maxfor the maximum negative-sequence current of this line end phase-to phase fault, safety factor get 1.2 ~ 1.3.

The setting value I of unbalanced fault protection II section ^iI _2.opwith operation time limit adjust by following formula:

In formula: I ⁱ _{under 2.op}for next circuit negative-sequence current I section setting value, K _2.bminfor negative-sequence current braning factor minimum value, safety factor get 1.1, Δ t gets 0.3 ~ 0.5s.

Unbalanced fault protection III section starting current adjust by following formula:

$I_{2.\mathrm{op}}^{\mathrm{III}}=0.2I_N$

In formula: I _nfor protection installation place rated load current.Adopt stairstepping operation time limit, upwards coordinate step by step by line end.

Positive sequence directional current protection I section setting value I ^Ι _1.opadjust by following formula:

$\left\{\begin{array}{c}{I^I}_{1.\mathrm{op}}=K_{\mathrm{rel}}^I{I}_{\min }^{\left(3\right)}\\ t_{\mathrm{dz}}^I=0\end{array}\right.$

In formula: safety factor get 0.9, for the minimum forward-order current of line end three-phase shortcircuit.

Positive sequence directional current protection II section setting value I ^iI _1.opadjust by following formula:

${I^{\mathrm{II}}}_{1.\mathrm{op}}=\frac{K_{\mathrm{rel}}^{\mathrm{II}}{K}_{\mathrm{ms}}}{K_{\mathrm{re}}}{I}_{L-\max }$

In formula, I _l-maxmaximum load current when expression system is normally run; Safety factor get 0.9; Self-starting COEFFICIENT K _msfor being greater than the constant of 1, concrete numerical value is determined by load character; K _refor drop-off to pick-up ratio.

Beneficial effect of the present invention is as follows:

(1) unbalanced fault protection only installs protection at mains side, forms simple, gets final product the alternate unbalanced fault of reaction system, and have very high sensitivity according to local electrical quantities.

(2) what unbalanced fault protection can solve that IBDG accesses that power distribution network brings loses selective problems, and setting value does not need along with whether the access of IBDG changes.

(3) forward-order current protection I section can protect this total track length, and in protection range any point break down all can instantaneous tripping, its setting value does not need along with whether the access of IBDG changes.

(4) protection scheme only need, in circuit mains side installing protection, form simple; Protection act clear principle is clear and definite, is easy to realize in Microcomputer Protection.

Accompanying drawing explanation

Fig. 1 P-Q control principle.

Fig. 2 IBDG transient magnetic.

Fig. 3 protection act theory diagram.

Embodiment

Below in conjunction with accompanying drawing and example, the present invention will be described.

Grid-connected IBDG DC voltage is approximately steady state value, and its transient characterisitics determine primarily of control mode.Current Control is as shown in Figure 1 the control strategy of current main flow, and directly the electric current of control IBDG exports, and its AC voltage is then complete to be determined by system.Three-phase grid-connected inverter adopts the line voltage vector control mode (voltageorientedcontrol, VOC) based on synchronous rotating frame, can realize the uneoupled control of real component and idle component, and namely under Park coordinate system, IBDG meets:

$\left\{\begin{array}{c}p=e_d{i}_d\\ q=e_d{i}_q\end{array}\right.---\left(1\right)$

In formula: p, q are meritorious, the idle output of IBDG; e _dfor points of common connection (point of common coupling, PCC) AC voltage; i _d, i _qfor output current is meritorious, idle component.

Actual line voltage is not ideal sine wave, and harmonic component makes phase-locked precision be affected, thus reduces the control performance of IBDG.The negative sequence component that system jam produces is at controlled quentity controlled variable i _d, i _qin can produce second harmonic, cause meritorious and idle output all containing harmonic.To this, can adopt the control program of two d-q axle, be positive sequence component and negative sequence component by system voltage Current Decomposition, controls respectively rotating and reverse under synchronous coordinate system.Because positive-negative sequence component is DC quantity under respective coordinate system, desirable control effects can be obtained by pi regulator, more effectively can eliminate negative phase-sequence in IBDG output current and harmonic components by filtering measures.Therefore, during system side generation phase-to phase fault, the fault current of IBDG is determined by the positive sequence component of PCC point voltage completely, can be equivalent to positive sequence fundamental frequency controlled current source and process.

There is transient process in instant of failure IBDG, reenters controllable state through 3 ~ 5 cycles, and its power stage gets back to reference value.Due to, the energy changing of IBDG has certain inertia, can think that the reference value of its active power does not change before and after fault; Its reactive power support can increase reactive current with support system voltage.Therefore, the IBDG fault stable state adopting two d-q to control is meritorious with reactive current is:

$\left\{\begin{array}{c}{i}_d^f==\frac{{U^{+}}_{\mathrm{PCC}-n}}{{U^{+}}_{\mathrm{PCC}-f}}{i}_{d-n}\\ i_q^f=i_{q-n}+K_q\frac{{U^{+}}_{\mathrm{PCC}-n}-{U^{+}}_{\mathrm{PCC}-f}}{{U^{+}}_{\mathrm{PCC}-n}}\end{array}\right.---\left(2\right)$

In formula: i _d-n, i _q-nmeritorious, reactive current during for normally running, U ⁺ _pCC-n, U ⁺ _pCC-ffor normally run and fault time PCC point positive sequence voltage, K _qfor reactive power support coefficient.

Analysis mode (2) is known, i _d-n, i _q-nand U ⁺ _pCC-nfor the operational factor under normal load conditions, K _qfor the controling parameters pre-set, in accident analysis, all can be considered constant, then the fault steady-state current of IBDG only and U ⁺ _pCC-frelevant, its large young pathbreaker is with U ⁺ _pCC-fchange and change.Therefore, IBDG can be equivalent to a three-phase positive sequence fundamental frequency controlled current source, as shown in Figure 2.

If the false voltage of PCC point then the fault steady-state current of IBDG is:

${\stackrel{\·}{I}}_{\mathrm{DG}-f}=I_m\∠(\mathrm{\δ}+\mathrm{\θ})---\left(3\right)$

In formula: $I_m=\sqrt{i_d^{f^2}+i_q^{f^2}},\mathrm{\θ}=\arctan (i_q^f/i_d^f).$

There is above-mentioned analysis known: when containing the power distribution network generation phase-to phase fault based on positive sequence component power limitation control IBDG, IBDG can be equivalent to three-phase positive sequence fundamental frequency controlled current source, is only included in positive sequence network, on the flow direction and the distribution not impact of electrical network negative-sequence current.Therefore, containing IBDG power distribution network in configurable syllogic unbalanced fault protection carry out unbalanced fault between reacting phase, both there is higher sensitivity, can solve again IBDG access power distribution network bring lose selective problems.

Accordingly, the present invention proposes following protection scheme:

1, arrange syllogic unbalanced fault protection at the mains side of every bar circuit, offside does not install protection.When being judged as troubles inside the sample space, by the circuit breaker of tripping circuit both sides while of remote signal, thus isolated fault.

Negative-sequence current I section setting value I ^Ι _2.opand operation time limit the principle pressing the not malfunction of bar line outlet unbalanced fault is adjusted.

$\left\{\begin{array}{c}{I^I}_{2.\mathrm{op}}=K_{\mathrm{rel}}^I{I}_{2.\max }\\ t_{\mathrm{op}}^I=0\end{array}\right.---\left(4\right)$

In formula: I _2.maxfor the maximum negative-sequence current of this line end phase-to phase fault, safety factor get 1.2 ~ 1.3.

The definite value I of unbalanced fault protection II section ^iI _2.opwith operation time limit coordinate with lower bar circuit I section.

In formula: I ⁱ _2.opbe down next circuit negative-sequence current I section setting value, K _2.bminfor negative-sequence current braning factor minimum value, safety factor get 1.1, Δ t gets 0.3 ~ 0.5s.

Unbalanced fault protection III section operation time limit follows ladder principle, starting current adjust by avoiding the maximum negative-sequence current of normal operation.

$I_{2.\mathrm{op}}^{\mathrm{III}}=0.2I_N---\left(6\right)$

In formula: I _nfor protection installation place rated load current.

2, because syllogic unbalanced fault protection can only reflect asymmetric phase fault, so the present invention adopts two-part positive sequence directional current protection to reflect three-phase symmetrical short circuit.So, two-part positive sequence directional current protection and syllogic unbalanced fault protection together constitute the phase fault main protection of power distribution network, and the logic matching relationship of the two as shown in Figure 3.After system short-circuit makes starting component action, if negative-sequence current is greater than threshold value ε with the ratio R of forward-order current, be then judged to be unsymmetrical short-circuit, open passive moods, locking positive sequence direction protection; Otherwise then locking passive moods, open positive sequence direction protection.During system generation line to line fault, R ⁽²⁾=1; When there is two-phase grounding fault, 0.5<R ^(1.1)=Z _{0 Σ}/ (Z _{2 Σ}+ Z _{0 Σ}) < 1.In order to ensure reliability, threshold value ε gets 0.2 ~ 0.4.In addition, may there is asymmetric process in short-term in the three-phase shortcircuit initial stage, add the time delay of 0.1s after starting component, ensures the accuracy that fault type judges can escape again transient process and the circuit breaker three-phase breakers asynchronous closing of IBDG.

3, owing to comprising IBDG in positive sequence network, its increasing-aid current makes the protection based on local electrical quantities be difficult to meet selectivity requirement, needs by certain means of communication.For this reason, the two-part positive sequence directional current protection of reaction three-phase shortcircuit is still installed in mains side, and offside does not install protection.I, II section of each positive sequence directional current protection all needs to form communication unit with the positive sequence directional current protection of lower bar circuit, and the actuating signal of following bar circuit positive sequence directional current protection I section or II section is as block signal.If this circuit I, II section action and do not receive block signal, be then judged as troubles inside the sample space, by remote signal tripping simultaneously bilateral circuit breaker, isolated fault.If this route protection receives block signal, be then shown to be outside protection zone and three-phase shortcircuit occurs, do not send trip signal.

Positive sequence directional current protection I section definite value is adjusted according to this line end three-phase fault action message.

$\left\{\begin{array}{c}{I^I}_{1.\mathrm{op}}=K_{\mathrm{rel}}^I{I}_{\min }^{\left(3\right)}\\ t_{\mathrm{dz}}^I=0\end{array}\right.---\left(7\right)$

In formula: safety factor get 0.9, for the minimum forward-order current of line end three-phase shortcircuit.

Positive sequence directional current protection II section setting value I ^iI _1.opadjust by following formula:

${I^{\mathrm{II}}}_{1.\mathrm{op}}=\frac{K_{\mathrm{rel}}^{\mathrm{II}}{K}_{\mathrm{ms}}}{K_{\mathrm{re}}}{I}_{L-\max }$

In formula, I _l-maxmaximum load current when expression system is normally run; Safety factor get 0.9; Self-starting COEFFICIENT K _msfor being greater than the constant of 1, concrete numerical value is determined by load character; K _refor drop-off to pick-up ratio.Adopt stairstepping operation time limit, upwards coordinate step by step by line end.

Claims (6)

1. a phase fault guard method for distribution network comprising inverse distributed power, comprises several aspect below:

(1) arrange syllogic unbalanced fault protection at the mains side of every bar circuit, offside does not install protection; When being judged as unsymmetrical short-circuit in district, by the circuit breaker of tripping circuit both sides while of remote signal, thus isolated fault.Wherein,

Negative-sequence current I section setting value I ^Ι _2.opand operation time limit the principle pressing the not malfunction of bar line outlet unbalanced fault is adjusted;

The setting value I of unbalanced fault protection II section ^iI _2.opwith operation time limit coordinate with lower bar circuit I section:

Unbalanced fault protection III section operation time limit follows ladder principle, starting current adjust by avoiding the maximum negative-sequence current of normal operation;

(2) short circuit of two-part positive sequence directional current protection reflection three-phase symmetrical is adopted, the phase fault main protection of power distribution network is together constituted by two-part positive sequence directional current protection and syllogic unbalanced fault protection, after system short-circuit makes starting component action, if negative-sequence current is greater than threshold value ε with the ratio R of forward-order current, then be judged to be unsymmetrical short-circuit, open passive moods, locking positive sequence direction protection; Otherwise then locking passive moods, open positive sequence directional current protection; Threshold value ε gets 0.2 ~ 0.4; In addition, after starting component, add the time delay of certain hour, to ensure the accuracy that fault type judges, and escape transient process and the circuit breaker three-phase breakers asynchronous closing of IBDG;

(3) the two-part positive sequence directional current protection reacting three-phase shortcircuit is still installed in mains side, and offside does not install protection; I, II section of each positive sequence directional current protection all needs to form communication unit with the positive sequence directional current protection of lower bar circuit, and the actuating signal of following bar circuit positive sequence directional current protection I section or II section is as block signal; If this circuit I, II section action and do not receive block signal, be then judged as troubles inside the sample space, by remote signal tripping simultaneously bilateral circuit breaker, isolated fault; If this route protection receives block signal, be then shown to be outside protection zone and three-phase shortcircuit occurs, do not send trip signal, wherein,

Positive sequence directional current protection I section setting value I ^Ι _1.opadjust according to this line end three-phase fault action message; II section setting value I ^iI _1.opadjusting by escaping maximum load current, adopting stairstepping operation time limit, upwards coordinating step by step by line end.

2. phase fault guard method according to claim 1, is characterized in that, negative-sequence current I section setting value I ^Ι _2.opand operation time limit adjust by following formula:

$\left\{\begin{array}{c}{I^I}_{2.\mathrm{op}}=K_{\mathrm{rel}}^I{I}_{2.\max }\\ t_{\mathrm{op}}^I=0\end{array}\right.$

In formula: I _2.maxfor the maximum negative-sequence current of this line end phase-to phase fault, safety factor get 1.2 ~ 1.3.

3. phase fault guard method according to claim 1, is characterized in that, the setting value I of unbalanced fault protection II section ^iI _2.opwith operation time limit adjust by following formula:

In formula: I ⁱ _{under 2.op}for next circuit negative-sequence current I section setting value, K _2.bminfor negative-sequence current braning factor minimum value, safety factor get 1.1, Δ t gets 0.3 ~ 0.5s.

4. phase fault guard method according to claim 1, is characterized in that, unbalanced fault protection III section starting current adjust by following formula:

$I_{2.\mathrm{op}}^{\mathrm{III}}=0.2I_N$

In formula: I _nfor protection installation place rated load current.

5. phase fault guard method according to claim 1, is characterized in that, positive sequence directional current protection I section setting value I ^Ι _1.opadjust by following formula:

$\left\{\begin{array}{c}{I^I}_{1.\mathrm{op}}=K_{\mathrm{rel}}^I{I}_{\min }^{\left(3\right)}\\ t_{\mathrm{dz}}^I=0\end{array}\right.$

In formula: safety factor get 0.9, for the minimum forward-order current of line end three-phase shortcircuit.

6. phase fault guard method according to claim 1, is characterized in that, positive sequence directional current protection II section setting value I ^iI _1.opadjust by following formula:

${I^{\mathrm{II}}}_{1.\mathrm{op}}=\frac{K_{\mathrm{rel}}^{\mathrm{II}}{K}_{\mathrm{ms}}}{K_{\mathrm{re}}}{I}_{L-\max }$

In formula, I _l-maxmaximum load current when expression system is normally run; Safety factor get 0.9; Self-starting COEFFICIENT K _msfor being greater than the constant of 1, concrete numerical value is determined by load character; K _refor drop-off to pick-up ratio.